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N-vinylation and N-allylation of 3,5-disubstituted pyrazoles by N–H insertion of vinylcarbenoids

  • Agagia Gill , Udo R. Werz and Gerhard Maas EMAIL logo
Published/Copyright: September 24, 2015
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Zeitschrift für Naturforschung B
From the journal Zeitschrift für Naturforschung B Volume 70 Issue 10

Abstract

A vinylcarbenoid approach toward N-functionalization of NH-pyrazoles is presented. The rhodium(II)-catalyzed reaction of methyl styryl-diazoacetate (1) or dimethyl 2-diazoglutaconate (3) with 3,5-disubstituted pyrazoles gave products of carbenoid N–H insertion in high combined yields, although regioselectivity issues posed by the pyrazole or the vinylcarbenoid moiety as well as positional and configurational isomerism concerning the C,C double bond of the latter led to product mixtures. The ambident reactivity of the vinylcarbenoid derived from 1 could be steered by the catalyst: while Rh2(OAc)4 yielded products of direct carbenoid insertion preferentially, silver(I) catalysis strongly favored reaction at the vinylogous site of the carbenoid resulting in an N-allylation of the pyrazoles.

Keywords: N–H insertion; pyrazoles; transition-metal catalysis; vinylcarbenoids; vinyldiazo compounds

1 Introduction

1H-Pyrazoles are among the most well-known aromatic azaheterocycles. Although compounds containing a pyrazole ring are not abundant in nature, a variety of bioactive pyrazoles are known [1]. Several pyrazole derivatives are in therapeutic use, in particular N,C,C-trisubstituted ones. Furthermore, pyrazole derivatives play a role in crop protection [2]. In the search of novel pyrazole derivatives with interesting bioactivities or for other applications, the synthetic chemist can rely on a range of synthetic methods, the most prominent of which are the reaction of 1,3-dicarbonyl compounds with hydrazine or substituted hydrazines, 1,3-dipolar cycloaddition reactions of alkynes or their synthetic equivalents and diazo compounds or nitrilimines, and several types of ring transformations [3].

A less common approach to pyrazoles starts from vinyldiazo compounds. The majority of known vinyldiazo compounds are characterized by their facile isomerization to 1H-pyrazoles through a thermally induced electrocyclic 1,5-cyclization leading to a 3H-pyrazole, followed by a sigmatropic shift to yield the aromatic 1H-pyrazole. While this long known transformation [4–6] constitutes a convenient pyrazole synthesis from diversely substituted vinyl-diazoacetates [7–11], it is often an unwanted side reaction in syntheses based on transition-metal catalyzed carbene transfer reactions from such vinyldiazo compounds to a broad range of substrates (cyclopropanation, insertion into N–H, O–H and C–H bonds and more [12]). Diazo compounds 1 and 3 are two examples which have recently been employed frequently for carbenoid transformations, and their isomerization to pyrazoles 2 and 4 (Scheme 1), respectively, has been noticed on several occasions (for examples, see refs. [8, 13] for 1 and refs. [9, 10] for 3). As one would expect, the amount of pyrazole increases, when the carbenoid transformation is carried out at elevated temperature or when it is slow at room temperature.

Scheme 1: Isomerization of vinyldiazo compounds 1 and 3 into 1H-pyrazoles 2 and 4.
Scheme 1:

Isomerization of vinyldiazo compounds 1 and 3 into 1H-pyrazoles 2 and 4.

In synthetic work with the styryl-diazoacetate 1, we observed sometimes the formation of minor products, which were identified as insertion products of the carbene moiety of 1 into the N–H bond of the tautomeric forms of pyrazole 2. This motivated us to explore whether this reaction behavior could be developed into a synthetically useful approach to N-vinylpyrazoles; the results of this study are reported herein.

The preparation of N-alkylpyrazoles from NH-pyrazoles or their deprotonated forms with alkyl electrophiles or diazomethane has been investigated in numerous studies (see, e.g. refs. [14–16]), and it has been recognized that for unsymmetrically substituted pyrazoles the regioselectivity (N1 vs. N2 alkylation), which depends on reaction conditions, nature of the alkylating reagent, electronic and steric factors, can often not be controlled satisfactorily [17]. Some highly regioselective N-alkylations are kown, however (for examples see refs. [18–22]).

Insertion of carbene moieties into the N–H bond of pyrazoles appears as another viable approach to N-substituted pyrazoles. Although there is ample precedence for the effective carbenoid insertion into N–H bonds of primary and secondary amines by transition-metal catalyzed reactions of diazo compounds (for selected papers, see refs. [23–27]) including styryl-diazoester 1 [28], analogous N–H insertion reactions of azoles are rare. With NH-pyrroles or NH-indoles, the electrophilic metal-carbene intermediates prefer to react at ring positions C-2 and C-3 [29–32]; the rhodium(II)-catalyzed N–H insertion of bis(methoxycarbonyl)carbene with 3-methylindole [33], an intramolecular analog thereof [34] and the reaction of methoxycarbonyl(trifluoromethyl)carbene with indole [35], all three occurring as low-yielding side-reactions, are exceptions. The N-(tert-butyl)hydroxylamine promoted, rhodium(II)-catalyzed insertion of the carbene unit of a styryl-diazoacetate into the N–H bond of 3-methylindole has been reported and has been extended to tryptophane labelling in proteins [36, 37].

In this paper, we show that vinyl-diazoacetates can be used for a carbenoid insertion into the N-H bond of pyrazoles to give N-vinyl and/or N-allyl substituted pyrazoles and we uncover the regioselectivity issues of these transformations.

2 Results and discussion

We have studied intermolecular carbenoid N–H insertion reactions of the two vinyl-diazoacetates 1 and 3 with two NH-pyrazoles, namely the symmetrical 3,5-diphenyl-1H-pyrazole (5) and the unsymmetrically substituted methyl 5-phenyl-1H-pyrazole-3-carboxylate (2). Only in the case of pyrazole 2, which can exist in two tautomeric NH forms, a regioselectivity issue is present and indeed gave rise to the formation of N1 and N2 substitution products. It should be noted at this point that 2 exists as the 3-CO2Me-5-Ph-tautomer in the solid state [38].

2.1 Reactions of styryl-diazoacetate 1

In preliminary experiments, we found that better product yields were obtained when the carbenoid reactions of styryl-diazoacetate 1 were conducted at 0 °C rather than at elevated temperatures. To compensate for the increased reaction times, a larger amount (4.4–5.6 mol%) than usual of the catalyst, dirhodium(II) tetraacetate, was applied.

From the reaction of styryl-diazoacetate 1 and pyrazole 5 at 0 °C, catalyzed by Rh2(OAc)4, four products of carbene insertion into the N–H bond of the pyrazole could be isolated after column chromatography in a combined yield of about 83 % (Scheme 2). The two major products are the expected carbene insertion product (E-6a, 43 %) and its double-bond shifted positional isomer (Z-6b, 18 %). In addition, pyrazole E-7a was obtained, which results from the so-called vinylogous reactivity of the metal-carbene complex derived from diazoacetate 1. The fourth product is most likely the carbene insertion product Z-7a (see below for the structural assignment).

Scheme 2: Rh(II)- or Ag(I)-catalyzed reactions of styryl-diazoacetate 1 and pyrazole 5.
Scheme 2:

Rh(II)- or Ag(I)-catalyzed reactions of styryl-diazoacetate 1 and pyrazole 5.

The vinylogous reactivity of rhodium carbenoids derived from vinyl-diazoacetates, including those without [39, 40] and those with [29, 32, 41, 42] a substituent at the vinyl terminus, is known and has been discussed. Among other factors, this vinylogous carbenoid reactivity is favored with bulky substrates and bulky ligands on the rhodium catalyst [29, 32], furthermore by the use of diruthenium(I,I) [43] and silver catalysts. Hu [41] and Davies [44] have reported, that the switch from Rh(II)- to Ag(I)-catalyzed reactions of vinyl-diazoacetates strongly enhances the vinylogous over the carbenoid reactivity. Thus, the high regioselectivity of the dirhodium(II) tetraacetate catalyzed O–H insertion with various primary alcohols at the carbenic site of the vinylcarbenoid is completely reversed in favor of the vinylogous site when the reaction is catalyzed by silver(I) triflate. With aniline as the substrate, the regioselectivity change is less expressed [44]. A DFT computational study has revealed that the vinylogous reaction proceeds through silver(I) vinylcarbene intermediates and not on a Lewis acid-catalyzed pathway [44].

In fact, when the reaction of diazoester 1 and pyrazole 5 was catalyzed with silver(I) triflate, the amount of the vinylogous carbene insertion product E-7a increased significantly (51 % yield after chromatographic work-up; 77 % yield based on consumed pyrazole 5), and the direct carbene insertion products were not found (Scheme 2). Under the reaction conditions (refluxing dichloromethane, excess of 1), a considerable amount of the diazo compound underwent cycloisomerization to give pyrazole 2.

The rhodium(II)-catalyzed reaction of styryl-diazoacetate 1 and the unsymmetrically substituted pyrazole 2 at 0 °C furnished the N–H insertion products E-8a and E-8b in high combined yield but with modest regioselectivity in favor of the 5-phenylpyrazole-3-carboxylate isomer (E-8a: E-8b = 1.5) (Scheme 3). When the reaction was catalyzed with silver(I) triflate, the vinylogous carbenoid reaction was triggered again and the regioselectivity of the N–H insertion was reversed in favor of the 3-phenylpyrazole-5-carboxylate isomer (E-9b: E-9a = 5.4). From the amount of pyrazole 2 recovered after the reaction, it can be concluded that ~40 % of diazoester 1 had undergone isomerization to form pyrazole 2 under the reaction conditions.

Scheme 3: Reactions of styryl-diazoacetate 1 with pyrazole 2.
Scheme 3:

Reactions of styryl-diazoacetate 1 with pyrazole 2.

Since pyrazole 2 is formed from styryl-diazoacetate 1 by thermal 1,5-electrocyclization, it should be possible to obtain the products of the subsequent carbenoid N–H insertion directly from 1 in a one-pot reaction, provided that the second reaction step can kinetically compete with the first one. When diazoester 1 was treated with a catalytic amount of Rh2(OAc)4 in refluxing 1,2-dichloroethane, the carbenoid pyrazole N–H insertion products E-8a (24 %) and E-8b (34 %) were indeed obtained in a medium combined yield after chromatographic work-up (Scheme 4). Besides, a mixture of unidentified decomposition products of the diazo compound was formed.

Scheme 4: Rh(II)-catalyzed decomposition of styryl-diazoacetate 1.
Scheme 4:

Rh(II)-catalyzed decomposition of styryl-diazoacetate 1.

2.2 Reactions with dimethyl 2-diazoglutaconate (dimethyl 2-diazopent-3-enedioate) (3)

The rhodium(II)-catalyzed reaction of diazodiester 3 with 3,5-diphenylpyrazole (5) yielded the carbenoid N–H insertion product Z-10 in excellent yield (Scheme 5). From the reaction of 3 with the unsymmetrical pyrazole 4, the analogous insertion products Z-11a and Z-11b were obtained in a high combined yield (83 %) (Scheme 5). The two N-substituted pyrazole isomers are formed with a higher regioselectivity (Z-11a: Z-11b = 2.6) than in the case of N–H insertion with styryl-diazoacetate 1; it appears that the higher electrophilicity of the rhodium carbene complex derived from 3 is responsible for the increased selectivity.

Scheme 5: Rh(II)-catalyzed reactions of pyrazoles 4 and 5 and diazoacetate 3.
Scheme 5:

Rh(II)-catalyzed reactions of pyrazoles 4 and 5 and diazoacetate 3.

The structure of the carbene moiety in 10 and 11 is the same in all cases. A double bond shift to the inserting carbon atom has occurred, resulting in an N-vinylation of the pyrazole. The Z-configuration of the olefinic double bond is confirmed by a crystal structure analysis of Z-11b and NOE experiments on Z-11a (see below).

2.3 Structural assignments

Several structural aspects of the new pyrazoles have to be considered. X-ray crystal structure determinations of E-8a and Z-11b, shown in Figs. 1 and 2, indicate the substitution sites in the pyrazole ring and the double bond configuration in the vinylic or allylic side chain for these two compounds.

Fig. 1: Molecular structure of E-8a in the solid state (ortep plot). Only one of the two symmetry-independent molecules in the unit cell is shown. Torsion angles (deg); the values for the other molecule are given after the slash: C2–C3–C9–C10 –38.0(8)/–38.2(8), C3–N1–C4–C15 121.2(6)/123.7(6), C2–C1–C7–O1 5.5(9)/3.6(8).
Fig. 1:

Molecular structure of E-8a in the solid state (ortep plot). Only one of the two symmetry-independent molecules in the unit cell is shown. Torsion angles (deg); the values for the other molecule are given after the slash: C2–C3–C9–C10 –38.0(8)/–38.2(8), C3–N1–C4–C15 121.2(6)/123.7(6), C2–C1–C7–O1 5.5(9)/3.6(8).

Fig. 2: Molecular structure of Z-11b in the solid state (ortep plot). Torsion angles (deg): C2–C3–C13–C14 –2.0(3), N1–N2–C4–C9 112.8(2), C2–C1–C11–O5 –4.5(3).
Fig. 2:

Molecular structure of Z-11b in the solid state (ortep plot). Torsion angles (deg): C2–C3–C13–C14 –2.0(3), N1–N2–C4–C9 112.8(2), C2–C1–C11–O5 –4.5(3).

Characteristic NMR chemical shifts of some of the prepared pyrazoles are shown in Fig. 3. In the 1H NMR spectra, the E configuration of the 1,2-disubstituted C=C bond of 6a, 7a, 8a, 8b, 9a, 9b is indicated by the large value of the olefinic 3J coupling constant (15.7–16.0 Hz); in Z-7a the 3J(cis) coupling constant amounts to only 11.4 Hz. On the other hand, the configuration at the N-vinyl double bond in compounds Z-6b, Z-10, Z-11a and Z-11b is less obvious from the NMR data. However, the observation of a NOE correlation between the CH2 and the pyrazole-H protons of 11a is expected only for the Z-configuration of the olefinic double bond and an almost perpendicular conformation at the N–Colefin bond (compare Fig. 2 for Z-11b). By analogy to Z-11a and Z-11b, the same configuration is also assumed for 6b.

Fig. 3: Selected 1H/13C NMR chemical shifts (δ/ppm, in CDCl3) of some pyrazoles.
Fig. 3:

Selected 1H/13C NMR chemical shifts (δ/ppm, in CDCl3) of some pyrazoles.

The identification of pyrazole Z-7a is based on several arguments. While the composition as a carbene insertion product is secured by mass spectra and elemental analysis, the positions of Ph and CO2Me at the allyl moiety are not immediately obvious from the 1H and 13C NMR spectra and furthermore, the signal assignment by 2D techniques is hampered mainly because two protons of the allyl moiety are covered by signals of the phenyl protons. The presence of an acrylate rather than a styryl moiety is indicated by the large chemical shift difference for the olefinic carbon atoms (δ = 119.5 and 145.4 ppm, Δδ = 25.9 ppm; Δδ ~11 pm is expected for styryl carbon atoms, see Fig. 3). The high chemical shift of the allylic proton (δ = 7.47 ppm) is unusual, but does not differ much from the value found for E-9b; it appears that in both cases, a conformation around the C(pyrazole)–C(allyl) bond is fixed which exposes the allylic C–H bond to the deshielding areas of magnetic anisotropy of the phenyl and/or pyrazole rings.

The substitution pattern of the pyrazole ring, i. e. the regioselectivity of the carbenoid N–H insertion, can be derived without doubt from the chemical shift of the pyrazole proton, which is deshielded in the CH=C–CO2Me as compared to the CH=C–Ph environment. Similarly, the 13C signal of the imine-type carbon atom is shielded when a phenyl group is attached and deshielded when a CO2Me group is attached, compared to the phenyl-substituted case. For both arguments, the data of Z-11b, the crystal structure of which was determined, can serve as a reference.

3 Conclusion

In this study we have shown that vinyl-diazoacetates 1 and 3 undergo an effective rhodium(II)-catalyzed carbenoid insertion into the N–H bond of pyrazoles. However, mixtures of several isomers were formed and had to be separated, which are the result of several selectivity issues. Thus, regioisomeric pyrazoles are formed when the pyrazole substrate is unsymmetrically substituted, and the vinylcarbenoid brings in additional regioselectivity problems, as it displays both the expected carbenoid and the vinylogous carbenoid reactivity. In addition, the C,C double bond in the vinylcarbenoid part introduces E,Z isomerism as well as positional isomerism. In the silver(I) triflate catalyzed reaction of styryl-diazoacetate 1, on the other hand, the complexity is reduced, because the vinylogous reactivity dominates by far over the direct carbenoid N–H insertion, and for the specific combination of 1 and pyrazole 2, the regioselectivity at the pyrazole ring is also increased.

4 Experimental section

4.1 General information

1H and 13C NMR spectra were recorded on a Bruker Avance 400 spectrometer (1H: 400.13 MHz; 13C: 100.62 MHz); δ values are reported in ppm and coupling constants are expressed in Hertz (Hz). The signal of the solvent was used as an internal standard: δ(CHCl3) = 7.26 ppm, δ(CDCl3) = 77.0 ppm. When necessary, 13C signal assignments were derived from C,H COSY, HSQC and HMBC spectra. IR spectra: Bruker Vector 22; wavenumbers (cm–1) and intensities (vs = very strong, s = strong, m = medium, w = weak, br = broad) are given. Elemental analyses: elementar vario MICRO cube; values are given in percent. Mass spectra: Finnigan MAT, SSQ-7000 (CI mode, 100 eV). Melting points were determined on a Büchi melting point B-540 apparatus. All reactions were carried out under an argon atmosphere, solvents were dried by standard methods and stored under argon. Column chromatography was performed on silica gel 60 (Merck, 0.063–0.200 mm).

4.2 Materials

(E)-Methyl 2-diazo-4-phenylbut-3-enoate (1) [45], methyl 5-phenyl-1H-pyrazole-3-carboxylate (2) [46] and 3,5- diphenyl-1H-pyrazole (5) [47, 48] were prepared by published procedures. (E)-Dimethyl 4-diazopent-2-enedioate (3) was prepared by analogy to the diethyl ester [49].

4.3 X-ray structure determinations

Suitable crystals were obtained by crystallization from ethyl acetate-cyclohexane (1:4) (for E-8a) or ethyl acetate (for Z-11b). Data collection was performed on a Stoe IPDS diffractometer. Software for structure solution and refinement: Shelxs/l-97 [50]; molecule plots: Ortep-3 [51]. In the refinement procedure, the hydrogen atoms were placed in geometrically calculated positions and treated as riding on their bond neighbors in the refinement. Further details are provided in Table 1.

Table 1

Crystal structure data for E-8a and Z-11b.

E-8aZ-11b
FormulaC22H20N2O4C18H18N2O6
Mr376.40358.34
Cryst. size, mm30.31 × 0.15 × 0.100.31 × 0.23 × 0.15
Temperature (K)190(2)190(2)
Crystal systemorthorhombictriclinic
Space groupPna21P 1̄
a, Å19.011(2)7.051(2)
b, Å10.825(1)11.615(3)
c, Å18.954(3)11.965(3)
α, deg9070.45(3)
β, deg9075.12(3)
γ, deg9078.81(3)
V, Å33900.7(9)886.2(4)
Z82
Dcalcd., g cm–31.2821.343
μ(MoKα), cm–10.0890.102
F(000), e1584376
hkl range±22, ±12, ±22±8, ±13, ±14
((sinθ)max, deg25.0325.02
Refl. measured272888428
Refl. unique/Rint)3458/0.0822960/0.0542
Param. refined509238
R(F)/wR(F2)a (all reflexions)0.0744/0.12530.0738/0.0823
x(Flack)b–
GoF (F2)a1.0100.852
Δρfin (max/min), e Å–30.45/–0.200.19/–0.16

aR(F) = Σ||Fo| – |Fc||/Σ|Fo|; wR(F2) = [Σw(Fo2 – Fc2)2/Σw(Fo2)2]1/2; GoF = [Σw(Fo2–Fc2)2/(nobs–nparam)]1/2; w = [σ2(Fo2) + (aP)2 + bP]–1, where P = (Max(Fo2, 0) + 2Fc2)/3; bcould not be determined reliably.

CCDC 1409521 (E-8a) and 1409522 (Z-11b) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

4.4 Reactions with styryl-diazoacetate 1

4.4.1 Rh(II)-catalyzed reaction of diazoester 1 with 3,5-diphenyl-1H-pyrazole (5)

A solution of pyrazole 5 (660 mg, 3.00 mmol) and styryl-diazoacetate 1 (606 mg, 3.00 mmol) in dichloromethane (20 mL) was cooled at 0 °C and Rh2(OAc)4 (66 mg, 0.15 mmol, 5 mol%) was added. The mixture was stirred at 0 °C until N2 evolution had ceased and the diazo compound was consumed (22 h, IR control). The solvent was evaporated in vacuo and the residue was fractionated by column chromatography (silica gel, ethyl acetate–cyclohexane (1:20) as eluant). The following fractions were obtained (in the order of decreasing Rf values): a) unidentified decomposition product(s) of 1 (20 mg); b) a yellow powder tentatively assigned as Z-7a (119 mg, 10 %) which contains minor impurities (1H NMR); c) E-7a (138 mg, 12 % yield) as a yellow powder; d) E-6a (434 mg, 37 %) as a yellow powder; e) 219 mg of an oily 1:2 mixture of E-6a (6 %) and Z-6b (12 %); f) 48 mg (4 %) of Z-6b as a resinous yellow oil; g) 34 mg of a mixture which after preparative thin-layer chromatography (ethyl acetate–cyclohexane (1:9) as eluant) yielded 20 mg (2 %) of Z-6b and 10 mg (1.5 %) of unconsumed pyrazole 5.

(E)-Methyl 2-(3,5-diphenyl-1H-pyrazolyl)-4-phenylbut- 3-enoate (E-6a): m. p. 123.6–124.1 °C. – IR (KBr): υ˜ = 700 (s), 759 (s), 771 (s), 808 (w), 885 (w), 956 (w), 974 (m), 1074 (w), 1125 (w), 1168 (m), 1198 (m), 1263 (m), 1294 (w), 1313 (w), 1372 (w), 1434 (m), 1458 (m), 1481 (m), 1550 (w), 1749 (s), 2959 (w), 3028 (w) cm–1. – 1H NMR: δ = 3.77 (s, 3 H, OCH3), 5.65 (dd, 3J= 7.6 Hz, 4J= 0.7 Hz, 1 H, NCH), 6.46 (d, 3J= 16.0 Hz, 1 H, CH=CHPh, 4J not resolved), 6.67 (s, 1 H, CHpz), 6.85 (dd, 3J= 16.0 Hz, 3J= 7.6 Hz, 1 H, NCCH=), 7.24–7.51 (m, 13 H, CHPh), 7.89–7.91 (m, 2 H, CHPh) ppm. – 13C NMR: δ = 53.0 (OCH3), 63.2 (NCH), 103.8 (CHpz), 123.2 (PhCH=CH), 125.9, 126.8, 127.8, 128.2, 128.5, 128.5, 128.8, 129.0, 129.1, 130.3, 133.3, 134.6 (CH=), 135.9, 145.8, 151.6, 169.2 (C=O) ppm. – MS: m/z= 395 (100 %). – Anal. for C26H22N2O2 (394.47): calcd. C 79.16, H 5.62, N 7.10; found C 78.93, H 5.65, N 6.93.

(Z)-Methyl 2-(3,5-diphenyl-1H-pyrazolyl)-4-phenylbut-2-enoate (Z-6b): – IR (KBr): υ˜ = 695 (m), 763 (s), 780 (m), 806 (m), ~1028−1080 (medium-strong broad absorption with several bands), 1204 (m), 1264 (s), 1299 (w), 1375 (w), 1436 (m), 1463 (m), 1485 (m), 1654 (m), 1725 (s), 2952 (w), 3028 (w) cm–1. – 1H NMR: δ= 3.43 (d, 3J= 7.6 Hz, 2 H, CH2), 3.64 (s, 3 H, OCH3), 6.83 (s, 1 H, CHpz), 7.06 (d, 2 H, J = 6.7 Hz, CHPh), 7.12–7.27 (m, 4 H, 3 CHPh and =CHCH2), ~7.20–7.45 (m, 8 H, CHPh), 7.91–7.93 (m, 2 H, CHPh). – 13C NMR: δ = 34.4 (CH2), 52.5 (OCH3), 103.4 (CHpz), 125.9, 126.7, 127.9, 128.1, 128.6, 128.7, 128.7, 128.8, 130.2 (NC=CH), 131.2, 133.0, 137.0, 143.7 (=CHCH2), 146.5, 152.5, 163.9 (C=O) ppm. – MS: m/z= 395 (100 %). – Anal. for C26H22N2O2 (394.47): calcd. C 79.16, H 5.62, N 7.10; found C 79.11, H 5.81, N 6.91.

(E)-Methyl 4-(3,5-diphenyl-1H-pyrazolyl)-4-phenylbut- 2-enoate (E-7a): m. p. 150.3–150.9 °C. – IR (KBr): υ˜ = 695 (m), 708 (m), 763 (s), 808 (w), 870 (w), 957 (w), 977 (m), 1039 (m), 1077 (m), 1154 (w), 1195 (m), 1226 (w), 1280 (s), 1346 (w), 1435 (m), 1452 (m), 1462 (m), 1486 (m), 1549 (w), 1644 (w), 1714 (vs), 2947 (w), 3059 (w) cm–1. – 1H NMR: δ = 3.73 (s, 3 H, OCH3), 5.83 (dd, 3J= 15.7 Hz, 4J= 1.4 Hz, 1 H, =CHCO2Me), 6.03 (dd, 3J= 6.3 Hz, 4J= 1.0 Hz, 1 H, NCH), 6.65 (s, 1 H, CHpz), 7.27–7.46 (m, 13 H, CHPh), 7.57 (dd, 3J= 15.7 Hz, 3J= 6.3 Hz, 1 H, NCCH=), 7.89 (d, J = 4.4 Hz, 2 H, CHPh) ppm. – 13C NMR: δ = 51.7 (OCH3), 62.9 (NCH), 103.7 (CHpz), 123.0 (=CHCO2Me), 125.7, 127.6, 127.7, 128.2, 128.5, 128.7, 128.8, 128.9, 129.1, 130.4, 133.4, 138.3, 145.4, 146.1 (NCCH=), 151.1 (C=N, pz), 166.4 (C=O) ppm. – MS: m/z= 395 (100 %). – Anal. for C26H22N2O2 (394.47): calcd. C 79.16, H 5.62, N 7.10; found C 79.01, H 5.70, N 6.96.

(Z)-Methyl 4-(3,5-diphenyl-1H-pyrazolyl)-4-phenylbut-2-enoate (Z-7a): IR (KBr): υ˜ = 693 (s), 761 (s), 872 (m), 11.85 (m), 1208 (s), 1401 (m), 1338 (m), 1461 (m), 1643 (m), 1713 (s), 2943 (w), 3037 (w) cm–1. – 1H NMR: δ= 3.62 (s, 3 H, OCH3), 5.98 (d, 3J = 11.4 Hz, 1 H, CHolef), 6.65 (s, 1 H, CHpz), 7.2–7.6 (m, 13 H, CHPh), 7.30 (dd, 1 H, CHolef), 7.47 (dd, 1 H, CHallyl), 7.90 (d, J = 7.4 Hz, 2 HPh) ppm. – 13C NMR: δ = 51.4 (OCH3), 58,1 (NCH), 103.6 (CHpz), 119.5 (CHolef), 125.6, 126.9, 127.6, 127.7, 128.54, 128.59, 128.69, 129.1, 130.3, 133.6, 139.8, 145.4 (CHolef), 145.6, 151.0 (C=Npz), 166.0 (C=O) ppm. – MS: m/z= 395 (100 %), 175 (78 %). – Anal. for C26H22N2O2 (394.47): calcd. C 79.16, H 5.62, N 7.10; found C 78.97, H 5.69, N 7.00.

4.4.2 Ag(I)-catalyzed reaction of diazoester 1 with pyrazole 5

To a solution of pyrazole 5 (220 mg, 1.00 mmol) and styryl- diazoacetate 1 (303 mg, 1.50 mmol) in dichloromethane (15 mL) was added silver(I) trifluoromethanesulfonate (38 mg, 0.15 mmol), and the mixture was heated at reflux, until the diazo compound had been consumed (8 h, IR control). After cooling, the solvent was evaporated in vacuo. The solid material, which was formed upon addition of ethyl acetate–cyclohexane (1:4) to the residue, was subjected to column chromatography (silica gel, ethyl acetate-cyclohexane (1:4)) to furnish: a) 202 mg (51 % based on initial 5) of E-7a as a colorless powder, 64 mg (29 %) of 3,5-diphenylpyrazole (5) and 114 mg (38 % based on 1) of pyrazole 2. The filtrate of the first precipitation was chromatographed separately and yielded unidentified decomposition products of 1 (73 mg), 3,5-diphenylpyrazole (5, 10 mg) and pyrazole 2 (3 mg).

4.4.3 Rh(II)-catalyzed reaction of diazoester 1 with methyl 5-phenyl-1H-pyrazole-3-carboxylate (2)

To a solution of pyrazole 2 (421 mg, 2.08 mmol) and styryl-diazoacetate 1 (421 mg, 2.08 mmol) was added Rh2(OAc)4 (39 mg, 88 μmol) in dichloromethane (15 mL), and the mixture was stirred at 0 °C for 19 h. After addition of another portion of Rh2(OAc)4 (13 mg, 29 μmol, 5.6 mol % in total), the mixture was again stirred at 0 °C for 20 h. The solvent was evaporated and the residue was taken up in ethyl acetate-cyclohexane (1:2), leaving behind 84 mg of pyrazole 2, which was filtered off. The filtrate was concentrated and subjected to column chromatography (silica gel, ethyl acetate-cyclohexane (1:2)). Three major fractions were obtained: a) 249 mg (32 %) of E-8b as a yellow oil; b) 385 mg (49 %) of E-8a as a colorless powder; c) another portion (60 mg, total amount 144 mg, “34 %”) of pyrazole 2 as a pale orange solid.

(E)-Methyl 1-(1-methoxycarbonyl-3-phenylprop-2-en-1-yl)-5-phenyl-1H-pyrazole-3-carboxylate (E-8a): m. p. 118.4–120.5 °C. – IR (KBr): υ˜ = 693 (w), 705 (w), 765 (m), 773 (m), 972 (w), 987 (w), 1173 (w), 1190 (w), 1222 (s), 1270 (m), 1386 (w), 1421 (w), 1433 (m), 1456 (m), 1728 (s), 1750 (s), 2953 (w), 3060 (w) cm–1. – 1H NMR: δ = 3.74 (s, 3 H, OCH3), 3.95 (s, 3 H, OCH3), 5.67 (dd, 3J= 7.9 Hz, 4J= 0.9 Hz, 1 H, NCH), 6.44 (d, 3J= 15.9 Hz, 1 H, =CHPh), 6.76 (dd, 3J= 15.9 Hz, 3J= 7.9 Hz, 1 H, NCCH=), 6.88 (s, 1 H, CHpz), 6.27–7.33 (m, 3 H, CHPh), 7.36–7.38 (m, 2 H, CHPh), 7.41–7.44 (m, 2 H, CHPh), 7.47–7.50 (m, 3 H, CHPh) ppm. – 13C NMR: δ = 52.1 (OCH3), 53.2 (OCH3), 64.0 (NCH), 109.3 (CHpz), 122.3 (NCCH=), 126.9, 128.5, 128.6, 129.0, 129.3 (2 C), 129.5, 135.6, 135.7 (=CHPh), 143.7, 145.8, 162.7 (C=O), 168.5 (C=O) ppm. – Anal. for C22H20N2O4 (376.41): calcd. C 70.20, H 5.36, N 7.44; found C 70.22, H 5.36, N 7.57.

(E)-Methyl 1-(1-methoxycarbonyl-3-phenylprop-2-en-1-yl)-3-phenyl-1H-pyrazole-5-carboxylate (E-8b): IR (NaCl): υ˜ = 693 (3), 737 (m), 762 (s), 823 (m), 877 (m), 917 (w), 965 (s), 1002 (m), 1029 (m), 1089 (m), ~1170–1280 (broad absorption, several overlapping strong bands), 1335 (s), 1369 (m), 1447 (br, s), 1506 (m), 1543 (m), 1606 (w), 1654 (w), 1721 (s), 1750 (s), 2850 (m), 2927 (m), 2953 (m), 3003 (m), 3027 (m), 3061 (m) cm–1. – 1H NMR: δ = 3.79 (s, 3 H, OCH3), 3.92 (s, 3 H, OCH3), 6.54 (dd, 3J= 8.0 Hz, 4J= 0.7 Hz, 1 H, NCH), 6.69 (d, 3J= 16.0 Hz, 1 H, =CHPh), 6.88 (dd, 3J= 15.9 Hz, 3J= 8.0 Hz, 1 H, NCCH=), 7.22 (s, 1 H, CHpz), 7.26–7.45 (m, 8 H, CHPh), 7.83–7.85 (m, 2 H, CHPh) ppm. – 13C NMR: δ = 52.1 (OCH3), 53.0 (OCH3), 65.5 (NCH), 108.7 (CHpz), 122.5 (NCCH=), 125.8, 126.9, 128.2, 128.3, 128.6, 128.6, 132.3, 133.1, 135.6 (=CHPh), 136.0, 150.7 (C=N, pz), 160.2 (C=O), 169.2 (C=O) ppm. – MS: m/z= 377 (100 %). – Anal. for C22H20N2O4 (376.41): calcd. C 70.20, H 5.36, N 7.44; found C 70.03, H 5.35, N 7.30.

4.4.4 Ag(I)-catalyzed reaction of diazoester 1 with pyrazole 2

A solution of pyrazole 2 (404 mg, 2.0 mmol) and styryl-diazoacetate 1 (404 mg, 2.0 mmol) in dichloromethane (20 mL) was cooled at 0 °C, silver(I) trifluoromethanesulfonate (51 mg, 0.2 mmol, 10 mol%) was added, and the mixture was stirred at 20 °C for 6 days. After evaporation of the solvent, the residue was taken up in ethyl acetate-cyclohexane (1:4), leaving behind 285 mg of pyrazole 2 which was filtered off. The filtrate was concentrated and separated by column chromatography (silica gel, ethyl acetate-cyclohexane (1:4)). Four fractions were obtained: a) 369 mg (49 %) of E-9b as a yellow oil; b) 52 mg (6.9 %) of E-9a as a yellow powder; 27 mg of a 1:2 mixture of E-8a (1.2 %) and E-9a (2.4 %); c) another 41 mg of pyrazole 2 as a colorless solid (total amount 326 mg, “81 %”).

(E)-Methyl 1-(3-methoxycarbonyl-1-phenylprop-2-en-1-yl)-5-phenyl-1H-pyrazole-3-carboxylate (E-9a): m. p. 133.4–134.2 °C. – IR (KBr): υ˜ = 698 (s), 740 (m), 769 (s), 828 (w), 950 (w), 996 (m), 1010 (m), 1114 (m), 1166 (m), 1178 (m), 1202 (s), 1215 (s), 1247 (s), 1310 (m), 1354 (m), 1376 (w), 1418 (m), 1430 (m), 1448 (s), 1471 (m), 1661 (m), 1718 (vs), 2951 (w), 2996 (w), 3133 (w) cm–1. – 1H NMR: δ = 3.72 (s, 3 H, OCH3), 3.94 (s, 3 H, OCH3), 5.77 (dd, 3J= 15.7 Hz, 4J= 1.5 Hz, 1 H, =CHCO2Me), 6.06 (d, 3J= 6.3 Hz, 1 H, NCH), 6.87 (s, 1 H, CHpz), 7.17–7.19 (m, 2 H, CHPh), 7.24–7.27 (m, 2 H, CHPh), 7.30–7.32 (m, 3 H, CHPh), 7.43–7.53 (m, 4 H, 3 CHPh and NCCH=) ppm. – 13C NMR: d = 51.8 (OCH3), 52.1 (OCH3), 63.6 (NCH), 109.4 (CHpz), 123.9 (=CHCO2Me), 127.4, 128.5, 128.9 (2 C), 129.3 (2 C), 129.4, 137.3, 143.6 (C=N, pz), 144.7 (NCCH=), 145.6, 162.8 (C=O), 166.0 (C=O) ppm. – MS: m/z= 377 (100 %). – Anal. for C22H20N2O4 (376.41): calcd. C 70.20, H 5.36, N 7.44; found C 70.23, H 5.44, N 6.92.

(E)-Methyl 1-(3-methoxycarbonyl-1-phenylprop-2-en-1-yl)-3-phenyl-1H-pyrazole-5-carboxylate (E-9b): IR (NaCl): υ˜ = 697 (m), 761 (m), 832 (w), 954 (w), 986 (w), 1029 (w), 1090 (m), 1170 (m), 1194 (m), 1226 (m), 1262 (s), 1315 (m), 1446 (s), 1495 (m), 1506 (m), 1541 (w), 1659 (m), 1722 (vs), 2951 (w), 3031 (w) cm–1. – 1H NMR: δ = 3.74 (s, 3 H, OCH3), 3.88 (s, 3 H, OCH3), 5.83 (dd, 3J= 15.7 Hz, 4J= 1.6 Hz, 1 H, =CHCO2Me), 7.19 (s, 1 H, CHpz), 7.23 (dd, 3J= 6.3 Hz, 4J= 1.5 Hz, 1 H, NCH), 7.26–7.45 (m, 8 H, CHPh), 7.64 (dd, 3J= 15.7 Hz, 3J= 6.3 Hz, 1 H, NCH=), 7.84–7.86 (m, 2 H, CHPh) ppm. – 13C NMR: δ = 51.7 (OCH3), 52.1 (OCH3), 64.0 (NCH), 108.6 (CHpz), 123.1 (=CHCO2Me), 125.7, 128.0, 128.2, 128.3, 128.6, 128.7, 132.3, 133.0, 138.0, 145.8 (CH=CHCO2Me), 150.6, 160.1 (C=O), 166.4 (C=O) ppm. – MS: m/z= 377 (100 %). – Anal. for C22H20N2O4 (376.41): calcd. C 70.20, H 5.36, N 7.44; found C 70.04, H 5.55, N 7.54.

4.4.5 Rh(II)-catalyzed decomposition of styryl-diazoacetate 1

To a solution of (E)-methyl 2-diazo-4-phenylbut-3-enoate (1, 404 mg, 2.0 mmol) in 1,2-dichloroethane (5 mL) was added Rh2(OAc)4 (8.84 mg, 20 μmol, 1 mol%) and the mixture was heated at reflux for 50 min, when the diazocompound had disappeared (IR control). After cooling to r.t., the solvent was evaporated in vacuo and the crude product mixture was separated by column chromatography (silica gel, ethyl acetate–cyclohexane (1:4)). Three fractions were obtained: 103 mg of a mixture of unidentified decomposition products from 1; b) a yellow oil of E-8b (127 mg, 34 %); c) a colorless powder of E-8a (89 mg, 24 %).

4.5 Reactions with (E)-dimethyl 4-diazopent-2-enedioate (3)

4.5.1 (Z)-Dimethyl 2-(3,5-diphenyl-1H-pyrazol-1-yl)pent-2-enedioate (Z-10)

A solution of diazoester 3 (442 mg, 2.4 mmol), 3,5- diphenylpyrazole (5, 440 mg, 2.0 mmol) and Rh2(OAc)4 (31 mg, 70 μmol) in dichloromethane (10 mL) was stirred at 0 °C for 45 h. Since the diazo compound was not yet consumed completely (IR control), another 16 mg (36 μmol, in total 4.4 mol%) of Rh2(OAc)4 was added and the mixture was heated at reflux for 10 h. The solvent was evaporated and ethyl acetate-cyclohexane (1:2) was added to the residue. Crude pyrazole 5 (29 mg) remained undissolved and was filtered off. The filtrate was subjected to column chromatography (silica gel, ethyl acetate-cyclohexane (1:2)), which yielded Z-10 (708 mg, 94 % based on pyrazole 5) as a yellow oil. – IR (NaCl): υ˜ = 696 (s), 760 (s), 883 (w), 919 (w), 956 (m), 989 (m), 1031 (w), 1051 (m), 1083 (w), 1174 (s), 1205 (s), 1263 (br, s), 1378 (m), 1437 (s), 1462 (s), 1486 (s), 1552 (m), 1606 (w), 1664 (m), 1735 (br, s), 2953 (m), 3062 (w) cm–1. – 1H NMR: δ = 3.28 (d, 3J= 7.2 Hz, 2 H, CH2), 3.61 (s, 3 H, OCH3), 3.64 (s, 3 H, OCH3), 6.79 (s, 1 H, CHpz), 7.28 (t, 3J = 7.2 Hz, 1 H, =CHCH2), 7.32–7.44 (m, 8 H, CHPh), 7.86–7.88 (m, 2 H, CHPh) ppm. – 13C NMR: δ= 33.4 (CH2), 52.2 (OCH3), 52.5 (OCH3), 103.5 (CHpz), 125.9, 127.7, 128.2, 128.6, 128.7, 128.8, 130.0, 132.8 and 133.0 (MeO2CC=CH and CPh), 135.6 (=CHCH2), 146.5, 152.8, 163.3 (C=O), 169.5 (C=O) ppm. – MS: m/z= 377 (100 %). – Anal. for C22H20N2O4 (376.41): calcd. C 70.20, H 5.36, N 7.44; found C 70.04, H 5.36, N 7.47.

4.5.2 Rh(II)-catalyzed reaction of diazoester 3 with methyl 5-phenyl-1H-pyrazole-3-carboxylate (2)

A solution of pyrazole 2 (355 mg, 1.76 mmol) and diazoester 3 (388 mg, 2.11 mmol) in dichloromethane (15 mL) was cooled at 0 °C, Rh2(OAc)4 (28 mg, 0.063 mmol) was added and the mixture was stirred at 20 °C for 18 h. After addition of another 14 mg (0.032 mmol) of Rh2(OAc)4, the reaction mixture was again stirred at 20 °C for 24 h. The solvent was evaporated and the residue was separated by column chromatography (silica gel, ethyl acetate-cyclohexane (1:2) to furnish three fractions: a) 118 mg (19 %) of Z-11b as a colorless powder; b) 51 mg of a mixture containing Z-11b (~27 mg, ~4 %); c) 377 mg (60 %) of Z-11a as a colorless powder.

(Z)-Dimethyl 2-[5-(methoxycarbonyl)-3-phenyl-1H- pyrazol-1-yl]-pent-2-enedioate (Z-11a): m. p. 97.8–99.2 °C. – IR (KBr): υ˜ = 699 (m), 763 (s), 782 (s), 812 (w), 884 (w), 947 (w), 990 (m), 1012 (s), 1052 (m), 1117 (m), ~1170–1270 (broad strong absorption with several overlapping bands), 1385 (s), 1452 (s), 1668 (m), 1733 (vs), 2955 (s), 3003 (m), 3137 (w) cm–1. – 1H NMR: δ = 3.09 (d, 3J= 7.2 Hz, 2 H, CH2), 3.62 (s, 3 H, CH2COOCH3), 3.66 (s, 3 H, Colef−COOCH3), 3.96 (s, 3 H, Cpz-COOCH3), 7.01 (s, 1 H, CHpz), 7.33–7.39 (m, 6 H, 5 CHPh and =CHCH2) ppm. – 13C NMR: δ = 33.1 (CH2), 52.16 (OCH3), 52.25 (OCH3), 52.72 (OCH3), 108.4 (CHpz), 127.8, 128.7 (MeO2CC=CH), 128.8, 129.2, 132.3, 137.2 (=CHCH2), 144.8, 146.7, 162.4 (C=O), 162.5 (C=O), 168.9 (C=O) ppm. – MS: m/z= 359 (100 %). – Anal. for C18H18N2O6 (358.35): calcd. C 60.33, H 5.06, N 7.82; found C 60.22, H 5.01, N 7.89.

(Z)-Dimethyl 2-[3-(methoxycarbonyl)-5-phenyl-1H- pyrazol-1-yl]-pent-2-enedioate (Z-11b): m. p. 104.5–106.1 °C. – IR (KBr): υ˜ = 688 (w), 761 (m), 775 (w), 1043 (m), 1084 (m), 1091 (m), 1177 (m), 1205 (m), 1234 (m), 1260 (s), 1353 (w), 1374 (m), 1437 (m), 1452 (m), 1512 (w), 1547 (w), 1664 (w), 1719 (s), 1729 (s), 1742 (s), 2995 (w) cm–1. – 1H NMR: δ = 3.25 (broadened d, 3J= 6.8 Hz, 2 H, CH2), 3.70 (s, 3 H, OCH3), 3.80 (s, 3 H, OCH3), 3.87 (s, 3 H, OCH3), 7.25 (s, 1 H, CHpz), 7.33–7.44 (m, 4 H, 3 CHPh and =CHCH2), 7.81–7.83 (m, 2 H, CHPh) ppm. – 13C NMR: δ = 33.2 (CH2), 52.19 (OCH3), 52.25 (OCH3), 52.67 (OCH3), 108.5 (CHpz), 125.8, 128.6, 128.7, 131.9, 133.4, 134.0 (C=CH), 135.5, 152.4 (C=N, pz), 159.3 (C=O), 163.0 (C=O), 169.6 (C=O) ppm. – Anal. for C18H18N2O6 (358.35): calcd. C 60.33, H 5.06, N 7.82; found C 60.25, H 5.17, N 7.78.

Corresponding author: Gerhard Maas, Institute of Organic Chemistry I, University of Ulm, 89081 Ulm, Germany, Fax: +49 (0)731 5022803, E-mail:

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Received: 2015-7-7
Accepted: 2015-7-29
Published Online: 2015-9-24
Published in Print: 2015-10-1

©2015 by De Gruyter

Articles in the same Issue

  1. Frontmatter
  2. In this Issue
  3. Review
  4. Equiatomic cerium intermetallics CeXX′ with two p elements
  5. Resin-assisted solvothermal synthesis of a manganese(II) coordination polymer with tetrachloroterephthalate
  6. Influence of metal ions on the formation of new metal complexes constructed from tetrachlorophthalic acid
  7. Synthesis, crystal structure and physicochemical characterization of a Hg(II) complex with 6-methoxyquinoline as ligand
  8. Ultrasound-assisted regioselective synthesis of aminomethylated daidzein derivatives via Mannich reaction
  9. I5– polymers with a layered arrangement: synthesis, spectroscopy, and structure of a new polyiodide salt in the nicotine/HI/I2 system
  10. 2,5-Bridged 1-Carba-arachno-pentaborane(10) Derivatives – Intermediates on the Way to Pentaalkyl-1,5-dicarba-closo-pentaboranes(5)
  11. N-vinylation and N-allylation of 3,5-disubstituted pyrazoles by N–H insertion of vinylcarbenoids
  12. Synthesis, characterization and molecular structure of a dinuclear uranyl complex supported by N,N′,N″,N′″-tetra-(3,5-di-tert-butylsalicylidene)-1,2,4,5-phenylenetetraamine
  13. Note
  14. 119Sn Mössbauer spectroscopy of solvothermally synthesized fluorides ASnF3 (A = Na, K, Rb, Cs)
https://doi.org/10.1515/znb-2015-0115
Keywords for this article
N–H insertion; pyrazoles; transition-metal catalysis; vinylcarbenoids; vinyldiazo compounds

Articles in the same Issue

  1. Frontmatter
  2. In this Issue
  3. Review
  4. Equiatomic cerium intermetallics CeXX′ with two p elements
  5. Resin-assisted solvothermal synthesis of a manganese(II) coordination polymer with tetrachloroterephthalate
  6. Influence of metal ions on the formation of new metal complexes constructed from tetrachlorophthalic acid
  7. Synthesis, crystal structure and physicochemical characterization of a Hg(II) complex with 6-methoxyquinoline as ligand
  8. Ultrasound-assisted regioselective synthesis of aminomethylated daidzein derivatives via Mannich reaction
  9. I5– polymers with a layered arrangement: synthesis, spectroscopy, and structure of a new polyiodide salt in the nicotine/HI/I2 system
  10. 2,5-Bridged 1-Carba-arachno-pentaborane(10) Derivatives – Intermediates on the Way to Pentaalkyl-1,5-dicarba-closo-pentaboranes(5)
  11. N-vinylation and N-allylation of 3,5-disubstituted pyrazoles by N–H insertion of vinylcarbenoids
  12. Synthesis, characterization and molecular structure of a dinuclear uranyl complex supported by N,N′,N″,N′″-tetra-(3,5-di-tert-butylsalicylidene)-1,2,4,5-phenylenetetraamine
  13. Note
  14. 119Sn Mössbauer spectroscopy of solvothermally synthesized fluorides ASnF3 (A = Na, K, Rb, Cs)
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