Volume 75, Issue 9

The continued growth of polyhedral borane chemistry is illustrated by the successful synthesis of [ closo -B 12 (OH) 12 ] 2 –by the H 2 O 2 oxidation of [ closo -B 12 H 12, ] 2 . The hydroxylated species reacts with common organic reagents to produce 12-fold degenerately substituted carboxylate esters, ethers, and carbamate esters. The ether derivatives undergo facile and reversible redox reactions in which the B12 scaffold serves as a one-or two-electron donor giving stable oxidation states described by [ closo -or hypercloso -B 12 OR) 12 ] n with n = −2,−1,and 0. The loss of polyhedral electrons is compensated by back-bonding from the attached ether oxygen substituents. Structural comparisons are presented along with representative reaction chemistry. Derivatives that bear functional groups on the termini of the 12-fold set of substituents provide a vast array of potential applications. To distinguish these new polyfunctional icosahedral species from dendrimers,they have been designated as closomers.

This paper reports extensions of the well-established field of phosphine–borane chemistry. Linked clusters, for example, {[(PPh 3 ) 2 (CO)OsB 4 H 7 -3-BH 2 -PPh 2 ] 2 [(Fe(C 5 H 4 ) 2 ]}, are formed in reactions of rigid backboned bidentate phosphines with {2,2,2-(PPh 3 ) 2 (CO)- nido -2-OsB 5 H 9 ]. Reaction of bidentate phosphines with the unsaturated clusters [8,8-PPh 3 ) 2 - nido -8,7-RhSB 9 H 10 ] and [9,9-(PPh 3 ) 2 - nido -9,7,8-RhC 2 B 8 H 11 ] leads to the isolation of novel species such as {1-(PPh 3 )[1,3-(µ-dppm)]- closo -1,2-RhSB 9 H 8 },with a dppm molecule bridging adjacent metal and boron vertices in the cage, [1,1-(ç 2 -dppe)-3-(ç 1 -dppm)- closo -1,2-RhSB 9 H 8 ], a mixed ligand system, and {9,9-ç 2 -[(ç 2 -BH 3 )PPh 2 PCH 2 PPh 2 ]- nido - 9,7,8-RhC 2 B 8 H 11 } which contains a bidentate dppm BH 3 moiety. The formation of bidentate phosphine-linked main group transition-metal moieties such as [BH 3 PPh 2 (CH) n PPh 2 IrCp*Cl 2 ] is also described as is an example of a bidentate phosphine-polyhedral borane adduct [NR 4 ][B 11 H 12 (dppm)] in which the borane cage is isoelectronic with [B 11 H 14 ] – . Finally, a novel oxidative coupling reaction promoted by Cp 2 ZrCl 2 / n -BuLi to form the new nonaborane cage system, arachno -B 9 H 11 (PPh 3 ) 2 , an unusual homolog of the well-known species B 10 H 12 (PPh 3 ) 2 , is reported.

A two-electron aromatic bis (tris-trimethylsilylmethylene)-substituted tetraborane(4) was found to be a useful precursor for the synthesis of two-electron aromatic tetraboranes(6), triboracyclopropanates,as well as tetraboranes(6) distorted toward triboracyclopropanates with boryl bridges. Bishomo two-electron aromatics with a borata bridge and a protonated borata bridge, respectively, are also presented.

The versatile reactivity of B(C 6 F 5 ) 3 in alkene polymerization reactions is summarized. Adduct formation with basic anions such as CN – and NH 2– gives extremely weakly co- ordinating diborates, which are the basis of some of the most active polymerization catalysts known to date. By contrast, the reaction of B(C 6 F 5 ) 3 with zirconium half-sandwich complexes leads to extensive C 6 F 5 transfer, including the surprising formation of borole-bridged triple decker complexes. Main group alkyls undergo such C 6 F 5 exchange reactions very readily unless donor ligands are present. Borate salts of new three-coordinate zinc alkyl cations proved to be highly effective catalysts for the ring-opening polymerization of epoxides and lactones.

The molecular shape and hydrophobicity of dicarba- closo -dodecaboranes may allow a new medical application as biologically active molecules. Recently, we have developed potent estrogen receptor (ER) agonists bearing carborane cage. The most potent compound (BE120) exhibited activity at least several times greater than that of 17 beta-estradiol in luciferase reporter gene assay and ER alpha binding. We also designed and synthesized estrogen antagonists on the basis of the structure of BE120, and we noticed the characteristic features of compound (BE360) having carborane cage and two phenols. The ER binding affinity of BE360 is comparable to that of estradiol. To examine in vivo estrogenic activity of the compound in bone, ovariectomized (OVX) mice were given BE360 or estradiol subcutaneously for 4 weeks. Treatment with BE360 at 1–30 µg/day dose-dependently restored bone loss in OVX mice to sham level without estrogenic action in uterus. These results suggest its possible application to osteoporosis as a new type of selective ER modulator.

In the conversion of nonaboranes B 9 H 13 SMe 2 to azanonaboranes B 8 NH 11 -NHR, one boron atom is lost. This boron atom was identified, and a pathway for the rearrangement of the cluster is proposed, based on the fate of covalently labeled boron atoms.

A rapidly developing area of organometallic chemistry centers on polynuclear metal coordination compounds having specified, highly symmetric geometries such as squares, triangles, rectangles, or rigid rods. Interest in such molecules arises in part from their unusual, aesthetically appealing structures, and on the possibilities for assembling them into extended systems having useful electronic, optical, magnetic, catalytic, or other properties. Given their characteristic thermal and oxidative stability, chemical versatility, and relatively low-bulk, metallocene-like steric requirements, small metallacarboranes are attractive candidates as building blocks for such structures. Recent work in our laboratory on the controlled substitution and linkage of these clusters has opened the way to some new metallacarborane “designer ”chemistry that is described in this article.

An overview of the syntheses, structures, and reactivity of the compounds formed by the incorporation of a number of d- and f-block metals into C 2 B 4 - and C 2 B 9 -carborane cages have been presented. In addition,the development of a safe, bench-scale preparation of the toxic and commercially unavailable polyhedral borane synthon, namely pentaborane(9), has also been described. Thus, this report discusses the latest developments leading to a sys- tematic synthetic approach to a number of carborane precursors and the subsequent reaction chemistry in the formation of a number of “carbons-apart ”metallacarboranes.

Several new organic transformations have been achieved utilizing boron halide reagents. Aryl aldehydes are conveniently converted to gem -dichloromethylbenzenes using boron trichloride. Aryl aldehydes are alkylated by alkylboron chlorides in a Grignard-like fashion to generate the corresponding arylalkanols or alkylboron chlorides. Aryl aldehydes react with divinylboron halides (generated via the haloboration of alkynes) to produce 1,5-di-halo-1,4-dienes in excellent yields.

For the structural extent of the polyhedral molecular chemistry of boron to rival that of the organic chemistry of the carbon hydrides, (a) the individual boron-containing polyhedra need to be fused together to make larger contiguous covalent assemblies, and (b) an intermolecular chemistry needs to be developed. Contemporary examples are given that illustrate novel chemistry in both these areas and that may point the way to some future developments.

Efficient cleavage of sterically hindered boronic esters of asymmetric diols to recover both the diol and a more reactive borane derivative has been a challenging problem. A remarkable borosilicate glass-catalyzed cleavage with thionyl chloride and imidazole proved less useful than hoped for. Potassium bifluoride converts boronic esters to diols and alkyltrifluoroborates. These with chlorosilanes yield alkyldifluoroboranes, which with alkyl azides form secondary amines in high enantiopurity.

The icosahedral aza- closo -dodecaboranes RNB 11 H 11 (R = H, Me, Ph) are opened by neutral bases L or anionic bases X – to give the nido -species RNB 11 H 11 L or [RNB 11 H 11 X] – , respectively, with a nonplanar pentagonal aperture; the N atom and a BHB hydrogen bridge are situated at opposite sides of the aperture. The BL or the BX vertex is found in the aperture either adjacent to the hydrogen bridge (type 1 ; C 1 ) or adjacent to nitrogen (type 2 ;C 1 ) or off the aperture adjacent to nitrogen on a mirror plane (type 3 ;C s ). At any rate, the isomer of type 1 is the primary product, which may rearrange to yield an isomer of type 3 via an isomer of type 2 . Working in deuteromethanol shows that the bridging H atom originates from the primarily attacked BH vertex. The process from RNB 11 H 11 to its base adduct of type 3 includes the opening and the closure of skeletal BN bonds and the jumping of the extra-H atom from endo into bridging positions and vice versa, whereas the base does not alter its position. The application of the opening process to a series of aza- closo -dodecaboranes with non-hydrogen boron ligands confirms that only atoms of the ortho -belt of the starting material are involved in structural changes. The elementary steps from the closo -species to the three isomers are identified as a [3c, 1c] collocation and subsequent [3c, 2c] translocations in the picture of molecular orbitals localized over three or two centers or to one center (lone pair).

The development of asymmetric synthesis during the past two decades aided organic chemists considerably in the synthesis of complex natural products. Organoborane chemistry continues to play an important role in asymmetric synthesis. One of the important reactions that has become very common in the arsenal of synthetic chemists is allylboration and related reactions. Another important reaction that has recently attained enormous importance in organic chemistry is the ring-closing metathesis (RCM) reaction. Indeed, a combination of allylboration and RCM reactions provides an excellent route to cyclic ethers, lactones, lactams, etc. Herein, we describe a sequential asymmetric allylboration and RCM reaction protocol that has been utilized for the synthesis of several alpha-pyrone-containing natural products,particularly biologically active molecules.

Dehalogenation of 1,2-dichloro-diborane(4) derivatives with Na/K alloy does not lead to planar cyclo -tetraboranes but to the blue puckered diisopropylamino compound and to the yellow 2,2,6,6-tetramethylpiperidino-tetraboratetrahedrane derivative, respectively. With smaller dialkylamino substituents, the formation of orange-red cyclo -hexaborane (BNMe 2 ) 6 and the green closo -hexaborane (BNEt 2 ) 6 is observed. When a 1:1 mixture of Me 2 NBCl 2 and [Me 2 N(Cl)B] 2 is dehalogenated, a small amount of colorless crystals of the planar diamond-shaped tetrabora- bicyclo -butane [(Me 2 N) 2 BB -BNMe 2 ] 2 is obtained. Its MO analysis reveals that eight framework electrons are used to form two 3c,2e sigma bonds, one 4c,2e pi bond, and one 4c,2e sigma bond along the edges. A new approach to reactive closo -dicarbapentaboranes involves the hydroboration of dichloroboryl- t butylacetylene with HBCl 2 leading to Me 3 CCH 2 C(BCl 2 ) 3 . On heating, it is transformed into the closo -dicarbapentaborane (Me 3 CCH 2 C) 2 (BCl) 3 , in which the chlorine atoms may be substituted to give new derivatives. By reacting B 2 Cl 4 with C 5 Me 5 -SiMe 3 the nido -1-borane-2,3,4,5,6-pentacarbahexa- borane(6)is stabilized by the Lewis acid BCl 3 to give (MeC) 5 B-BCl 3 . Hydroboration of 3,4-bis(isopropylidene)-1,3-diborolanes leads to the formation of nido -2,3,5-tricarbahexaboranes(7),which may be deprotonated to give the corresponding anions isolobal with C 5 H 5 - . Heterotricarbahexaboranes are obtained when a 3,4-bis(dichloroboryl)-2,5-hexadiene derivative is reacted with heptamethyldisilazane and hexamethyl-disilthiane to give the corresponding 1,2,5-azadiborolane and thiadiborolane. Their hydroboration leads to the aza-and thia- nido -dicarbahexaboranes, respectively.

New decaborane-based, single-source molecular and polymeric precursors to boron carbide have been developed that enable the formation of boron carbide in processed forms, including nanofibers, nanocylinders, and nanoporous materials.

A review is presented on the new routes to the synthesis of the monocarbaboranes [ arachno -6-CB 9 H 14 ] – , arachno -4-CB 8 H 14 , and nido -1-CB 8 H 12 , that now have become excellent starting materials for the preparation of a family of seven-, eight-, and nine-vertex closo -monocarbaborane anions [2-CB 6 H 7 ] – , [1-CB 7 H 8 ] – , and [4-CB 8 H 9 ] – . These anions exhibit high stability and have a good chance to become new candidates for weakly coordinated anion chemistry. Discussed are also reactions leading new families of phosphacarboranes that contain one, two, and three phosphorus atoms in the cage. These compounds, namely closo - 2,1-PCB 8 H 9 , closo -6,1-PCB 8 H 9 , nido -7,8,9-PC 2 B 8 H 11 , nido -7,8,11-PC 2 B 8 H 11 , and nido - 7,8,9,10-P 3 CB 7 H 8 ,are structural analogs of the corresponding carboranes and are likely to exhibit similar chemical behavior.

New applications in which the carborane and metallacarborane species play a crucial role have been recently developed. In these applications, clusters display many particular characteristics that do not find parallel in their organic counterparts. The o -carborane can modulate the C···C distances depending on the substituents bonded to the cluster carbon atom and can lead to the formation of uncommon halogenophosphane charge-transfer “spoke ” complexes with iodine due to their electron-withdrawing capacity. Besides, metallacarboranes have been used as doping agents in organic conducting polymers and as catalysts in Kharasch reactions.

This paper outlines the development of our knowledge and understanding of the structures and bonding of boron cluster compounds, with particular reference to the evolving complementary roles localized bonding and molecular orbital treatments have played in providing simple rationalizations of their polyhedral molecules.

Although supraicosahedral (hetero) boranes have long been of interest to theoreticians, the area is under-developed from a synthetic viewpoint. The synthesis of supraicosahedral carboranes by reduction then capitation ( RedCap ) of C 2 B 10 species is attractive, but unsuccessful as long as the cage carbon atoms are free to separate in the reduction step. Studies on 4,1,6-MC 2 B 10 13-vertex metallacarboranes have shown that the partial degradation of such species can be a facile process, in spite of the fact that the binding energy of the metal atom to the carborane framework can be at least as high as that of a {BH}fragment. These findings support the general concept of the kinetic instability of 1,6-C 2 B 11 species, explaining why a supraicosahedral carborane could not be made from 1,2-C 2 B 10 H 12 . However, tethering together the two cage C atoms with a C 6 H 4 (CH 2 ) 2 strap ultimately allowed the synthesis of the first supraicosahedral carborane. This species has a henicosahedral geometry, and there is evidence that a facile rearrangement from kinetic to thermodynamic isomer has occurred. The RedCap synthesis of this unprecedented cluster has the potential to be applied successively, yielding 14-,15-,16-, etc. vertex carboranes, the larger of which may be sufficiently kinetically stable to exist without a C,C tether.

The controlled synthesis of carborane anions is a difficult challenge and is of great interest to synthetic chemists. By varying the bridge length of cage carbons-linked o -carboranes, the two cage carbon atoms are locked in place during the reactions, leading to the controlled synthesis of ortho -, meta - and para -isomer of nido -carborane dianions. The isomers of arachno -carborane tetraanions can also be prepared in the controlled manner via chang- ing the electronic configuration of the transition-metal ions or the bridge length. This article provides an overview of our recent work on this subject.

The evaluation of the Gd-carborane DTPA complex as a magnetic resonance imaging (MRI) and boron carrier agent was carried out in vivo. The MRI revealed that the Gd-carborane DTPA was metabolized slower in the body in comparison with Gd-DTPA.

Enolization–aldolization of conjugated ketones, enantioselective synthesis of benzofuryl beta-amino alcohols, and synthesis of p -dihydroxyborylphenylalanine (BPA) and its analogs are described. Aldolization of benzaldehyde with lithium dienolates derived from unhindered conjugated cyclohexenones favored anti- selectivity, whereas syn selectivity was favored for hindered cyclohexenones. Anti-aldols were preferentially formed from dienolborinates derived from conjugated cyklohexenones, however,competing aldolization at the 2-position was observed for hindered ketones. Benzofuryl beta-amino alcohols were prepared using as a key step the enantioselective reduction of the corresponding alpha-bromoacetylbenzofurans with (–)- B- -chlorodiisopinocampheylborane. Ionic liquids were used as solvents for the synthesis of BPA by the Suzuki cross-coupling reaction. The reaction time is short, and a solution of the catalyst in the ionic liquid can be recycled.