Volume 78, Issue 7

This volume summarizes the present status of research in the field of organic and inorganic boron chemistry, presented by the invited speakers at the 12th International Meeting on Boron Chemistry (IMEBORON-XII), held in Sendai, Japan, 11-15 September 2005. IMEBORON-XII consisted of 1 plenary lecture, 22 invited lectures, 24 keynote lectures, 48 short oral presentations, and 99 poster presentations. In all, 280 chemists contributed to a truly international meeting, with participants representing China, Czech Republic, Germany, Israel, Japan, Mexico, Poland, Russia, South Korea, Spain, Sweden, the United Kingdom, and the United States. The presentations at IMEBORON-XII covered all aspects of boron chemistry including theoretical studies, synthetic methods of organic and inorganic boron compounds, novel molecular structures, application of organic and inorganic boron compounds to organic synthesis as catalysts or as reagents, medicinal applications, and creation of new materials (liquid crystals, supramolecular clusters, nanocylinders, molecular electronic devices, nanomachines, ceramics, etc.). Not only the distinguished senior members of the boron community, but also young boron chemists took an active part in the conference. Not only traditional boron chemistry, but also new evolving research areas of boron chemistry were presented. Accordingly, I feel that a new generation of both researchers and research fields is coming in boron chemistry. A selection of the invited contributions to IMEBORON-XII is presented in the 14 papers in this issue. The importance of scientific exchange in this field was recognized during IMEBORON-XII. Therefore, the continuation of this series of conferences was discussed, and the venue for IMEBORON-XIII in 2008 will be organized by Prof. F. Teixidor at the Institut de Ciencia de Materials de Barcelona, C.S.I.S., Campus U.A.B., Ballaterra, Spain. Yoshinori Yamamoto Chairman of IMEBORON-XII

Interest has recently intensified in the search for molecular motors and actuators capable of delivering useful work to nanodevices under the control of electrochemical or photochemical power sources. While many of these man-made molecular machines are designed to deliver rectilinear motion, very few are proposed for the controlled delivery of rotary motion on the time scale characteristic of intramolecular rearrangements. The adaptation of commo -bis-dicarbollide metallacarborane structures to the possible design and synthesis of such rotary molecular motors is now under investigation. Progress toward this goal will be reported.

The study of compounds containing boron continues to have an important impact on virtually every area of chemistry. One of the few areas in which compounds of boron have been neglected is crystal engineering, which seeks to develop and exploit an understanding of how the structure and properties of crystals are related to the individual atomic or molecular components. Although detailed predictions of crystal structures are not yet reliable, crystal engineers have developed a sound qualitative strategy for anticipating and controlling structural preferences. This strategy is based on the design of special molecules, called tectons, which feature carefully selected cores and multiple peripheral functional groups that can direct association and thereby place neighboring molecules in predetermined positions. Recent work has demonstrated that molecular crystals with unique properties can be constructed logically from tectons with boron in their cores or sticky sites of association. In particular, the -B(OH) 2 group of boronic acids engages in reliable patterns of hydrogen bonding, and its use as a sticky site in tectons has emerged as an effective tool for controlling association predictably. In addition, replacement of tetraphenylsilyl or tetraphenylmethyl cores in tectons by tetraphenylborate leaves the overall molecular geometry little changed, but it has the profound effect of introducing charge. Tectons derived from tetraphenylborate can be used rationally to construct porous charged molecular networks that resemble zeolites and undergo selective ion exchange. In such ways, boron offers chemists exciting new ways to engineer molecular crystals with predetermined structures and properties.

The reactions of alkynes with nido- dimetallaboranes containing monocyclopentadienyl metal fragments are highly dependent on metal identity. Typical metals from groups 6 and 7 do not react under mild conditions, Rh gives exclusively alkyne cyclotrimerization via a cluster-catalyzed reaction, and Ru reacts predominantly via insertion to give new ruthenacarborane clusters. The facile reaction observed for Ru permits the characterization of intermediates exhibiting novel structural features, e.g., external cluster-bridging monoborane or alkylidene fragments. Despite the large differences in reactivity, circumstantial evidence suggests the alkyne adds to and opens the metallaborane to an arachno- structure. The metal present determines the character of the metallaborane, which in turn determines whether further reaction occurs. For Ru, it is the hydrogen-rich character fostered by the metal fragment that leads to the copious chemistry observed.

A summary of the latest results in the authors' laboratories covering three areas is presented. The results of the controlled syntheses of oxide ion encapsulating lanthanacarboranes, by the introduction of stoichiometric amounts of H 2 O in the synthesis reactions, and the strictures of the resulting compounds were discussed. The scope and limitations of a general method for the synthesis of the open pentadienyl complexes were presented. In addition, the results of a systematic study of the reactions of the [2, n -(SiMe 3 ) 2 - nido -2, n -C 2 B 4 H 4 ] 2- ( n = 3, 4) with the late transition-metal salts MCl 2 (M = Fe, Co, Ni) were discussed.

The preparation and chemistry of bis- and tris-mercaptoborates based on closo -B 12 and closo -B 10 structures is described. These species undergo a number of reactions such as halogenation and alkylation without changing the closo -borane core. Addition of the third dimethylsulfide substituent to the closo -boranes is thought to proceed via an unusual cationic intermediate.

In contrast to reactions that have been observed in traditional organic solvents, decaborane olefin-hydroboration and alkyne-insertion reactions proceed in ionic liquid (IL) solvents without the need of a catalyst. These reactions now provide important new, high-yield synthetic pathways to functionalized decaborane and ortho -carborane clusters. As illustrated by the synthesis of n -B 18 H 22 , ILs can also provide an inert reaction medium for carrying out dehydrocondensation reactions leading to higher fused cage compounds.

It is shown that reactions of triallylborane with pyrrole, pyridines, isoquinolines, lactams, 1-pyrroline, and acetylenes offer versatile methodology for the construction of various bicyclic and polycyclic nitrogen compounds, some of which are skeletally related to important classes of alkaloids. Optically active 3-borabicyclo[3.3.1]non-6-enes are useful precursors for synthesis of chiral 3-aza- and 3-thiabicyclo[3.3.1]non-6-enes, as well as derived chiral cyclohexenoid systems. The convenient methodology for the transformation of 1-boraadamantanes into 1-azaadamantanes is also discussed.

Direct C-H borylation of aromatic compounds catalyzed by a transition-metal complex was studied as an economical protocol for the synthesis of aromatic boron derivatives. Iridium complexes generated from Ir(I) precursors and 2,2'-bipyridine ligands efficiently catalyzed the reactions of arenes and heteroarenes with bis(pinacolato)diboron or pinacolborane to produce a variety of aryl- and heteroarylboron compounds. The catalytic cycle involves the formation of a tris(boryl)iridium(III) species and its oxidative addition to an aromatic C-H bond.

The utilization of aminoborane derivatives in amination reactions such as Strecker-type aminative cyanation, Mannich-type reaction, and reductive amination is described. Bis(dialkylamino)cyanoboranes and bis(dialkylamino)boron enolates underwent the concurrent transfer of the amino group and either the cyano or the enoxy group from the boron to carbonyl carbon atom in their reaction with aldehydes, leading to the formation of α-amino nitriles and β-amino ketones, respectively. Bis(dialkylamino)borane derivatives that lack the additional nucleophilic groups on the boron atoms were found to serve as effective generators of iminium ions from aldehydes. In the presence of these aminoboranes, reactions of aldehydes with ketene silyl acetals or silyl enol ethers take place, giving Mannich-type products, i.e., β-amino esters and ketones, selectively. Using diisopropylamino-substituted boron compounds, which are designed as "universal" iminium ion generators, reactions of free sec -amines, aldehydes, and ketene silyl acetals proceed efficiently, giving β-amino esters in which the amino groups are derived exclusively from the free amines. The use of the universal iminium ion generator is also effective for reductive amination of aldehydes, in which NaBH 4 is used as a hydride donor.

The asymmetric allylboration of N -H aldimines, generated from either N -trimethylsilyl or N -diisobutylalanyl precursors, with B -allyl-9-borabicyclo[3.3.2]decane is described. The desired homoallylic amines are obtained efficiently (60-90 %) and selectively (60-89 % ee). A non-oxidative work-up procedure has also been developed for this new method, which permits the recovery of the air-stable pseudoephedrine (PE) complex (50-70 %), which is conveniently converted back to B -allyl-9-borabicyclo[3.3.2]decane (98 %) with allylmagnesium bromide in ether for its reuse in the asymmetric allylboration process. Additional studies were conducted to better understand these processes and the origin of the observed enantioselectivity.

Among the plethora of protocols for the preparation of chiral homoallylic amines, the use of boron-based reagents remains relatively undeveloped. However, the recent advances in the use of α-pinene-based versatile reagents for the synthesis of such amines confirmed that very high enantioselectivity and outstanding diastereoselectivity can be readily achieved. Addition of the "allyl"boron reagents to various N-substituted imines provided the desired amine products in high yields and high to very high ee. The discovery that an addition of 1.0 equiv of methanol or water to the "allyl"boration reaction with N -masked imines is critical allowed for higher yields and noticeably improved ee. The use of N -aluminoimines, which are not only easy to prepare by a partial reduction of nitriles, but are also relatively stable for both enolizable and non-enolizable substrates, considerably expanded the scope of the reactions. In this review, the developments in the syntheses of chiral homoallylic amines using organoboranes, with the particular accent on the reagent-controlled reactions, are summarized. Additionally, the novel methodology for the crotyl- and alkoxyallylboration of imines using trialkylboron "ate" complexes is described.

Novel organoboron polymers containing transition metals (Ni, Pd, Pt) and pyrazaboles in the main chain were prepared by Cu-catalyzed coupling polymerization. The Ni- and Pt-containing polymers showed MLCT (metal-to-ligand charge transfer) absorption, whereas the Pd-containing polymer did not demonstrate the absorption. Only the Pd-containing polymer exhibited the emission spectrum with a vibronic structure. These differences in the optical properties were studied in detail.

For the molecular design of new π-electron materials, the incorporation of main group elements into the π-conjugated frameworks is a powerful approach to modifying the nature of the parent π-conjugated systems. In particular, the group 13 boron is of interest, since the boron element has several characteristic features, such as an effective orbital interaction with the π-conjugated frameworks through the vacant p-orbital (i.e., p π -π* conjugation), high Lewis acidity, and trigonal planar geometry. By exploiting these features of the boron atom, we have designed and synthesized several types of new π-electron materials, including trianthrylborane- or dibenzoborole-based π-conjugated systems as a new fluoride ion sensor, boryl-substituted thienylthiazole as a new building unit for electron-transporting materials, and B , B ', B ''-trianthrylborazine (B 3 N 3 )-based materials as a model of the bundled system of π-conjugated frameworks.

Novel methods are described for the preparation of alkyldimethylamine cyanoboranes and β-hydroxylalkyldimethylamine cyanoboranes by C-lithiation of trimethylamine cyanoboranes followed by reaction with alkyl halides, aldehydes, and ketones. Lithiation of the monobromo derivatives of amine cyanoboranes led to the synthesis of the first examples of diborane derivatives of amine cyanoboranes. Bromo derivatives of amine cyanoboranes and amine carboxyboranes have been synthesized by new simple and efficient methods. Amine fluorocyanoboranes and amine fluorocarboxyboranes, new classes of compounds, have been prepared from the bromo precursors by fluorine/bromine exchange using fluorinating reagents such as AgF and Et 3 N.3HF. Eight different derivatives of oxazaborolidines were synthesized and evaluated for their affect on Streptococcus mutans viability, adhesion, and biofilm formation using 3 [H]-thymidine labeled bacteria, and fluorescent stained bacteria. This is the first reported antibacterial activity of this class of compounds. The minimal inhibitory concentration (MIC) values ranged from 0.26 to 10 mM. Structure-activity relationship was observed. The B-butyl moiety of the oxazaborolidines contributed an anti-adhesion effect for all derivatives, while its effect diminished when the boron atom was incorporated in a fused heterocyclic ring. The B-phenyl group induced bacterial adhesion in all tested compounds. In a separate study for boronated saccahrides and enzymatic inhibition, the complex formation between N -butylboronic acid and a series of monosaccharides was investigated by 1 H, 13 C, and 11 B NMR spectroscopy and gas chromatography-mass spectrometry (GC-MS). Then, a series of boronic acid compounds with protease inhibition properties were prepared. The effect of added mono-, di-, and polysaccharides on the inhibitory activity of these compounds was studied. Potassium organotrifluoroborates were found to be reversible competitive inhibitors of α-chymotrypsin and trypsin. Based on 19 F NMR, it was speculated that they inactivate the enzymes as a result of the formation of hydrogen bonds between fluorine atoms of the inhibitors and the serine protease.

Differential scanning calorimeters (DSCs) are widely used for temperature, heat capacity, and enthalpy measurements in the range from subambient to high temperatures. The present recommendations describe procedures and reference materials (RMs) for the calibration of DSCs. The recommendations focus on the calibration of the response of the instrument and on the estimation of the measurement uncertainty. The procedures for temperature, enthalpy, and heat-flow rate calibration are given in detail. Calibration on cooling has also been considered. Recommended RMs are listed, and the relevant properties of these materials are discussed.