Boron, diboron compounds

This chapter will concentrate primarily on features of the chemistry of diboron compounds of particular interest from the organometallic point of view. While our limited scope does not permit a comprehensive review of all aspects of diboron chemistry, we will initially survey some general features of the subject, with emphasis on unique synthetic aspects and on properties particularly characteristic of compounds containing a simple electron-pair bond between boron atoms. Previously, we published several review articles provided some aspects on boron chemistry,8 16 and including natural boron-containing compounds.17,18... 

Synthesis of diboron compounds may involve either reductive coupling reactions of monoboron derivatives to form the boron-boron bond,19 or reactions of compounds possessing preformed B2 fragments.20... 

Boron compounds, subvalent, 193-194 Boron-containing compounds, See also Diboron compounds Boryl complexes, 165 Borylene complexes bridged, 166-176 molecular structure data, 172 reactivity, 170-171 spectroscopic and structural aspects, 172-176... 

Synthesis of specific diboron compounds may involve either (1) reductive coupling reactions of monoboron derivatives to form materials containing the boron-boron bond, or (2) reactions of compounds possessing preformed B2 fragments. Since the formation of the boron-boron bond is in a sense the central synthetic difficulty in diboron chemistry, it is of interest to consider in some detail the approaches of various workers to this problem. 

Little or no detailed physical information is available concerning diboron compounds with organic groups bonded to boron. Most of the available... 

Most workers have presumed that the addition reaction proceeds via TT-donation from the olefin into the vacant/) orbitals of the diboron compound (55). Stereochemical evidence (discussed more fully below. Section III,B) is in general agreement with such a four-center mechanism, and there is as yet no convincing evidence for an alternative. The involvement of the boron p orbitals is clearly indicated by the lack of reactivity of B2Cl4-base complexes as well as by the failure to obtain addition with compounds such as the aminodiborons in which the coordinative saturation at the boron atoms can be removed or reduced by pir-pn bonding. 

The six-membered ring heterocyclics containing two adjacent boron atoms, and are thus diboron derivatives, have been prepared and the shifts compared with the corresponding five-membered ring borolanes. There was a surprising degree of similarity between the pairs of compounds (40 and 41 X = 0 or S and Y = C1 or NMe2). The shifts of these and other diboron compounds are shown in Table X. 

Diboron compounds are a part of a relatively unexplored class of compounds. Their main use is for p-boration reactions where a boron centre is rendered nucleophilic with the use of a metal catalyst or a Lewis base to form a sp -sp diboron compound. The reactivity of these... 

Compounds 167-171 outlined in Fig. 43 form another series of diboronic acids that form complexes with mono- and disaccharides. In these cases the asymmetrical immobilization of chromophoric functional groups, e.g., aromatic rings in 167-170 or Fe -complexation with the related boronate 171, can be analyzed by circular dichroism measurements [256-262]. 

The metal catalysed hydroboration and diboration of alkenes and alkynes (addition of H-B and B-B bonds, respectively) gives rise to alkyl- or alkenyl-boronate or diboronate esters, which are important intermediates for further catalytic transformations, or can be converted to useful organic compounds by established stoichiometric methodologies. The iyn-diboration of alkynes catalysed by Pt phosphine complexes is well-established [58]. However, in alkene diborations, challenging problems of chemo- and stereo-selectivity control stiU need to be solved, with the most successful current systems being based on Pt, Rh and An complexes [59-61]. There have been some recent advances in the area by using NHC complexes of Ir, Pd, Pt, Cu, Ag and Au as catalysts under mild conditions, which present important advantages in terms of activity and selectivity over the established catalysts. 

The widespread use of aryl boronic acids or aryl boronates in various metal-catalysed C-C bond-forming reactions has created a substantial demand for these versatile nncleophiles. A general procedure for the preparation of these compounds, based on a NHC/Pd catalysed coupling, has been reported by Fiirtsner and co-workers nsing aryl chlorides and the tetraalkoxy diboron derivative 27 as conpling partners. Very good yields were obtained in several cases especially when electron poor aryls were employed [169]. Milder reaction conditions can be achieved when diazonium salts are used instead of chlorides [170] (Scheme 6.51). 

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