Diborane derivative

Substituted diboranes derived from the hydroboration of 1,3-butadiene l,2-tetramethylenediborane(6), 1 and l,2-bis(tetramethylene)-diborane(6), 2 undergo symmetrical and unsymmetrical cleavage reactions 12). 

The EPR spectra of the NHC boryl radicals that were generated through HAT to the ferf-butoxyl radical clearly show the delocalized 7i-type nature of these intermediates postulated to be essential by calculations [10, 12]. It was also demonstrated that the decay of the EPR signals could be fitted to a second-order decay having 2kt = 9 x 106 M-1 s-1. In agreement with this kinetic analysis, the NHC boryl radicals ultimately dimerize to give bis-NHC diborane derivatives. With the aid of EPR spectroscopy it was also established that the NHC boryl radicals readily abstract bromine atoms from primary, secondary, and tertiary alkyl bromides. However, chlorine atom abstraction is much slower and useful only for benzyl chloride. 

Nevalainen, V. Quantum chemical modeling of chiral catalysis. Part 17. On the diborane derivatives of chiral oxazaborolidines used as catalysts in the enantioselective reduction of ketones. Tetrahedron Asymmetry 1994, 5, 395-402. 

The reagent actually exists as the diborane derivative (1) rather than as the monomer.1 The Chemical Abstracts name for this reagent is tetra-3-pinanyldiborane. It has been referred to as Pn4B2H2. 

This diborane derivative is thermally unstable [reaction (29)]. 

The reaction of Li2AsPh, obtained from phenylarsine, with the diborane derivative B2(NMe2)Bt2, which is a stable... 

Compounds containing A1—C Bonds.— The A1- C coupling constants in dimeric AlMcs and its derivatives show remarkable similarities to B- H coupling constants in diborane derivatives. This arises from the similarity in the electronic structures of the two series, and can be accounted for by the Fermi-contact mechanism for spin-spin coupling. ... 

When a mixture of triphenylborane and diborane is heated to about 80° C in both the presence (65) and absence (66) of ether, 1,2-diphenyl-diborane is the only phenyl-substituted diborane obtained. This is in striking contrast to the above alkyl exchange in which all diborane derivatives except the 1,2-dialkyldiborane are found. 

Thus, far, all of the known carboranes have been derived either directly or indirectly from reactions of alkynes with various boron hydrides. In these reactions only the carbons participating in the triple bond of the alkyne become skeletal atoms in the carborane. No routes to the hypothetical four-carbon carboranes have been suggested. However, the dehydrogenation of the cyclic diborane derivatives prepared from butadiene 98, 100) should lead to C4BHS and C4B2H6. Unfortunately, such a conversion may require electric discharge conditions, in which case extremely low yields can be expected. 

G in the presence of a catalytic amount of a Lewis base such as dimethylether, (GH2)20. In addition to the gas-phase pyrolysis of diborane, can be prepared by a solution-phase process developed at Union Garbide Gorp. Decaborane is a key intermediate in the preparation of many carboranes and metaHa derivatives. As of this writing, this important compound is not manufactured on a large scale in the western world and is in short supply. Prices for decaborane in 1991 were up to 10,000/kg. 

Boron Hydrides" under "Boron Compounds" in ECT 1st ed., Vol. 2, pp. 593—600, by S. H. Bauer, Cornell University "Boron Hydrides and Related Compounds" under "Boron Compounds," Suppl. 1, pp. 103—130, by S. H. Bauer, Cornell University "Diborane and Higher Boron Hydrides" under "Boron Compounds," Suppl. 2, pp. 109—113, by W. J. Shepherd and E. B. Ayres, Gallery Chemical Company "Boron Hydrides" under "Boron Compounds" in ECT 2nd ed., Vol. 3, pp. 684—706, by G. W. Campbell, Jr., U.S. Borax Research Corporation "Boron Hydrides and their MetaHo Derivatives" under "Boron Compounds," in ECT 3rd ed., Vol. 4, pp. 135—183, by R. W. Rudolph, The University of Michigan. 

The most successful of the Lewis acid catalysts are oxazaborolidines prepared from chiral amino alcohols and boranes. These compounds lead to enantioselective reduction of acetophenone by an external reductant, usually diborane. The chiral environment established in the complex leads to facial selectivity. The most widely known example of these reagents is derived from the amino acid proline. Several other examples of this type of reagent have been developed, and these will be discussed more completely in Section 5.2 of part B. 

Olefins may be obtained by elimination from the organoboron intermediates formed from enol derivatives and diborane. " As in the reaction with a,jS-unsaturated ketones (section IX), the conversion is carried out in two parts first formation of the diborane adduct and second, decomposition in refluxing acetic anhydride. 

Condensation of adipic acid derivative 17 with phenylethylamine in the presence of carbo-nyldiimidazole affords the bis-adipic acid amide 18. The synthesis is completed by reduction of the carbonyl groups with diborane followed by demethylation of the aromatic methoxy groups with hydrogen bromide the afford dopexamine (19) [3]. 

A mixture of d- and l- hexoses also results from the hydroboration of these 5-enes. Hydroboration results in anti-Markownikoff, cw-hydration of the double bond and the amount of each hexose formed varies according to the nature of the substituent groups. For example, hydroboration (23) of methyl 6-deoxy-a-D-ryZo-hex-5-enopyranose (3) affords methyl a-D-glucopyranoside and methyl / -L-idopyranoside in the ratio of 1 2.5 respectively whereas hydroboration of the fris-trimethylsilyl ether of 3 afforded them in the ratio 1 0.6 respectively. The hydroboration method can be used to achieve specific labelling of hexoses with tritium methyl-/ -L-idopyranoside[5-H3] and methyl a-D-glucopyranoside [5-H3] were thus prepared (23). Similarly, hydroboration of the D-Zt/ro-hex-5-eno derivative (14) with diborane-H3 followed by removal of the isopropyli-dene group, afforded methyl a-D-mannopyranoside [5-H3] and methyl / -L-gulopyranoside [5-H3] in the ratio of 1 2 respectively (23). 

A. Pelter, Reductions Involving Diborane and Its Derivatives, Chem. Ind. 1976, 888. 

Table 4.3 provides some data on the regioselectivity of addition of diborane and several of its derivatives to representative alkenes. Table 4.3 includes data for some mono- and dialkylboranes that show even higher regioselectivity than diborane itself. These derivatives are widely used in synthesis and are frequently referred to by the shortened names shown with the structures. 

In accord with the resonance structure drawn, there is little B=B bonding in this diborane(4) derivative and the B—B distance is found to be 1.859 A,68 which is considerably longer than even the normal expected value of 1.7 A for a boron-boron single bond.67 This is consistent with the normal repulsion of negative charges on adjacent atoms not stabilized by 7r-bonding. 

However, the successful isolation of an iminoborane derivative in the reaction of trichloroacetonitrile with diborane indicated that the stability of imino-boranes is not only a function of the nature of the borane but also of that of the imine 16>. 

The compound C13CH=NBH2 is obtained upon reaction of trichloroaceton-itrile with diborane during the synthesis of 1,3,5-trichloroethylborazine (Eq. (13) 16)) it is undoubtedly not monomeric as indicated by its i.r. spectrum (v BH2 cm-1, v C=N 1705 cm-1 22>) Most likely the material is dimeric and it is difficult to isolate in pure form due to the ready conversion to the bor-azine derivative. 

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