Rearrangements of organoboranes

Table 1. Thermal rearrangement of organoboranes resulting from the hydroboration of tetrasubstituted cyclopentene derivatives of type 34.

The activation of non-activated C-H bonds is an important research field 14), In most cases, transition metal complexes have been used for this purpose 14-19). In this chapter, we wish to describe a stereoselective allylic C-H activation involving the thermal rearrangement of organoboranes (Scheme 7) 20-22). The observed stereochemistry may be best explained by a dehydroboration-rehydroboration mechanism, but mechanistic studies indicate that a more complex pathway involving a second molecule of BH3 may be involved. 

Complexation is believed to be involved in the first step of the hydrobo-ration of alkenes and alkynes rearrangement reactions of organoboranes most likely involve intermediates of a rc-complex type. However, the stability of such complexes is generally too low to allow their isolation (112). However, evidence for 7t-complex formation has been obtained by the device of anchoring the alkene function to the metal atom in question by means of a... 

The synthetic value of the reaction lies in the modification of these organoboranes. The commonest reaction involves the decomposition of the borane by alkaline hydrogen peroxide. The highly nucleophilic hydroperoxide anion attacks the electron-deficient boron with the formation of an ate complex. Rearrangement of this leads to the formation of a borate ester which then undergoes hydrolysis to an alcohol in which an oxygen atom has replaced the boron (Scheme 3.15). The overall outcome of this reaction is the anti-Markownikoff hydration of the double bond. The regiochemistry is the reverse of the acid-catalysed hydration of an alkene. The overall addition of water takes place in a cis manner on the less-hindered face of the double bond. 

The reaction of alkenes with borane, monoalkyl and dialkylboranes leads to a new organoborane (see 15-16). Treatment of organoboranes with alkaline H2O2 oxidizes trialkylboranes to esters of boric acid." This reaction does not affect double or triple bonds, aldehydes, ketones, halides, or nitriles that may be present elsewhere in the molecule. There is no rearrangement of the R group itself, and this reaction is a step in the hydroboration method of converting alkenes to alcohols (15-16). The mechanism has been formulated as involving initial formation of an ate complex when the hydroperoxide anion attacks the electrophilic boron... 

Compare this rearrangement with a possible rearrangement path of organoborane complexes. (See Sect. II.D. 1.)... 

Organoboranes can be used for the synthesis of various organic compounds containing an amino group . The most common procedures involve the reaction of organoboranes with azides, with hydroxylamine derivatives, or with chloroamines. All amination reactions proceed via intermediate formation of a borate complex followed by anionotropic rearrangement to form the new C—N bond (Equation (21)). 

A more convenient procedure for the synthesis of (113) from (5) was later elaborated <9UOM(4l2)l>. It is based on the intramolecular version of the reaction of organoboranes with organic azides. The THF complex of 1-boraadamantane (5a) is treated with iodine in the presence of an excess of sodium azide. The iodine atom in the intermediately formed borabicycle (118) undergoes an easy nucleophilic substitution by azide ions. The subsequent anionotropic rearrangement in the borabicyclic azide (119) leads (after the oxidation of the reaction mixture) to aminoalcohol (120), which is smoothly converted to 1-azaadamantane (113). The yield of (113) in this synthesis is 40-45% based on (5a), or 20-22% based on triallylborane (Scheme 44). 

The carbonylation of organoboranes presumably proceeds via a weak complex RjB-CO which rearranges in a similar way to an acylborane. This can be converted as shown into a variety of organic products. In several of these reactions, however, only one or two of the three R groups on boron are used. 

The boron-zinc exchange is an unique way for preparing chiral secondary alkylzinc reagents which are configurationally stable over a wide temperature scale. Coupled with the thermal rearrangement of tertiary organoboranes, a broad range of open-chain and cyclic polyfunctional molecules have been prepared. In addition, several examples of a diastereoselective remote C-H activation have been studied. 

Ring expansion of 1,3,2-diazaboracycloalkanes with phenyl isocyanate and isothiocyanate has resulted via insertion of two atoms of the double bond system into the ring. Similarly, three new heterocyclic organoboranes have been produced from heterocyclic aminoboranes and organic isocyanates. The synthesis and thermal rearrangement of boron carbamoylaminopyridin-ates of the type (30) and (31) have been discussed. ... 

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