B-alkylboracyclanes

One of the first synthetic applications of organoboranes in radical chemistry is the conjugate addition to enones (Scheme 23, Eq. 23a) and enals reported by Brown [58-61]. Addition to -substituted enones and enals are not spontaneous and initiation with the oxygen [62], diacetyl peroxide [63], or under irradiation [63] is necessary (Eq. 23b). A serious drawback of this strategy is that only one of the three alkyl groups is efficiently transferred, so the method is restricted to trialkylboranes derived from the hydroboration of easily available and cheap alkenes. To overcome this limitation B-alkylboracyclanes have been used but this approach was not successful for the generation of tertiary alkyl radicals [64,65]. 

However, conjugate additions with trialkylboranes are inefficient in that only one of the three organic groups is transferred. The inefficiencies of group transfer are surmounted by the use of either the B-alkylboracyclanes (55)37 37b or B-alkyldiphenylboranes (56),37c in which the B-alkyl groups arc preferentially transferred under radical conditions. The B-alkylboracyclanes are obtained by sequential... 

Under similar conditions, B-alkylboracyclanes also undergo alkyl exchange. The heterocyclic ring, being exceptionally stable, does not take part in the reaction (8). 

Fig. 1. Exchange of alkyl groups between B-alkylboracyclanes with 5-, 6-, and 7-membered rings. Column 2 meters, squalane on Chromosorb 0.3-0.4 mm carrier gas helium, 100 ml/min inlet pressure 0.7 atg temperature 100°C sample size 10-15 /ixl.

Simple boron heterocycles, such as B-alkylboracyclanes, give equilibrium... 

Bis(boracyclyl)alkanes 23, 27), formed by the hydroboration of diolefins with B2H6 23) at room temperature or from trialkylamine boranes at high temperatures 27), isomerize in the same way as B-alkylboracyclanes when heated. The equilibrium mixture of B2(C H2 )3 (XVIII) ( = 5, 6) was determined, before and after thermal isomerization, by oxidation and hydrolysis to diols 2J). 

Exact determination of the thermal equilibrium mixture from (XVIII), and also, in general, for mixtures of compounds with more than one boron atom, is more difficult than for simple B-alkylboracyclanes, because direct gas chromatographic separation is almost impossible, due to the low vapor pressures and the high thermal instability of the compounds. 

B-alkylboracyclanes (XXXVII) may be readily prepared by reaction of dienes with trialkylamine-borane, in the presence of trialkylboranes, at 120-140° C (5). 

The B-alkylboracyclanes are the most stable products under these conditions, because an alkyl exchange is possible in the presence of BH compounds. Therefore, above 120° C, polymeric organoboranes are transformed, partly because of the dehydroboration-hydroboration equilibrium of B-alkylboracyclanes and other lower organoboranes, e.g.,... 

Trialkylboranes, R3B(R > C2), react with dienes (27) above 120° C to give displacement (15, 33, 57) of the alkyl groups B-alkylboracyclanes can be isolated from the mixture, e.g.. 

In addition to five- and six-membered rings, traces of a seven-membered ring can be found. The isomeric mixture is similar to that gotten from thermal isomerization of B-alkylboracyclanes from 1,5-hexadiene and trialkylboranes or alkyldiboranes. Oxidation and hydrolysis give primarily 1,5-hexanediol, along with the 1,4- and 1,6-diols. The side chain R is mostly... 

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