Trialkylboranes, “conjugate addition

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]. 

Brown proposed a mechanism where the enolate radical resulting from the radical addition reacts with the trialkylborane to give a boron enolate and a new alkyl radical that can propagate the chain (Scheme 24) [61]. The formation of the intermediate boron enolate was confirmed by H NMR spectroscopy [66,67]. The role of water present in the system is to hydrolyze the boron enolate and to prevent its degradation by undesired free-radical processes. This hydrolysis step is essential when alkynones [68] and acrylonitrile [58] are used as radical traps since the resulting allenes or keteneimines respectively, react readily with radical species. Maillard and Walton have shown by nB NMR, ll NMR und IR spectroscopy, that tri-ethylborane does complex methyl vinyl ketone, acrolein and 3-methylbut-3-en-2-one. They proposed that the reaction of triethylborane with these traps involves complexation of the trap by the Lewis acidic borane prior to conjugate addition [69]. 

The reaction between trialkylboranes and enones has found some interesting synthetic applications. An example is the preparation of prostaglandin precursors from exo-methylene cyclopentanone, generated in situ from a Mannich base. After dehydrogenation, a second conjugate addition of tri-octylborane was used to introduce the w-chain (Scheme 25) [70]. 

PTT = phenyl trimethylammonium perbromide Scheme 25 Conjugate addition of a functionalized trialkylborane... 

Toru has investigated the stereoselectivity of the conjugate addition of trialkylboranes to 2-arylsulfinylcyclopentenones. Excellent stereocontrol is achieved with different alkyl radicals (Scheme 27) [73-76]. In the acyclic series, the lack of diastereoselectivity in the addition step and a competitive Pummerer rearrangement have limited the synthetic potential of this reaction [77]. 

Brown and co-workers have established the feasibility of the conjugate addition of trialkylboranes to a,jS-unsaturated ketones (eq. [36]) (56), and one investigation has addressed the question of... 

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... 

Mixing trialkylboranes with a, 3-unsaturated carbonyl compounds in the presence of water gives conjugate addition of one of the alkyl groups from the boron. The carbonyl compound can be an aldehyde (R = H) or a ketone (R = alkyl). 

Addition to ot, -acetylenic esters and ketones (3, 108 6, 163-164). The addition of organocopper reagents to conjugated acetylenic carbonyl compounds is usually not stereospeciflc, although cis-addition predominates. Japanese chemists have found that the stereoselectivity in the reaction with alkylcopper reagents is markedly enhanced by use of the complex RCu BRj. Thus the reaction of dimethyl acetylenedicarboxylate with n-butylcopper complexed with tri-n-butylboron (or triethylboron) results in exclusive formation of the cis-adduct. In the absence of a trialkylborane the cis- and adducts are formed in the-ratio 85 15. 

Homolytic cleavage of the carbon-boron bond of a trialkylborane can be promoted by oxygen. The so-formed alkyl radical can be used in synthesis (for radical reactions, see Section 4.1). Indeed, triethylborane in air can be used to generate radicals from precursors such as alkyl iodides or selenides. Hydroboration followed by addition of an a,(3-unsaturated aldehyde or ketone leads to transfer of an alkyl group from the boron atom via an alkyl radical intermediate. The reaction takes place by addition of the alkyl radical to the conjugated system to form an enol borinate, hydrolysis of which gives the aldehyde or ketone product (5.37). 

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