Boranes conjugate addition

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 conjugate addition of phosphorus nucleophiles of various oxidation states and in neutral or metallated form constitutes an efficient and well-known method for C—P bond formation [30]. In the case of phosphanes as nucleophiles especially, the corresponding phosphane-borane adducts have been used in 1,4-additions to Michael acceptors. Following the idea to use a chirally modified phosphorus nucleophile in asymmetric Michael additions to aromatic nitroalkenes, we synthesized the new enantiopure phospite 45 starting from TADDOL (44) with nearly quantitative yield. Due to the C2 symmetry, of the... 

A large number of aza crowns are made by these routes. Stepwise addition of CH2CH2NHTs groups occurs readily by reaction of the appropriate tolu-enesulfonamide with 2-bromoacetamide followed by borane reduction and A-tosylation. If a trimethylene spacer is desired, then reaction of the tosyla-mide with acrylonitrile followed by reduction and tosylation gives good yields of the desired product (Scheme 1.2). An example of such a sequence is provided by the synthesis of the symmetrical [24]-N6 cycle, 4 (Scheme 1.3).9 In this particular case, introduction of the desired C3 spacer was undertaken on the. electrophilic component by conjugate addition of the tosylamide with methyl acrylate followed by reduction with LiAlH4 and mesylation. 

Conjugate Addition of Boranes to Activated Double Bonds Hydro-alkyl-addition (overall transformation)... 

Reactions of chiral allylic boranes with carbonyl compounds Reactions of chiral allyl boranes with imines Asymmetric Addition of Carbon Nucleophiles to Ketones Addition of alkyl lithiums to ketones Asymmetric epoxidation with chiral sulfur ylids Asymmetric Nucleophilic Attack by Chiral Alcohols Deracemisation of arylpropionic acids Deracemisation of a-halo acids Asymmetric Conjugate Addition of Nitrogen Nucleophiles An asymmetric synthesis of thienamycin Asymmetric Protonation... 

This commoner type of reaction involves the attack of carbon or heteroatom nucleophiles onto carbonyl compounds, by direct or conjugate addition, and onto imines. Because we have just dealt with boranes we shall start with the reaction of allylic boron compounds with such electrophiles. You might strictly not call this nucleophilic attack. 

Conjugate addition of nitroalkanes to a,6-unsaturated carbonyl compounds has been achieved using basic alumina, in the absence of a solvent.36 3-Nitropropanal has been prepared by conjugate addition of nitrite anion to acrolein reduction to 3-nitropropanol was effected with borane-dimethyl sulphide.36 -Nitro-acids and u-nitro-esters have been prepared by ring cleavage of 2-nitrocycloalkanones (Scheme 163).362... 

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