Trialkylboranes with acrolein

Reaction with trialkylboranes. 3-Butyne-2-one, like acrolein (this volume), does not react spontaneously with trialkylboranes, but in the presence of catalytic amounts of oxygen it reacts readily to give a,/3-unsaturated methyl ketones (2)... 

Reaction with trialkylboranes. The reaction of trialkylborancs with acrolein (2, 15) and with methyl vinyl ketone (2. 284-185) yields, respectively, aldehydes and methyl ketones. However, the reaction is not directly applicable to all a,/3-unsaturated carbonyl compounds. Thus /ram-crotonaldehyde (I) and mesityl... 

Borane, monoalkylboranes and dialkylboranes (such as thexylborane and 9-BBN) were shown to be powerful reducing agents for carbonyl derivatives (sec. 4.6.A). When the B—H unit is removed, as in trialkylboranes, addition to the alkene moiety of a,p-unsaturated ketones or aldehydes occurs. An alkyl group of the borane is transferred to the terminal position of the alkene moiety (1,4-addition) and boron is transferred to the oxygen, giving a boron enolate (sec. 9.4.D). Initial hydroboration of acrolein with tricyclopentylborane gave boron enolate 196.1 0... 

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]. 

Homopropargylic alcohols ot-allenic alcohols. The synthesis of alkylallenes (5, 397) by the reaction of trialkylboranes and lithium chioropropargylide, ClCH2C=CLi (1), has been extended to these two systems. Thus addition of acrolein to the organoborane a can lead to either 2 or 3, depending on the temperature at which a is kept before reaction with the aldehyde (equation I). If the aldehyde is... 

Conversion of olefins into esters. Brown1 has extended the one-carbon homologation of olefins with carbon monoxide and the three-carbon homologation of olefins with acrolein to a two-carbon homologation with ethyl bromoacetate. The olefin is converted into the trialkylborane in THF at 0° by addition of the calculated quantity of diborane in THF. An equimolecular quantity of ethyl bromoacetate is added, followed by an equimolecular quantity of potassium f-butoxide in t-butanol. The reaction is apparently complete immediately. Ethyl chloroacetate can also be used, but the reaction is somewhat slower and gives slightly lower yields. 

A trialkylborane. for example (3) resulting from reaction of hexene-1 (1) with diborane (2. generated from NaBH4-l- BFa). undergoes remarkably fast 1,4-addition to acrolein to produce the enol borinate (4). which on hydrolysis gives the aldehyde (5).1 The Brown group found it more convenient to add water to a solution of the... 

Even acrolein, which reacts with remarkable speed with hindered trialkylboranes, fails to react with B-R-9-BBN. See Brown HC, Rogid MM, Rathke MW, Kabalka GW (1969) J Am Chem Soc 91 2150... 

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