Ketones, reaction with allylboranes

This topological rule readily explained the reaction product 211 (>90% stereoselectivity) of open-chain nitroolefins 209 with open-chain enamines 210. Seebach and Golinski have further pointed out that several condensation reactions can also be rationalized by using this approach (a) cyclopropane formation from olefin and carbene, (b) Wittig reaction with aldehydes yielding cis olefins, (c) trans-dialkyl oxirane from alkylidene triphenylarsane and aldehydes, (d) ketenes and cyclopentadiene 2+2-addition, le) (E)-silyl-nitronate and aldehydes, (f) syn and anti-Li and B-enolates of ketones, esters, amides and aldehydes, (g) Z-allylboranes and aldehydes, (h) E-alkyl-borane or E-allylchromium derivatives and aldehydes, (i) enamine from cyclohexanone and cinnamic aldehyde, (j) E-enamines and E-nitroolefins and finally, (k) enamines from cycloalkanones and styryl sulfone. 

Among these species, Mikhailov [2] and Brown [8] have shown that allylboranes, especially those with low-molecular-weight, nonsterically hindered allyl groups are readily hydrolyzed and produce olefins rather than expected allylic alcohols under usual alkaline hydrogen peroxide oxidation conditions. Consequently, it is appropriate to convert the allylborane to a derivative that would give an unambiguous product on oxidation. Such a derivative is prepared by the reaction of allylborane with ketone, such as acetone, prior to oxidation [9]. The other potential adducts (I, 111, and IV) are inert to acetone under these conditions. After oxidation of this derivatized mixture, the quantity ofhomoallylic alcohol determines the amount of the attack at the terminal carbon of the allenic system (allylborane), the quantity of ketone determines the extent of internal attack (vinylborane), and the quantity of unreacted allene reveals the amount of dihydroboration (0% allene = 0% dihydroboration, 50% allene = 100% dihy-drob oration). The positions of the diol reveal the point of attack in dihydroboration. 

Ever since the discovery that allylboranes could add in a nucleophilic fashion to aldehydes and ketones in 1964,carbonyl allylboration reactions have been thoroughly utilized in organic chemistry. Modification of allylic boron reagents and the substrates with which they can react has been the focus of many research groups over the past three decades. The application of allylbor-onates in the context of aldehyde allylation, which results in the formation of homoaUylic secondary alcohols via an allyl transfer reaction with aldehydes, has become an invaluable tool to synthetic chemists (Figure 3). ... 

Boron enolates react with aldehydes and ketones under neutral conditions to give intermediates which hydrolyze to aldol products. The reaction proceeds via a cyclic transition state (Equation B5.2) and is analogous to the allylborane reactions discussed above. 

The synthesis of methyl ketone 281 began with the reaction between the tetra-substituted allylborane 279 and 2,3-0-isopropylidene-D-glyceraldehyde 48. The resulting homoallylic alcohol 280, obtained in 73% yield and excellent selectivity (exact ratio not defined) [231], was converted in two steps to the methyl ketone 281. Aldol condensation between the lithium enolate of 281 and aldehyde 278 (structure shown in Scheme 11-12) gave, after protection of the initial adduct, the Felkin diastereomer 282 as the only reported product in 54% yield. This adduct... 

Allylboranes such as 9-allyl-9-BBN react with aldehydes and ketones to give allylic carbinols. The reaction proceeds through a cyclic transition state. Bond formation takes place at the y carbon of the allyl group, and the double bond shifts. ... 

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