Allylboration aldehydes, ketones

The B-allyl-9-BBN derivatives formed by hydroboration of allenes are utilized for addition of the allyl group to the carbonyl moiety of aldehydes, ketones, and other carbonyl derivatives (allylboration). Since allenes are easily prepared from corresponding olefins [12, 13], the route the olefin-allene-allylborane-carbonyl addition adduct is a general allylation procedure, which provides an attractive alternative to Grignard-based sequences for the syntheses of complex structures. 

The allylboration of a-oxocarboxylic acids, a-amino ketones and a- and //-hydroxy aldehydes and ketones has also been examined [119]. A representative example of the reaction of an a-oxocarboxylic acid with E- or Z-2-butenylboronates is shown in Scheme 10-72. Clearly, the carboxylic acid -substituents exert a significant effect on the regio- and diastereoselectivity of these reactions. The reactions are proposed to occur via the bicyclic transition structure shown. The reac-... 

Allylboration with 9-BBN derivatives (see B-Allyl-9-borabi-cyclo[33.1]nonane) is an efficient process, resulting in the smooth formation of homoallyUc alcohols (eq Alkynylbo-ranes also undergo 1,2-addition to both aldehydes and ketones. As with other reactions producing B-alkoxy-9-BBN byproducts, the conversion of these to alcohols with Ethanolamine also results in the formation of an alkane-insoluble 9-BBN complex which is conveniently removed, thereby greatly simplifying the workup procedure. 

Cyclopropanated allylboration reagents have been used to homoallylate and homocrotylate aldehydes. A zinc-mediated a-prenylation of aldehydes and ketones has been reported conventional conditions have yielded mainly the y-adduct to date. " A racemic allene has been hydroborated in an enantioconvergent manner the enantioenriched crotylborane then reacts with aldehydes in high ee to give ( j-stannyl-homoallylic alcohols. A copper-catalysed y-selective and stereospecific allylic alkylation of ketene silyl acetals has been reported. ... 

Allylboration of aldehydes and ketones with B-crotyl-9-BBN affords only the rearranged product (Eq. 6.8 Table 6.7) [1]. Monomeric formaldehyde gives 2-methyl-3-buten-1 -ol. 

Certain functionalities like halides can be tolerated allylboration with unhindered aldehydes and ketones are far faster than those with esters and amides. The former groups are readily allylborated in the presence of latter. 

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). ... 

A review of the current status of the applications of chiral organoboranes derived from a-pinene for the synthesis of fluoroorganic molecules has been made. Our three-pronged approach for the synthesis of asymmetric fluoro-organics involve (i) asymmetric reduction of fluoiinated ketones, (ii) asymmetric allylboration of fluoiinated aldehydes, and (iii) asymmetric enolboration-aldolization of fiuoro-ketones and aldehydes. It appears that the presem e of fluorite atom(s) in the molecule influences the stereoctemi< outcome in asymmetric reduction and asymmetric enolboration-aldolization using cMorodiisopinocampheylborane. 

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