B-allyl-9-BBN

Although the first two additions proceed under relatively mild conditions, temperatures of over 100 °C are required to effect the third addition, and so 9-BBN derivatives often prove convenient substrates for the reaction. An example of the use of B-allyl-9-BBN is depicted in Equation B5.1. 

Less is known about the reactions of allylboron compounds however, an ate complex between B-allyl-9-BBN and NaOMe afforded allylarenes in high yields within 0.5-1 h (Equation (209)).898... 

B-allyl-9-BBN reacts vigorously with acid chlorides and acid anhydrides slowly with carboxylic acid esters and N,N-dimethylbenzamide (Eq. 47)98). 

Hydroboration of allenes. With few exceptions, the 9-BBN hydroboration of allenes affords B-allylic-9-BBN derivatives. In contrast, the reactions of allenes with disiamylborane or dicyclohexylborane afiord predominantly vinylic boranes. With unsymmetrical allenes, 9-BBN binds to the less substituted carbon atom. Allene itself affords a 1,3-dibora derivative. The B-allylic-9-BBN derivatives are useful reagents for the allylic boration of carbonyl compounds. ... 

Allyllithium gives even higher yields of B-allyl-9-BBN than allylaluminum sesqui-bromide. However, the stoichiometry of the reaction must be controlled to avoid the formation of a diallyl ate complex, and the isolation of product is not simple. Alkylations of B-methoxy-9-BBN with the RMgX reagents meet with limited success. Only the methyl group is quantitatively transferred. Lower yields are obtained with other groups , e.g., n-Bu (42%), Ph (27%). 

In contrast to HjB-THF and other dialkylboranes, 9-BBN shows a strong tendency to form B-allylic-9-BBN derivatives in the hydroboration of aliphatic allenes. This tendency is retained even when the allene system is unfavorably substituted. Lower selectivity may be expected for cyclic allenes, for 1,2-cyclononadiene gives 17% of B-cyclononen-l-yl-9-BBN and 83% of B-cyclononen-2-yl-9-BBN. The reaction can be... 

Brown [7, 8] have studied the directive effect of addition of 9-BBN (1 equiv) toward B-allyl-9-BBN. Oxidation of the bis adduct, formed after rapid hydroboration (<30 min), gives 91% of 1,3-propanediol, and no 1,2-propanediol is detected. This reveals that adduct formed is 1,3-bis-boron derivative. 

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 B-allyl-9-BBN derivatives react with carbonyl compounds to afford borinate esters, which on protonolysis using triethanolamine afford the corresponding homoallylic alcohols. Consequently, the allylboration sequence provides a synthetically useful alternative to the familiar Grignard s synthesis of these alcohols. However, the protonolysis by triethanolamine causes a problem in the isolation of homoallylic alcohol from the thick, air-sensitive, boron-containing mixture. Fortunately, treatment of pentane solution of borinate esters of 9-BBN with 1 equiv of ethanolamine results in the rapid formation of a fluffy white precipitate of ethanolamine 9-BBN adduct, which can be easily removed (Eq. 6.7) [1]. 

Table 6.8 Allylboration of acetone with unsymmetrical B-allyl-9-BBN [1]...

B-Allyl-9-BBN reacts vigorously with acid chlorides (Scheme 6.4) and acid anhydrides (Scheme 6.5), slowly with carboxylic acid esters (Scheme 6.6), and N,N-dimethylamide (Scheme 6.7). Acid chlorides, esters, and N,N-dimethylam-ides react with 2 equiv of the allylborane acid anhydrides utilize 4 equiv (Table 6.9) [1]. In case of acid chloride, sticky gel is formed due to the generation of hydrogen chloride resulting from the protonolysis of chloroborane. This difficulty is circumvented by the addition of 1 equiv of lithium isopropoxide before adding 2 equiv of ethanolamine. 

Allylboranes are extremely labile and undergo allyl transfer to carbonyl groups even at very low temperatures. In this regard, it is interesting to note that Fiirstner and Seidel were able to carry out chemoselective cross-coupling of B-Allyl-9-BBN 141 with aryl bromides 140, 143 (Scheme 28.39). ... 

PdCl2(dppf), MeOK SCHEME 28.39 Cross-coupling with B-allyl-9-BBN. 

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