Allylboronates pinacol esters

Allylboronates prepared from simple diols display appreciable reactivity, but eyelie boronate derivatives prepared from 1,2- or 1,3-diols display considerably less. The commonly employed pinacol esters are among the least reactive members of this class. 2-Allyl-3-methyl-l,3,2-oxaza-... 

Scheme 20 Reactivity of 29 in Suzuki-Miyaura coupling of allylboronic acid pinacol ester derivatives...

Reactions of allylboronates (87)-(89) and aldoximes, imines and sulfenimides have been described. These reactions are considerably slower than those of aldehydes, and consequently reagents (87) and (88) are generally used in preference to the less reactive pinacol ester (89). The reactions of imines and (87) proceed at room temperature, while those of (88) and oximes and sulfenimides generally require heating in refluxing CCU or toluene. The reaction of (88) and oximes can be performed at room temperature if a pressure of several kilobars is applied. ... 

For a study of the dependence of reactivity of allylboronates on the diol unit, see W. R. Roush, L. Banfi, J. C. Park and L. K. Hoong, Tetrahedron Lett., 1989, 30, 7305. While pinacol esters are commonly employed owing to their stability, these derivatives are considerably less reactive than other esters. 

The aoss-coupling reaction of bis(pinacolato)diboron with allyl acetates provides the pinacol esters of allylboronic acids in high yield (Scheme The reaction is effectively catalyzed by Pd(dba)2 in DMSO at 50 °C. The reaction more or less accompanies the formation of biallyl, the amount of which markedly depends on the kind of leaving group of the al-lyhc substrates. E)- and (Z)-Cinnamyl acetate and a-phenylallyl acetate aU provide trans-cinnamylboronic acid ester selectively and in almost the same yield, which suggests that the reaction proceeds via ir-allylpalladium as a common intermediate. 

The desired diene-containing compounds such as 48 can also be synthesized using the Suzuki-Miyaura cross-coupling reaction. For example, cross-coupling of the aromatic diiodide 49 with allylboronic acid pinacol ester 50 readily furnished 48 [23]. After exposure of this diene 48 to an RCM reaction, followed by DDQ oxidation, the naphthalene 51 was produced in good yield (Scheme 17.8). 

SCHEME 17.8 Synthesis of substituted naphthalenes from 1,2-diiodobenzenes—application of allylboronic acid pinacol ester. (Adapted with permission from Ref. [10]. Copyright (2009) American Chemical Society). 

Conjugate addition of allylboronic acid pinacol ester to nons)mimet-ric dialkylidene ketones with an in situ generated phosphonite nickel catalyst. 

The intramolecular allylboration of an aldehyde function leads selectively to cir-disubstituted cyclic ethers. It has been shown that both the reactive aldehyde and the allylboronate moiety can be initially generated in situ in a masked form and then liberated simultaneously by hydrolysis of the precursor functions <1997JA7499>. This methodology was successfully applied to the one-pot synthesis of the oxocene 82, a precursor of (-l-)-laurencin (Scheme 13). A DIBAL reduction of the Weinreb amide 80, metalation with r f-butyllithium, borylation with the pinacol borate ester, and, finally, liberation of both the aldehyde and the allylboronate function by aqueous pH 7 buffer solution generated the reactive 81, which cyclized in 38% overall yield to the oxocene 82. Only the all-cis-diastereomer is formed, which means that the cyclization proceeds under high asymmetric induction from the resident stereogenic center present in 80. 

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