Trans-vinylboranes

The reactivity of l-silyl-l-boryl-2-alkenes 68 obtained by gem-silylboration of allyl-chloride were studied by Hiyama et al. [36]. Allylsilanes 68, engaged in SMS reactions, afford the desired trans-vinylboranes 69 with good yields (Scheme 13.29). 

Vinyl halides and aryl halides, both of which contain a halogen X bonded directly to an sp hybridized carbon, are most often used, and the halogen is usually Br or I. The Suzuki reaction is completely stereospecific, as shown in Example [3] a cis vinyl halide and a trans vinylborane form a cis,trans-l,3-diene. 

Vinylboranes, which have a boron atom bonded to a carbon-carbon double bond, are prepared by hydroboration using catecholborane, a commercially available reagent. Hydrobo-ration adds the elements of H and B in a syn fashion to form a trans vinylborane. 

The use of 1-naphthaldehyde instead of benzaldehyde renders the distillative separation of trans-vinylboranes, a greatly simplified process in most cases. 

Devise a synthesis of the given trans vinylborane, which can be used for bombykol synthesis (Figure 26.1). All of the carbon atoms in the vinylborane must come from acetylene, 1,9-nonanediol, and catecholborane. 

It is not possible to synthesize diene D using a Suzuki reaction with ethynylcyclohexane as starting material. Hydroboration of ethynylcyclohexane adds the elements of H and B in a syn fash bn affording a trans vinylborane. Since the Suzuki reaction is stereospecific, one of the double bonds in the product must therefore be trans. 

Furthermore, such boranes should undergo more facile cis, trans-isomerization to yield 33 as the contribution of 31c becomes more important. To our knowledge, the ease of isomerizing (Z)-vinylboranes (31a) thermally to their -isomers (33) has not yet been evaluated (Eq. 10), possibly because structures 31a are not readily available. 

Halogenation of vinylboranes generates vinyl halides. Both cis and trans halides are available by modification of the reaction conditions. When trans-alkenyl boronic acid (73, derived from 1-octyne) was treated with iodine and sodium hydroxide, trans-1-iodo-l-octene (74) was formed in 90%. When the boronic acid was treated with iodine and then with base, the (Z)-alkenyl iodide (75) was produced.Vinylboranes derived from internal alkynes lead to cis-trans mixtures with both of these procedures. Boronic acids derived from alkynes and catecholborane give the (Z)-bromide on addition of bromine followed by sodium hydroxide. ... 

Thexylborane in tetrahydrofuran added dropwise at 0° to a soln. of 1-hexyne in the same solvent, stirred an additional hr. at 0-5°, the resulting vinylborane soln. treated with solid trimethylamine oxide at sudi a rate that the temp, remains between 0 and 5°, stirred an additional hr. at this temp., 3 N NaOH added followed by dropwise addition of iodine in tetrahydrofuran -> cis,trans-S,l-dodecadiene. Y 65%. cw,cw-Isomers can be prepared with diisobutylaluminum hydride. F. e. s. G. Zweifel, N. L. Polston, and C. C. Whitney, Am. Soc. 90, 6243 (1968). 

The fast rate of reduction of ArCHO by 3 as compared with simple B-alkyl-9-BBN derivatives is because of added stability imparted to the transition state of the empty p orbital on the boron atom in the additional 9-BBN substituent [10, 13]. The stabilization also leads the reaction to occur at lower temperature than for example, with B-s-Bu-9-BBN, a feature that favors a more selective process. As noted above 1,3-repulsions for the Me-C-C-(9-BBN) array are greater than for Me-C-C-Me, and this aspect disfavors the alternating transition state, leading to cis-vinylborane. Consequently, the combination of stereo- and electronic factors favor the transition state, and the whole process makes the preparation of trans 2 from 3 a highly efficient and selective method [8]. 

Unique to E-vinyl derivatives of 9-BBN is their Grignard-like addition to aldehydes [6] to afford stereodefined alcohols. trans-E-Vinyl-9-BBN synthesis via dehydroborylation [7] process is extended to the synthesis of trans-2-tnmeth-ylsilyl vinylborane [8]. trans-2-Trimethylsilylvinyl-9-BBN adds cleanly to aldehydes (Eq. 6.17) to provide pure frans-3-trimethylsilyl(TMS) allylic alcohols in excellent yields (Table 6.18) [8]. ... 

The concept of using an excess of the alkyne to enhance vinylborane formation is not workable for an a,co-diyne if both alkyne moieties are to be hydrobo-rated. However, for example with 4 1 9-BBN l,8-nonadiyne stoichiometry, the l,l,9,9-tetra-9-BBN adduct is cleanly formed. The treatment of tetraborane adduct with 2 equiv of PhCHO results in the clean formation of trans,tram-, 9-di-9-BBN-l,8-nonadiene which on deuterolysis with AcOD affords the pure tranSytrans-didevLterated diene in 64% overall yield (Scheme 24.8) [27]. 

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