Alkenes from alkenylboranes

The stereodefined alkenyl halides are of prime importance due to the recent developments of di- or trisubstituted alkene synthesis by cross-coupling reactions between organometallics and alkenyl halides catalyzed by transition metal compounds 171). These alkenyl halides can be conveniently obtained from alkenylboranes or alkeneboronic acids. B-Alk enylcatecholboranes undergo rapid hydrolysis when stirred with excess water at 25 °C (Eq. 109)102). The alkeneboronic acids are usually crystalline solids of low solubility in water and can be easily isolated and handled in air without significant deterioration. 

The preparation of heteroatom-substituted alkenes from the corresponding gem-dihalides and aldehydes was also mediated with chromium(II) chloride. In Scheme 8.46, representative examples of the preparation of alkenylborane, [51] -silane, [52] and -stannane [53] are shown. In each case, the high -selectivity is observed. As these compounds are very important substrates for Suzuki-, Hiyama-, and Stille coupling, the stereoselective formation of these compounds heightens the value of the chromium(II)-chloride-mediated reactions. 

However, in the reaction of 1-alkenylboranes with aryl- or 1-alkenyi iodides. 2-aryl-l-alkenes 648 are obtained as the main products. When Pd metal produced from Pd(OAc)2 as a catalyst and EtjN as a weak bu.se are u.sed. abnormal products are formed. On the other hand, normal products 649 are obtained by using NaOH[5l7]. 

In general, hydroboration—protonolysis is a stereoselective noncatalytic method of cis-hydrogenation providing access to alkanes, alkenes, dienes, and enynes from olefinic and acetylenic precursors (108,212). Procedures for the protonolysis of alkenylboranes containing acid-sensitive functional groups under neutral or basic conditions have been developed (213,214). 

Several methods for stereoselective alkene synthesis are based on boron intermediates. One approach involves alkenylboranes, which can be prepared from terminal alkynes. Procedures have been developed for the synthesis of both Z- and E-alkenes. 

The Zweifel trans-alkene synthesis from dialkyl-1-alkenylboranes involves the hy-droboration of 1-halo-l-alkynes with dialkylborane (Eq. 69) 126). It is of interest that the base produces a transfer with inversion 126 127). 

The currently available methods for the synthesis of the title compounds are confined to the preparation of homo-1,1-dihalo-1-alkenes 180 while only a few reports are available for mixed 1,1-dihalo-1-alkenes of defined stereochemistry 18u. As the hy-droboration reaction proceeds in a stereospecific manner, the hydroboration-oxi-dation-bromination-debromoboration sequence of 1-chloro-l-alkynes produces selectively (Z)-l-bromo-l -chloro-l-alkenes (Eq. 116),82>. The oxidation with anhydrous trimethylamine oxide of the alkenylborane prior to the addition of bromine is necessary to avoid the competing transfer of one of 1,2-dimethylpropyl group from boron to the adjacent carbon atom. Similar reaction sequence provides 1,1-dibromo-l-alkenes (Eq. 117)182). 

Cross-coupling of aryl halides and 1-alkenylboranes. Aryl iodides and bromides couple efficiently with 1-alkenylboranes (prepared from acetylenes and catecholborane, 4, 70) in the presence of this Pd(0) complex and a base (NaOC2Hs) (equation I). The reaction proceeds with retention of configuration with respect to the alkenylborane to form only (E)-alkenes. Yields are typically 81-100%. One example is reported where the alkenylborane was prepared with disiamylborane (yield 41%)."... 

E)- and (ZyAlkenes. Alkenylboranes (1), prepared by hydroboration of 1-alkynes, are converted into (E)-alkenes (2) by reaction with palladium(II) acetate (1 equiv.) and triethylamine (0.7-1.0 equiv.). The reaction is believed to involve c/s-acetoxypalladation, 4higration with inversion of R from boron to carbon, and cis i9-elimination (equation I). ... 

Treatment of the intermediate alkenylborane, such as 52 or 53, with iodine in the presence of a base (such as sodium hydroxide or methoxide) forms, stereose-lectively, a Z-1,2-disubstituted or trisubstituted alkene. Transfer of one alkyl group from boron to the adjacent carbon atom occurs stereospecifically, resulting, after anti elimination of boron and iodine, in a new alkene in which the two substituents of the original alkyne become trans to each other (2.53, 2.54). ... 

Hydroboration of alkenes and alkynes is an established preparative method of alkyl- and alkenylboranes. Arylboranes, arylboronic acids and their esters (boronates) are prepared from aryllithium or Grignard reagents. 

Hydroboration of alkenes with diborane (e.g. BH3 THF or Me S BH ) gives mono-, di- or trialkylboranes, depending on the steric bulk of the alkene. Mono-and disubstituted alkenes generally yield R3B, trisubstituted (R BH) and tetrasubstituted (RBHJ. Some useful bulky hydroborating agents can be prepared from substituted alkenes (Table 3.6). These show greater selectivity than diborane itself (Table 3.7). Their reactions with alkynes can stop at the alkenylboranes, whereas diborane itself gives dihydroboration. 

However, the method has serious drawbacks. During fluorination of the alkenyhnetal species, an alkene is formed as a by-product and it is difficult to separate this from the fluoroalkene [19-22]. Moreover, the fluorination of alkenylmetal species such as alkenylsilane and alkenylborane proceeds non-steieoselectively, and a mixture of stereoisomers is formed [17, 18]. Recently Ritter et al. reported that the fluorination of alkenylboronic acid 9 with SelectfluorT proceeds steieoselectively and (E)- -fluoroalkene 10 can be obtained stereoselectively [23] (Scheme 4). The use of AgOTf is critical for the stereoselective synthesis of fluoroalkene and the formation of undesired alkene was not observed under these conditions. The alkenylsilver species 11 was postulated as an intermediate. 

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