Alkenyl silanols

Cyclic sUyl ethers/ Alkenyl silanols cyclize in the same manner as the all-carbon analogues. 

Aryl and alkenyl silanols react smoothly with alkenes. The presence of the OH group on silicon is important and silanols are reactive without activation by... 

Oxidations. Cu(OAc)2 has been used as a reoxidant in Pd catalyzed reactions of aryl and alkenyl boronic acids with alkenes and alkynes, and aryltins, and aryl or alkenyl silanols with electron deficient olefins (eq 26). This Mizoroki-Heck type reaction supposedly proceeds through a Pd(II)-boron transmetallation step, followed by addition across the double (triple) bond and final... 

Scheme 4. Scope of arylation and alkenylation procedure. Conditions (a) substrate (0.02 m), silanol (2equiv.), Pd(OAc)2 (4mol%), benzoquinone (4mol%), Cu(OAc)2 (2equiv.), DMF, 100°C.

Denmark and his co-workers recently reported a series of the palladium-catalyzed coupling reactions of alkenylsilicon compounds with aryl iodides. Thus, ( )- or (Z)-alkenylsilacyclobutanes, which are readily prepared by the reaction of the corresponding alkenylaluminum compounds with chlorosila-cylobutanes or the reduction of alkynylsilacyclobutanes, respectively, were found to be active nucleophiles to react with aryl iodides in 10 min at room temperature in most cases (Eq. 13) [17]. Later, it was clarified that silacyclobu-tanes were first converted into alkenyl(propyl)silanols by hydrolysis under the reaction conditions [18], and these were shown to be truly active species. Alkenylsilanols actually react with aryl iodides under the similar conditions (Eq. 14) [19,20]. Independent study by Hiyama and Mori revealed that silver(l) oxide also is an excellent activator for the palladium-catalyzed coupling of alkenylsilanols with an aryl iodide (Eq. 15) [21]. Very recently Denmark... 

In 1999, Demnark disclosed that silacyclobutane derivatives (34) were very effective coupling partners for the transfer of alkenyl and aryl groups. The original theory was that the release of strain energy upon treatment with fluoride to make the silicon pentacoordinate is what made these species so active. However, later studies showed that under the conditions of the cross-coupling reactions, silacyclobutanes formed silanols (35) and disoloxanes (36), which themselves proved... 

The TBAF-promoted coupling of pyridylsilanes to alkenyl and aryl iodides works equally well to form (F)-165 and (F,Z)-166. As with previously reported coupUng reactions of silanols, the reactions employing 2-pyridylsilanes are also compatible with several functional groups, including an ester (( ,Z)-166) anda phenol ((F)-169). 

Hydroxysilyl groups can also act as directing groups in the alkenylation. As the substituents are removable, the sequence of alkenylation/desilylation of silanol substrates leads to the (semi-) one-pot syntheses of 2-alkenylphenol [26] and 2-alkenyltoluene derivatives [27] (Scheme 18.26 and Scheme 18.27). 

Alkenylsilanols or isopropoxysilanes have been recently used for cross-coupling reactions of aryl or alkenyl halides under mild conditions. The palladium-catalyzed reaction of silanols has been optimized in the presence of Pd(dba)2, but in some cases the use of dimer (1) gave better results.The reaction of (Z)-vinylsilanols with vinyl iodides led to (Z,Z)-diene derivatives in 68% yield (eq 84). The stereoselectivity of the reaction was found to he 92.4% in favor of the (Z,Z)-isomer. 

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