Silanols, reaction with palladium

The analysis of silanolate order with a stoichiometric amount of palladium with respect to 2-iodothiophene also revealed a first-order dependence of reaction rate on silanolate concentration. This critical experiment rules out the possibility that an activated complex involving another molecule of silanolate (with i to generate a pentacoordinate silicon) is formed. In the latter case, second-order behavior would have been expected. 

An asymmetric variant of this reaction was developed using chiral Pd complex 111 with either silanes or disiloxanes [66-68]. Both relative and absolute stereochemistries were controlled in this system and good yields (60-85%) were obtained after oxidation (Eq. 18). Formation of the silane-containing product was inhibited by the presence of water due to competitive formation of the palladium hydrides and silanols [68]. The use of disiloxanes as reductants, however, provided expedient oxidation to the alcohol products without decreasing the isolated yields enantioselectivity was 5-15% lower in this more robust system [66]. Benzhydryldimethylsilane proved to be a good compromise between high yield and facile oxidation [66]. Palladium com-... 

This surface reaction would involve a change in the cyclopentadienyl hapaticity prior to SiO-H activation by the metallic species. Loss of cyclopentadiene by reductive elimination would then occur to provide an allyl palladium(ll) surface species, probably stabilized by a silanol group, in which the oxygen atom acts as a 2e donating ligand. However, when the temperature is raised significant carbon contamination has been evidenced by TPD and TPO experiments. These results are consistent with the absence of further SiO-H activation to eliminate propene [57]. 

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... 

Organometallic cross-coupling reactions provide a regiocontrolled method for the introduction of substituents to the indole ring. Palladium-catalzyed cross-coupling of 2-indolyldimethylsilanols have been utilized in the synthesis of 2-arylindoles <04OL3649>. For example, treatment of indole-2-silanol 206 and aryl iodides 207 with a palladium catalyst, copper iodide, and sodium f-butoxide provided 2-arylindoles 208. 

Clearly, the promoter-less coupling discussed previously follows a pathway that is mechanistically distinct from all the other reported palladium-catalyzed silanol cross-couplings, namely, that of a Heck-type process. Another use of organosi-lanols in coupling reactions that proceed by a unique pathway has been reported by Sames et al. [48] In studies with the ortho-t-butylaniline substrate 75 (arising... 

A significant recent development with this reaction was reported in 2009 by Springs group [103], who showed that vinyldisiloxanes - which equilibrate with the corresponding silanolates under basic conditions and subsequentiy undergo palladium-catalyzed cross coupling with aryl iodides or aryl bromides. The substituted vinyldisiloxanes act as masked silanolate precursors. The advantage of this method is that fluoride activation is not required. The reaction conditions are shown in Scheme 1.37. The study unfortunately was limited in scope to the formation of stilbene derivatives. 

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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