Silylenes with palladium

Disubstituted silole derivatives are synthesized by the palladium-catalyzed reaction of (trialkylstannyl)di-methylsilane with terminal alkynes (Equation (107)).266 The mechanism is supposed to involve a palladium silylene complex, which is generated via /3-hydride elimination from LJ3d(SiMe2H)(SnBu3) (Scheme 62). Successive incorporation of two alkyne molecules into the complex followed by reductive elimination gives rise to the silole products. 

From (trialkylstannyl)dimethylsilane and terminal alkynes this method gives 3,4-disubstituted siloles (28, R = H, R = alkyl, aryl or alkoxy groups) in moderate to good yields44. Catalysis by Pd is better than by Ni or Rh. The proposed mechanism involves a palladium-silylene species 30 as an intermediate. This reacts with an alkyne giving successively a palladasilacyclobutene (31) and a palladacyclohexadiene (32). In the final... 

The reaction of a silirene with an alkyne in the presence of a palladium catalyst allows cyclization of two molecules of the alkyne with the silylene, as in equation 9 above. For example, Seyferth and coworkers have prepared the silole 33 in 80% yield from 1,1-dimethyl-2,3-bis (trimethylsilyl) silirene and phenylacetylene (equation 10)45. Without catalyst, this reaction yielded the silole 34 and the ene-yne 35, resulting respectively from ring expansion and cleavage by PhC=CH of the silirene. Under UV irradiation, 35 alone was formed. 

Palladium forms with both silylenes 83 and 84 silylene-bridged dinuclear complexes. The dinuclear Pd(0) complex 110 (R = Ph) was found to be active in Suzuki and Stille cross-coupling reactions <2001CC2372, 2002JOM141>. NMR investigations indicate that also homoleptic silylene complexes of Pd(0) such as Pd(83)(, [Pd(83)2]2, and Pd(84)4 exist these complexes are, however, very labile <2002JOM141>. 

Oxidative addition of Si-Si bonds onto palladium(O) has long been presumed to be involved in a number of palladium-catalyzed bis-silylation reactions of unsaturated carbon compounds. The oxidative addition and its reverse reaction, i.e., reductive elimination, may be in rapid equilibrium, whose direction is influenced by the structure of disilanes and ligands on the palladium atom. In spite of early reports on the formation of bis(organosilyl)palladium(II) complexes [14,15], a well-characterized complex was first synthesized in 1992 by reaction of hydro disilanes with hydridepalladium complex, probably through initial activation of Si-H bond followed by silylene migration (see Sect. 2.3) [16]. Since... 

Similar generation of palladium-silylene complexes may be involved in reaction of oligosilanes with aryl isonitriles giving four-membered rearranged products 95, although the mechanism has not yet been clarified (Eq.49) [81,86]. 

It was reported that a substituent such as 8-dimethylaminonaphth-l-yl group, which may be able to coordinate with the generated silylene intramolecularly, facilitates the nickel and palladium-catalyzed extraction of silylene species from trisilane 96 (Eq. 50) [87]. 

In reactions that are formally analogous to hydrosilylation, transition-metal complexes catalyze the insertion of unsaturated hydrocarbons into other Si-X (X = C, Si, Sn, etc.) bonds. Palladium catalysts seem to work best in many of these reactions. The work of Kumada and coworkers has already been referred to in connection with metal-catalyzed silylene transfer to alkynes (see equation 47)95"97. Sakurai s group has shown that the cyclic disilane 55 will add to alkynes in the presence of a palladium catalyst (equation 110, see also equation 80). The unstrained disilane Me3SiSiMe3 undergoes a similar reaction... 

Noteworthy is that the spirocyclic trisilane 4, in which both the five-membered rings contain an Si—Si bond, reacted with Pd(CN-t-Bu)2 to give the dinuclear palla-dium(II) complex 5 having t-silylene and pi-isocyanide ligands coordinated to the two palladium atoms (Scheme 4). Interestingly, an X-ray analysis showed that the two palladium atoms are separated by 2.75 A, which is the same as the shortest metal-metal contact in metallic palladium. 

The synthesis of the first homoleptic palladium complexes of saturated and unsaturated jY-heterocyclic silylenes, 72, has been reported.Reaction of Pd(PBu% with excess saturated silylene gave the tetrakis-silylene, and reaction with excess unsaturated carbene gave the tris-silylene. The complexes were identified in solution by NMR. On working up the solutions in the presence of displaced phosphine, the dinuclear complex 73 was obtained and identified by X-ray crystallography. The two silylene ligands bridge the two palladiums. Complex 74 was also identified in solution but could not be isolated. 

An ab initio computational study has predicted that 87r-homologs (129 and 130) of planar aromatic carbenes and silylenes will form twisted 6 2-geometries, providing palladium complexes with Mobius-aromatic chiral character, and present an opportunity to design potentially chiral monodentate metal coordination. ... 

The transition-metal-catalyzed [2 + 2 + 1] cycloaddition of two alkynes and heteroatom sources is a useful method for the synthesis of five-membered heterocycles. For example, a silylene species reacts with two alkynes 64 in the presence of nickel or palladium catalyst to afford substituted siloles 65 and 66. Various silylene equivalents, such as disilanes 67 [24], silacyclopropenes 68 [25], 69 [26], cyclotrisilanes 70 [27], alkylidenesilacyclopropanes 71 [28], silacyclopropanes 72, and 73 [29], have been developed as shown in Scheme 6.21. However, the utility for organic synthesis has been limited, due to the difficulty of those organosilane syntheses and the narrow alkyne scope. 

In 2008, Suginome et al. reported that diarkylamino-substituted silylboranes can be used as an easily accessible silylene equivalent [31]. The palladium-catalyzed [2-I-2-I-1] cycloaddition of terminal alkynes 77 with silylboranes 78 afforded 2,4-disubstituted silylboranes 79 as a major regioisomer (Scheme 6.23). 

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