Catalytic dehydrogenative coupling

Catalytic dehydrogenative coupling of organohydrosilanes, a common method for the formation of linear polysilanes, can also be applied to cyclopolysilane synthesis. Thus, benzylsilane reacts to give all-fraws-hexabenzylcyclohexasilane in the presence of dimethyltitanocene (equation 3). However, extremely long reaction times are required and only moderate yields are obtained15. 

SCHEME 16. Catalytic dehydrogenative coupling of diorganotin dihydrides... 

SiH-functional poly(silylated) aromatic hydrocarbons are important starting materials for the preparation of arene-bridged polysilanes through thermal or catalytic dehydrogenative coupling reactions. 

The trimetallic Fe-Pd-Sn complex 19-16 has been isolated as an intermediate in the catalytic dehydrogenative coupling of tin hydrides.94... 

Styrene, the best known representatives of the aromatic olefin class, is readily obtained by cometathesis reaction of stilbene with ethene in the presence of alumina-supported molybdenum oxide catalysts. In a two-step process [27] stilbene is first produced from toluene by catalytic dehydrogenative coupling over Pb0/Al203, followed by codisproportionation to styrene, with ethene, over M0O3/AI2O3 ... 

Very recently, interesting catalytic conversion of alcohols to esters, acids, and amides by hydrogen transfer to a sacrificial acceptor was reported [64, 65]. Low turnover catalytic dehydrogenative coupling reaction of amines with alcohols to form amides with H2 liberatiOTi under basic conditions was also reported very recently [66-71]. 

Hydride species were also formed in the dehydrogenative coupling of hydrosilanes with DMF [45]. The catalytic system is applicable to tertiary silanes, which are known to be difficult to be converted into disiloxanes (Fig. 17). The catalytic reaction pathway involves the intermediacy of a hydrido(disilyl)iron complex... 

In the course of studying the reactions of Si-H compounds with dialkyltitanocenes, with a view to the synthesis of new hydridosilyltitanocene complexes, we adventitiously discovered that phenylsilane undergoes facile, quantitative dehydrogenative coupling to a linear poly(phenylsilylene) under the catalytic influence of dimethyltitanocene. The ease with which this reaction proceeds initially induced us to underestimate the significance of the observation. 

Polysilazanes that can serve as precursors for silicon carbonitride have been prepared using a Ru3(CO)i2-catalyzed Si-H/N-H dehydrogenative coupling process by workers at SRI [21]. Thus the ammonolysis product of CH3SiHCl2, whose composition approximates [CH3Si(H)NH]n, could be crosslinked by heating at 40 °C with a catalytic quantity of Ru3(CO)i2. Other polysilazanes were prepared by this procedure ... 

The properties of siloxide as ancillary ligand in the system TM-O-SiRs can be effectively utilized in molecular catalysis, but predominantly by early transition metal complexes. Mono- and di-substituted branched siloxy ligands (e.g., incompletely condensed silsesquioxanes) have been employed as more advanced models of the silanol sites on silica surface for catalytically active centers of early TM (Ti, W, V) that could be effectively used in polymerization [5], metathesis [6] and epoxidation [7] of alkenes as well as dehydrogenative coupling of silanes [8]. 

The same catalytic system as described for the CDC of amines and ni-troalkanes, complemented with C0CI2 as a co-catalyst, also proved efficient for the allylic alkylation via cross-dehydrogenative coupling between various cycloalkenes and diketones (Eq. 9). Again, the exact mechanism or role of the organic peroxide are not known to date, but the formation of water probably provides the thermodynamic driving force for these reactions [121,122]. 

Semicarbazones of peptide aldehydes are stable to hydrogenolysis and various coupling procedures (azide and mixed anhydride). Deprotection of the Z-protected amino acid semicarbazones, such as Z-Phe-H semicarbazone, by catalytic dehydrogenation gives the deprotected derivatives in good yields,these can be coupled to peptide azides or peptide acids using the mixed anhydride procedure. The semicarbazone is readily deprotected with 37% formaldehyde/HCl to give the peptide aldehyde. 

Table 9 Catalytic rates for the dehydrogenative coupling of PhSiH3 catalyzed by the nickel complex [(l-Melnd)Ni(PPh3)Me] 159 in the presence of the phosphine-alane (Me2PCH2AlMe2)2 2 and a Lewis base...

A further method for the synthesis of the title compounds with only hydrogen as byproduct is the base-catalyzed dehydrogenative coupling (index D) of ammonia and tris(hydridosilylethyl)boranes, B[C2H4Si(R)H2]3 (R = H, CH3). Initially, the strong base, e.g. n-butyl lithium, deprotonates ammonia. The highly nucleophilic amide replaces a silicon-bonded hydride to form a silylamine and lithium hydride, which then deprotonates ammonia, resuming the catalytic cycle. Under the conditions used, silylamines are not stable and by elimination of ammonia, polysilazane frameworks form. In addition, compounds B[C2l-L Si(R)H2]3 can be obtained from vinylsilanes, H2C=CHSi(R)H2 (R - H, CH3), and borane dimethylsulfide. 

---