Catalytic dehydrocoupling

With diphosphanes recently Stephan et al. reported an intriguing Al and P based macrocyclic structure [37]. A zirconium based catalyst precursor first was employed in the catalytic dehydrocoupling of the primary bidentate phosphane to give the tetraphosphane 6, (Scheme 4). The function of 6 as a molecular building block has been demonstrated by its reaction with MMe3(M = Al, Ga). Although, the gallium derivative 7 has not been... 

Catalytic dehydrocoupling, as shown in reaction (3), was pioneered by Fink (6-10) and later studied by Andrianov et al (43). Although Fink described the synthesis of the first polysilazanes, (9) reaction (23), this route to oligo- and polysilazanes remained unexplored... 

Gauvin, Franjois, Harrod, John F., and Woo, Flee Gweon, Catalytic Dehydrocoupling A General Strategy for the Formation of Element—Element Bonds. .. 42 363... 

Harrod-type catalytic dehydrocoupling method using early transition metal catalysts (see COMC II (1995), chapter Organopolysilanes, p 99, and earlier in this review, Section 3.11.4.1.3. (i)).69,75 Si-H bonds are susceptible to free radical attack, and use of this was made in the free radical substitution of 38 to prepare a number of oxy-functionalized polysilanes, as shown in Scheme 25.185,186... 

Catalytic dehydrocoupling can also be used for the efficient generation of heterocyclic systems by use of a suitable precursor incorporating a hetero-nuclear bond between two p-block elements. For example, the thermal dehydrocoupling of Ph2PH BH3 at 170 °C yields a mixture of the cyclic trimer (Ph2PBH2)3 (2.7) and tetramer (Ph2PBH2)4 (2.8) in an 8 1 ratio. The same two... 

In contrast, previous attempts at catalytic dehydrocoupling of germanes have led to the formation of only low molecular weight oligomers.145... 

The coupling of a trialkylsilane and an amine with loss of H2, catalyzed by palladium on carbon, was first reported by Sommer and Citron in 1967.178 More recent work by Laine and Blum has involved the application of catalytic dehydrocoupling of compounds containing Si-H and N-H bonds to form aligo- and polysilazanes. These polymers, with silicon-nitrogen bonds in the backbone, are useful precursors to silicon nitride. In the presence of Ru3(CO)i2, silicon-nitrogen bonds are cleaved and reformed... 

Polysilazanes can also be synthesized by catalytic dehydrocoupling (equation 4)28,29. The reaction works best with monosubstituted silanes (MeSiH3 is flammable) however, as with the ammonolysis reactions, molecular weights are normally less than 2000 Da. Hence, as-formed oligomers are not useful precursors. However, combinations of ammonolysis followed by catalytic dehydrocouphng provide access to useful precursors as discussed in the SiCN section. 

Mu, Y. Harrod, J. F. Synthesis of Poly(methylsilyene) by Catalytic Dehydrocoupling with Cp2MMe2 (M = Ti, Zr) Catalysts, (J. F. Harrod, R. M. Laine, Eds.) Inorganic and Organometallic Oligomers and Polymers, Kluwer Academic Publishers, Netherlands, 1991. p. 23. 

Catalytic Dehydrocoupling A General Strategy for the Formation of Element-Element Bonds... 

Given the diverse range of catalysts and experimental conditions under which catalytic dehydrocoupling occurs, it is likely that any one, or several, of the mechanistic types described earlier may be important under a particular set of conditions. It is therefore of paramount importance that the particular conditions be carefully considered when making comparisons between different sets of experimental results. 

Spectroscopic evidence shows that the structure shown on the left is closer to the actual structure. The existence of this compound encourages speculation as to the existence of a silicon analogue, or the participation of such ene complexes in catalytic dehydrocoupling cycles. To date, no evidence to this effect has been reported and the best hypothesis is that the phosphine polymerization goes by a tr-bond metathesis cycle, with or without one-electron redox steps.130... 

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