Methane, first cycle yield

Considering that (methyldichlorosilyl)bis(dichlorosilyl)methane 139 has two Si-H bonds, these workers first attempted to prepare the 1,3-disilacyclopentane. Surprisingly, l,3-disilacyclopent-4-ene 141 was obtained in very good yield when tetrakis(triphenylphosphine)palladium was used as a catalyst. On the basis of the results, a plausible catalytic cycle for this dehydrogenative double-silylation reaction was proposed (Scheme 17). 

Scheme 8 presents a plausible mechanism for the intermolecular hydroamination of terminal alkynes promoted by the organothorium complex 1. The first step in the catalytic cycle involves the N-H a-bond activation of the primary amine by the organothorium complex yielding the bisamido-amine complex Cp2 Th(NHR )2 (H2NR ) (28) and two equivalents of methane (step 1). Complex 22 was found to be in rapid equilibrium with the corresponding bis(amido) complex 18 (step 2) [57, 60]. An additional starting point involved a similar C-H activation of the terminal alkyne with complex 1 yielding methane and the bis(acetylide) complex 17 (step 3). 

This allylic bromination with NBS is analogous to the methane chlorination reaction discussed in Section 6.3 and occurs hy a similar radical chain reaction mechanism. As in methane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br-radical, which cycles hack into the first step and carries on the chain. The Br2 results from reaction of NBS with the HBr formed in the first step. 

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