Catalytic metathesis alkenes binding

Scheme 2. Catalytic cross-metathesis binding of terminal alkenes (A) and alkynes (B) to allyldimethylsilyl polystyrene.

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

A catalytic tandem cyclopropanation-ring-closing metathesis of dienyne 80 led to derivative 81 in good yield (Scheme 30 <2004JA9524>). For internal alkynes, carbene-mediated ring-closing enyne metathesis was observed. Less favorable alkyne binding leads to preferential reactions of the metal carbene with the 1-alkene moiety. 

The mechanism and scope of rare-earth metal-catalyzed intramolecular hydrophosphination has been studied in detail by Marks and coworkers [147,178-181]. The hydrophosphination of phosphinoalkenes is believed to proceed through a mechanism analogous to that of hydroamination. The rate-determining alkene insertion into the Ln-P bond is nearly thermoneutral, while the faster protolytic o-bond metathesis step is exothermic (Fig. 22) [179,181]. The experimental observation of a first-order rate dependence on catalyst concentration and zero-order rate dependence on substrate concentration are supportive of this mechanism. A notable feature is a significant product inhibition observed after the first half-life of the reaction. This is apparently caused by a competitive binding of a cyclic phosphine to the metal center that impedes coordination of the phosphinoalkene substrate and, therefore, diminishes catalytic performance [179]. 

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