Agostic intermediate 50 formation

The availability of an allylic hydrogen through the formation of the 114 agostic intermediate or an enyl ([Pg.526]

Formation of a -agostic intermediate has been implicated for hydride-olefin insertion (Eq. 2.21) [62]. 

Route I (oxidative addition) involves a concerted oxidative addition process with the formation of metal-hydride species A. Alternatively, an electrophilic attack by the metal center on the aryl ip o-carbon may afford a metal arenium (Wheland) complex B followed by proton loss. In the agostic C-H bond activation route, the six-membered transition state C including a hydrogen-metal interaction has been found to initiate the C-H activation process, leading to an agostic intermediate D and acting simultaneously as an intramolecular base for deprotonation. 

As for the computer chemistry of cyclometalation reactions, the reaction with the most representative substrate, A,A-dimethylbenzylamine in a palladium compound, was studied. This reaction proceeds very easily, and its intermediate state, or agostic interaction, is therefore not actually isolated. As shown in Scheme 6.3, however, the activation energy for the agostic interaction is only 13 kcal/mol. It may be pointed out that the acyl group assists the formation of the agostic interaction in the reaction, as exhibited by the agostic intermediate 6.6 shown in Scheme 6.3 [24]. 

Scheme 1.14 Reversible formation of the agostic intermediate 50. (Adapted from Heinrich, H. et al. Chem. Commun., 1296-1297, 2001. With permission.)... 

The alkyne-to-vinylidene tautomerization processes on various transition metal centers have also been discussed. Three different pathways for the formation of vinylidene from p -acetylene on electron-rich transition metals were the most theoretically studied. Most studies suggested that the favorable pathway proceeded via an intermediate with an agostic interaction between the metal center and one C—H bond followed by a 1,2 hydrogen shift (the bl+b2 pathway shown in Scheme 4.5). The reverse process, the vinylidene-to-p -acetylene tautomerization, was also discussed. It was found that complexes with electron-poor metal centers were able to mediate the reverse process. 

In the first step, a cis 1,2-insertion of cyclopentene takes place the intermediate derived from this insertion is sterically too demanding to permit insertion of another monomer molecule and is isomerised before another cyclopentene molecule is inserted. Since only /1-hydrogen atoms are available for the stabilisation of the active species (jS-agostic interaction), /1-hydride transfer, followed by monomer rotation or migration, and then reinsertion occur, leading to 1,3-linked polymer chains. The rotation around the Zr olefin n bond results in the formation of cis-1,3-poly(cyclopentylene) [scheme (3a)] [20] while the n face migration of the olefin across the nodal plane of the double bond results in the formation of trans-1,3-poly(cyclopentylene) [scheme (3b)] [19]. 

Other mechanistic studies on carbonyl insertions of molybdenum(II) complexes, RMo(f7 -C5H5)(CO)3, postulated formation of an acyl intermediate stabilized by interaction of the hydrocarbon portion of the acyl group with the metal, similar to the well-established agostic interaction). 

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