Agostic intermediates

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

Agostic interactions shown in structures 99-101 were found to be involved in selective H-D exchange processes with R-OD (R = D, CHj, CDs) catalyzed by complexes 98. process induced by complex 98a was shown by spin saturation transfer experiments to be highly selective, with only one of the methylene protons participating in the exchange (H in bis-agostic intermediate 99 is not involved). 

For the Rh-P,P system several stationary points involved in bond activations were identified. The C-H agostic intermediate structure (137) is a common intermediate and the other two structures are transition states leading to the two products [Eq. (6.124)]. Along the entire reaction pathway both chelating phosphorus atoms remain bound to the transition metal atom. C-H activation is always the kinetically favored process (AA = -Skcalmol ) and the C-C activation product is the more stable one (AA = -Skcalmol ). 

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

Most reactions occur through the agostic intermediate, so the leaving groups have to be els to one another in the complex. Sometimes the reduced metal reacts... 

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 6.3 Easy cyclometalation reactions with a benzylamine proceed via agostic interaction as shown in agostic intermediate 6.6 Equation (6.4) Chelation-Assisted Carbon-Halogen Bond Activation by a Rhodium(I) Complex ...

In the cyclometalation reaction of 2-(dimethylamino)pyridine with an iridium complex ([H2lr(OCMe2)2L2]BF4 (L = PPh,) as shown in Eq. (6.9), the agostic intermediate 6.11 observed by NMR is predicted (DFT(B3PWW91) computation) to give C-H oxidative addition to form an alkyl intermediate 6.12. Loss of H2 leads to the fully characterized alkyl product 6.13, which loses acetone to give the alkyli-dene product 6.14 by rapid reversible a-eUmination [29]. 

FIGURE 11.4 The Grubbs experiment. Since tbe a-CH bonds of the metal alkyl are not involved in the Cossee mechanism (Eq. 11.58), we expect a 50 50 mixture of isotopomers, as observed in some situations. On the modified Green-Rooney mechanism shown here, we would expect a preferential binding of C—H over C—in the agostic intermediate, which leads to a non-50 50 ratio as observed for certain systems. 

Lledos [26] reported on a similar mechanism, also with [Pd(OAc) J, for the selective cyclopaUada-tion of iminophosphoranes RjP=NCH2/fr. The energy profile found for the exo cyclometalation (Fig. 25.16) exhibited an agostic intermediate (5) in analogy to the mechanism described by Macgregor and coworkers (see Fig. 25.11). 

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

Figure 9 Two views of the structures of the reactant, nr-complex, transition state, and y-agostic intermediates in the initiation process of ethylene polymerization with the substituted real diimine Ni catalyst...

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