4-Center transition states

In classical mechanisms for C—H bond activation either C—H rr-bond donation or cyclic. 4-centered transition states are important, but these are precluded in the porphyrin systems and the mechanism proposed for activation of CH4, toluene, and Hy by the Rh porphyrin radicals is a new mechanistic possibility. 

The mechanism of ketone addition has been studied in detail for the reaction of MesAl with benzophenone [50]. With a 1 1 ratio, a coordination complex MeaAl Ph2CO is formed first, and this subsequently rearranges to the adduct, presumably via a 4-centered transition state. With an excess of Me3Al, a path involving two Me3Al units seems to be preferred, and a 6-centered transition state (similar to structure 6 studied theoretically for dinuclear olefin insertion [36, 37]) has been proposed for this alternative path. Studies using more complicated systems also suggest that, if a 6-centered transition state is available, this is preferred over a 4-centered transition state [51]. 

Once again it would be extremely difficult if not impossible to account for such large A factors in terms of the relatively tight 4-center transition state of reaction H for which if anything a very small A factor of the order of 106 to 10 liter/mole-sec. might be expected. Compared to the H-abstraction reactions of radicals (Tables I and II) for which A = 108-5 liter/mole-sec., the value A = 10 2 is high by about 103-6. 

We now propose that the proton transfer process could be occurring via an intramolecular 4-centered transition state between the newly formed silyl ketal (Figure 10). A similar 4-centered transition state has been previously proposed by Sommer and Fujimoto to explain retention of the stereochemistry at an asymmetric silicon center during the exchange of alkoxide groups (Scheme 37).34 Brook and co-workers also proposed a 4-centered transition state for the thermal rearrangement of (3-ketosilanes (Scheme 38).35... 

Scheme 37. Exchange of alkoxy groups via a 4-centered transition state.

Scheme 38. 4-Centered transition state showing the rearrangement of p-ketosilanes.

The decomposition of metal dimethyl complexes may proceed via a 4-center transition state ... 

D(C1-C02R) = 56 kcal.mole" would satisfy the observed kinetics. This value (which seems too low by about 5-lOkcal.mole" ) is certainly a lower limit because surface initiation reactions are undoubtedly also important. The Arrhenius /4-factors observed for the normal elimination reactions to olefin, HCl, and CO2 fluctuate around the transition state estimates and do so probably as a result of experimental errors and reaction complexities. Note that the chloroformic acid, which is the primary elimination product, is very unstable at reaction temperatures and rapidly decomposes, probably by a 4-center transition state, to give HCl -I- CO2 (ref. 159). The experimental reaction rates of the chloroformate ester eliminations are two powers of ten faster than those for the corresponding formate and acetate esters. This is reasonable since electron withdrawing substituents at the (C-1) position accelerate the decompositions. It seems likely, then, that the normal uni-molecular eliminations and the free radical chain decompositions are competitive processes in these chloroformate ester reactions. 

Three internal rotations are restricted in this 4-center transition state. 6-center ... 

Five internal rotations are restricted in the 8-center transition state however, three bonds (instead of two as in the former cases) are broken. Although this process produces the wrong products, this is not necessarily a serious problem (see below). Transition state estimates of the 4-factors favor the 6-center displacement process although 4-center transition states had formerly been proposed It has been noted that the decomposition rate coefficient reported for acetic anhydride, the product of the methylene diacetate reaction, was an order of magnitude larger than that for methylene diacetate. This seemed to suggest errors in one or both sets of rate data. The problem has been resolved however, from an examination of the thermodynamics of the acetic anhydride decomposition ... 

In studies on stoichiometric reactions of hydrosilanes with zirconocene and hafirocene derivatives containing M-Si or M-H bonds, it was found that o-bond metathesis processes readily occur. Thus, 4-center transition states containing a d metal center, hydrogen, and silicon easily form and mediate facile bond-making and bond-breaking processes involving these components. 

The results are interpreted in terms of a tight, cyclic-4-centered transition state such as ... 

While there was initial debate between Pasto5 and Brown6 about the mechanism of the hydroboration reaction, Brown s proposed mechanism6 is now generally accepted as the most likely pathway. Brown suggested that the reaction proceeds via an equilibrium between BH3 THF (5) and free BH3 (6). The free BH3 then rapidly adds B-H across a 71-system of the olefin (7) in an anti-Markovnikov fashion via an asynchronous, 4-centered transition state (8) to afford hydroboration product 9. 

Zr catalysts. The Zr pre-catalyst is believed to generate a metal hydride species that participates in a step growth process which involves 4-centered transition states (Seheme 19)." ... 

In 1985, Watson and ParshaU [14] reported the first example of a o-bond metathesis. The interchange between [Cp MCHj] (M = Y and Lu) and C-labeled methane was observed. They snggested a concerted 4-centered transition state, as depicted in Figure 25.7. 

It is known, however, that oxidative addition is impossible with d metal complexes, because they do not have the required non-bonding electrons to form the two bonds with the new ligands resulting from this reaction. As shown in the scheme below (left), an intermediate H2complex is first formed, then a square, 4-center transition state forms that can then follow two topologically symmetrical paths come back to the starting a H2 complex or proceed forward to the transient a alkane complex that finally releases the alkane. This mechanism follows o-bond metathesis. 

Once the pre-catalyst formed, this species reacts with a hydrosilane via a a bond metathesis reaction to yield the active copper hydride species. As shown in Scheme 3, the formation of a cr bond between the copper and hydrogen atoms occurs through transmetalation [46, 72, 110] passing by a 4-center transition state. 

Once activated, the copper hydride species enters the catalytic cycle. The proposed mechanism for the hydrogenation of ketones occurs through a two-step cycle as presented in Scheme 4. A first step concerns the formation of a copper alkoxide, through a a metathesis similar to that observed for the activation of the pre-catalyst. In a second step, another 4-center transition state between this alkoxide and a hydrosilane leads to a silylated ether, and the reactivated catalyst. The alcohol can be recovered through hydrolysis of the silylated ether. Due to the fact the alkoxide complex is not observed experimentally, the reduction of the ketone is suggested to be the rate-limiting step. 

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