Solvento intermediate

This problem requires a modified approach which Gray 16) has solved in the case of substitution in square planar complexes. He uses the fact that bases, like hydroxide, substitute very slowly but will immediately deprotonate, and hence stabilize, a protonic solvento intermediate. This elegant approach cannot be applied to the octahedral cobaltammines whose reaction rate with such bases is very high. 

Our alternative approach has been to synthesize the solvento intermediate and then study its reactions in isolation. We thereby hope to show that its reactivity and steric course is inconsistent with the postulate stating that it is an intermediate in the substitution reactions. Complexes of the type cis- and trans-[Co en2 CH3OH q]+2 (7)f and cis-[Co en2 (CH3)2SO Cl]+2 32) have been prepared. We have shown in the first case that the lability of the coordinated methanol does not sufficiently explain the nonappearance of the solvento complex in the reactions of cis- and trans-[Co en2 Cb]"1" in methanol unless it is not an intermediate in the reaction. In dimethyl sulfoxide solution, cis- and trans-[Co en2 Cb] have been shown to isom-erize to an equilibrium mixture that also contains the solvento intermediate 32). 

Detailed kinetic studies indicate that about 80% of the time isomerization goes via the solvento intermediate. But this is not a rate-determining solvolysis, rather a temporary diversion of the intermediate of a dissociative reaction. Watts 33) has recently prepared the dimethylformamide complex, [Co en2 DMF Cl]+2 and has shown that it cannot be an intermediate in the isomerization of cis- and trans-[Co en2 Cl2]+ in dimethylformamide. 

Scheme 2, which represents a simple extension of Scheme 1, includes the reverse hydrolysis (a competing anation reaction). Application of the steady-state approximation to the solvento intermediate [ML3S] leads to rate law (7) (assuming a contribution from the ligand pathway also), which is adhered to when k. i[X] is significant. 

Complications of this type were apparent as early as 1962 in the reaction of [PtBr(dien)]N02 with pyridine (29). The reaction, which proceeds mainly by the solvolysis route, slows as [Br ] builds up, due to the competing back-reaction. [PtBr(dien)]+ reacts with the nu-cleobase inosine (Ino) mainly by the solvolysis pathway and is likewise inhibited by bromide (30). The observed rate law fits Eq. (7) with X = Br and Y = Ino. By means of stopped-flow methods, reactions between the palladium analog [PdBr(dien)] and Ino have also been followed (31). Due to the low reactivity of Ino, its affinity for the solvento intermediate is similar to that of Br", and in this case a third complicating feature, the rare operation of a reverse reaction to the ligand-dependent route, is also apparent. Equations (8-10) show the reactions from which rate law 11 is derived. 

The mechanism for square-planar substitutions can be written as in Scheme 1, if the reactions are assumed to be associative. If the solvento intermediate is present... 

For instance, platinum(ii) aqua-complexes have now been shown to react readily with thiocyanate and ethene, contrary to the conclusions in some of the previous studies. So far, there seems to be no example of a solvento intermediate ML3S reacting more slowly with the nucleophile than the parent complex ML3X. 

Kelm and co-workers have carefully analysed the data of Romeo et al. for isomerization and solvolysis. They conclude that the rate law reported by these authors (involving a mass-law retardation for addition of halide) can also be described by a mechanism involving a rapid reversible solvolysis step followed by a rate-determining isomerization ( d) of the solvento intermediate, according to Scheme 4. A prerequisite for this interpretation is that kc and k-c>ka, which... 

The first suggestion that ligand loss was important in reductive eliminatimi reactions from Pt(lV) complexes dates back more than 40 years. In 1969, Ettore observed that added iodide inhibited the reductive elimination of Phi from L2PtPh2l2 in methanol and suggested iodide loss to form a six-coordinate solvento intermediate [13]. Several years later, a five-coordinate intermediate was proposed by Puddephatt in studies of C-C reductive eliminations from Pt(IV) complexes (see Sect. 2.1.1) [15]. Since then, the involvement of five-coordinate intermediates has been supported so consistently in both experimental (e.g., [10,13-42]) and computational studies (e.g., [29, 30, 43 8]) of alkyl C-C, C-H, and C-X reductive elimination that it is now accepted as the norm in mechanistic schemes for reductive elimination from Pt(IV). 

The non-co-ordinating properties of some organic solvents can be useful in mechanism elucidation, in that solvento-intermediates are ruled out. Replacement of one Lewis base by another in alkylcobaloximes, R[Co], e.g. reaction (1), is a dissoci-... 

The suggested mechanism here is slow formation of a solvento (solv) species [Pt(L)(solv)Cl2] with subsequent slow displacement of L by bipyridyl. For L = ethylene there is evidence for a labile intermediate [Pt(C2H4)(bipy)Cl]+, but not, in acidic aqueous methanol, for a solvento-intermediate. ... 

The higher catalytic activity of the cluster compound [Pd4(dppm)4(H2)](BPh4)2 [21] (20 in Scheme 4.12) in DMF with respect to less coordinating solvents (e.g., THF, acetone, acetonitrile), combined with a kinetic analysis, led to the mechanism depicted in Scheme 4.12. Initially, 20 dissociates into the less sterically demanding d9-d9 solvento-dimer 21, which is the active catalyst An alkyne molecule then inserts into the Pd-Pd bond to yield 22 and, after migratory insertion into the Pd-H bond, the d9-d9 intermediate 23 forms. Now, H2 can oxidatively add to 23 giving rise to 24 which, upon reductive elimination, results in the formation of the alkene and regenerates 21. 

The relative reaction rates and the stability of the aquo complex make it possible to identify the aquo complex as an intermediate and study the individual acts separately. However, if the solvento complex were less stable and the anation rate much faster than the solvolysis, it would not be possible to observe this intermediate, and the process would be kinetically indistinguishable from a unimolecular dissociative process. Both processes would exhibit overall first-order kinetics and the usual mass-law retardation and other competitive phenomena characteristic of an extremely reactive intermediate. 

Normal associative ligand substitutions at these sterically hindered compounds are dominated by the solvento (Ai) route, and scope for forming the CB of aquo intermediates at high pH [(Eq. (16)] was recognized as a potential mechanistic complication (86a). 

To effectively model the asymmetric hydrogenation reaction, we must look at the mechanism carefully. The first step involves the displacement of solvent and the coordination of the enamide to produce the two diastereomers (Fig. 3) (17-20). It appears as though the enamide-coordinated diastereomers are in rapid equilibrium with each other through the solvento species (Fig. 4). This square planar rhodium(I) cation is then attacked by dihydrogen to form an octahedral rhodium(III) complex (Fig. 4). Hydrogen then inserts into the Rh-C bonds, and the product is reductively eliminated (Fig. 4). From a molecular mechanics standpoint we have three entities to model the square planar rhodium(I) solvento species and the two intermediates (square pyramidal dihydrogen complex and the octahedral dihydride). 

Figure 9 shows a schematic representation of the potential energy profiles for a two-step C-X reductive elimination from a center with step a i being the rate limiting (solid line). A one-step C-X reductive elimination with an external nucleophile Z is also presented (dashed line). The two-step reaction path in Fig. 9 includes a five coordinate intermediate or a six-coordinate solvento-complex 4 and leads to a lower energy transition state TSs compared to the transition state TSse that corresponds to a direct nucleophilic attack of Z at the metal-bound carbon of the starting six coordinate compound. 

An interesting pertinent question to answer is whether or not concerted three-center C-0 elimination reactions proceed via a dissociation - elimination mechanism involving a formally five coordinate intermediate or its solvento adduct such as 6 (Fig. 11, solid line) or a direct elimination from the six coordinate starting complex (Fig. 11, dashed line) takes place. 

Fig. 11 A schematic representation of the potential energy profiles for a concerted three-center C-X reductive elimination of an organic product R-Z via a cationic intermediate 6 solid line) and directly from the six coordinate reactant dashed line). Sol designates a potential solvento ligand...

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