Protonolysis transition state

Protonolysis. Simple trialkylboranes are resistant to protonolysis by alcohols, water, aqueous bases, and mineral acids. In contrast, carboxyUc acids react readily with trialkylboranes, removing the first alkyl group at room temperature and the third one at elevated temperatures. Acetic and propionic acids are most often used. The reaction proceeds with retention of configuration of the alkyl group via a cycHc, six-membered transition state (206). 

R,R-diphenyl ethylene carbonate CR,R-DPEC)) with a racemic zirconaaziridine. (C2-symmetric, cyclic carbonates are attractive as optically active synthons for C02 because optically active diols are readily available through Sharpless asymmetric dihydroxylations [67].) Reaction through diastereomeric transition states affords the two diastereomers of the spirocyclic insertion product protonolysis and Zr-mediated transesterification in methanol yield a-amino acid esters. As above, the stereochemistry of the new chiral center is determined by the competition between the rate of interconversion of the zirconaaziridine enantiomers and the rate of insertion of the carbonate. As the ratio of zirconaaziridine enantiomers (S)-2/(R)-2 is initially 1 1, a kinetic quench of their equilibrium will result in no selectivity (see Eq. 32). Maximum diastereoselec-tivity (and, therefore, maximum enantioselectivity for the preparation of the... 

However, intramolecular nucleophilic participation by the conjugate base during protonolysis of a C—Hg bond is questionable. A study of the acidolysis of the carbon-mercury bond in unsymmetrical di-alkylmercurials rather suggests that the reaction proceeds via a three-center transition state.In any case, substantial kinetic and stereochemical evidence has led to the idea that reaction occurs by a concerted, front side attack with a transition state that involves a pentacoordinate carbon center. In some cases unimolecular mechanisms, SeI, also have been observed. 

Unlike the metals in equations 8.57 and 8.58, the metal in equation 8.5983 is an early -transition metal. The OA-RE pathway is not possible from such a complex, because it is difficult to remove electrons from a d° metal, yet protonolysis occurs again with retention. The mechanism for this reaction probably involves path b, a concerted SE2 reaction involving a 3-centered transition state, 28. This mechanism seems to be general for electrophilic cleavages involving the early transition metals because the HOMO is centered at the M-C oxidative pathways are not possible.84... 

Pioneering experimental findings by Marks and coworkers [97,103,114] followed by theoretical analysis [109] allowed elucidation of the mechanism of aminoalkene hydroamination/cyclization (Fig. 13). The reaction is considered to proceed through a rare-earth metal amido species, which is formed upon protonolysis of a rare-earth metal amido or alkyl bond. As discussed in the previous section, the first step of the catalytic cycle involves insertion of the alkene into the rare-earth metal amido bond with a seven-membered chair-like transition state (for n = 1). The roughly thermoneutral [103,109] insertion step is considered to be rate-determining, giving rise of a zero-order rate dependence on substrate concentration and first-order rate dependence on catalyst concentration. 

Phosphine-terminated polyethylenes have been prepared in related organolanthanide-catalyzed reactions [31, 32]. After initiation by insertion of ethylene into the Ln-P bond of a phosphide complex, polymerization occurs by repeated ethylene insertion. Termination results from protonolysis of the growing polymer chain via a four-centered a-bond metathesis transition state, as seen in Scheme 19, to give a polymeryl-phosphine and regenerate the lanthanum phosphide complex (Scheme 20). 

The mechanistic alternative of an aUcoxy-alumination, followed by protonolysis of a C-Al bond was investigated by calculation. The transition state for alkoxy-alumination was at an accessible level, but the proto-de-alumination step was kineti-cally hindered, excluding this alternative mechanism [56]. This example places once more emphasis on the earlier notation that oxy-metaUation/proto-de-metaUation sequences should not be posmlated without specific evidence (Sect. 2.1.2). 

Suggest mechanisms for the two transformations. Of the two, amination, in our view, has the more straightforward mechanism. Protonolysis begins with coordination of the carbonyl oxygen of the carboxylic acid to the boron R-H bond formation is then believed to occur via a cyclic transition state. 

The reactivity of [Ir(GO)2l3Me] with other species has also been investigated, in particular, reactions leading to methane, a known byproduct of iridium-catalyzed carbonylation. Methane formation occurs on reaction of [Ir(GO)2l3Me] either with carboxylic acids or with H2 at elevated temperature. In both cases, the reaction is inhibited by the presence of GO, suggesting that GO dissociation from the reactant complex is required. For the protonolysis reaction with carboxylic acids, a mechanism was proposed (Scheme 8(a)) in which the acid coordinates to a vacant site created by GO loss, and methane is then liberated via a cyclic transition state. The hydrogenolysis reaction, which leads cleanly to [Ir(GO)2l3H] , could proceed via oxidative addition of H2 or an rf-Hz complex as shown in Scheme 8(b). 

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