Oxidative Addition Step

In the absence of olefin complexes, such as [RhL (olefin)Cl], are formed. With moderate binding olefins, such as cyclohexene, the formation of [RhLj(olefin)Q] complexes is an unimportant side reaction during olefin hydrogenation. However, olefins fliat bind with relatively high equilibrium constants, such as ethylene, act as competitive inhibitors of the dihydride path by forming stable [RhLj(olefin)Q] complexes. 

The rate law in Equation 15.18 quantitatively accounts for the observed rate behavior for the hydrogenation of cydohexene. Since the migratory insertion reaction k is the slowest step in the catalytic cycle, under conditions of constant H, pressure, high olefin concentrations ( 1 M), and no added phosphine, the rate reaches a limiting value determined by the rate of the migratory insertion step. 

Cationic Rhodium Compiexes Containing Aromatic Phosphines 

The cationic rhodium complexes described earlier in this chapter catalyze the hydrogenation of olefins through a manifold that is different from that followed by neutral rhodium complexes. The synthetic studies of Osborn show that complexes of the formula [RhLjSJ undergo oxidative addition of hydrogen to form octahedral dihydride complexes, but kinetic studies by Halpem on the catalytic reactions indicate that the olefin binds before hydrogen adds. 

Turnover-limiting step N . H2 coordination or n -k migratory insertion 

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The intramolecular Heck reaction presented in Scheme 8 is also interesting and worthy of comment. Rawal s potentially general strategy for the stereocontrolled synthesis of the Strychnos alkaloids is predicated on the palladium-mediated intramolecular Heck reaction. In a concise synthesis of ( )-dehydrotubifoline [( )-40],22 Rawal et al. accomplished the conversion of compound 36 to the natural product under the conditions of Jeffery.23 In this ring-forming reaction, the a-alkenylpalladium(n) complex formed in the initial oxidative addition step engages the proximate cyclohexene double bond in a Heck cyclization, affording enamine 39 after syn /2-hydride elimination. The latter substance is a participant in a tautomeric equilibrium with imine ( )-40, which happens to be shifted substantially in favor of ( )-40. 

A plausible mechanism accounting for the catalytic role of nickel(n) chloride has been advanced (see Scheme 4).10 The process may be initiated by reduction of nickel(n) chloride to nickel(o) by two equivalents of chromium(n) chloride, followed by oxidative addition of the vinyl iodide (or related substrate) to give a vinyl nickel(n) reagent. The latter species may then undergo transmetala-tion with a chromium(m) salt leading to a vinyl chromium(m) reagent which then reacts with the aldehyde. The nickel(n) produced in the oxidative addition step reenters the catalytic cycle. 

An oxidative addition step is involved in the reaction between trans-(EtjP)2lr(CO)Cl and the mercurials Hg(MMe3)2 (M = Si, Ge) (141). 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

A mechanistic pathway is proposed based upon the observed regioselectivities and other results that were obtained during the exploration of the scope and limitations of the Alder-ene reaction.38 Initially, coordination of the alkene and alkyne to the ruthenium catalyst takes place (Scheme 5). Next, oxidative addition affords the metallocycles 42 and 43. It is postulated that /3-hydride elimination is slow and that the oxidative addition step is reversible. Thus, the product ratio is determined by the rate at which 42 and 43 undergo /3-hydride elimination. 

While the transmetalation step is often the rate-determining step for Pd-catalyzed reactions with organometallics, the oxidative addition step is often the rate-determining step in the Heck reactions, although olefin insertion can be rate-limiting in some cases — this is why the Heck reactions of tri- and tetra-substituted olefins sometimes proceed slower than those of di-substituted and terminal olefins. 

A one-pot synthesis of 3,3-disubstituted indolines was achieved by taking advantage of a sequential carbopalladation of allene, nucleophile attack, intramolecular insertion of an olefm and termination with NaBPh4 (Scheme 16.6) [10]. First, a Pd(0) species reacts with iodothiophene selectively to afford ArPdl, probably because the oxidative addition step is facilitated by coordination with the adjacent sulfur atom. Second, the ArPdl adds to allene, giving a Jt-allylpalladium complex, which is captured by a 2-iodoaniline derivative to afford an isolable allylic compound. Under more severe conditions, the oxidative addition of iodide to Pd(0) followed by the insertion of an internal olefm takes place to give an alkylpalladium complex, which is transmetallated with NaBPh4 to release the product. 

For the thioboration reactions of alkynes preferentially catalyzed by Pd(0) instead of Pt(0), the reaction mechanism involves a metathesis-like process. The reason for not having an oxidative addition step can be related to the electron richness of the alkylthio group, which prevents the oxidative addition of thioborane to the metal center. Because of the preference for having a metathesis-like process, Pd becomes a better candidate due to its relative less electron richness in comparison to Pt. 

As for the oxidative addition (step i), both cyclic and acyclic hydrogen phosphonates, 4a and 4b, react with Pd(0) to generate the corresponding adducts 12a and 12b (Scheme 29). Although five-membered 4a is somewhat more reactive, this small difference in reactivity does not account for the lack of catalytic addition of 4b. 

A subsequent study using neopentane as the alkane substrate gave evidence in support of the same mechanism, and also allowed resolution of near-coincident y(CO) absorptions due to [Cp Rh(CO)Kr] (1946 cm ) and [Cp Rh(CO)(di2-neopen-tane)] (1947 cm ) [18]. Further studies were able to quantify the reactivity of [Cp Rh(CO)Kr] towards a range of alkanes [20]. It was found that binding of the alkane to Rh becomes more favorable, thermodynamically, as the alkane size is increased, but that the rate of the C-H oxidative addition step shows less variation with linear alkane chain length. No reaction with methane was observed, which was explained by the ineffective binding of methane (relative to excess Kr) to Rh. 

At constant CO partial pressure, the rate determining step is a function of the HX bond strength. In the case of HSnR, the rate determining step is CO dissociation, Equation 9. For HSiR and H, it is the oxidative addition step. Equation 10. ... 

Racemic a-phenylethyl bromide is carbonylated under phase-transfer conditions with 5 N NaOH and dichloromethane containing bis-(dibenzylideneacetone)Pd(O) and a chiral 2-substituted 3,1,2-oxaza-phospholane to give a-phenylpropionic acid in moderate ee (Scheme 83) (196). The reaction involves kinetic resolution of the bromide with a discriminative slow oxidative addition step. 

It is assumed that the mechanism of the palladium-catalyzed cross-coupling reactions of iodonium salts involves the initial oxidative addition step, followed by ligand coupling at the iodine and then at the palladium centers analogously to the mechanism shown in Scheme 31 [63,66]. 

In nucleophilic catalysis the metal ion is generally in a low oxidation state, and this type is common in organometallic catalysis, for example, in many processes involving an oxidative addition step (Eq. 3). 

This mechanism differs from the commonly accepted mechanism of catalytic asymmetric hydrogenation, because the accepted one proposed an irreversible oxidative addition (the oxidative addition step should have the largest energy barrier), but it does agree with recent experimental results that show irreversible and stereodetermining migratory insertion [78]. 

The general catalytic cycle for this carbonylation coupling reaction is analogous to direct carbon-heteroatom coupling [scheme (39)] except that carbon monoxide insertion takes place after the oxidative addition step and prior to the nucleophilic attack of the amine [scheme (40)] ... 

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