Related Stoichiometric Reaction

As we have seen, reviewing catalytic S-X bond activations, some reactions complete their catalytic cycles by C-S bond-forming reductive elimination from C-M-S complexes. Hartwig et al. have reported on the mechanism of the C-S bond-forming reductive elimination from Pd(L)(R)(SR ) 122 (Eq. 7.72) [69]. 

They demonstrated that electron-deficient R groups and electron-rich R substituents at S accelerated the reductive elimination. They proposed 123 (Lj = DPPE, R = Ph, R = Ar) as a transition state, where R acts as an electrophile and thiolate as a nucleophile. The Hammet plot for the reductive elimination showed that the resonance effect of the substituent in R determines the inductive effect of the R group, and the effect in SR showed an acceptable linear relationship with the standard o-values. The relative rate for sulfide elimination as a function of the hybrid valence configuration of the carbon center bonded to palladium followed the trend sp sp spl 

Throughout the entire S-X bond activation and the corresponding mechanistic studies, a general feature of the reactivity of S-M bonds was gradually revealed. 

OjiMA in The Chemistry of Organic Silicon Compound . S. Patai, Z. Rappoport, (eds.) Wiley-Intersdence Chichester, 1989 Chapter 25. 

CoLLMAN, L. S. Hegedus, J. R. Norton, R. G. Finke, Principles and Application of organotransition metal Chemistry University Sdence Books Mill Valley, CA, 1987 pp 355. 

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Intermediate 2 can coordinate an additional alkyne (or olefin) to give 5 after insertion into the Ni-C bond (Scheme 7). Reductive elimination affords the corresponding cyclodecatriene derivative. Several related stoichiometric reactions with certain Ni complexes and alkynes or allene have been observed, thus confirming the proposed catalytic cycle (route ( )) [48] (eqs. (17) and (18)). 

Principle mechanisms of pyrone formation are summarized in Scheme 2. The probable pathway via Structures 10-11-12 is based on related stoichiometric reactions with model complexes [19] and X-ray structural investigations on precatalysts, and is consistent with the experimental details. In 11, the sp center next to nickel is suitable for the insertion of further alkynes, yielding the intermediate 12. Reductive elimination of the product 9 and addition of a further alkyne molecule closes the cycle. Analogous complexes to the intermediate 11 were shown to be versatile stoichiometric reagents for transformations to unsaturated acids and esters, by the groups of Hoberg, Dinjus, and Walther [20]. 

The main focus of this chapter is homogeneous catalysis which involves the cleavage of C-C=0 and C-CN bonds. Catalytic reactions are categorized in the following order (i) type of bond cleaved (ii) mechanism (oxidative addition, others, and unclear) and (iii) strategy (directed, non-directed, and others). Related stoichiometric reactions are also given when necessary. For catalytic reactions via decarboxylation of a-keto carboxylic acids, refer to Chapter 4. 

It is otherwise for complex reactions, for which the rate equation may or may not be simply related to the overall stoichiometric reaction. For example, the rate equation for the alkaline hydrolysis of ethyl acetate, which is a complex reaction (see Section 1.2),... 

Beyond palladium, it has recently been shown that isoelectronic metal complexes based on nickel and platinum are active catalysts for diyne reductive cyclization. While the stoichiometric reaction of nickel(O) complexes with non-conjugated diynes represents a robust area of research,8 only one example of nickel-catalyzed diyne reductive cyclization, which involves the hydrosilylative cyclization of 1,7-diynes to afford 1,2-dialkylidenecyclohexanes appears in the literature.7 The reductive cyclization of unsubstituted 1,7-diyne 53a illustrates the ability of this catalyst system to deliver cyclic Z-vinylsilanes in good yield with excellent control of alkene geometry. Cationic platinum catalysts, generated in situ from (phen)Pt(Me)2 and B(C6F5)3, are also excellent catalysts for highly Z-selective reductive cyclization of 1,6-diynes, as demonstrated by the cyclization of 1,6-diyne 54a.72 The related platinum bis(imine) complex [PhN=C(Me)C(Me)N=Ph]2Pt(Me)2 also catalyzes diyne hydrosilylation-cyclization (Scheme 35).72a... 

Today, iridium compounds find so many varied applications in contemporary homogeneous catalysis it is difficult to recall that, until the late 1970s, rhodium was one of only two metals considered likely to serve as useful catalysts, at that time typically for hydrogenation or hydroformylation. Indeed, catalyst/solvent combinations such as [IrCl(PPh3)3]/MeOH, which were modeled directly on what was previously successful for rhodium, failed for iridium. Although iridium was still considered potentially to be useful, this was only for the demonstration of stoichiometric reactions related to proposed catalytic cycles. Iridium tends to form stronger metal-ligand bonds (e.g., Cp(CO)Rh-CO, 46 kcal mol-1 Cp(CO)Ir-CO, 57 kcal mol ), and consequently compounds which act as reactive intermediates for rhodium can sometimes be isolated in the case of iridium. 

The procedures are simple and give essentially stoichiometric reactions which can be carried out in the absence of a solvent. The systematic nature of these syntheses relates to the observation that hydride ion can be abstracted from certain boron hydride anions to give as one of the final products a neutral boron hydride which contains one more boron atom than the anionic starting material. These reactions are described below. 

This set of relations between reaction orders and stoichiometric coefficients defines what we call an elementary reaction, one whose kinetics are consistent with stoichiometry. We later wiU consider another restriction on an elementary reaction that is frequently used by chemists, namely, that the reaction as written also describes the mechanism by which the process occurs. We will describe complex reactions as a sequence of elementary steps by which we will mean that the molecular collisions among reactant molecules cause chemical transformations to occur in a single step at the molecular level. 

Abstract The purpose of this chapter is to present a survey of the organometallic chemistry and catalysis of rhodium and iridium related to the oxidation of organic substrates that has been developed over the last 5 years, placing special emphasis on reactions or processes involving environmentally friendly oxidants. Iridium-based catalysts appear to be promising candidates for the oxidation of alcohols to aldehydes/ketones as products or as intermediates for heterocyclic compounds or domino reactions. Rhodium complexes seem to be more appropriate for the oxygenation of alkenes. In addition to catalytic allylic and benzylic oxidation of alkenes, recent advances in vinylic oxygenations have been focused on stoichiometric reactions. This review offers an overview of these reactions... 

Mori has reported the nickel-catalyzed cyclization/hydrosilylation of dienals to form protected alkenylcycloalk-anols." For example, reaction of 4-benzyloxymethyl-5,7-octadienal 48a and triethylsilane catalyzed by a 1 2 mixture of Ni(GOD)2 and PPhs in toluene at room temperature gave the silyloxycyclopentane 49a in 70% yield with exclusive formation of the m,//7 //i -diastereomer (Scheme 14). In a similar manner, the 6,8-nonadienal 48b underwent nickel-catalyzed reaction to form silyloxycyclohexane 49b in 71% yield with exclusive formation of the // /i ,// /i -diastereomer, and the 7,9-decadienal 48c underwent reaction to form silyloxycycloheptane 49c in 66% yield with undetermined stereochemistry (Scheme 14). On the basis of related stoichiometric experiments, Mori proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of dienals involving initial insertion of the diene moiety into the Ni-H bond of a silylnickel hydride complex to form the (7r-allyl)nickel silyl complex li (Scheme 15). Intramolecular carbometallation followed by O-Si reductive elimination and H-Si oxidative addition would release the silyloxycycloalkane with regeneration of the active silylnickel hydride catalyst. 

The preceding stoichiometric reactions are clearly closely related to the catalytic hydroformylation of epoxides. Somewhat less clear is their relation... 

Any hnear combination of steps in the reaction scheme that leads to an overall stoichiometric reaction that converts reactants and products gives rise to a relationship of thermodynamic consistency. Specifically, if stoichiometric coefficients of the linear combination of steps i that lead to an overall stoichiometric reaction, then the values of AH and AS- for these steps are related by the following equations ... 

The reaction rate equations used in SMBR modeling are commonly of the power law type, and relate the reaction rate r, of component i to its stoichiometric coefficient Vi, to the reaction rate constant kR, and to the solid-phase concentrations of the reactants and the products, qR and qp, respectively ... 

An interesting relation has been pointed out by Shaffer.19 Among the reactions in which the only change is the transfer of electrons from one ion to another the bimolecular reactions are immeasurably fast, but the reactions which require triple or quadruple collisions for the stoichiometrical reaction are usually slow. For example the reaction... 

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