Computational studies elimination reactions

Palladium(II) complexes provide convenient access into this class of catalysts. Some examples of complexes which have been found to be successful catalysts are shown in Scheme 11. They were able to get reasonable turnover numbers in the Heck reaction of aryl bromides and even aryl chlorides [22,190-195]. Mechanistic studies concentrated on the Heck reaction [195] or separated steps like the oxidative addition and reductive elimination [196-199]. Computational studies by DFT calculations indicated that the mechanism for NHC complexes is most likely the same as that for phosphine ligands [169], but also in this case there is a need for more data before a definitive answer can be given on the mechanism. 

Let us consider the general trends of the reactivity of C-C, C-S, and C-Q (Q = Cl, Br, I) bonds towards oxidative addition and reductive elimination (Scheme 7-25). In many cases, either C-C bond-forming reductive elimination from a metal center (a) or the oxidative addition of a C-Q bond to a low-valent metal center is a thermodynamically favorable process (c). On the other hand, the thermodynamics of the C-S bond oxidative addition and reductive elimination (b) lies in between these two cases. In other words, one could more easily control the reaction course by the modulation of metal, ligand, and reactant Further progress for better understanding of S-X bond activation will be achieved by thorough stoichiometric investigations and computational studies. 

Olefination Reactions Involving Phosphonium Ylides. The synthetic potential of phosphonium ylides was developed initially by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond. The mechanism originally proposed involves an addition of the nucleophilic ylide carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes direct formation of the oxaphosphetane by a cycloaddition reaction.236 There have been several computational studies that find the oxaphosphetane structure to be an intermediate.237 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.238 Betaine intermediates have been observed only under special conditions that retard the cyclization and elimination steps.239... 

A similar borderline system, fran -bis[2-(2-chloroethyl)pyridine]palladium chloride (1), has been prepared and structurally characterized by X-ray spectroscopy and computational study.2 A study on the elimination reaction of (1) induced by quinuclidine in acetonitrile has been performed (Scheme 1). The results suggest that the initial product of elimination is a palladium complex of vinylpyridine and that displacement from this complex is partially rate determining in the formation of the uncoordinated product. Despite experimental efforts, it was not possible to distinguish between two possible mechanisms, E2 concerted or ElcB. 

Computational studies concerning theoretical approaches to the intrinsic basicity of neutral nitrogen bases have been reported, including those of phos-phoranimines. The non-ionic phosphazene bases BEMP (112), BTPP (113) and (114, R = Ph) appear to be excellent catalysts for the Michael addition reactions. Thus the yield of the coupling reaction of ethyl isocyanoacetate with l,2-bis(4-bromomethylphenyl)ethane is increased by the addition of the phosphazene base BEMP. Polymer-supported BEMP (P-BEMP) has been applied for the allylation of 2H-benzo[d]l,3-dioxolan-5-ol by allyl bromide. " Cyclodehydration of 1,2 diacylhydrazines by tosyl chloride in the presence of P-BEMP leads to excellent yields of 1,3,4,-oxadiazoles. Addition of P-BEMP also improves the yield of the Hofmann elimination step in the synthesis of tertiary mines using REM resin (polymer-bound acrylate ester). ... 

This chapter will examine structure correlations pertinent to carbonyl substitution-elimination reactions as well as results of computational studies of potential energy surfaces and transition-state structures for such reactions. We shall then assess the current standing of the original postulates as well as that of the derivative models employed in the discussion of stereoselection and regioselectivity. 

The reaction sequence includes (1) an oxidative addition of C-X bond of the aromatic substrate Ar-X to a Pd° center, (2) a substitution of OR for X in the LnPd (Ar)X intermediate, and (3) a reductive elimination of the C-O bond from the Pd center. The ability of palladium(ll) alkoxides bearing p-hydrogen atoms to undergo p-hydride elimination imposes some limitations on the type of alkoxide groups that can be involved in these C-O coupling reactions [2]. The C(sp )-0 reductive elimination reactions from Pd and Pt centers have also been studied computationally [4]. The reactions were suggested to proceed via a concerted three-center mechanism. 

A DFT computational study of the reaction of Br2 with aromatics (benzene, naphthalene, anthracene, and phenanthrene) in the absence of a catalyst and in simulated CCI4 solution suggests that simple addition can proceed with a barrier that is lower than that for direct substitution. This rather heretic study further suggests that substitution products also can arise from stepwise Br2 addition-HBr elimination routes." ... 

Several observations led to the proposal that some of the catalysts containing metals other than platinum do not react by the Chalk-Harrod mechanism. First, carbon-silicon bond-forming reductive elimination occurs with a sufficiently small number of complexes to suggest that formation of the C-Si bond by insertion of olefin into the metal-silicon bond could be faster than formation of the C-Si by reductive elimination. Second, the formation of vinylsilane as side products - or as the major products in some reactions of silanes with alkenes cannot be explained by the Chalk-Harrod mechanism. Instead, insertion of olefin into the M-Si bond, followed by p-hydrogen elimination from the resulting p-silylalkyl complex, would lead to vinylsilane products. This sequence is shown in Equation 16.39. Third, computational studies have indicated that the barrier for insertion of ethylene into the Rh-Si bond of the intermediate generated from a model of Wilkinson s catalyst is much lower than the barrier for reductive elimination to form a C-Si bond from the alkylrhodium-silyl complex. ... 

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