Elimination reactions theoretical studies

A comprehensive experimental and theoretical study was undertaken on the reaction, which was shown to be a concerted reductive elimination process kinetic studies were consistent with reductive elimination, and DFT calculations on complex 3 (Fig. 13.1) supported an associative reductive elimination pathway with a small... 

Theoretical studies of catalytic alkane-dehydrogenation reactions by [(PCP )IrH2], PCP rf-C6H3(CH2P112)2-l, 3 and [cpIr(PH3)(H)]+, suggest that they proceed through similar steps in both cases namely (i) alkane oxidation, (ii) dihydride reductive elimination, (iii) /3-II transfer from alkyl ligand to metal, (iv) elimination of olefin.402 The calculated barriers to steps (i), (ii), and (iv) are more balanced for the PCP system than for cp(PH3). 

The second fixation reaction that has been the subject of theoretical study is the nickel(0)-catalyzed coupling reaction of C02 with acetylene (equation 1), which was theoretically investigated with the ab initio SD-CI method [25]. The theoretical calculations clearly showed that if the nickel(O) moiety was eliminated from the reaction system, the activation barrier increased very much, and that the C-C bond formation between C02 and acetylene was accelerated by the charge-transfer from the nickel(O) d orbital to the orbital resulting from the combination between n orbitals of acetylene and CO2. Actually, the HOMO contour map of Ni(PH3)(C02)(C2H2) clearly displays this orbital mixing in the transition state. 

Recent theoretical studies of reductive elimination from MeaCu-S in the presence of BF3 suggest that reaction rate of the conjugate addition can increase if one of the Me groups is detached from the copper(III) to bind with a boron atom (Scheme 10.12) [129]. 

The thermodynamic stabilities of phenonium ions relative to the parent have been determined in the gas phase by measuring the position of the equilibrium between (46) and (47)7 The results followed a Yukawa-Tsuno relationship with a p value of -12.6 and an r+ value of 0.62, the general behaviour being more like benzenium ions than benzyl cations, with tt-delocalization less effective than in benzyl cations. A theoretical study of the elimination of molecular H2 from the benzenium ion C6H7+ shows that the barrier to this process appears to be very small.The gas-phase Friedel-Crafts alkylation reaction of CF3C6L6+ (L = H or D) with C2L4 is accompanied by isotopic scrambling, which has been used to elucidate the mechanism of this process. A theoretical calculation shows that the lifetime of triplet phenyl cation must be very short. ... 

A theoretical study of the 5 n2 reaction of MeBr with MeaCuLi.LiCl has addressed two mechanistic possibilities a simple S 2 reaction with the carbon nucleophile (eq. 4) and the S 2 reaction with the copper atom (followed by rapid reductive elimination of a triallylcopper(III) intermediate in a manner which forms only the cross-coupling product RR ) (eq. 5). ... 

Ab initio methods using the 6-31 + G basis sets have been used in a theoretical study of competing, S n2 and El reactions of NCCH2CH2CI with HO and HS in the gas phase. The antiperiplanar elimination transition state, which is favoured over those for n2 and El(gauche) reactions, is more lcB-like than that for the slower El anti) reaction of ethyl chloride. 

Important features favoring the reductive elimination reaction have been discussed based on theoretical and experimental studies [128-130]. The reductive elimination of an azohum salt from a palladium NHC alkyl complex (Fig. 13) proceeds under direct formation of Pd° in an exothermic process with a low activation barrier [128]. The coligands at the palladium atom play an important role. It has been shown that the Caikyi-Pd-CNHC angle becomes more acute during the reductive elimination to allow for an optimal orbital overlap of the groups to be... 

The reactions of nitrones with 165 have been described (277-279). In the approach described by Koskinen and co-workers (279), the bulky nitrone 166 was used in a reaction with 165 to give a 20 1 mixture of 167 and an unidentified diastereomer (Note Opposite enantiomers are shown here). Reactions of less bulky nitrones gave lower selectivities (277,278). Kim et al. (280,281) described reactions of 165 with silyl nitronates (Scheme 12.52). The configuration of the direct isoxazolidine products was not determined. Instead, diastereoselectivities of 66-88% de of 169 were found after elimination of the silyloxy group. The reaction of various nitrile oxides proceeded to give the same isoxazoline products 169 as obtained for nitronates (Scheme 12.52). For the reactions of 165 with various alkyl and aryl nitrile oxides 170, the products 169 were obtained with diastereoselectivities of 62-90% de (282-286). In a theoretical study, it was proposed that the... 

There have been both experimental and theoretical studies to probe the degree of concertedness in gas-phase substitutions as shown in Scheme 1. Is (2) an intermediate with a finite lifetime, or are the addition and elimination steps concerted so that (2) is a transition state Experimental molecular beam studies on the femtosecond time-scale have been reported for the reaction of chloride ions with the iodobenzene cation to yield chlorobenzene and iodine. The results show an 880 fs reaction time for the elimination process, indicating a highly non-concerted process, so that here the er-complex is an intermediate rather than a transition state.12 The reactions of halobenzene cations with ammonia have been interpreted in terms of the formation of an addition complex which may eliminate either halogen, X, or hydrogen halide, HX, depending on the nature of the halogen.13... 

Zwitterions [32] and biradicals [33] have been also proposed as intermediates. These mechanistic possibilities were insufficient for rationalizing the lack of correlation between the reaction rates and solvent polarity [14,31], An early theoretical study by Goddard [33] favored a biradical intermediate. However, isotope effect studies [34], the lack of Markovnikov-type directing effects [14,31], and the fact that radical scavengers have no effect on the reaction eliminated a biradical mechanism for the singlet-oxygen-alkene ene reaction. 

Each reaction type (oxidative addition, reductive elimination, etc.) was studied according to the electronic configuration (at this time only the even dn configurations have been considered), the coordination number, and the coordination geometry. The matrices that we have composed for the evaluation (Matrices 1, 2, 3, 4, 5, 6, and 7 see also Section I,C), show the structure d"-ML for which the reaction is allowed or forbidden. We must note that, in most of the cases, the rules that we present derivefrom theoretical studies found in the literature and that exceptions certainly exist. Another difficulty in this reaction evaluation is the importance of the coordination geometry (15b), related to the spin state (low or high), the choice of which is particularly difficult. 

Yim and Liu [145] used a combined ab initio molecular dynamics (MD) study to reveal several new mechanistic paths that are energetically favored over dissociation of the C-N02 bond. Their study indicated dissociation of NTO via an aci-nitro tautomer followed by ring scission to a ketinimine intermediate as the most favorable pathway (Scheme IX), requiring only 38 kcal/mol as compared to the 62.5 kcal/mol needed for cleaving the C-N02 bond. Kohno et al. [151] conducted a theoretical study on the decomposition of the NTO dimer. They reported that HONO elimination is the last step of a cascade of reactions with a total barrier of 88 kcal/mol. 

The oxidative addition of 2-methylimidazolium salts has been studied from the theoretical point of view [ 146,147]. hi fact, the reaction is the reverse of the ubiquitous reductive elimination reaction observed for hydrocarbonyl-Pd- NHC complexes [ 151,152]. The DFT studies predict that the reaction of the 2-methylimidazolium salt with the model complex Pt(PH3)2 is exothermic. The reductive elimination hydrocarbonyl-Pd - NHC complexes has also been studied in detail by DFT calculations [153]. 

As shown in Scheme 42, selenoxides are crucial intermediates in the selenoxide elimination reactions. These are syn-eliminations which proceed via an intramolecular mechanism to yield alkenes as reaction products. The regioselectivities of these eliminations are dependent on the nature of the substituent Y in the / -position as shown in Scheme 44.286 The mild reaction conditions for these elimination reactions make them highly useful in organic synthesis and theoretical studies on this reaction have been carried out as well.286... 

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