Alkanes - computational studies

Rate parameters for these classes a isomerizations are extremely scarce. Our computational studies, however, suggest that for 1,2 hydrogen shift in alkanes, barrier heights should be about 25-40 kcal/mol, the same order of magnitude as the strain energy associated with the formation of three-centered rings. In Table XI some values reported in the literature are also presented (Dente and Ranzi, 1983). 

Computational studies performed on model complexes in collaboration with Hall and coworkers suggest that alkane borylation may occur by a ej-bond metathesis pathway (Scheme 3) [48]. The proposed mechanism for the borylation of alkanes by 1 begins with elimination of HBpin to generate the 16-electron complex Cp Rh(Bpin)2. This complex then forms a <7-complex (3) with the alkane. The vacant p-orbital on boron then enables cr-bond metathesis to generate a o-borane complex (4). Reductive elimination of the alkylboronate ester product and oxidative addition of B2pin2 then regenerate 1. 

Many computational studies of the permeation of small gas molecules through polymers have appeared, which were designed to analyze, on an atomic scale, diffusion mechanisms or to calculate the diffusion coefficient and the solubility parameters. Most of these studies have dealt with flexible polymer chains of relatively simple structure such as polyethylene, polypropylene, and poly-(isobutylene) [49,50,51,52,53], There are, however, a few reports on polymers consisting of stiff chains. For example, Mooney and MacElroy [54] studied the diffusion of small molecules in semicrystalline aromatic polymers and Cuthbert et al. [55] have calculated the Henry s law constant for a number of small molecules in polystyrene and studied the effect of box size on the calculated Henry s law constants. Most of these reports are limited to the calculation of solubility coefficients at a single temperature and in the zero-pressure limit. However, there are few reports on the calculation of solubilities at higher pressures, for example the reports by de Pablo et al. [56] on the calculation of solubilities of alkanes in polyethylene, by Abu-Shargh [53] on the calculation of solubility of propene in polypropylene, and by Lim et al. [47] on the sorption of methane and carbon dioxide in amorphous polyetherimide. In the former two cases, the authors have used Gibbs ensemble Monte Carlo method [41,57] to do the calculations, and in the latter case, the authors have used an equation-of-state method to describe the gas phase. 

With the development of powerful methods for molecular orbital calculations (e.g. DFT) (see Molecular Orbital Theory), several computational studies of the C-H oxidative addition (see Alkane Carbon-Hydrogen Bond Activation) process have been undertaken. One has used CpRh(PH3) to model the reactive intermediate Cp Rh(PMe3) proposed for the reaction shown in equation (13). The results... 

In the period covered by this edition, the hypothesis of a concerted process in the C-H insertion reaction has been challenged by a stepwise mechanism with groups both pro and contra (Scheme 1). A number of computational studies appeared applying different levels of theory on different model systems. A minireview covering selective alkane C-H bond functionalizations has also appeared <2004MI247>. 

One proposal for the catalytic cycle involves an Ir(III)-dihydride intermediate that forms after OA of H2 onto an Ir(I)-alkene complex. Experimental results seem to support this cycle,36 but computational studies suggest that the cycle involves Ir(III) and Ir(V) intermediates.37 The details of neither proposal have been elucidated. Work continues to expand the scope of this reaction to include asymmetric hydrogenation of any unfunctionalized alkene that could yield a chiral alkane. 

Ramalingam and coworkers have performed detailed computational studies at various levels of theory with respect to the insertion of HCIC and CbC singlet carbenes into the carbon-hydrogen bond of small alkanes and identified two insertion modes (Fig. 6.8)." Of these, the a approach has been found to be preferred over the n mode at all levels of theory because the k attack leads to the eclipsed conformation (compare transition state 143 to 144, which is a secondary saddle point). In the initial phase of the insertion process there is a net charge flow from the alkane to the carbine. Barriers for the insertion... 

Ren, B. (2002a) Application of novel atom-type A1 topological indices to QSPR studies of alkanes. Computers Chem., 26, 357-369. 

A computational study of the activation of alkane (methane, ethane, propane, and butane) C-H bonds by the metallocarbene homoscorpionate [Cu=C(H)(CC>2CH3)(Tp)] (Fig. 2.133) and [Cu=C(H)(C02CH3)(TpBr3)] has been performed with DFT Becke3LYP calculations. A low-barrier transition state where the key bond-breaking and bond-forming processes take place in a concerted way has been postulated. The transition state has several possible conformations.549... 

Fox, S. J., Gourdain, S., Coulthurst, A., et al. (2015). A Computational Study of Vicinal Fluorination in 2,3-Difluorobutane Implications for Conformational Control in Alkane Chains. Chemistry -A European Journal, 21(4), 1682-1691. 

More specifically, calculations have suggested that the ruthenium-alkyl complex in Equation 6.53 reacts with arene to exchange covalent ligands by a process closely related to a a-bond metathesis mechanism. Computational studies of the reactions of a simple iridium-alkyl and alkoxo complex with alkanes to generate new metal-alkyl complexes have also suggested that a mechanism is followed that involves many of the characteristics of a classic a-bond metathesis transition state. However, calculations of the mechanisms of these two processes imply that the transition state contains some degree of M-H bonding. 

Abstract The Shilov system, a mixture of di- and tetravalent chloroplatinate salts in aqueous solution, provided the first indication of the potential of organotransition metal complexes for activating and functionalizing alkanes under mild conditions the participation of higher-valent species plays a crucial role. In this chapter, we discuss the experimental and computational studies that have led to detailed mechanistic understanding of C-H activation and functionalization by both the original Shilov system and the many subsequent modifications that have been developed, and assess the prospects for practical, selective catalytic oxidation of alkanes using this chemistry. 

Computational studies have shown that alkane metathesis cannot occur via the o-bond metathesis between the C-C a-bonds and the M-C a-bonds originally proposed [101]. Experimental evidence has also suggested that the reaction mechanism must involve alkene metathesis as the key step and alkylidene hydrido metal complexes as associated intermediates [92, 93, 102]. To date, only few computational studies on alkane metathesis have been reported [103-106]. 

Zheng X, Blowers P (2005) A computational study of alkane hydrogen-exchange reactions on zeolites. J Mol Catal A Chem 242(1-2) 18-25... 

To further examine the nucleophilic side of the CH activation continuum, the our group and the Cundari group undertook a computational study [59] of alkali metal amide and alkaline earth metal amide CH activation reactions with alkanes. This study is directly related to classic superbase chemistry using cesium cyclohexylamide-type reagents to induce CH bond deprotonation of alkanes [60]. 

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