Computational studies allylation

The reactions occur preferentially though an endo TS in which the sulfur substituent is oriented toward the allylic group.273 Computational studies (MP2/6-31G ) found the endo TS to be favored over the exo by 1.5-2.2. kcal/mol.274... 

Another computational study examined the effect that the boron ligands might have on the reactivity of allyl derivatives.50 The order found is shown below and is related to the level of the boron LUMO. The dominant factor seems to be the ir-donor capacity of the ligands. The calculated order is consistent with experimental data.51... 

Substituents at the allene in general and methyl groups in particular favor /3-addition of radicals, which leads to the formation of allylic and therefore stabilized intermediates. Results from computational studies predict that CH3 addition to Cp of... 

We performed a computational study [69] to assess which interaction (H bonding, metal-alcoholate formation, or metal-alcohol coordination between the allylic hydoxyl moiety and the Re complex) affects the TS and to determine which oxygen of the Re peroxo moiety acts as H-bond acceptor in the case of an H-bonded TS. A summary of the results with propenol as model allylic alchohol is presented in the following. 

A recent study has indicated that the skeletal rearrangement step in the B12-catalysed isomerization of methylmalonyl-CoA to succinyl-CoA occurs not by a radical pathway but by an anionic or organocobalt pathway. A computational study of the isomerization of allyl alcohol into homoallyl alcohol by lithium amide has pointed to a process proceeding via a transition state in which the proton is half transferred between carbon and nitrogen in a hetero-dimer. l,l-Dilithio-2,2-diphenylethene... 

Density functional theory computational studies have been used to determine die importance of secondary orbital interactions for the stability of transition-state structures for die 4 + 2-cycloaddition of furan with cyclopropene.175 Kinetic studies of die 2 + 4-cycloaddition of 2-cyclopropylidene acetates with furan and dimethylful-vene suggest a mechanism involving diradicals or zwitterions as intermediates.176 Cyclopropene, produced by die reaction of allyl chloride with sodium bis(bimediyl-silyl)amide, reacts with 1,3-diphenylisobenzofuran to produce both endo- and exo-Diels-Alder cycloadducts isolated for the first tune.177... 

A computational study has been carried out593 on the rearrangement reactions of methylsilacyclopropane, and the pathway shown in Scheme 118 has been put forward594 to account for the formation of product (459) from the reaction of Me2AlCl with the (2 + 2)-cycloadduct (458) between naphthoquinone and allyl trimethylsilane. 

Although Brown and co-workers proposed a six-membered transition state for the asymmetric allylboration reaction in which the aldedyde oxygen initially coordinates to boron followed by an internal transfer of the allyl group from boron to the carbonyl carbon,8 a quantitative analysis to explain the enantioselectivity was not available until 1993, when Gennari et al. conducted a computational study to rationalize the enantiofacial selectivity of Brown allylation9 (Scheme 3.1g). Calculation predicts that transition state A, in which the allyl group attacks the si-lace of the aldehyde, is favored over transition state B by 2.12kcal/mol. 

In an effort to elucidate the electronic effects in the stereochemistrydetermining transition states, Gung and co-workers conducted a computational study in 2002.21 The calculations carried out on the allylation reaction between... 

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). ... 

Pokin, A. A., Kushko, A. O., Kiri], A. V., Yurchenko, A. G., Schleyer, P. v. R. Direct Transformations of Ketones to y-Unsaturated Thiols via [2,3]-Sigmatropic Rearrangement of Allyl Sulfinyl Carbanions A Combined Experimental and Computational Study. J. Org. Chem. 2000,... 

A more recent experimental and computational study of rf-allyl-Pd cationic complexes confirmed the tendency of nucleophiles to attack the terminal positions of the allyl ligand as long as jx-accepting ligands are present.54 If a-donating ligands are attached to palladium, which would pump electron density into the allyl ligand, attack occurs mainly at the C-2 position. Equations 8.37 and 8.38 summarize these results. 

Branchadell, V., Crevisy, C., Gree, R., From Allylic Alcohols to Aldols by Using Iron Carbonyls as Catalysts Computational Study on a Novel Tandem Isomerization Aldolization Reaction, Chem. Eur. J. 2004, 10, 5795 5803. 

As computational methods for describing excited states have been refined, additional understanding of the structures has developed. Relatively early computational studies provided some indication of the geometries associated with the butadiene excited states.The ground state has a maximum at a twist of 90° about the C(l)-C(2) bond. This structure, which can be approximately described as a singlet methylene-allyl diradical, is found at about 2.3 eV and is more stable than a structure with 90° twist at both terminal groups (3.1 eV). There is no major pyramidalization of the methylene groups in this second structure. The spectroscopic (Franck-Condon) state is about... 

The pattern that emerges from the experimental and computational studies of the conjugated dienes and trienes is the involvement of CIs having certain features in common. The singlet CI for the dienes appears to be a tetraradicaloid with the potential for several re-bonding schemes. In the absence of steric problems, it is structurally compact. The hexatriene-cyclohexadiene system also appears to involve a tetraradicaloid structure, one component of which is an allylic system. These structures can account for the major product types in both systems. 

The computational studies disclosed that Diels-Alder-like transition states could account for the high diastereoselectivity obtained in these allylic substitution reactions (eqn. (2) in Scheme 6.6). 

The two plausible mechanisms for the DAAA are either via inner-sphere or outer-sphere attack of the allyl group (Scheme 4.25). As mentioned previously the different selectivity observed by Trost between the Li-enolate and the enol carbonate is suggestive of an inner-sphere mechanism via a Pd-enolate rather than an outer-sphere mechanism as is typically seen in other 7i-allyl alkylations [48]. The limited enolate scrambling observed in the DAAA would be consistent with an inner-sphere mechanism. A series of computational studies were carried out by Stoltz which suggest that the inner-sphere mechanism is lower in energy that the... 

Axial preferences in the transition state of addition of 3,3-disubstituted allyl-zinc species to aldehydes have been rationaUzed by computational studies of model compounds. (g)... 

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