Hyperconjugation stereochemistry

The hydrogen atom migration observed on thermolysis of is reminiscent of 1,2-hydrogen atom migrations in carbene chemistry (45,46,47). The stereochemistry of such processes is now relatively well-understood and involves initial hyperconjugative interaction between a gauche C-H bond and the carbene unoccupied p atomic orbital, followed by a low activation energy 1,2 shift (eq.(6)) ( 7,48,49,50). 

Can Hyperconjugation in a 1,3-Diradical Control the Stereochemistry of Cyclopropane Ring Opening and Make a Singlet the Electronic Ground State of the Diradical ... 

Many such examples are known. In most cases where the stereochemistry has been investigated, retention of configuration is observed,225 but stereoconvergence (the same product mixture from an E or Z substrate) has also been observed,226 especially where the carbanionic carbon bears two electron-withdrawing groups. It is not immediately apparent why the tetrahedral mechanism should lead to retention, but this behavior has been ascribed, on the basis of molecular orbital calculations, to hyperconjugation involving the carbanionic electron pair and the substituents on the adjacent carbon.227... 

As with alkenes, in general, anti-addition is often the course of reaction, especially when halonium ions are involved109-112. However, as mentioned earlier, syn addition can take place in the bromination of /Tsilylslyrenes. This stereochemistry is explained by stabilization of the open-chain carbocation by the aromatic group, compared to the cyclic bromonium ion. In this case the conformer 83 has the maximum hyperconjugative stabilization, and is formed by the least motion rotation about the carbon-carbon bond. 

Recently, density functional calculations were performed to determine the nature and stereochemistry of the olefin insertion into the Cu-B bond of (NHC)Cu boryl complexes (NHC = iV-heterocyclic carbene). The theoretical calculations confirm that the mechanism of insertion involves a nucleophilic attack of the boryl ligand on the coordinated olefin. Furthermore, the hyperconjugation of Cu-C (bond angles, which was also experimentally confirmed by the X-ray diffraction studies of these boryl-copper complexes <2007OM2824>. 

The stereoselectivity of cyclopropane ester formation could also be effected by using reagents supported on linear or cross-linked polymers. The most important effects were noted with chloromethylated polymers cross-linked with divinylbenzene. The role of hyperconjugation in determining the stereochemistry of nucleophilic cyclopropanation of electrophilic alkenes has been studied and predicted In terms of equation 57 the... 

The stereochemistry of exo closure in a case like the radical 7.92, giving the cis product 7.94 (cis. trans 72 28), is controlled by the usual preference for the resident substituent to adopt an equatorial orientation 7.93 and for the chain of atoms to adopt a chair-like conformation.1031 In the case of a radical like 7.95, however, there is a clearly contrathermodynamic preference for the formation of the cis product 7.97.1032 It has been suggested that the endo-like transition structure 7.96 might have an attractive secondary orbital interaction from the filled hyperconjugative orbitals with the n orbital, rather like that used to explain the endo rule for Diels-Alder reactions. The preference for cis products like 7.97 does not extend to all substituents on the radical centre, and Z-substituents often lead to trans products, possibly because they lack the same set of orbitals, but probably also because they are larger than a methyl group. 

A theoretical approach to the role of hyperconjugation in determining the stereochemistry of nucleophilic epoxidation has proved to be consistent with experimental observations11". Standard ab initio methods were used to calculate the barriers to rotation for the carbanion (98) generated by ZO attack on the alkene (Z and Y various types of sustituent). These calculations taken together with the nucleofugality of Z and the electron-withdrawing ability of Y allow predictions in accord with experiment. 

The Cram model does not predict the correct stereochemistry of reduction products derived from cyclohexanone derivatives. anti-Periplanar hyperconjugative effects have been invoked to explain the observed results. The Felkin-Ahn model suggests an interaction such as that shown in 297. 91 An alternative proposal was put forth by Cieplak, shown in 298, 92 and it was found that the incipient bond (the one being formed) was electron deficient, in line with Cieplak s proposal. Cieplak s model has led to some controversy, and a... 

The combination of experimental and calculated determination of NMR chemical shifts and spin-spin coupling constants allows the characterization of the -l-cyclopropyl-2-(triisopropylsilyl)-ethyl cation (1) and an unequivocal assignment of its stereochemistry. Due to the stabilizing p-hyperconjugative interaction of both the strained cyclopropyl C-C bonds and the P-C-Si bond with the vacant orbital at Ca the rotation around the Ca-Cp and Co-Cp bonds is frozen under the experimental conditions between -150°C and -80°C. 

Saners, R. R. A natural bond orbital analysis of carbanions (2012). Computational and Theoretical Chemistry, 999,43. Apeloig, Y., Kami, M., Rappoport, Z. (1983). Nncleophibc attacks on carbon-carbon double bonds. Part 29. Role of hyperconjugation in determining the stereochemistry of nucleophilic epoxidation and cyclopropanation of electrophilic olefins. Journal of the American Chemical Society, 105(9), 2784—2793. 

Since current qualitative MO theoretical models seemingly provide answers to these questions, we shall be able to compare the conclusions of MOVB theory with the conclusions of these models and, thus, define the present limits of our understanding of chemical bonding and the new horizons opened by MOVB theory. Our first task is then to describe exactly what the currently popular models of chemical bonding predict with regards to the stereochemistry of N2H, H2O2 and their derivatives. In more specific terms, we want to first examine how the hyperconjugation 5... 

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