Four-center

Hasegawa, M. Four-Center Photopolymerization in the Crystalline State. Vol. 42, pp. 1 —49. 

All three- and four-center two-electron integrals, which are by far the most numerous of the two-electron integrals, are neglected. 

The first three types of integrals involve one or two centers. The fourth type of integral involves up to four centers. 

It has been suggested that a second mechanism, involving a four-center transition state, is also possible Bellus, D. Schaffner, K. Hoigne, J. Helv. Chim. Acta, 1968, 51, 1980 Sander, M.R. Hedaya, E. Trecker, D.J. J. Am. Chem. Soc., 1968, 90,7249 Bellus, D. Ref. 413. 

Hasegawa, M., Suzuki, Y., Suzuki, F. and Nakanishi, H. (1972). Four-Center Type Photopolymerization in the Crystalline State, vol. 5, pp. 143-203. Kodansha, Tokyo 

The dynamic olefin insertion process has been modeled using various quantum mechanical methods. A concerted four-center mechanism involving a frontal copla-nar attack of the C=C unit on the Zr-H bond of 1 is associated with a low activation energy of 0-15 kcal mol and has been proposed for the reaction of ethylene (Scheme 8-2) [37]. 

The influence of the solvent on the decomposition rate of phenylpentazole (see Table II) supports the one-step four-centered 1,3-flssion of phenylpentazole according to 22. The correlation of the polarity of the transition states of 1,3-additions, which resemble the assumed transition state of the phenylpentazole decomposition (22), andsolvent effects has recently been discussed by Huisgen.  

Recognition of the fact that tin(IV) enolates exist predominantly as the carbon-bound enolate has led to the alternative suggestion that a four-center transition state, such as L could also rationalize the reversal of diastereoselectivity upon changing the enolate counterion from aluminum to tin26-44. 

There are some addition reactions where the initial attack is not at one carbon of the double bond, but both carbons are attacked simultaneously. Some of these are four-center mechanisms, which follow this pattern  

Epoxides can be converted to alkenes by treatment with triphenylphosphine or triethyl phosphite P(OEt)3. The first step of the mechanism is nucleophilic substitution (10-50), followed by a four-center elimination. Since inversion accompanies the substitution, the overall elimination is anti, that is, if two groups A and C are cis in the epoxide, they will be trans in the alkene  

A simplified mechanism for the hydroformylation reaction using the rhodium complex starts by the addition of the olefin to the catalyst (A) to form complex (B). The latter rearranges, probably through a four-centered intermediate, to the alkyl complex (C). A carbon monoxide insertion gives the square-planar complex (D). Successive H2 and CO addition produces the original catalyst and the product  

In 1965, however, the computational resources needed for the full SCF approach were not yet available. Practical MO theories therefore still needed approximations. The main problem is the calculation and storage of the four-center integrals, denoted (fiv I Aa), needed to calculate the electron-electron interactions within the 

In contrast to the extensive body of work on the preparation of these zinc carbenoids, few investigations are on record concerning the mechanism of the Furu-kawa method for carbenoid formation. Two limiting mechanisms can be envisioned - a concerted metathesis via a four-centered transition structure or a stepwise radical cleavage-recombination (Scheme 3.11). 

The formation and decomposition of benzenediazoazide and phenylpentazole can be described by a mechanism alternative to the one discussed in Section III, A, 1 [Eq. (3)]. In contrast to the tacitly assumed independent formation and decomposition of phenylpentazole, e.g. one-step four-centered processes as described by 21 and 22, 

In a few cases, SnI reactions have been found to proceed with partial retention (20-50%) of configuration. Ion pairs have been invoked to explain some of these. For example, it has been proposed that the phenolysis of optically active a-phenyl-ethyl chloride, in which the ether of net retained configuration is obtained, involves a four-center mechanism  

Thermal rearrangement of trans-l,2-dibromo compounds is known in the literature (refs. 6-10). In all case studies only one pair of bromine in each organic molecular was studied. Bellucci (ref. 10), for example, studied the kinetics of such trans-l,2-cyclo alkanes as cyclopentane, hexane, octane, etc. The intermediates suggested as an explanation for the experimental results are bromonium bromide I in polar solvents and four center transition state II in non-polar solvents. 

It has generally been assumed that phosphorous oxychloride-pyridine dehydrations, the elimination of sulfonates, and other base catalyzed eliminations (see below) proceed by an E2 mechanism (see e.g. ref. 214, 215, 216). Concerted base catalyzed eliminations in acyclic systems follow the Saytzelf orientation rule i.e., proceed toward the most substituted carbon), as do eliminations (see ref 214). However, the best geometrical arrangement of the four centers involved in 2 eliminations is anti-coplanar and in the cyclohexane system only the tran -diaxial situation provides this. 

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