Four-centered transition

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). 

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. 

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 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. 

The degree of vibrational excitation in a newly formed bond (or vibrational mode) of the products may also increase with increasing difference in bond length (or normal coordinate displacement) between the transition state and the separated products. For example, in the photodissociation of vinyl chloride [9] (reaction 7), the H—Cl bond length at the transition state for four-center elimination is 1.80 A, whereas in the three-center elimination, it is 1.40 A. A Franck-Condon projection of these bond lengths onto that of an HCl molecule at equilibrium (1.275 A) will result in greater product vibrational excitation from the four-center transition state pathway, and provides a metric to distinguish between the two pathways. 

M amines competed effectively with 55.5 M H O and 0.1 M OH for the nitrosating agent and suggested that possibly more reactive isomers of N O and N 0. are generated by the gaseous NO and NO components Here, -nltrosamines result from reaction of the unsymmetrical tautomer (ON-NO ), whereas the symmetrical tautomer (O N-NO ) produces an N-nitramine possibly via a four-center transition state. The results for N O may be xplalned similarly in terms of the corresponding ON-nO.ON-ONO tautomers. This conclusion has a prece-... 

The H—D exchange between triethyltin deuteride and di-isobutylaluminum hydride has been studied by Neumann 84). A four-center transition state has been proposed for this reaction... 

However, when (+)-methylneophylphenyltin deuteride, (+)-(56) ([ot] s + 10.7) is kept in the dark mixed with five equivalents of diethylaluminum hydride for ten hours at room temperature in benzene, optically inactive (72) is formed 44). (In the absence of (Et2AlH)2 less than 3 % of (12) is racemized under these conditions). The four-center transition state is therefore very unlikely. 

In the second step, the n complex becomes the addition product by passing through a four-center transition state in which the boron atom is partially bonded to the less substituted carbon atom of the double bond. 

Addition takes place through the initial formation of a n complex, which changes into a cyclic four-center transition state with the boron atom adding to the less hindered carbon atom. The dashed bonds in the transition state represent bonds... 

A theoretical study of the reaction mechanism for addition of organozincate complexes to aldehydes was recently performed using density functional theory.298 It has been suggested that the addition takes place through formation of a four-centered transition state and, therefore, it can be considered a typical nucleophilic reaction. 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

As a model for the insertion process in the polymerization of ethylene, the reaction of Cp ScMe with 2-butyne was investigated. The reaction was revealed to have a relatively small enthalpy of activation and a very large negative entropy of activation a highly ordered four-centered transition state (117) was proposed [111, 112]. 

The mechanism suggested for this reaction is represented in Scheme 21. The involvement of a four-centered transition state is generally accepted for many related (2+2) exchange reactions (1,63). 

In 1998, Kubota, Nakamura and coworkers stndied the addition of substituted (alkoxy)allyllithium and zinc reagents to cyclopropenone acetaP. They found that the regioselectivity of the (alkoxy)allyllithiation is dependent on the substituent, while the diastereoselectivity remains constantly high (>97%). This is supported by their theoretical study (at the B3LYP/6-31G level. Scheme 3) which reveals that a (hydroxy)allyllithium species of 7r-allylmetal nature can react with cyclopropene via two [2 + 2]-type four-centered transition states of similar energies leading to a- and y-adducts. 

SCHEME 3. (Alkoxy)allylmetalation of cyclopropene, the four-centered transition states leading to both Qf-adduct and y-adduct are shown. 

Hydroboration is a sterospecific syn addition. The addition occurs through a four-center transition state with essentially simultaneous bonding to boron and hydrogen. Both the new C—B and C—H bonds are, therefore, formed from the same side of the double bond. In molecular orbital terms, the addition is viewed as taking place by interaction of the filled alkene n orbital with the empty p orbital on boron, accompanied by concerted... 

Figure 8-11 shows the proposed mechanism for isoselective propagation. Monomer coordinates at the vacant site of titanium, resulting in a four-center transition state and subsequent insertion of monomer into the polymer-transition metal bond. The insertion is referred to as migratory insertion since the polymer chain migrates from its original site to that occupied... 

There is still ample room for the development of more effective catalysts for C H insertion. To date, enantiomeric excesses are too low to draw any firm conclusions about the geometry of the C- H insertion process. A tentative suggestion, based on modeling50 of the results above, is that the reaction passes through a four-center transition structure, with significant C—C as well as Rh-H bonding. 

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