Four centred intermediates

A mechanism for this reaction involving nucleophilic attack of the ylide on the cyanide group and formation of the P=N linkage via a four-centred intermediate was formulated. The structure of this phosphazene was confirmed by its synthesis from the vinyl azide, Ph(N3)C=CHPh, and triphenylphosphine. Phosphoranes stabilized by electron-withdrawing... 

The reactions of triphenylphosphazenyl derivatives with many other electrophilic species have been described. With sulphur tetrafluoride, sulphur imines are formed, possibly via the four-centred intermediate (24) ... 

Kinetic studies of the reaction of Z-phenyl cyclopropanecarboxylates (1) with X-benzylamines (2) in acetonitrile at 55 °C have been carried out. The reaction proceeds by a stepwise mechanism in which the rate-determining step is the breakdown of the zwitterionic tetrahedral intermediate, T, with a hydrogen-bonded four-centre type transition state (3). The results of studies of the aminolysis reactions of ethyl Z-phenyl carbonates (4) with benzylamines (2) in acetonitrile at 25 °C were consistent with a four- (5) and a six-centred transition state (6) for the uncatalysed and catalysed path, respectively. The neutral hydrolysis of p-nitrophenyl trifluoroacetate in acetonitrile solvent has been studied by varying the molarities of water from 1.0 to 5.0 at 25 °C. The reaction was found to be third order in water. The kinetic solvent isotope effect was (A h2o/ D2o) = 2.90 0.12. Proton inventories at each molarity of water studied were consistent with an eight-membered cyclic transition state (7) model. 

An ab initio study on the unimolecular elimination reactions of methacrylonitrile has revealed a direct four-centre elimination of HCN and three-centre elimination of H2 channels.17 A methylcyanoethylidene intermediate has also been identified. 

The alkene metathesis reaction was unprecedented - such a non-catalysed concerted four-centred process is forbidden by the Woodward-Hoffmann rules - so new mechanisms were needed to account for the products. Experiments by Pettit showed that free cyclobutane itself was not involved it was not converted to ethylene (<3%) under the reaction condition where ethylene underwent degenerate metathesis (>35%, indicated by experiments involving Di-ethylene) [10]. Consequently, direct interconversion of the alkenes, via an intermediate complex (termed a quasi-cyclobutane , pseudo-cyclobutane or adsorbed cyclobutane ) generated from a bis-alkene complex was proposed, and a detailed molecular orbital description was presented to show how the orbital symmetry issue could be avoided, Scheme 12.14 (upper pathway) [10]. 

It appears, therefore, that the formation of phenylpentazole proceeds independendy of the pentazene (Scheme 4) possibly as a one-step, four centre process through intermediate 262. The decomposition of the pentazole is also thought to involve a similar intermediate (263). This is supported by the inverse relationship between the rate... 

Emissions from both the and the previously unreported lli states of the IF molecule have been observed in the gas-phase reaction of L with F2 at low pressure a four-centre complex has been proposed as the reaction intermediate. A combined theoretical-experimental programme has been conducted to establish techniques for the study of excited-state transitions in Ij and IC1. Experimental techniques based on two-step excitation using two synchronized, tunable lasers have been developed, and successfully applied to excited-state fluorescence measurements on ICl. lodine(i) chloride adsorbed on silica gives the same Raman spectrum as that obtained from adsorbed l2. ... 

C2H2F. In the reaction O + 1,1-C2H2F2, the OCH2CF2 intermediate is favoured and four-centre elimination of HF results [analogous to pathways (a), (b) and (c) in Fig. 14], giving an inversion ratio (Ni/Nq) that is closer to the statistical limit. 

The neglect of the off-diagonal four-centre integrals is either called the Complete Neglect of Differential Overlap (CNDO), or Zero Differential Overlap (ZDO). Models that retain some other four-centre integrals are termed Intermediate Neglect of Differential Overlap (INDO). 

A stepwise mechanism with a rate-limiting leaving group expulsion from the intermediate was proposed for the reactions of Z-aryl dimethyl- (43a), methylphenyl-(43b), and diphenyl-phosphinates (43c) with substituted anilines in DMSO at 333 K, based on the positive cross-interaction constants, The steric effects of the two ligands (R, R ) play a role in determining the reactivity of the phosphinates, but are relatively small compared to other phosphinate systems. A dominant frontside nucleophilic attack involving a hydrogen-bonded four-centre-type transition state (44) was proposed... 

Tetrahedral Anions. The hydrolysis of the [BHJ anion in aqueous DMSO has the kinetic form for a general-acid-catalysed reaction, but with the catalytic coefficients and k-s.+ very different to the values observed for aqueous media. This difference is ascribed to the inability of DMSO to act as a proton relay system. A series of molecular orbital calculations suggest that BH5 could be a metastable intermediate in the hydrolysis of [BH They indicate the presence of identifiable BH3 and Hg subunits in the structure. The authors conclude, however, that limitations in the theoretical methods used do not yet allow experimental and computational results to be reconciled (at least to within a few kcal). The acid-cat ysed ammonolysis in liquid ammonia is first-order in both [NHJ+ and [BHJ, but is appreciably slower than the corresponding reaction in water the rate is markedly decreased by increases in ionic strength. This may be due to the participation of a water molecule in the latter case. The rates of reduction of a variety of substrates by [BHJ have been reported, and for the reduction of ketones it is suggested on the basis of H/T isotope effects that a four-centre transition state is involved. 

The role of HMPA as solvent in the addition of organolithium reagents to enones has been explored by applying a multinuclear NMR technique to quantify the amount of solvent-separated ion pairs (SIP) in solution and to corrolated this with changes in regioisomeric and diastereomeric product ratios.Contact ion pairs (CIP) have been found to react exclusively by 1,2-addition, presumably via a four-centre transition state as hypothesized. However, the situation for SIPs is more complicated and clean 1,4-addition occurs only in the absence of lithium catalysis. Well stabilized anions react by 1,2- and 1,4-addition in the absence of HMPA, when lithium catalysis is possible and SIPs are energetically accessible intermediates. 

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