Transition states four-centred

Ferrocenyltriphenylphosphonium perchlorate (84) has been synthesized from iodoferrocene, tetrakis(acetonitrile)copper(i), and triphenylphosphine in nitromethane. The authors suggest that iodoferrocene first forms the complex (85) which then breaks down via a four-centred transition state to (84). 

Dialkyl dimethyl phosphoramidites (16) react with j8-propiolactone to give the phosphoramidate (17) and the phosphonate (18), A kinetic study suggests a mechanism involving initial attack of phosphorus at saturated carbon to give (17), while a four-centred transition state (19) is invoked to explain the formation of (18). 

A summary of the kinetic parameters is given in Table 11. With the exception of 3,3,3-trifluoropropyltrifluorosilane the decompositions are first-order homogeneous processes. The 2,2-difluoro compounds decompose by a four-centre transition state ... 

The four centre transition state requires the addition of both boron and hydrogen to the same face of the molecule. 

According to the Keele theory the primary reaction between the perchloric acid and the monomer is also (1), but thereafter the growth is by a ring-expansion reaction (4) in which no free end is ever formed this goes by way of a four-centred transition state and gives the cyclic product (IV) ... 

B) Cleavage of ring structure and change of the electrons via a four centre transitional state... 

The interaction between two adjacent bulky groups can depend on steric factors which are not necessarily related to the stability of the radicals produced on homolysis. It is estimated from linear free energy relationships that only 65-70% of the ground-state strain energy is relieved in the transition state for homolysis of a bond between two quaternary centres (Ruchardt and Beckhaus, 1980, 1986). Thus steric constraints to delocalization in the radicals produced may persist. A pertinent example is 2,3-di(l-adamantyl)-2,3-dimethylbutane [123] which has four such centres, linked by the long C-C bonds characteristic of this sort of structure. The strongest... 

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. 

A theoretical study of the reaction of water and methanol with HNCO has led to a prediction of a four-centred transition state for both reactions. The interactions of water and of alcohols with alkyl isocyanates have been the subject of both experimental and theoretical study. In the case of hydration, evidence for initial interaction of water and water clusters (n = 1-3) across the N=C bond of the alkyl isocyanate... 

Unimolecular pyrolysis of the tautomers of monothioformic acid (two conformers of thiol- and two conformers of thiono-) have been studied by ab initio methods with STO-3G and 6-31 G basis sets. The barrier heights for dehydrogenation (via a four-centre transition state) and dehydrogensulfldation (via a three-centre transition state) of thiol formic acid are 67.47 and 67.09 kcalmol" respectively. Dehydration of 5-cw-HCSOH occurs via a three-centre transition state with an activation energy of 81.18 kcalmoG this is much greater than for dehydration of the s-trans form, which occurs via a four-centre transition state with a barrier of only 68.83 kcalmol" ... 

Thermochemical parameters estimated by semiempirical AMI calculations have been found to support the proposal that isobutene formation on gas-phase thermolysis of iV-methyl-A-phenyl-fert -butylsulfenamide and morpholinyl-ferf -butylsulfenamide occurs by a unimolecular mechanism involving a four-centre cyclic transition state and co-formation of the corresponding thiohydroxylamines." ... 

The investigation of factors affecting facial selectivity in the hydroboration of steroidal -alkenes revealed the facial (a vs /3) stereoselectivities of hydroboration of androst-5-enes (69) and B-norandrost-5-enes (70) do not parallel the difference between the calculated force-field energies for a- and jS-cyclobutane models (71)-(74). This finding appears to suggest that the facial selectivity is not determined by the four-centre transition state but by the relative ease of formation of the initial tt-complex. ... 

Reaction (166) is also catalyzed by acidic rhodium chloride solutions under similar conditions to those used for ruthenium. Aquachloro complexes were again implicated and the most active species was [RhCl5(H20)]3. The hydration step was thought to involve a four-centred transition state (134).616 Here also, inhibition at low and high chloride ion concentration occurred. 

The transition-state structures for fluorination, chlorination and bromination were obtained by ab initio MO calculation82. Chlorination and bromination were found to proceed via three-centred geometries (cyclic halonium ions) leading to awfi-addition. In contrast, fluorination involves a four-centred transition state which is consistent with the observed yyw-stereoselectivity82. 

However, the mechanism is not limited to four-centred transition states, and cyclic six-centred transition states formed by synchronous electrophilic substitution and internal coordination have been postulated7, e.g. 

The nomenclature used in describing bimolecular electrophilic substitutions involving cyclic transition states reflects, in part, the above-mentioned difficulty. Ingold3 has adopted the nomenclature of Winstein et al.1 and refers to such substitutions as SEi, but to the present author this is not a particularly appropriate choice since it does not indicate the bimolecular nature of the substitution. Dessy et al.8 have used the term SF2 to describe a mechanism, such as that in reaction (5), in which a four-centred transition state is formed, but not only is such a term too restricted, it also provides no indication that the mechanism is one of electrophilic substitution. The view of Reutov4 is that the cyclic, synchronous mechanism is very close to the open mechanism and that both can be described as SE2 mechanisms. Dessy and Paulik9 used the term nucleophilic assisted mechanisms to describe these cyclic, synchronous mechanisms and Reutov4,10 has recently referred to them in terms of internal nucleophilic catalysis , internal nucleophilic assistance , and nucleophilic promotion . Abraham, et al,6 have attempted to reconcile these various descriptions and have denoted such mechanisms as SE2(cyclic). 

The two-alkyl exchange (15) has also been studied by Dessy et al.29,33 the reported rate coefficients at 25 °C and the activation parameters are collected in Table 7. Dessy and Lee33 suggested that the dialkylmercurys were attacked by mercuric iodide in dioxan to give a four-centre transition state (XII) (of the SE2(cyclic) type) or a transition state (XIII) derived from an ion-pair attack, viz. 

Jensen and Rickborn35 have criticised the use of relative reactivities of mercuric salts in reactions such as (15) as a basis for the deduction of reaction mechanism. They point out that, whereas cyclic transition states involving mercuric halides as electrophiles must, of necessity, be four-centred (e.g. (XIV)), the electrophiles mercuric acetate and mercuric nitrate could give rise to six-centred transition states (e.g. (XV)) that might be energetically more favoured than the four-centred. 

Hence the observation that, for example, mercuric acetate reacts with a given substrate in a given solvent faster than does mercuric bromide can be interpreted in at least two ways (i) the mechanism of reaction is SE2(open) and mercuric acetate is a more powerful electrophile than is mercuric bromide, and (ft) the mechanism of reaction is SE2(cyclic) and mercuric acetate is better able to act as a bridging group in a six-centred transition state than is mercuric bromide in a four-centred transition state. The possibility that the two salts might be reacting by different mechanisms must also be considered. 

It may be noted that Reutov normally writes the transition states of these symmetrisations without incorporation of any ammonia molecules. Although he specifically states46 that ammonia molecules can be incorporated, Reutov regards such incorporation as not affecting the fundamental cyclic four-centred aspect of the transition state. 

Enthalpies of activation, transition-state geometries, and primary semi-classical (without tunneling) kinetic isotope effects (KIEs) have been calculated for 11 bimolecu-lar identity proton-transfer reactions, four intramolecular proton transfers, four nonidentity proton-transfer reactions, 11 identity hydride transfers, and two 1,2-intramole-cular hydride shifts at the HF/6-311+G, MP2/6-311+G, and B3LYP/6-311+-1-G levels.134 It has been found that the KIEs are systematically smaller for hydride transfers than for proton transfers. The differences between proton and hydride transfers have been rationalized by modeling the central C H- C- unit of a proton-transfer transition state as a four-electron, three-centre (4-e 3-c) system and the same unit of a hydride-transfer transition state as a 2-e 3-c system. 

Stone Wales (1986) considered the ring rearrangement shown in figure 3a. They concluded that as a concerted process it has a Hiickel four-centre transition state and thus will have a substantial activation barrier. The existence of such an activation barrier has been confirmed by the calculations of Yi Bernholc (1992) who found activation energies in excess of 500 kJ mol-1 (5 eV). 

The characteristic features of hydroboration are consistent with a concerted four-centre transition state carrying charges on the participating atoms (Figure Bl.l) and this model adequately rationalizes the majority of hydroboration results. 

Note that the mechanism of incorporation of the coordinating epoxide into the Mt-X active bond actually occurs in a multicentred transition state but not in a four-centred transition state. This is implied by the participation of another metal atom of the same or different catalyst molecule in this transition state. As a consequence, an inversion of the configuration at the nucleophilically attacked carbon atom of the coordinated epoxide takes place [63,68,71,116 121]. 

Since olefin insertion into the metal carbon bond has been established to be of the cis type, it has been considered to proceed by a concerted mechanism involving the formation of a four-membered transition state. However, various models of active centres and of the insertion mechanism have been proposed for olefin polymerisation systems with coordination catalysts. 

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