Four-centre transition states

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... 

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" ... 

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. 

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. 

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]. 

The mechanism (Following fig.) involves the alkene n bond interacting with the empty p orbital of boron to form a n complex. One of BH3 s hydrogen atom is then transferred to one end of the alkene as boron itself forms a o bond to the other end. This takes place through a four-centred transition state where the alkene s n bond and the B-H bond are partially broken, and the eventual C-H and C-B bonds are partially formed. There is an imbalance... 

We know that this is not the whole story because of the stereochemistry. Hydroboration is a syn addition across the alkene. As the addition of the empty p orbital to the less substituted end of the alkene gets under way, a hydrogen atom from the boron adds, with its pair of electrons, to the carbon atom, which is becoming positively charged. The two steps shown above are concerted, but formation of the C-B bond goes ahead of formation of the C-H bond so that boron and carbon are partially charged in the four-centred transition state. 

Chamberlin and coworkers proposed a similar chair-like transition state 29 to account for the stereoselectivity of the carbolithiation reaction of vinyllithium 10 to 1224. The observed diastereoselectivity is consistent with a four-centre transition state where a preferred coplanar approach of the carbon-lithium bond to the double bond would give the obtained major product (Scheme 8). 

The kinetics of reduction of IS cyclohexanones by Li(Bu 0)3AlH were investigated and found to follow a simple second-order process. The rate constants were determined at various temperatures and were found to exceed the corresponding values for sodium borohydride. Activation parameters were derived for the reduction, which was nearly isoenthalpic, and rate differences were attributed to the en-tropic contribution. The results were consistent with a simple four-centred transition state, but additional work is required for a definitive conclusion. 

In an earlier paper Barton and Onyon considered the unimolecular mechanism of dehydrochlorination to be of more universal application than the radical chain mechanism and postulated that a chloro-compound will decompose by a radical chain mechanism only so long as neither the compound itself nor the reaction products will be inhibitors for the chains . On the basis of this postulate the authors correctly predicted the mechanism of decomposition of a number of chlorine compounds. The postulate does not hold well for bromine compounds which show a greater tendency to decompose via radical chain mechanisms. However, from their early studies on 2-bromopropane 2-bromobutane, t-butyl bromide, and bromo-cyclohexane, Maccoll et a/.234,235,397,410,412 concluded that these compounds also decompose unimolecularly via a four-centre transition state similar to that proposed by Barton and Head. 

Apart from the four-centre transition state and its variants just discussed, a six-centre state has been proposed to account for the relatively facile decomposition of l-chloro-c/j-2-butene and a-chloroorthoxylene. 

For isopropyl chloroformate decomposition in the gas phase at 240 °C these two reactions proceed at almost equal rates . The products can be accounted for in terms of a four-centre transition state (1) for the substitution reaction, and a six-centre state (2) for the elimination reaction... 

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