Four-membered transition state

The formation of the less favored trisubstituted isomer (37) occurs by the usual intramolecular axial proton transfer from the 6 position, whereas that of the tetrasubstituted isomer (38) involves the intramolecular proton transfer of the stereoelectronically less favored equatorial proton either via a four-membered transition state (39) or a six-membered transition state (40). 

In the absence of counteranions, as in irradiation induced polymerization, the transition state for transfer is least favorable, which leads to highest molecular weights. Kennedy35 has proposed a possibility of four membered transition state of transfer, which can take place in the absence of counteranion assistance. 

Notice that it all happens in one concerted process (via a four-membered transition state). Let s take a close look at this first step, and consider the regiochemistry and stereochemistry. 

The elimination of the original CHj group from ionized n-butanoic acid, 38, was shown to proceed also mainly via a two-step process. The reaction is initiated by a hydrogen migration via a four-membered transition state 38->39, which... 

Astonishingly, the study of the mechanism of formaldehyde loss from anisole revealed two different pathways for this process, one involving a four- and one a five-membered cyclic transition state (Fig. 6.37). [129] The four-membered transition state conserves aromaticity in the ionic product, which therefore has the lower heat of formation. Prompted by the observation of a composite metastable peak, this rather unusual behavior could be uncovered by deconvolution of two different values of kinetic energy release with the help of metastable peak shape analysis (Chap. 2.8). 

This reaction can be viewed as an internal hydrogen abstraction which takes place through a highly strained four-membered transition state instead of the usual six-mem bered one. The formation of the enol is wavelength dependent and is retarded by triplet quenchers.58-80 In water, excitation into the second excited singlet of biacetyl formed the enol with a quantum yield of 0.10, while excitation into the first excited singlet formed the enol with a quantum yield of 0.01, possibly via a higher excited state formed by a triplet-triplet reaction. Therefore, the authors conclude that the second excited triplet is the state that isomerizes to the enol. 

Irradiation of pyruvic acid (26) in the vapor phase produces acetaldehyde and carbon dioxide.85 The most reasonable mechanism for the reaction appears to be a concerted decarboxylation via a cyclic four- or five-membered transition state. Although the four-membered transition state has an analogy... 

Biacetyl (5) is reported to undergo enolization when irradiated in water, methanol, or hexane solution, presumably via a highly strained four-membered transition state instead of the usual six-membered one.40-43 From the wavelength dependence of enol formation, it was concluded that enolization occurred from the second triplet state. 

In these solvents heterolytic activation can occur through a chain of hydrogen-bonded solvent molecules connecting the H2 proton with the basic center (for instance via a six-membered transition state) instead of an -bond metathesis [2+2] of the hydrogen molecule through a four-membered transition state (Scheme 2). 

It requires a four-membered transition state involving a ring of the past asymmetric center, the gegen ion, and the two atoms of the new monomer. The presence of the established asymmetric center in the cyclic four membered ring determines the stereochemistry of the new asymmetric center. Steric or electronic repulsion of the substituents in the transition state causes isotactic steric control. 

The results in Table 3 were explained as shown in Scheme 4. From the fact that no kinetic isotope effect was observed in the reaction of phenyl-substituted disilenes with alcohols (Table 1), it is assumed that the addition reactions of alcohols to phenyltrimethyl-disilene proceed by an initial attack of the alcoholic oxygen on silicon (nucleophilic attack at silicon), followed by fast proton transfer via a four-membered transition state. As shown in Scheme 4, the regioselectivity is explained in terms of the four-membered intermediate, where stabilization of the incipient silyl anion by the phenyl group is the major factor favoring the formation of 26 over 27. It is well known that a silyl anion is stabilized by aryl group(s)443. Thus, the product 26 predominates over 27. However, it should be mentioned that steric effects also favor attack at the less hindered SiMe2 end of the disilene, thus leading to 26. 

If proton transfer occurs from C-4 to radical 267, then 270 would result directly. The later, more direct pathway, unfortunately proceeds through a four-membered transition state which would be predicted to occur at a much slower rate than the six-membered TS leading to 269. However, no evidence for unreacted alkene 273 resulting from expulsion of Fe(IV)=0 from 269 was evidenced,109 suggesting that the path through radical 268 was not followed in this example. 

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. 

The inversion of the configuration at the carbon atom of the epoxide ring where cleaved in the course of the ring opening during polymerisation indicates that the monomer complexed with the metal atom is attacked from the back side by the nucleophilic substituent X [scheme (1)]. If front side nucleophilic attack of this substituent occurred on the coordinated monomer, i.e. via the four-membered transition state as in scheme (2), no inversion but rather the retention of the configuration at the epoxide ring carbon atom where cleaved should be observed however, this is not the case. 

Benson and Bose16 have proposed a semi-ion-pair transition state mechanism for organic halides pyrolysis in the gas phase. On the basis of this model involving a four-membered transition state, O Neal and Benson17,18 have presented calculations of energies and entropies of activation which were in good agreement with the experimental values. 

Hiberty40 used the single determinant ab initio molecular orbital theory to study the unimolecular HC1 elimination from ethyl chloride. The calculations of three potential energy surfaces corresponding to -elimination, planar //-elimination, and nonplanar anti-elimination were performed and the dehydrochlorination process was predicted to be syn, and to proceed via a planar four-membered transition state. According to these estima-... 

The thermal decomposition of l-(l-chloroethyl)-2-methylbenzene in the gas phase was examined at 285-325 °C41. Attempts to prove experimentally that the reaction proceeds via a six-membered transition state with the o-Me group as in l-(l-chloromethyl)-2-methyl-benzene42 were negative. In fact, the reaction proceeds through a four-membered transition state mechanism with moderate charge separation. 

Apparently, the carboxylic acid proton of pKa-4.8 assists readily the halogen expulsion as compared with the H atom of the CH3 group (pKa - 48). In order to support this conclusion, the H of the COOH was replaced by a methyl group to give methyl 2-halopropi-onates. The comparative kinetic data are shown in Table 13. The methyl haloesters pyrolyze much slower than the free acids and they probably eliminate HX through a normal four-membered transition state. 

In a study of the effect of alkyl and polar groups on the gas-phase pyrolyses of a-substi-tuted ethyl chlorides70, pinacolyl chloride was found to produce dimethylbutenes by kinetic control. The main alkene product, i.e. 3,3-dimethyl-1-butene, was formed through the normal four membered transition state for HC1 elimination. However, the formation of... 

The reaction was found to be homogeneous and unimolecular, and it followed a first-order rate law. The rate coefficient was given by the equation kx = 10(14 26 0 20)exp [(-251,000 2000)/8.3147"] s-1. Olefin formation is impossible and only CH3C1 was obtained. The reaction rate is 4000 times slower than the overall rate of ethyl chlorofor-mate decomposition190, and the mechanism proposed involves a cyclic four-membered transition state. 

The cis-trans alkene isomerization observed in the transformation of conjugated alkenes gave rise to proposals of several different reaction mechanisms. Four different pathways for the epoxidation of alkenes by the manganese metal-oxo species have been proposed (Scheme 9) in order to explain the experimental results. The two concerted pathways (b,c) involve formation of a three-membered or four-membered transition state that involves the alkene and either the transferring oxygen or the Mn=0, respectively, of the formal Mn(V) species. The other pathways are stepwise mechanisms one proceeds via bond formation between the alkene and the... 

These observations, along with kinetic isotope effect studies and Hammett correlation studies, support a concerted elimination by a-bond metathesis involving a four-membered transition state (Eq. 2) [23]. A large kinetic isotope effect is observed for the loss of methane from methyl amide complexes lb and lh (Eqs. 3 and 4), comparable to those observed by Buchwald and coworkers for formation of zirconium rj2-thioaldehyde complexes [25] and by Bercaw and coworkers for formation of tantalum rf- imine complexes [5a] through similar transition states. 

It is clear that oxidative addition of H2 to these high-valent compounds cannot occur the catalytic cycle is most likely to involve four-membered transition states... 

Sn-C-bond being more reactive than the Si-C-bond. In contrast, in olefin 10 the next 1,1-organoboration is favourable (since hindered four-membered transition state is necessary for 1,2-organoboration). Both sequences afford heterocycle 11. 

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