Five-membered cyclic transition

The site selectivity in this reaction Is excellent -70 - 80% yield of (3) - and It depends on the stability of cyclic transition state (6). This looks at first like a six-membcrcd ring, but N—H—CII2 is linear and shorter than two bonds so that it is somewhere between a five and a six-membered ring.Cyclisation to (3) Is via a conventional five-membered cyclic transition state. Problem ... 

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). 

An ab initio RHF/3-21 G study has shown that the decomposition of 3-hydroxy-3-methylbutan-2-one is a concerted process with hydrogen transfer and bond breaking via a five-membered cyclic transition state.AMI and PM3 methods using UHF calculations were applied to study the thermolysis of 2-cyanofuroxan. The reaction proceeds via a two-step pathway in which the second step is rate determining. The effect of solvent in the thermal decomposition reaction of fran -3,3-dimethyl-5,6-tetramethylene-l,2,4-trioxacyclohexane was studied. ... 

As a consequence of the conformational mobility of the thiepane 1-oxide ring (115) it was possible to form the necessary planar five-membered cyclic transition state for a thermal E elimination reaction (equation 19). The acyclic sulfenic acid intermediate was not isolated but rearranged to c/s-2-methylthiane 1-oxide by an intramolecular cyclization mechanism (75TL2235). 

Sometimes, alcohols can direct the oxidation of alkenes, resulting in highly stereoselective formation of tetrahydrofurans by the action of Collins reagent. Thus, 1,2-diols can form cyclic chromate esters that can intramole-cularly oxidize alkenes, positioned so as to allow the operation of five-membered cyclic transition states.119... 

A unimolecular elimination involving a semi-polar five-membered cyclic transition state (81) (Scheme 9, R1 = Ph, R2 = R3 = H) appears to account for the formation of benzaldehyde, CO, and H2O on eliminative fragmentation of mandelic acid in the gas phase.44 The same type of transition state has been proposed for gas-phase pyrolysis of ROCH2COOH (R = MeO, EtO, and Ph O) with corresponding formation of ROH, CO, and formaldehyde the rate of reaction is little dependent on R.45... 

The [2,3]-Wittig Rearrangement is a [2,3]-sigmatropic reaction, a thermal isomerization that proceeds through a six-electron, five-membered cyclic transition state. A general scheme for [2,3]-sigmatropic reactions is given here ... 

The thermal decomposition (pyrolysis) of alkylaryl selenoxides (selenoxide pyrolysis) to an alkene and an aryl selenic acid Ar—Se—OH often takes place even at room temperature (Figure 4.10). This reaction is one of the mildest methods for introducing a C=C double bond by means of a /3-elimination. The mechanism is described by the simultaneous shift of three electron pairs in a five-membered cyclic transition state. One of these electron pairs becomes a nonbonding electron pair on the selenium atom in the selenic acid product. The Se atom is consequently reduced in the course of the pyrolysis. 

At this point in our studies we favor a five-membered cyclic transition state, quite analogous to the four-center transition state (Reaction 5), except that no o- bonds of the attacking electrophile are broken. The fourth bonding center is the carbon sp3 orbital as in structures 25 and 26. 

Such a five-membered cyclic transition state has been proposed for the triiodide cleavage of carbon-mercury bonds (2). While such a structure is unlikely for the linear I3 ion, it should easily accommodate the ozone molecule whose bonding angle is 116° (12). Of course the geometrical plausibility of Structure 27 depends on the degree of carbon-mercury... 

Figure 1 Native chemical ligation (NCL) between two unprotected peptide segments. The initial transthioesterification reaction leads to an intermediate that undergoes an S to N-acyl shift via a five-membered cyclic transition state and generates a native amide bond at the ligation site.

The last step shows intramolecular transfer of a proton because five-membered cyclic transition states are readily achieved. The process could also be represented by two intermolecular steps... 

Still-Wittig rearrangement of the nonracemic (/ )-propargylic ethers (100a) and (100b) has been found to give the (/ )-allenes (100c) and (lOOd) with complete asymmetric transfer (equation 25a). The stereochemistry is consistent with a five-membered cyclic transition state. 

In those versions of the Ei mechanism that involve a four-or five-membered cyclic transition state, there is a requirement that all the atoms are co-planar or... 

Pyrolytic reactions often occur in the gas phase, without the addition of another reagent. For example, heating carboxylate esters readily produces an alkene and a carboxylic acid. There is a requirement for a c/s-(3-hydrogen in order for this reaction pathway to proceed, because the reaction proceeds via a six-membered cyclic transition state. This cyclic pyrolytic elimination is labelled the Ei mechanism, which stands for intramolecular, or internal, elimination. In those versions of the Ei mechanism that involve a four- or five-membered cyclic transition state, there is a requirement that all the atoms are co-planar or virtually so. 

For four- or five-membered cyclic transition states, pyrolysis only leads to beta-elimination if the C -H and C -X bonds can eclipse so as to ensure the necessary proximity for reaction. The syn nature of the amine oxide decomposition has been demonstrated by elimination from the diastereoisomeric pairs of the 1,2-diphenylpropyl system. In the cyclohexyl series, an eclipsed transition state is only achieved if elimination proceeds through the boat conformer and consequently the pyrolysis of 1-methylcyclohexylamine oxide gives an almost quantitative yield of the less stable exo-olefin. This orientation contrasts markedly with that of the six-membered cyclic transition states of the corresponding esters and xanthates which yield predominantly the endo-olefin, syn-clinal stereochemistry being preferred. For all the other ring... 

We won t discuss the mechanisms of the first two steps—selenylation and oxidation you are welcome to work them out For the selenoxide elimination step, stereochemical evidence suggests a five-membered cyclic transition state, as shown below ... 

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