Six Minor Pathways

A nucleophile adds, forming a trigonal bipyramidal pentacovaient intermediate that then ejects the leaving group in the microscopic reverse of the nucleophilic addition reaction. The pentacovaient intermediate is often short-lived, and this is shown by enclosing it in brackets. The pentacovaient intermediate bonds that are colinear are called axial (or apical) bonds, whereas the three bonds that lie in the plane perpendicular are called equatorial. Three common elements that can react by this pathway are silicon, phosphorus, and sulfur. Hydrolysis of phosphate esters like RNA occurs by this path. 

Energetics. Pentacoordinate intermediates are possible for third-row or higher elements, like Si, P, and S, that are larger and have available d orbitals. Do not use this path for second-row elements like carbon. 

Overlap Attacking Nu forms an axial bond therefore, by microscopic reversibility, the departing L must leave from an axial position. The orbital overlap is similar to that shown for the Sn2 (Fig. 7.5), but the structure in the center of the figure is now an intermediate rather than a transition state. 

Leaving group good to fair. If the leaving group is very good, the intermediate would be expected to have a short lifetime and may become just a transition state (Sn2). 

Steric access usually not a problem since bond lengths are longer. 

---

A more significant difference between 1 -methylpyrazole and isothiazole photochemistry, however, appears to be the minor role of the N-2-C-3 interchange pathway in isothiazole chemistry. Thus, although N2-C3 interchange is a major transposition pathway in pyrazole chemistry, it is only a minor pathway upon irradiation of phenylisothiazoles in benzene solution. In fact, 4-phenylisothiazole (55), the compound most expected to react via the N-2-C-3 interchange pathway, was the only isomer that did not yield a transposition product upon irradiation in benzene solution (Scheme 27). This is not due to the photostability of the compound. Indeed, 55 is the most reactive of the six isomers. Nevertheless, even after consumption of 85% of 55, no phototransposition product could be detected. 

Other Systems.— The addition of formic, acetic, and chloroacetic acids to quadri-cyclane at 20 °C affords the corresponding esters of exo-2-norbornenol and nortri-cyclenol. Solvent effects and the results obtained using the 0-deuteriated carboxylic acids are interpreted in terms of the six-centre addition mechanism (97) the minor pathway to nortricyclyl products, in the absence of added mineral acid, is the result of leakage from (97) to the norbornenyl cation. ... 

Scheme 20 [103] shows that the polymerization of 131 proceeds via a stable tris(alkylthio)carbenium ion 135. This species can be attacked in three different ways by the second monomer, producing 134, 133, and 132 via paths a, a, and b, respectively. Kinetically, path a should dominate over the others. Path a is one of the possible pathways, but presumably it is of minor importance. Although the conversion of 134 to 132 cannot be ruled out, 132 is more likely to be formed via path b, as is the case with six-membered spiro orthocarbonates. At high... 

For the allyl ether/Me3Si+ system the usual ether reaction , i.e. loss of an olefin via a four-centre elimination, is a minor process. The two major decomposition pathways occur through six-membered transition structures (reactions 115, 116) and, obviously, involve the allylic double bond. Quite remarkable is the methyl transfer 293- 295. 

According to the observed photochemical products and the results of deuterium labeling studies, the six isomeric phenylisothiazoles and phenyl-thiazoles can be organized into a tetrad of four isomers that interconvert mainly via P5, Pg, and P, transposition pathways and a dyad in which 5-phenylthiazole 52 transposes via P5 and P7 pathways to 4-phenylisothiazole 48 (Scheme 8), the only isomer that did not yield a transposition product upon irradiation in benzene solution. With one minor exception, no interconversions between the tetrad and dyad were observed. In that case, in addition to transposing to members of the tetrad, 5-phenylisothiazole 49 also transposed to 5-phenylthiazole 52, the first member of the dyad, in less than 1 % yield. This conversion was assumed to occur via a P4 permutation process. 

---