One-step elimination

The M-17 peak with the expected metastable ion was found to be a significant feature of the mass spectra of all substituted mono-TV-oxides examined and is assigned to a one-step elimination of the hydroxyl radical. For quinoxaline dioxides the M-16 peak is more important and is due to the preferential loss of an oxygen atom from the molecular ion.269... 

The formation of 1,6-anhydro-j3-D-glucopyranose from phenyl 1 -deoxy-1 -thio-jS-D-glucopyranoside (XXXVI) indicates that the thio-phenyl group was eliminated as such, and that the oxygen bridge of the 1,6-anhydride probably arose from the hydroxyl group on C6. This would be a possible result in a one-step elimination, but a necessary result in the two-step mechanism. 

According to the definition given above, E2 eliminations are one-step eliminations. Still, in an E2 transition state the C—H bond can be broken to a different extent than the C—Het bond. If the C—H bond is broken to a greater extent than the C—Het bond, we have an E2 elimination with an Elcb-like distortion of the transition state geometry. Such transition states exhibit characteristic partial charges. In the Elcb-like distorted E2 transition state, a... 

The entries in rows 3-5 in Table 14.1 refer to one-step eliminations/rearrangements of this type in which oxenium ions are avoided. 

Such observations are best understood in terms of what Biinncit (p. 478) has called the variable transiUon state theory of E2 elimination. Wc are speaking, remember, of a one-step elimination both the C--H and C X bonds are being broken in the same transition state. But there is a whole spectrum of F2 transition states which differ in the relative extent to which the two bonds are broken. 

A one-step elimination mechanism via TSse was suggested to be not viable for a series of complexes/ac-L2PtMe3(Z), where L2 = dppe (7) or dppbz (8) and Z = OAc or OAr (Fig. 5) [23, 24]. It was shown that additives of nucleophiles in the form of -Bu4N(0Ac) (Z = OAc) or [K 18-crown-6] (OAr) (Z = OAr) in THF solutions of the complexes above do not affect the rate of the C-O elimination from the Pt center. On the basis of the absence of a linear dependence between the C-O elimination rate and concentration of nucleophile additives in solution as well as some other considerations, it was concluded that the reaction does not follow a one-step mechanism including a direct nucleophilic attack upon six coordinate starting complex (Fig. 9, dashed line) but rather follows a two-step nucleophilic mechanism (Fig. 4, path a Fig. 9, solid line). 

Add two curved arrows to Figure 14.5 to complete the one-step elimination with a strong base. 

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