Antiperiplanar lone pair

It has also been called the antiperiplanar lone-pair hypothesis (ALPH). For a reinterpretation of this factor in terms of the principle of least nuclear motion (see 15-10), see Hosie,L. Marshall,P.J. Sinnott,M.L.y. Chem. Soc.,Perkin Trans. 2,1984,1121 Sinnott, M.L. Adv. Phys. Org. Chem., 1988, 24, 113. 

Stereoelectronic effects in chemical reactivity The bond-lengthening and -weakening influence of an antiperiplanar lone pair leads to strong stereoelectronic effects on chemical reactivity.97 In molecule 28a with lone-pair-bearing atom D adjacent to an A—B bond, a vicinal nD—s-cab hyperconjugative interaction can be associated (cf. Example 1.4 and Section 3.3.1) with a partial admixture of the alternative resonance structure 28b,... 

The picture is less clear when free electron pairs on other heteroatoms (O, S, etc.) are involved. There is only one clear-cut case providing evidence concerning endocyclic sulfoxides The unusually large shieldings of carbon atoms in gauche position relative to an axial oxygen atom in thiane oxides 79, 80, 82, and 83 (p. 252) (200,201) have been partially ascribed to an upfield-shifting effect of the equatorial antiperiplanar lone pair (cf. Section III-C). 

Failures of the antiperiplanar lone pair hypothesis (ALPH). 120... 

Reinterpretation of apparent kinetic antiperiplanar lone pair effects in terms of the principle of... 

Scheme2.7. Antiperiplanar lone pairs weaken C-H bonds and reduce their...

Numerous other examples have, moreover, been reported in which bonds with antiperiplanar lone electron pairs react more slowly than, or at rates similar to, comparable bonds without antiperiplanar lone pairs [52, 63-67]. 

Ratcliffe, A. J. Mootoo, D. R. Andrews, C. W. Fraser-Reid, B. Concerning the antiperiplanar lone pair hypothesis oxidative hydrolysis of conformationally constrained 4-pentenyl glycosides./. Am. Chem. Soc. 1989, 111, 7661— 7662. 

Graczyk, P. P. Mikolajczyk, M. Inapplicability of the antiperiplanar lone pair hypothesis to C-P bond breaking and formation in some S-C-P+ systems./. Org. Chem. 1996, 61, 2995— 3002. 

Hosie, L. Marshall, P. J. Sinnott, M. L. Failure of the antiperiplanar lone pair hypothesis in glycoside hydrolysis. Synthesis, conformation, and hydrolysis of a-D-xylopyranosyl and a-D-glucopyranosyl pyridinium salts. /. Chem. Soc. Perkin Trans. 2 1984,1121-1131. 

Typically, for the methoxlde elimination step the transition state which has an antiperiplanar lone pair to the methoxide leaving group is ca. 11 kcal/mol lower in energy than the transition state without this app lone pair (Fig.l). 

The hypothesis gained considerable plausibility by appearing to be a simple extension of what was already accepted. Thus, an antiperiplanar lone pair of electrons can be considered to play essentially the same role as the pair of electrons in the breaking C—H bond in an E2 elimination, and the strong preference for anti-stereochemistry in such reactions had long been known. Moreover, the frontier-orbital explanation of the anomeric effect was receiving support at the expense of the antecedent electrostatic explanation, and the ALPH merely applied the same ideas to transition states. 

It is thus clear that in general, hydrolysis and related reactions of acetals do not proceed through transition states whose geometries are such as to present the departing group with an antiperiplanar lone pair of electrons. 

If the origin of the effects held to support ALPH were indeed electronic in origin, one would expect the differential effect of going from one antiperiplanar lone pair to two to be far less marked than the effect of going from no antiperiplanar lone pairs to one. Thus one would expect the effects to be more marked in reactions of sp3 carbon centres at the aldehyde level of oxidation than at the acid level of oxidation, whereas the reverse is the case. There is also, for this author at least, the difficulty inherent in ALPH of reconciling the physical picture of a lone pair of electrons driving off the... 

In fact, although in some radical reactions there appear to be antiperiplanar lone pair effects, they are always modest, and, where they are not equally well considered as least motion effects, they arise from the intrinsic bias of the system examined. In this section systems supposedly giving rise to ALPH effects will be treated in order of increasing complexity. 

The aluminum-mediated Petasis-Ferrier rearrangement is a stepwise [1,3]-sigmatropic process. The first step is the coordination of the Lewis-acid to the 0-atom of the enol. Coordination to the ether O-atom is reversible and nonproductive. Cleavage of the adjacent C-O-bond, assisted by the antiperiplanar lone pair of the etheral O-atom, stereospecifically gives rise to an oxocarbenium enolate species, which cyclizes to the desired oxacycle. The rate difference in the rearrangement for the five- versus six-membered series can be explained by the more facile 6-(enolendo)-endo-trig cyclization. The last step is the intramolecular equatorial hydride delivery. 

The antiperiplanar, lone-pair hypothesis is based on the presumption that the lifetime of a tetrahedral intermediate in hydrolysis is shorter than the average time of rotation about the C-O bond that is, the barrier of conformational transition exceeds the reaction barrier. However, a simple intermediate (hydrogen orthoester) has been detected, and on this basis, the antiperiplanar, lone-pair theory was questioned.Similarly, studies on the hydrolysis of saccharide derivatives 4os indicated that the departure of... 

Although interactions with the antibonding orbital are implicitly invoiced in the antiperiplanar, lone-pair hypothesis, additional, coexisting, lone-pair interactions are the basis of another qualitative approach to rationalizing many reactions of the monosaccharides. This approach utilizes the fact that the energy of the most reactive MO s increases with their interactions. 

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