Antiperiplanar lone pair hypothesis

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. 

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

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. 

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. 

Figure 3.10 Ideas behind the theory of stereoelectronic control or antiperiplanar lone pair hypothesis (ALPH) Antiperiplanar (sp ) lone pairs are shown shaded.

PAPH phosphorus antiperiplanar lone-pair hypothesis... 

Many examples of stereoelectronic effects have been proposed in numerous areas of organic chemistry. The textbook example is perhaps the requirement for the anti conformation of the electrons of the scissile C—H bond with the leaving group in the E2 elimination reaction. However, over the past decade, the term stereoelectronic effect has become synonymous with an effect otherwise termed the kinetic anomeric effect or the antiperiplanar lone-pair hypothesis. While it is quite erroneous to label this hypothesis as the stereoelectronic effect , the fact that this situation has come about does serve to emphasize the ascendency of this hypothesis in the minds of many organic chemists. 

Each orthoester having three oxygen atoms has six sites (labeled a - f) that are available for protonation by H30, yielding in principle six transition structures per orthoester. Study of all these possible sites of protonation by AMI semiempirical Hamiltonian revealed that some oxygen lone pairs could not be attacked by H30 for steric reasons. However, rotation of the methoxy group about the C1-01 bond could relieve these unfavorable interactions and thus make these hindered sites available for protonation. Thus, protonation of orthoester 215 at the endo site c was equivalent to protonation at site e (the other equivalent positions are indicated in Fig. 8.48). In the case of six-membered orthoester 215 where all bonds are staggered, use of antiperiplanar lone pair hypothesis allows straightforward identification of the most... 

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