And pyramidal inversion

This chapter assesses the ability of both molecular mechanics and quantum chemical models to correctly assign the lowest-energy conformational arrangements in flexible molecules as well as aceount for energy differences between alternative conformers. It also assesses the performance of different models with regard to the calculation of barriers to single-bond rotation and pyramidal inversion. 

Barriers to single-bond rotation and pyramidal inversion derive principally from microwave spectroscopy, from vibrational spectroscopy in the far infrared and (for the larger barriers) from NMR. Although the number of systems for which data are available is limited (and the systems themselves primarily limited to very small molecules), in some cases barriers are known to high accuracy (to within 0.1 kcal/mol). 

MMFF mechanics is not well suited to this problem. SYBYL mechanics is unsuitable for both single-bond rotation and pyramidal inversion barriers. 

Hoge G (2004) Stereoselective cyclization and pyramidal inversion strategies for P-chirogenic phospholane synthesis. J Am Chem Soc 126 9920-9921... 

The principles of pseudorotation have an important application in the explanation of the reaction mechanisms of many phosphorus compounds (e.g. Chapter 13.3). Because of pseudorotation and pyramidal inversion possibilities, trigonal bipyramidal and pyramidal phosphorus compounds are said to be stereochemicaUy non-rigid. There is evidence that pseudorotation processes occur in arsenic compounds and a few other non-pnictide compounds such as Fe(CO>5. 

Phosphorus is m the same group of the periodic table as nitrogen and tricoordi nate phosphorus compounds (phosphines) like amines are trigonal pyramidal Phos phmes however undergo pyramidal inversion much more slowly than amines and a number of optically active phosphines have been prepared... 

Although unsynunetrically substituted amines are chiral, the configuration is not stable because of rapid inversion at nitrogen. The activation energy for pyramidal inversion at phosphorus is much higher than at nitrogen, and many optically active phosphines have been prepared. The barrier to inversion is usually in the range of 30-3S kcal/mol so that enantiomerically pure phosphines are stable at room temperature but racemize by inversion at elevated tempeiatuies. Asymmetrically substituted tetracoordinate phosphorus compounds such as phosphonium salts and phosphine oxides are also chiral. Scheme 2.1 includes some examples of chiral phosphorus compounds. 

Whereas the barrier for pyramidal inversion is low for second-row elements, the heavier elements have much higher barriers to inversion. The preferred bonding angle at trivalent phosphorus and sulfur is about 100°, and thus a greater distortion is required to reach a planar transition state. Typical barriers for trisubstituted phosphines are BOSS kcal/mol, whereas for sulfoxides the barriers are about 35-45 kcal/mol. Many phosphines and sulfoxides have been isolated in enantiomerically enriched form, and they undergo racemization by pyramidal inversion only at high temperature. ... 

An accurate MO study of the inversion barrier in dimethyl sulphoxide9 showed that the height of the calculated barrier is much more sensitive to the overall quality of the basis set, and to geometry optimization, than to the presence of 3d functions. This study predicts an S—O bond lengthening to 1.55 A, and the best estimate of the barrier is 39.9 kcal mol 1. This was the difference in energy between optim um planar and pyramidal... 

Both thermal- and acid-induced equilibrations of 3,3-disubstituted thietane oxides were very slow (K 10-5 s-1)194. The results suggest that thietane oxides are similar to the various acyclic sulfoxides with respect to the rates of thermally induced pyramidal inversion at sulfur238, and that this inversion process, therefore, does not interfere significantly in the above exchange/racemization studies. 

Compounds With Tervalent Chiral Atoms. Atoms with pyramidal bonding might be expected to give rise to optical activity if the atom is connected to three different groups, since the unshared pair of electrons is analogous to a fourth group, necessarily different from the others. For example, a secondary or tertiary amine where X, Y, and Z are different would be expected to be chiral and thus resolvable. Many attempts have been made to resolve such compounds, but until 1968 all of them failed because of pyramidal inversion, which is a rapid oscillation of the unshared pair from one side of the XYZ... 

In molecules in which the nitrogen atom is at a bridgehead, pyramidal inversion is of course prevented. Such molecules, if chiral, can be resolved even without the presence of the two structural features noted above. For example, optically active 12 (Trdger s base) has been prepared. Phosphorus inverts more slowly and arsenic still more slowly." Nonbridgehead phosphorus," arsenic, and antimony compounds have also been resolved... 

The SnI reactions do not proceed at bridgehead carbons in [2.2.1] bicyclic systems (p. 397) because planar carbocations cannot form at these carbons. However, carbanions not stabilized by resonance are probably not planar SeI reactions should readily occur with this type of substrate. This is the case. Indeed, the question of carbanion stracture is intimately tied into the problem of the stereochemistry of the SeI reaction. If a carbanion is planar, racemization should occur. If it is pyramidal and can hold its structure, the result should be retention of configuration. On the other hand, even a pyramidal carbanion will give racemization if it cannot hold its structure, that is, if there is pyramidal inversion as with amines (p. 129). Unfortunately, the only carbanions that can be studied easily are those stabilized by resonance, which makes them planar, as expected (p. 233). For simple alkyl carbanions, the main approach to determining structure has been to study the stereochemistry of SeI reactions rather than the other way around. What is found is almost always racemization. Whether this is caused by planar carbanions or by oscillating pyramidal carbanions is not known. In either case, racemization occurs whenever a carbanion is completely free or is symmetrically solvated. 

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