Inversion, pyramidal

The first experimental determination of the inversion barrier of a tertiary arsine was reported in 1971 . The kinetics of racemization of (/ )-(—)- and (S)-( + )-ll, resolved by the metal complexation method, at 217.6 +0.3 °C in decalin (sealed tube) was determined polarimetrically in the 310-350 nm region. From the kinetic data, by substitution into the Eyring equation, the free energy of activation, AG was calculated to be 175 + 2kJmol at 217.6°C. This energy value corresponds to a half-life for racemization of the arsine of ca 740 h at 200 °C. It had been reported previously that resolved ethylmethylphenylarsine and methyl(n-propyl)phenylarsine showed no detectable loss of optical activity over 10 h at 200 On the basis of photoracemization studies 

Cyclic (4p-2p)n conjugation contributes to the lowering of the inversion barrier in 167, however, where the value AG = 145kJmol at 151 °C was determined by an analysis of kinetic data for the equilibration of a 72 28 diastereomeric mixture of the arsindole . The effect will be maximal in the planar transition state. Total line-shape analysis of the temperature-dependent silyl-methyl NMR resonances in 168 indicated a cooperative effect between cyclic (p-p)n delocalization and substituent atom electronegativity, since the diminution in barrier height was greater than that afforded by either effect alone. 

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Trigonal pyramidal molecules are chiral if the central atom bears three different groups If one is to resolve substances of this type however the pyramidal inversion that mterconverts enantiomers must be slow at room temperature Pyramidal inversion at nitrogen is so fast that attempts to resolve chiral amines fail because of their rapid racemization... 

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... 

Tricoordmate sulfur compounds are chiral when sulfur bears three different sub stituents The rate of pyramidal inversion at sulfur is rather slow The most common compounds m which sulfur is a chirality center are sulfoxides such as... 

Thiophenium fluorosulfonate, 1,2,3,4,5-tetramethy 1-ylide, 4, 724 Thiophenium salts aromaticity, 4, 724 proton abstraction, 4, 766 pyramidal inversion barrier, 4, 724 structure, 4, 715 synthesis, 4, 723-724 Thiophenium salts, 1-alkyl-solvolysis, 4, 766 UV spectra, 4, 766 Thiophenium salts, aryl-synthesis, 4, 726... 

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. ... 

One consequence of tetrahedral geometry is that an amine with three different substituents on nitrogen is chiral, as we saw in Section 9.12. Unlike chiral carbon compounds, however, chiral amines can t usually be resolved because the two enantiomeric forms rapidly interconvert by a pyramidal inversion, much as an alkyl halide inverts in an Sfg2 reaction. Pyramidal inversion occurs by a momentary rehybridization of the nitrogen atom to planar, sp2 geometry, followed by rehybridization of the planar intermediate to tetrahedral, 5p3 geometry... 

Figure 24.1 Pyramidal inversion rapidly interconverts the two mirror-image (enantiomeric) forms of an amine.

Pyramidal inversion (Section 24.2) The rapid stereochemical inversion of a trivalent nitrogen compound. 

Lmine(s)—cont d primary. 916 properties of, 920 purification of. 923-924 pyramidal inversion in, 919-920 reaction with acid anhydrides, 807 reaction with acid chlorides, 803-804... 

The numerous examples of optically active sulfoxides reflect their configurational stability. Optically active sulfoxides resist thermal racemization by pyramidal inversion, so... 

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. 

Prostaglandins 624, 725, 960 Prostanoids 620 Protonation 565-567, 1049 photochemical 882 Pseudopotential methods 15, 16 Pummerer rearrangement 240, 243, 470, 843 Pyramidal inversion 602, 604 Pyrazolenines 749 Pyridazine oxides 640 Pyridine aldehydes, synthesis of 310 Pyridine oxides 640 Pyrolysis 102-105 of sulphones 110, 679-682, 962 of sulphoxides 739, 740 Pyrroles 265, 744... 

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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