Phosphorus inversion

Similar observations are possible with acyclic Pv compounds if the substituents of phosphorus are different and also rather bulky (and more especially if chelation can occur to stabilise the conformers).(1969,110) In (CH3)2NP(0)F(C4H9-f), the methyl groups are non-equivalent with 37(P-N-CH3) of 8-2 and 0-0 Hz.(1965,8) The phosphorus inversion remains the dominating phenomenon here. (1969>in>... 

Incorporation of d-functions on the inverting N site is especially important. Such functions contribute more to the pyramidal GS than to the TS and thus stabilize the GS with respect to the TS. In the absence of -functions on N, the inversion barrier in NH3 is found to be much too small and H2N—CN and H2N—SiHs are found to be more stable in the planar form inclusion of -functions (and p on H) leads to the correct NH3 barrier 158> and to pyramidal nitrogen sites in HaN—CN and H2N— SiH3 (Table 8 144>). Of course, the relative error introduced by the absence of -functions is smaller the higher the barrier. This may explain in part why the barriers calculated for aziridine and oxaziridine without including -functions are in satisfactory agreement with experimental values. In PH3, the phosphorus inversion barrier changes from 30.9 to 37.2 kcal/mole on inclusion of two sets of -functions 159>. 

All these ligands (I-III) are pyramidal at phosphorus, but with varying degrees of sp hybridization. Due to their reduced phosphorus inversion barrier phospholes are more planar than typical phosphines and their phosphorus lone pair possesses less s-character than does the phosphorus in either I or II. Thus from a frontier orbital point of view since the lone pair is likely the HOMO for all these ligands, we would anticipate that I and II might be poorer donors than III. Likewise, III is considerably less bulky than either I or II. In sum then, if cyclic conjugation is not large in III, its donor ability should approximate those of I and II. 

The latter fact and the energetically disfavored process via heterolytic bond cleavage of the SiPTe skeleton may rather imply a nonclassical conversion process in which both bonds to the Te atom (Si-Te, P-Te) were simultaneously cleaved and reformed after phosphorus inversion. Remarkably, the dissociation energy for the latter process takes only about 20 kcal mol [25]. This observation strongly supports the description of such small ring compounds as 7t-complexes in terms of the Dewar-Chatt-Duncanson model (see Scheme 5). 

A mixture of ais- (5) and tvans- (6) 1-phenyl-4-tert--butylphosphorinane 1-oxide, m.p. 160-161° and 88.5-95°, respectively, is obtained on boiling 1,1-diphenyl-4-tert--butylphosphorinanium bromide with M sodium hydroxide for a long time. The isomers have been separated and converted by phenylsilane reduction into cis- and trans-l-phenyl-4-tei f--butylphosphorirane, each with a b.p. 110-120 /0.1 mm. 1,1--Diphenyl-4-tf ans-butylphosphorinanium bromide on treatment with aqueous sodium hydroxide yields a mixture of (5 60%) and (6 40%) (K.L. Maris et at, J. org. Chem., 1977, 42, 1306). Studies of phosphorus inversion in and conformational analysis of ais- and l-phenyl-4-tef t-butylphosphorinane have been... 

Two kinds of information about nucleotidyltransferases and phosphotransferases are obtained by use of substrates or substrate analogs with chiral P. The stereochemical course of phosphoryl transfer and nucleotidyl transfer gives important information about the reaction mechanism. If inversion of configuration at phosphorus is observed, it may be concluded that an uneven number of displacements at phosphorus occurs in the reaction mechanism. If retention of configuration at phosphorus is observed, it may be concluded that the mechanism entails an even number of displacements at phosphorus. Inversion corresponds to the single-displacement mechanism of Eq. (2), and retention indicates a mechanism such as that of Eq. (3) or Eqs. (4a) and (4b). 

Ab initio MO calculations, including contributions of the electron correlation by a second-order Moller-Plesset (MP2) perturbation approach, predict PH3 (Cg to invert by a vertex inversion process through a planar trigonal transition state (Dg, ) rather than by an edge inversion process through a planar, T-shaped transition state (Cgv) [1, 2] for a topological electron density analysis of tricoordinate phosphorus inversion processes, see [3]. 

The interplay between the rates of phosphorus inversion (which interconverts 2 and 2 ) and the rates of electrophilic attack to each diastereomer of 2 will dictate the enantiomeric excess of 3. 

A mixture of all four possible stereoisomers of 9 was detected by NMR, which is not consistent with the existence of 8 as single diastereomer and suggests it to be a mixture of two of them rapidly interconverting via phosphorus inversion. 

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