Spirocyclic phosphoranes

Pseudorotation.—A number of spirocyclic phosphoranes possess square-pyramidal structures rather than the trigonal-bipyramidal structures previously assumed, and this could have important consequences on the interpretation of their variable-temperature spectra. There is, as yet, no evidence that acyclic or monocyclic phosphoranes favour the square-pyramidal geometry, and the variable-temperature XH... 

Under UV radiation, the phosphorane azide 28, which is one of the first thermally stable derivatives in which A5-phosphorus is bonded to five nitrogen atoms, is quantitatively converted into the new spirocycle phosphorane 29 (equation 13)66. 

The reaction of monoammonium perfluoropinacolate with 2-amino-1,3,2-dioxaphospholane (52) provides a spirocyclic phosphorane 53 with hydrogen attached to phosphorus, with evolution of ammonia occurring 263). 

The compounds 57 (60,61) formed according to Eq. (43) are listed in table II the substituent Z normally occupies the second axial position, and the perfluoropinacolyl bridge has—with exceptions—axial-equatorial conformation. Spirocyclic phosphoranes 60 are shown in Table III. 

Several new spirocyclic phosphoranes (80a-e) have been isolated and examined by X-ray crystallography. For (80a-c), X was found to be apical but for... 

The addition-elimination reaction proceeds via addition of a nucleophile at phosphorus to generate a five-coordinate intermediate or activated complex. TTie phosphorus atom is readily able to use its d orbitals to expand its coordination number and it is argued that an attacking nucleophile such as OH" adds to the four-coordinate phosphorus to form a pentacoordinate phosphorane (143). It appears that generally the pentacoordinate intermediate will have a trigonal bipyramidal structure, although spirocyclic phosphoranes seem to favor square pyramidal structures (80). 

Bicvclic and Tricyclic Phosphoranes - The work of Schmutzler et al. reported in references 25 and 26, also includes the preparation and characterisation of a number of spirophosphoranes. For example the reaction of (60) with (66a-e) produced a range of spirocyclic phosphoranes (67a-e). Likewise the reaction of (66a-e) with two moles of (63) produced a series of... 

The lack of direct evidence for TBP intermediates has allowed the proposal of alternative pentacoordinate intermediates and of hexacoordinate intermediates in nucleophilic substitution reactions at tetrahedral phosphorus. Pentacoordinate phosphorus species (spirocyclic phosphoranes) have been observed in which the most stable form adopts the square pyramidal (SP) geometry (Howard et ai, 1973). Such SP species have been postulated as intermediates in substitution reactions at tetrahedral phosphorus (Boudreau et ai, 1975). 

The S 2(P) reaction proceeds by addition of a nucleophile at phosphorus to generate a five-coordinate intermediate or transition state. The phosphorus atom can readily expand its coordination number by use of its d-orbitals. An attacking nucleophile such as hydroxide adds to four coordinate phosphorus to give the pentacoordinate phosphorane [10]. Normally the phosphorane will have a trigonal bipyramidal stereochemistry, but spirocyclic phosphoranes appear to favour square pyramidal structures [11]. 

With tris(dimethylamino)phosphine and ethanolamine or carboxylic add hydrides, spirocyclic phosphoranes are obtained. 

Apparently, the spirocyclic skeleton which can very conveniently accomodate to the angle requirements of the trigonal-bipyramidal phosphorane structure28 66 precludes here the potential decomposition to the tetrahedral ylide 95, which would be much more strained. 

Reactions of oc-diketones and o-quinones with phosphorus compounds in the syntheses of P-heterocycles (cyclophosphates, phosphorans, spirocyclic compounds) 02CRV629. 

A number of new five-co-ordinate spiro-phosphoranes such as (102), obtained from P(NMe2)s and ephedrine," " and (103)" have also been synthesized. In compound (104), replacement of the dimethylamino-group with both mono- and di-hydroxy species has been studied,and i.r. investigations on a number of similar compounds, but containing P—H bonds point to the establishment of the equilibrium in equation (51) between the phosphorus(v) spirocyclic form and an... 

---