Phosphoranes Hydroxyphosphoranes

In addition to the possibility of detecting their presence as intermediates during certain reactions, some hydroxyphosphoranes can be isolated as salts or can be detected as unstable intermediates in the course of a reaction178 thus, the phosphorane 102 was prepared179 and the structure of its triethylammonium salt determined by XRD180. 

Pentacoordinate hydroxyphosphoranes are likely intermediates or transition states in substitution reactions at tetracoordinate phosphorus (1). Recently, stable hydroxyphosphoranes (2, 3) and their conjugate bases - metal phosphoranoxides (J, 5) have been isolated. Spectroscopic evidence (J - 7) for equilibria between P(IV) compounds and hydroxyphosphoranes have been reported. Observation (8) and isolation (9) of P(IV) TBP phosphoranide species have also been announced. All these phosphoranes are stabilized by several features dominated by their spirobicyclic nature. 

The phosphites 3 and 4 are oxidized by chlorine and bromine to furnish stable covalent phosphoranes. No Arbuzov reaction was observed. The trichlorophosphorane 5 can be reacted with again to form a spirophosphorane 7 which is hydrolyzed without ring cleavage to give an extremely stable hydroxyphosphorane or a phosphoraneoxide anion . . ... 

Further results on the acidities of hydroxyphosphoranes have been published. The p/STa values of the dicarboxy compound (86) were in the range 0—4 depending on the solvent, that of (109 X = CF3) at 2, whilst calculations on tetrahydroxyoxyphosphorane indicated that they should have p Ta values of 10—20. The acidity of the hydroxyphosphorane (110) has been compared with other acidic phosphoranes. ... 

The five coordinate spirophosphoranes have played an important role with respect to understanding the stereochemistry of four coordinate phosphorus reactions which proceed via phos-phorane intermediates. Variable temperature NMR studies have established the apicophilicities of a wide range of groups (i.e. relative preferences of a series of groups to occupy an apical position rather than an equatorial position of a trigonal bipyramidal (t.b.p.) conformation of a phosphorane). The spiro ring system has been used to stabilize transient reaction intermediates of acyclic compounds, for example hydrolysis intermediates such as hydroxyphosphoranes. 

Chlorospiro-phosphoranes, under the action of Lewis acids such as SbCls, may be converted to the quasiphosphonium salt, for example (16) <88Zaac(561)49>. Hydroxyphosphorane (42 R = OH, Y = CMej) is converted to the spiroquasiphosphonium triflate (14) by the action of trifluoro-methanesulphonic acid <78JA5229> this in turn can be reduced by LAH to the phosphoranide (42 R = electron pair, Y = CMcj). 

This preliminary study was expanded by use of higher-level STO-3G ab initio calculations for similar dimethoxy hydroxyphosphorane rotamers (Gorenstein et ai, 1979). In this study, the three rotameric phosphoranes [56], [57] and [58] app lone pairs shaded), representing phosphorus TBP intermediates, were geometry-optimized. The results of energy and Mulliken population analysis (given in Table 3) confirm the previous results at the CNDO/2 level. 

Cyclic Phosphoranes - The reaction of dioxetanes (28a-c) with ylides (29a-d) gave phosphonium alkoxides (30a-f) in equilibrium with the pentacoordinate dioxa-2,5-phosphorinanes (31a-f).16 Hydrolysis via the hydroxyphosphoranes (32a-f) gave the phosphine oxides (33a-f). 

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