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These fragmentations were accomplished thermally (110" in toluene). We later found that the fragmentations could be conducted at room temperature by subjecting the precursors to ultraviolet light (254 nm) [2], and we generated both alkyl metaphosphates and N,N-dimethylmetaphosphoramidate (Me2N-P02) by this method. Other studies have led to the generation of alkyl metathiophosphates [3]; applications of the 3-coordinate species for special phosphorylations have also been described [4]. We have continued to develop the chemistry of the 2,3-oxaphosphabicyclo[2.2.2]octene ring system, and we report in this paper some new observations on the synthesis and stereochemistry of the system, as well as on some kinetic studies of the thermal fragmentation that provide compelling proof that a free metaphosphate is indeed released. Additional proof for a free metaphosphatt, has come from the use of chiral P-alkoxy derivatives of the bridged ring system, which are useful in proving the planarity of the species, but also reveal that an oxy function properly placed may undergo anchimeric participation with the metaphosphate function.In Scheme 1 is outlined the general synthetic method used for the construction of the requisite ring system. Phosphole oxide intermediates are first generated and instantly trapped by Diels-Alder reaction with a maleimide derivative to give a product with the 7-phosphanorbornene framework. No other dienophile has yet been found to be superior 0 1991 VCH Publishei-s, Inc

1,6-Dihydrophosphinine I -oxides with 4-chloro-3(or 5)-methyl substituents give cycloadducts with dimethyl acetylenedicarboxylate having the 2-phospha-The fragmentation of bridged, strategically unsaturated heterocyclic systems is a powerful technique for the generation of small molecules with multiple bonding at the heteroatom. We have exploited this method in several ways to generate phosphorus species with multiple bonds. We have, for example, successfully generated the unsaturated, 3-coordinate fragment -POz attached to carbon [1,21, alkoxy [ 1,3], or amino groups [3-51, employing both thermal and photochemical fragmentation of derivatives of the 2,3-oxaphosphabicyclo[2.2.2]octa-5,7-diene ring system. The thermal fragmentation has been shown to occur by a concerted retro-cycloaddition [6] that accomplishes extrusion of the P-0 bridging unit with the generation of a new P=O bond.

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Differential thermal analysis and differential scanning calorimetry showed that the title compounds decomposed at 240-260°C with release of the fragment RP(0) = CH,. Mass spectral studies also showed this to be a fragmentation pathway. The extruded methylenephosphine oxide could be trapped with ethanol, hydroquinone, or by reaction with the surface OH groups of silica gel.