1.2- Oxaphospholenes

R =Pr R =H). Finally, terminally-disubstituted compounds yield 1,2-oxaphospholenes (175) exclusively. 

In exploring various synthetic methods for the preparation of 1,2-oxaphospholene 2-oxides we investigated the reaction of diacetone alcohol (4-hydroxy-4-methyl-2-pentanone) with methyl and phenylphosphonous dichloride, which was reported (1) to proceed as shown in eq (1) to give 2,3,3,5-tetramethyl-l,2-oxaphos-phol-4-ene 2-oxide or the corresponding 2-phenyl derivative. 

Fig. 2. Molecular structure of a fused 1,2-oxaphospholene-4 and 1,2,5-dioxaphot-phorinene-3 ring system with pentavalent phosphorus, from X-ray crystallographic analysis.

Scheme 3 contains several of the fundamental structures with penta-valent phosphorus, among them phosphorane itself, PH6 (21), and pentahydroxyphosphorane, P(OH)6 (26). The latter is the hydrate of phosphoric acid H3P04+H20 - P(OH)5. A hydrate of methylphos-phonic acid (CH3)(HO)4P (29), is also included as a model for its esters. Mono- and di-ionized forms, (27), (28), (30), are also given, since their stabilities in various isomeric forms provide important data concerning the role of intermediate oxyphosphoranes in the chemistry of phosphoric acid and its derivatives. A model compound of the 1,2-oxaphospholene ring (31), is provided, since this system, in the form of several derivatives, will be discussed extensively in Section VIII. 

The two tetraoxyphosphoran.es (82) and (83) in Scheme 10 have in common a cis relationship of two identical substituents on the five-membered ring. There are some similarities between (82), (83), and the derivative of the 1,2-oxaphospholene (70) of Scheme 8, since in both cases, the five-membered rings (including its carbon-substituents) have... 

The TBP-phosphorus atom with three equivalent ligands can no longer be a chiral center. As a consequence, the positional exchange of ligands in the 1,2-oxaphospholene (64) (Scheme 15) reported a few years... 

Details have appeared of the formation of cyclopropanes from arylidene-malononitriles and the biacetyl-trimethyl phosphite adduct. 1,2-Oxaphospholens.— Among a 3-unsaturated ketones used in the formation of 1 1 adducts with tervalent phosphorus compounds are the sulphone (42) ... 

The 1,2-oxaphospholens from trimethyl phosphite and but-3-en-2-one (44) and benzylideneacetylacetone (45) are powerful D-alkylating agents, converting acids and phenols into esters and ethers, respectively. Mesiloic acid is esterified at room temperature. The mechanism is probably as shown in (46). 

Unsaturated ketones were also used in such cyclizations, but if the ketone was prone to isomerization under acidic conditions, the final product possessed a 2-oxo-1,2-oxaphospholene structure [114]. However, both the above procedures lead to the desired compounds in yields of less than 20%. At the same time, phosphoryl-substituted... 

A similar palladium-catalysed approach was used to obtain fused benzophos-phacyclanes 87 (Scheme 53) [119]. In both cases, the palladium-catalysed procedure allows creation of five- to seven-membered heterocycles in reasonable yields however, the best results were achieved for the corresponding 1,2-oxaphospholanes. Furthermore, 2-oxo-1,2-oxaphospholenes 88 can be advantageously used as precursors for the synthesis of substituted phostones 89 (Scheme 54) [120],... 

The usual product, after pyrolysis, is a 1,2-oxaphospholen 2-oxide (32), although an alcoholic work-up yields an acyclic phosphinate (33) (Scheme 10). Just in case these advances in our current mechanistic interpretation of these reactions (c/. ref. 40) leads to complacency, a further Russian paper has described the reactions of acetone and other simple ketones with chlorodiphenylphosphine or chlorodiethylphosphine. These yield the a-chloroalkylphosphine oxide (34), or the derived oxide (35), and not the oxide (36), which is known to be formed from mesityl oxide (28) and chlorodiethylphosphine. a-Halogenoalkylphosphine oxides are generally produced when aldehydes are heated with a wide range of halogenophosphines, and the formation of such an adduct from a ketone is a novel result. 

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