Dioxaphospholanes, conformation

Conformation problems in five- and six-membered rings containing phosphorus continue to attract attention and, using n.m.r., i.r., and dipole-moment measurements, studies have been reported on 1,3,2-dioxaphospholan systems (118), > 1,3,2-dioxaphosphorinane systems... 

Cyclic Esters of Phosphorous Acid.—A large number of 2-substituted-4-methyl-l,3,2-dioxaphospholans (88) have been prepared and their stereochemistry and conformations investigated by 1H and 31P n.m.r.69 Unlike the corresponding 1,3-dioxans, the tra/w-isomer (88a) is favoured in all cases, and each isomer is best described in terms of two rapidly equilibrating half-chair conformers with the 4-alkyl group pseudo-axial or pseudo-equatorial. 

Much work has been done on configuration and conformation, e.g. of 2-methoxy-2-oxo-4,5-diphenyl-l,3,2-dioxaphospholanes with 170 and lsO labelled reagents (81CC245). A large number of 2-dimethylamino-l,3,2-dioxaphospholanes or 3-methoxy-2-methyl-l,3,2,-oxazaphospholanes are mentioned in a recent publication, in which spiro compounds of the general structure (108) were prepared by addition to diethyl azodicarboxylate (equation (69)) (80PS(8)147>. 

Mechanistically, the requisite betaine intermediates [from dioxaphospholanes N and O] adopt either the chair ( C -> 4) or twist-boat ( 85, 82) conformations so that the C-O" and -O-+PPh3 groups can assume the requisite pre-transition state andperiplanar arrangement for suitable displacement of Ph3PO. From the results of molecular modeling studies (/.e., MacroModel II) on both the chair e.g., IC4) and twist-boat betaine e.g., 85, 82) conformers, the small energy differences between them suggest that the twist-boat ( 85 and 82) betaine conformers may also be... 

Fig. 8. Envelope (a) and half-chair (b) conformations of cycle in 2-chloro-l,3,2-dioxaphospholan...

Alternative conformational isomers of phospholanes (13.104) and phosphetanes (13.105) are to be expected. IR absorption spectra indicate that both isomers are present in the dioxaphospholanes (13.108), down to -160°C, but in general, compounds of this type appear to be conflgurationally stable (6.958). 

As mentioned earlier, by using the dipole moment method and different quantum-chemical methods, the conformational analyses of 6rs(2-phe-nylalkyl)phosphine selenides, and of l,3,2-dioxaphospholan-2-yl 2,2,2-trifluoroacetate (7) and 4,5-benzo-l,3,2-dioxaphosphol-2-yl 2,2,2-tri-fluoroacetate (8) were carried out. The experimental dipole moments... 

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