Conformations of six-membered ring phosphorus heterocycles. I. Ring conformations and phosphorus configurations of isomeric six-membered ring phosphites

Abstract

A six-membered ring phosphorochloridite (5-tert-butyl-2-chloro-l,3,2-dioxaphosphorinane) and a geometrically isomeric pair of six-membered ring phosphites (cis-and Zram-5-/er/-butyl-2-methoxy-l ,3,2-dioxaphosphorinane) have been synthesized, and their conformational and configurational properties have been investigated. The m-phosphite is shown to be the more stable isomer by about 1.4 kcal/mol at 25°and by nmr analysis to be a chair-form conformer with the ieri-butyl equatorial and the methoxy axial. The predominate conformer of the trans isomer is the chair form with both substituents axial. The axial preference of the methoxy substituent in the phosphite series, as in the corresponding cyclic sulfites, is found to be greater than for the 5-te/V-butyl-2-methoxy-1,3-dioxanes. A true boat form for the other conformer(s) of the tra«s-phosphite is ruled out, but present data indicate both chair and twist conformers to be worthy of further consideration.

Structural studies of phosphorinanes

“…Conversely, variable temperature experiments and I spectral analysis of compounds £ (26), ^ (9, 10, 11, 26) 17b (13,14), 2]^ (17), 32b (20), 3^ (19,20), and 34b (20) indi cated changes in nmr spectral parameters consistent with con formational changes generally assumed to be chair-chair interconversions as indicated above for I and II.…”

“…Isomers of the chloro compounds 3^ (12), (12), 22 (13,14), 32 (12,19,20,28,30), 40 (9,22,23), although expected, have not been observed. Furthermore, pnmr spectra of the 2-chloro compounds ^ (9, 11), 2^ (9), 30 (29), and 41 (9) and to a greater extent those of the 2-bromo compounds 1^ (9) and £7 reversed for the b isomer as expected for equatorial and axial phosphoryl groups, respectively.…”

“…By similar reasoning the a,b isomers of compounds 2^ (9,24), 25 (9), 26 (9), 27 (9), 28 (25), 29 (29), 30 (29) and 31 (29), have been assigned as differing in C5 configuration with con sistent phosphorus configuration for each pair, whereas compounds ^ (13,14), ^ (15), (15,16), 32 (20,28), 33 (19,20,30), 34 (20), 35 (20), 38 (9,21,22) and 42 (22) are isomeric at phosphorus with consistent carbon configuration.…”

“…Analyses of nmr spectra of compounds 1^-3^ (8), 5 (9, 10), 7 (9, 10), 8 (11), n (9, 10, 11), 0^ (9, 10, 11), 13 (9, 10, 11), 15 (12), 1^ (12), ]^a (13,14), 18a,b (15), 19a,b (15,16), 21a (17), 22a,b (18), 23 (13,14), 26a,b (9, 10), 27a,b (9, 10), 37 (12,19,20), 38b (9,10,21,22), 42 (22), ^ (22), and ^ (9, 10, 11) have been accomplished by observation at one temperature only and all have been assigned a chair conforma tion, Spectra of compounds £ (9,10,11,23), £ (10,22), ^a,b (9,10,24), 25^a,b (9, 10), 28a,b (25), 32a (20), 33a (19,20), 34a (20), 35 (20), 38^a (9,10,21,22), ^ (9,10,21,22) and £1 (9,10) have been obtained at various tempera tures and concluded to exist in either a single rigid chair conformation or an equilibrium of two chair conformations indicated below for I and II which does not change appreciably within t...…”

“…The stereochemistry of C4 and C5 in 41 would also be difficult to change owing to the presence of a six-membered ring fused at these positions. The energetic balance of isomeric compounds ^ (28), ^ (13,14), ^ (15), ^ (15,16), 21 (17) and £2 (18) are more difficult to predict and C5 versus phosphorus isomerism for ^ has not been determined. Generally the tert-butyl group prefers an equatorial orientation.…”