Conformational analysis of sulfur-carbon-phosphorus anomeric interactions. 3. The conformational preference of the diphenylphosphinoyl group in cyclohexane

Tandem chain extension–Mannich reaction: an approach to β-proline derivatives

“…The opportunity, therefore, exists for the Z-phosphinoyl imine to react with the Z enolate to provide the syn product. Alternatively, the steric size of the diphenylphosphinoyl groups 18 as presented in an E-imine may encounter excessive 1,3-diaxial steric interaction when reacting with a Z-enolate. Alleviation of this steric interaction by reaction through a twist boat conformation would also account for formation of the syn product.…”

Second‐row anomeric interactions: The involvement of phosphorus

“…Because the magnitude of the anomeric effect is frequently defined as the difference between the free energy difference for the equilibrium studied and the conformational energy value for the same substituent in cyclohexane [5], a study was undertaken to estimate the conformational preference of the diphenylphosphinoyl group in cyclohexane [9] (Equation 4).…”

“…Application of the Hill equation [ 113 to the precise structural data of 1-axial provided AW = 1.25 kcal/mol, to which was added the entropy term, TAS" = (298°K) (2.6 cal/deg O K ) = 0.78 kcal/ mol; therefore, the reference AGO (steric AGO) from which to estimate the magnitude of the anomeric effect (Equation 6) was estimated as -2.03 kcall mol. Then, since anomeric effect = -AG'&.ic + AGEbserved (6) the magnitude of the anomeric effect would amount to 3.03 kcal/mol [9]. 2. It has been shown [12] that the AGO (2-tertbutyl) in 1,3-dithianes is about 60% of AGO (tertbutyl) in cyclohexane.…”

“…2. It has been shown [12] that the AGO (2-tertbutyl) in 1,3-dithianes is about 60% of AGO (tertbutyl) in cyclohexane. Thus, applying Franck's methodology [13], the expected size of the diphenylphosphinoyl group in 1 is 60% of 2.74 kcal/ mol [9], which affords an anomeric effect equal to 1.0 + (0.60 x 2.74) = 2.63 kcal/mol [14, 151. In conclusion, estimates (1) and (2), although admittedly crude, afford quite similar values for the magnitude of the anomeric effect present in 1: close to 3 kcal/mol, which is one of the largest yet recorded [ 161.…”

Theoretical Examination of the S–C–P Anomeric Effect

“… The magnitude of the anomeric effect can be estimated by comparison of the conformational preference of the substituent in the heterocyclic system relative to the conformational preference of the same substituent in cyclohexane (its A value) . Thus, consideration that the diphenylphosphinoyl group’s A value is worth 2.74 kcal/mol suggests an anomeric effect worth 2.71 – (−2.74) = 5.45 kcal/mol in 1 (in chloroform). By the same token, since the calculated conformational preference of the trimethylphosphonium group in 1,3-dithiane is −0.96 kcal/mol and the calculated A value of the same group is −4.04 kcal/mol (this work), this affords a value of 3.08 kcal/mol for the magnitude of the S–C–P anomeric effect in 4 (in chloroform).…”