Circumambulatory rearrangements

The Circumambulation of a Phosphirane: Taking 9‐Phenyl‐9‐phosphabicyclo[6.1.0]nona‐2,4,6‐triene for a “Walk”

“…Keywords: density functional calculations · phosphorus heterocycles · sigmatropic rearrangement Figure 4. Relative (U)B3LYP6-31G* and CASPT2 (8,8) [13] (in italics) enthalpies at 298 K (in kcal mol À1 ) for the [1,5] shift of hydrocarbon 3 C.…”

“…[3] "Walk" rearrangements are also known for 1, 2, and 5, with sulfur, nitrogen, or oxygen as the migrating atom, [1] but not with phosphorus, though a degenerate rearrangement along part of a ring has been reported for a triphospha analogue of 1, with a weak phosphorus-phosphorus bond. [4] Whereas thio, [5] oxa, [6] amino, [7] and phospha analogues [8] of 3 are known, the potential for "walk" activity has not been recognized for any.…”

“…Similar barriers of 37.8 (TS 1,5 (a)) and 35.9 kcal mol À1 (TS 1,5 (b)) are predicted at the UB3LYP level of theory using a spin-projection method to obtain proper open-shell singlet energies. [16,17] The origin of the diradical [1,5] shift for the hydrocarbons has been attributed to the too long distance between the migrating methylene carbon and the opposite ring carbon to allow for a pericyclic process. [13] Evidently, the longer CÀP bonds and the more diffuse character of phosphorus underlie the pericyclic nature of the [1,5]-rearrangement of 3 P into 7 P. The exo conformer of 3 P', which shows excellent agreement with the X-ray structure of 3 P, readily inverts to the lessstable (0.5 kcal mol À1 ) endo form (3 P'(exo)!TS inv !…”

“…The [1,5]-rearrangement is pericyclic without diradical character for transition state TS 1,5 . In sharp contrast, the more-demanding conversion of hydrocarbon 3 C into 7 C occurs in a diradical manner, as inferred from the 15 N/N scrambling during the [1,5] shift of singly 15 N-labeled 9,9-(CN) 2--3 C. [13] The reaction path for 3 C was reported to have two similar high-lying transition states (TS 1,5 (a) 40.0 and TS 1,5 (b) 39.2 kcal mol À1 at the CASPT2(8,8) [13] level of theory) without an intermediate (Figure 4), enabling both retention and inversion of configuration of the migrating carbon atom. Similar barriers of 37.8 (TS 1,5 (a)) and 35.9 kcal mol À1 (TS 1,5 (b)) are predicted at the UB3LYP level of theory using a spin-projection method to obtain proper open-shell singlet energies.…”

The Circumambulation of a Phosphirane: Taking 9‐Phenyl‐9‐phosphabicyclo[6.1.0]nona‐2,4,6‐triene for a “Walk”

“…With 4n + 2 electrons involved, (Scheme 1, 4-6) the migrating group should retain its configuration, causing rearrangements between two isomeric structures that reflect a non-least motion pathway (Figure 1 b), but such systems often follow a symmetry-forbidden stereochemical path with diradical character. [3] "Walk" rearrangements are also known for 1, 2, and 5, with sulfur, nitrogen, or oxygen as the migrating atom, [1] but not with phosphorus, though a degenerate rearrangement along part of a ring has been reported for a triphospha analogue of 1, with a weak phosphorus-phosphorus bond. [4] Whereas thio, [5] oxa, [6] amino, [7] and phospha analogues [8] of 3 are known, the potential for "walk" activity has not been recognized for any.…”