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

Abstract

Walking rings: At room temperature, the P‐R entity of a bicyclic phosphirane (see picture) “walks” along the perimeter of the larger ring. Although this concerted rearrangement is similar to that of its hydrocarbon analogue, the additional 1,5‐sigmatropic shift follows a different mechanism.

Kinetic Control in the Regioselective Alkylation of Pterin Sensitizers: A Synthetic, Photochemical, and Theoretical Study

“…The [1,3]‐ and [1,4]‐methyl shifts require very high activation energies. Thus, it is logical to conclude that the O ‐ and N ‐alkyl pterins, for example, 3 and 4 , are non‐interconverting species, unlike walk processes observed for molecular rearrangements , such as the walk rearrangement of bicyclo[2.1.0]pent‐2‐ene .…”

Synthesis and Reactivity of the Phosphorus Analogues of Cyclopentadienone, Tricyclopentanone, and Housene

“…Although diphosphahousene 5 (P 2 ‐ E ) bears six inequivalent tert ‐butyl groups, only 4 different 1 H and 13 C{ 1 H} NMR resonances were found at room temperature. Upon cooling to −80 °C, the expected 6 sets of t Bu signals were observed, indicating that 5 is dynamic at room temperature and undergoes a facile degenerate [1,3] sigmatropic shift (see Scheme ), which was supported by theory (Δ E ≠ =17.5 kcal mol −1 at ωB97X‐D/6‐31G(d,p)), and also found for triphosphahousene H…”

“…[20] Heating 4 for 20 hours in refluxing toluene afforded 5,which displays two different 31 P{ 1 H} NMR resonances and was isolated as ap ale yellow solid in 97 %y ield (Scheme 3; d 31 P{ 1 H} = 53.5 (P1), À169.1 ppm (P2), 1 J P, P = 171.0 Hz). Although diphosphahousene 5 (P 2 -E)bears six inequivalent tert-butyl groups,only 4different 1 Hand 13 C{ 1 H} NMR resonances were found at room temperature.U pon cooling to À80 8 8C, the expected 6s ets of tBu signals were observed, indicating that 5 is dynamic at room temperature and undergoes afacile degenerate [1,3] sigmatropic shift (see Scheme 3), [21] which was supported by theory (DE°= 17.5 kcal mol À1 at wB97X-D/6-31G(d,p)), [13,22] and also found for triphosphahousene H. [8] To gain more insight into the mechanism of the unusual 4-5 rearrangement, we resorted again to DFT calculations [13] and found that 4 undergoes a[2+ +2] cycloaddition between the P = Pa nd C = Cb onds affording P-bridged phosphabicyclo-[1.1.0]butane 9 as intermediate (DE = 2.7 kcal mol À1 , DE°= 31.4 kcal mol À1 ;Scheme 4). Although diphosphahousene 5 (P 2 -E)bears six inequivalent tert-butyl groups,only 4different 1 Hand 13 C{ 1 H} NMR resonances were found at room temperature.U pon cooling to À80 8 8C, the expected 6s ets of tBu signals were observed, indicating that 5 is dynamic at room temperature and undergoes afacile degenerate [1,3] sigmatropic shift (see Scheme 3), [21] which was supported by theory (DE°= 17.5 kcal mol À1 at wB97X-D/6-31G(d,p)), [13,22] and also found for triphosphahousene H. [8] To gain more insight into the mechanism of the unusual 4-5 rearrangement, we resorted again to DFT calculations [13] and found that 4 undergoes a[2+ +2] cycloaddition between the P = Pa nd C = Cb onds affording P-bridged phosphabicyclo-[1.1.0]butane 9 as intermediate (DE = 2.7 kcal mol À1 , DE°= 31.4 kcal mol À1 ;Scheme 4).…”

Possibility of [1,5] Sigmatropic Shifts in Bicyclo[4.2.0]octa-2,4-dienes

“…Sigmatropic [1,n] carbon migrations (Scheme 3, I, R=alkyl) on the other hand, do not generally involve concerted transition states because the overlap of the orbitals in the transition structure is usually too weak, 24 but are believed to occur via intermediate singlet-state biradicals. 25,26 Exceptions, involving pericyclic transition states with good overlap, are the [1,5] sigmatropic migration in 1,3-cyclopentadienes [27][28] and the so-called "walk rearrangements" [29][30][31][32][33] of bicyclo[n. 1.0]polyenes for which the thermally allowed process should occur with inversion of configuration at the migrating carbon atom. 26,[34][35] Walk rearrangements are [1,5] systems have received a lot of attention in both experimental and computational studies, 16,[30][31][36][37][38][39] as they have been observed to proceed with inversion at the migrating center, indicating an orbital-symmetry forbidden rearrangement.…”