Theoretical Investigation of the Photochemical C²–C⁶ Cyclisation of Enyne–Heteroallenes

Abstract

Herein we discuss computations that explain experimental results regarding a highly efficient triplet analogue of the C(2)-C(6) cyclisation of enyne-heteroallenes recently discovered by Schmittel and co-workers.1 To shed some light on the reasons for the differences found between enyne-carbodiimides, enyne-ketenimines and enyne-allenes, we have computed the reaction profiles of the C(2)-C(6) and of the C(2)-C(7) cyclisations for various model compounds, assuming that the reactions take place on the lowest-lying triplet surfaces. Our results nicely explain the differences and the unexpected high efficiency found for the enyne-carbodiimides. The differences between enyne-carbodiimides and enyne-ketenimines prove to be due to differences in the shapes of the corresponding triplet surfaces. In contrast to the enyne-carbodiimides, for which our calculations predict that a direct cyclisation to the biradical intermediates should occur after the vertical excitation, the enyne-ketenimines relax into a local minimum on the triplet surface. As a consequence, further reaction channels are opened. Our computations indicate that enyne-allene compounds do not react because the necessary excitation energy lies outside the range of the employed triplet photosensitizer. Finally, the close agreement between our results and the experimental findings indicates that the underlying reasons for the differences in the photochemical behaviour of enyne-carbodiimides, enyne-ketenimines and enyne-allenes are related to differences in the electronic structures of the parent systems, while substituent effects are less important.

Photochemical Myers−Saito and C2−C6 Cyclizations of Enyne−Allenes: Direct Detection of Intermediates in Solution

“…These cyclizations occur very effectively along the C 2 −C 6 , but interestingly not along the C 2 −C 7 (Myers−Saito) path. Theoretical studies by Engels explained the preference for the C 2 −C 6 route and suggested, as did our experimental results, that the process is initiated by triplet sensitization. Moreover, for a photochemical enyne−allene cyclization to be successful, the study recommended to avoid benzannulated derivatives due to their high excitation energy .…”

The thermal C2–C6 (Schmittel)/ene cyclization of enyne–allenes – crossing the boundary between classical and nonstatistical kinetics

“…As to what concerns the latter processes, they may constitute an interesting terra incognita for nonstatistical dynamic effects as well. Because the photochemical initiation of the C 2 –C 6 cyclization of enyne–allenes via triplet excited states has equally been identified to be controlled by nonstatistical effects, the well established photocyclization of enyne–(hetero)allenes may open further interesting venues for studying nonstatistical behavior.…”

(Hetero)aromatics from dienynes, enediynes and enyne–allenes

“…[29][30][31] Heteroatomsubstituted enyne-allene analogues undergo similar cyclizations, as in the case of the aza-Myers reaction 32 or in the Moore cyclization of enyne-ketenes (1d, Scheme 2d). 33 Similar processes take place in enyne-isocyanates, 34 enyne-ketenimines 35 and enynecarbodiimides. 36 The Schmittel reaction (occurring in enyneallenes via a C 2 -C 6 mode) gives a non-aromatic biradical, [37][38][39][40] but several examples describe a subsequent intramolecular reaction involving a phenyl-substituent, likewise leading to benzocondensed aromatics.…”