A Hetero Diels–Alder Concerted vs. Aldol Stepwise Mechanism in the Cyclization of Silyloxyazadienes with Aldehydes: A Theoretical Study

Abstract

The Diels–Alder reaction of 3‐silyloxy‐2‐azadiene with formaldehyde has been studied at the B3LYP/6‐31G* level of theory and the role played by a Lewis acid evaluated. It is shown that the reaction is preferred when the Lewis acid coordinates to the aldehyde oxygen. This coordination allows for a concerted as well as a stepwise mechanism for the cyclization. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

The Participation of 2H‐Pyran‐2‐ones in [4+2] Cycloadditions: An Experimental and Computational Study

“…The higher CT and global reactivity indices confirmed the polar nature of the reaction and identified ynamine 2 as a nucleophilic species in these polar DA reactions. The observed mechanism has parallels in the stepwise mechanism studied by Jorgensen et al in the enzymatic environment of the macrophomate synthase, where evidence was presented that the Diels-Alder transition state is less stable than the Michael-aldol transition state, [10c] as well as in recent examples of the competition between a polar Diels-Alder and a two-step reaction described by Bongini and Panunzio et al [28] The unexpected formation of the minor products 4a-d during the reaction of 1 with 2 can be rationalized by considering the low torsion barrier around the newly formed C-C bond, which enables two different cycli-zation modes. Singleton et al [29] have employed a dynamic trajectory approach [30] to point out the possibility of nonstatistical recrossing to explain the yield of minor products.…”

Schiff Bases: A Short Survey on an Evergreen Chemistry Tool

“…They have been found synthetic applications in the last few decades as relatively stable analogues of the corresponding Schiff bases. Their elaborations to azadiene have been fully explored by the Barluenga [60,61,62], Ghosez [63,64] and Panunzio groups [65,66,67,68,69]. Generally speaking, they are monomeric compounds reasonably stable under anhydrous conditions.…”

Reactions of Aldehydes and Ketones and their Derivatives

“…349 3-Silyloxy-2-aldehydes can undergo hetero-Diels-Alder reaction with aldehydes to give useful heterocycles. 350 A model reaction, H 2 C=C(OSiH 3 )-N=CH 2 with formaldehyde, has been explored theoretically. Lewis acids such as boron trifluoride catalyse the reaction by coordinating to the aldehyde oxygen, making the aldehyde more electrophilic.…”