A Rh(I)‐Catalyzed Cascade Cyclization of 1,5‐Bisallenes and Alkynes for the Formation of cis‐3,4‐Arylvinyl Pyrrolidines and Cyclopentanes

Abstract

The cascade cyclization reactions of 1,5bisallenes grant access to a great variety of products by precisely tuning the catalyst system and/or the reagents involved. Herein, we present our findings that 1,5-bisallenes react with two molecules of dimethyl acetylenedicarboxylate to afford, in a completely diastereoselective manner, cis-3,4-arylvinyl pyrrolidines and cyclopentanes. DFT calculations have been used to postulate a mechanism for the developed reaction which encompasses a [2 + 2 + 2] cycloaddition reaction of the two alkynes and the external double bond of one allene, followed by a cycloisomerization involving the internal double bond of the second allene.

Rh(I) Complexes with Hemilabile Thioether-Functionalized NHC Ligands as Catalysts for [2 + 2 + 2] Cycloaddition of 1,5-Bisallenes and Alkynes

“…The reaction with L1 afforded three products, which were identified by NMR spectroscopy. In striking contrast to our previous studies with the same substrates (Scheme b), a mixture of diastereoisomers trans and cis 3a in 35% yield with a ratio of about 5:1 was formed by a [2 + 2 + 2] cycloaddition reaction between the two internal double bonds of the two allenes of 1 and the alkyne (entry 1), also in contrast to the case of Ma (Scheme a) in which both an internal and an terminal double bond were involved in the reaction . The molecular structure and stereochemistry of the major diastereoisomer 3a , which was found to be trans , was confirmed by X-ray crystallographic analysis (Figure ).…”

“…It should be noted that bisphosphine ligands such as BINAP, Tol-BINAP, BIPHEP, DPEphos, Xantphos, and XPhos were not able to provide a defined product upon reaction of 1a and 2a as investigated in a previous study of our group. 10 The scope of the reaction was then evaluated (Scheme 4). Methyl, ethyl, and tert-butyl acetylenedicarboxylates 2a, 2b, and 2c were tested in the cycloaddition affording excellent yields and high trans/cis ratios 34 of the corresponding cycloadducts 3a−3c.…”

“…It should be noted that bisphosphine ligands such as BINAP, Tol-BINAP, BIPHEP, DPEphos, Xantphos, and XPhos were not able to provide a defined product upon reaction of 1a and 2a as investigated in a previous study of our group. 10 …”

“…Further mechanistic study by DFT calculations showed that the reaction took place through a rhodium-catalyzed cycloisomerization/Diels–Alder cascade encompassing oxidative coupling of the rhodium to the central carbon atoms of both allenes (intermediate I ) followed by a β-hydride elimination (intermediate II ) and reductive elimination of the rhodium to afford a non-isolable cycloheptatriene derivative III , which gave a further Diels–Alder reaction with the alkene (Scheme a). In contrast, when an alkyne was used as a third component using the same catalytic system as before, 1,5-bisallenes reacted with two molecules of the alkyne to afford cis -3,4-arylvinyl pyrrolidines and cyclopentanes in a totally diastereoselective manner . Here again, DFT calculations allowed us to unveil the course of the reaction, which involves a [2 + 2 + 2] cycloaddition between two molecules of the alkyne and the terminal double bond of one of the two allenes of the bisallene (intermediate IV ) followed by a cycloisomerization reaction involving the internal double bond of the second allene unit (intermediate V ).…”

“…In contrast, when an alkyne was used as a third component using the same catalytic system as before, 1,5-bisallenes reacted with two molecules of the alkyne to afford cis -3,4-arylvinyl pyrrolidines and cyclopentanes in a totally diastereoselective manner. 10 Here again, DFT calculations allowed us to unveil the course of the reaction, which involves a [2 + 2 + 2] cycloaddition between two molecules of the alkyne and the terminal double bond of one of the two allenes of the bisallene (intermediate IV ) followed by a cycloisomerization reaction involving the internal double bond of the second allene unit (intermediate V ). Finally, a β-hydride elimination step followed by reductive elimination afforded the final product ( Scheme 2 b).…”