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

Abstract

The [2 + 2 + 2] cycloaddition of 1,5-bisallenes and alkynes under the catalysis of Rh(I) with hemilabile thioetherfunctionalized N-heterocyclic carbene ligands is described. This protocol effectively provides an entry to different trans-5,6-fused bicyclic systems with two exocyclic double bonds in the cyclohexene ring. The process is totally chemoselective with the two internal double bonds of the 1,5-bisallenes being involved in the cycloaddition. The complete mechanism of this transformation as well as the preference for the trans-fusion over the cis-fusion has been rationalized by density functional theory calculations. The reaction follows a typical [2 + 2 + 2] cycloaddition mechanism. The oxidative addition takes place between the alkyne and one of the allenes and it is when the second allene is inserted into the rhodacyclopentene that the trans-fusion is generated. Remarkably, the hemilabile character of the sulfur atom in the N-heterocyclic carbene ligand modulates the electron density in key intermediates, facilitating the overall transformation.

Intermolecular Allene–Alkyne Coupling: A Significantly Useful Synthetic Transformation

“…Very recently, an additional effort by the same research group was made on designing a Rh(I) catalytic system with a hemilabile thioether-functionalized N-heterocyclic carbene to synthesize trans-5,6-fused bicyclic systems with two exocyclic double bonds in the cyclohexene ring (41g) using a catalyst loading amount of 5 mol % (Scheme 41B). 96 Interestingly, this catalytic system exhibits unique chemoselectivity favoring a [2 + 2 + 2] cycloaddition in which the two internal double bonds of 1,5-bisallenes are involved in cycloaddition with one molecule of ADC (2b). Via DFT calculations, the authors proposed a reaction mechanism, which is a typical [2 + 2 + 2] cycloaddition mechanism, where the first oxidative addition step takes place between the alkyne and distal double bond of one of the allenes via the S-chelated Rh-complex 41h.…”

Recent Progress in Developing Thioether-Containing Ligands for Catalysis Applications

“…Very recently, within the framework of a collaboration between the Roglans and Bellemin-Laponnaz groups, a Rh(I) complex with hemilabile thioether-tunctionalized NHC was developed. 61 The rhodium complex X catalyzed [2 + 2 + 2] cycloaddition of 1,5bisallenes and alkynes to form bicyclic trans-3,6dimethylenecyclohex-1-ene trans-51 with high selectivity and good yield (Scheme 27). Noteworthy the cycloisomerization/Diels-Alder cascade product 52 was not observed under these conditions.…”

“…The Sadamantyl functionality modulates electron density by facilitating oxidative cyclometallation through its coordination and the final reductive elimination step through its dissociation from the rhodium center. 61 The coordination of sulphur atom imposes a new geometry for the Rh complex which brings the ligands closer to each other and, which will subsequently impose the formation of the trans-fused rhodabicyclo intermediate via the transition state TStrans (Scheme 27). Pàmies, Diéguez and co-workers developed chiral Ir/thioether-NHC (Y) complexes with a six-membered (κ 2 -C,S)-chelate and applied them to asymmetric hydrogenation reactions.…”

Aluminum Complexes Bearing Aminoquinoline Ligands with Different Pendant Donors: Remarkable Impact of Hemilability on the Ring-Opening Polymerization of ε-Caprolactone

“…Aliphatic polyesters have received increasing attention because of growing concerns for a sustainable environment. − Among them, poly(ε-caprolactone) (PCL) displays appropriate degradability, excellent biocompatibility, and interesting permeability toward water and small molecules, which make it one of the most popular materials for medical devices, tissue engineering, and drug delivery. , The superior properties of PCL highly rely on a well-defined structure, predictable molar mass ( M n ), low dispersity ( Đ ), and desirable end groups and are best achieved by ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) under mild conditions. − Although metal-based catalysts are predominant in this field because of their high activity, selectivity, and diversity, − competitive organocatalysts and dual catalytic systems have recently emerged. − Among numerous metal-based catalysts, Al complexes are especially attractive because of the large abundance of this element in the earth’s crust and its low cost and high Lewis acidity. , Moreover, both the catalytic activity and selectivity of Al complexes can be much enhanced by tuning the coordination environment of the metal center by introducing suitable ancillary ligands. − This accounts for the development of numerous bidentate or multidentate ligands containing different chemical functionalities such as hard (O– or N–) and soft (S– or P–) donors. Such hybrid ligands often display hemilabile properties of catalytic relevance owing to their ability to facilitate substrate coordination by opening a coordination site and to stabilize reactive intermediates in the transition state. − This concept is now widely applied in a number of catalytic reactions, including alkylation, cross-coupling, carbonylation, hydroformylation, hydrogenation, olefin polymerization, and ring-opening metathesis polymerization. − However, there are only a few examples of ROP catalysts supported by hemilabile ligands. For instance, the cationic amino-phenolate Al complex (Chart...…”