The Role of Spacer Length in Macrocyclization Reactions Under Confinement

Abstract

We studied the influence of the distance of olefin metathesis catalysts from the inner surface of a mesoporous support on macrocyclization and Z-selectivity under confinement. For these purposes, the cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) catalysts [Mo(N-(2-^tBu-C₆H₄)(1-mesityl-3-(3-trimethoxysilylprop-1-yl)-imidazol-2-ylidene)(CHCMe₂Ph)(MeCN)Br⁺ B(Ar^F)₄⁻] Mo2, [Mo(N-(2-^tBu-C₆H₄)(1-mesityl-3-(3-trimethoxysilylprop-1-yl)-imidazol-2-ylidene)(CHCMe₂Ph)(MeCN)OTf⁺ B(Ar^F)₄⁻] Mo3, [Mo(N-(2,6-Me₂-C₆H₃)(1-mesityl-3-(3-trimethoxysilylprop-1-yl)-imidazol-2-ylidene)(CHCMe₂Ph)(MeCN)Br⁺ B(Ar^F)₄⁻] Mo5, and [Mo(N-(2,6-iPr₂-C₆H₃)(1-mesityl-3-(3-trimethoxysilylprop-1-yl)-imidazol-2-ylidene)(CHCMe₂Ph)(MeCN)⁺Br B(Ar^F)₄⁻] Mo7 (B(Ar^F)₄ = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), all containing a trimethoxysilylpropyl tether, were selectively immobilized inside the mesopores of SBA-15. Under confinement, both macro(mono)cyclization (MMC) and Z-selectivity were higher than in solution but lower than with catalysts directly bound to the surface of the mesoporous supports. These findings are in agreement with existing theoretical models on substrate and product distribution in mesopores, which suggest that the highest substrate concentration is found at the pore wall and that it increases with decreasing pore diameter.