Deprotonation of [Mo(COMe)(CO)2(PPh2H)(η-C5H5)] and reaction with activated alkynes: controllable formation of vinylphosphine and η3-acryloyl ligands

Abstract

Deprotonation of the secondary phosphine ligand in the complex [Mo(COMe)(CO)₂(PPh₂H)(η-C₅H₅)] 1 with DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) at −78 °C followed by reaction with DMAD (dimethyl acetylenedicarboxylate, MeO₂CCCCO₂Me) afforded the vinylphosphine complex trans-[Mo(COMe)(CO)₂{PPh₂C(CO₂Me)CHCO₂Me}(η-C₅H₅)] 2a after protonation. The crystal structure of this compound confirms that the phosphine ligand is formed exclusively as the Z isomer. A similar reaction employing methyl propiolate afforded an analogous product [Mo(COMe)(CO)₂(PPh₂CHCHCO₂Me)(η-C₅H₅)] 2b with complete regioselectivity but less stereoselectivity in that three isomers (trans-E, trans-Z and cis-E) are formed in a ratio of 9.4∶2.8∶1. Deprotonation of 1 at room temperature, which has previously been shown to form the anion [Mo(CO)₂(PPh₂COMe)Cp]⁻ by migration of the acetyl group to phosphorus, followed by treatment with DMAD and rapid addition of acid produces the acryloyl complex [Mo{COC(CO₂Me)CHCO₂Me}(CO)(PPh₂COMe)(η-C₅H₅)] 3a, accompanied by the chelating vinyl species [Mo{C(CO₂Me)CHCOOMe}(CO)₂(η-C₅H₅)] 4. A similar reaction with methyl propiolate gave [Mo(η³-COCHCHCO₂Me)(CO)(PPh₂COMe)(η-C₅H₅)] 3b but in this case the isomeric vinyl complex trans-[Mo(CHCHCO₂Me)(CO)₂(PPh₂COMe)(η-C₅H₅)] 5 was formed as the minor product. Finally, deprotonation of 1 at room temperature followed by treatment with DMAD without subsequent addition of acid gives the metallacycle [Mo{PPh₂COCHC(CO₂Me)}(CO)₂(η-C₅H₅)] 6.