Stoichiometric cyclotrimerisation of chiral alkynes at a ruthenium centre: preparation of optically active (η6-arene)(η4-cycloocta-1,5-diene)ruthenium(0) complexes

The chiral alkynes (S)-MeCH(R)CCH, 2 (R=Et, 3-methyl-1-pentyne, a; iPr, 3,4-dimethyl-1-pentyne, b; tBu, 3,4,4-trimethyl-1-pentyne, c), containing a stereogenic centre in α position to the triple bond, react at room temperature with the complex Ru(η6-naphthalene)(η4-COD), 1, to give the corresponding optically active complexes Ru{η6-(S)-1,3,5-C6H3[CH(Me)R]3}(η4-COD), 6, and Ru{η6-(S)-1,2,4-C6H3[CH(Me)R]3}(η4-COD), 7, the η6-1,3,5-arene regioisomer being the prevalent product. With (S)-2a, a mixture of 6a and 7a (6a/7a=90:10) is obtained and, with the more sterically demanding alkynes (S)-2b and (S)-2c, the regioselectivity to the corresponding complexes 6b and 6c is almost complete. This synthetic procedure does not involve the stereogenic centres on the alkynes and it proceeds with complete stereoselectivity.