Stoichiometric cyclotrimerisation of chiral alkynes at a ruthenium centre: preparation of optically active (eta(6)-arene)(eta(4)-cycloocta-1,5-diene)ruthenium(0) complexes

The chiral alkynes (S)-MeCH(R)-C drop CH. 2 (R = Et, 3-methyl-1-pentyne, a; 'Pr, 3,4-dimethyl-1-pentyne, b;'Bu, 3,4,4-trimethyl-1-pentyne, c), containing a stereogenic centre in alpha position to the triple bond, react at room temperature with the complex Ru(eta (6)-naphthalene)(eta (4)-COD). 1. to give the corresponding optically active complexes Ru (eta (6)-(S)-1, 3, 5-C6H3[CH(Me)R](3)} (eta (4)-COD), 6, and Ru(eta (6)-(S)-1,2,4-C6H3[CH(Me)R](3)}(eta (4)-COD), 7. the eta (6)-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.