SYNTHESIS, STRUCTURE, AND REACTIVITY OF CHIRAL RHENIUM ALKYNE COMPLEXES OF THE FORMULA [(ETA-5-C5H5)RE(NO)(PPH3)(RCCR')]+BF4-
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Reactions of [(6-C5H6)Re(NO)(PPh3)(ClC6H6)]+BF4 and RCCR (R = R = H (a), CH3 (b), CH2CH3 (c), C6H5 (d); R/R = C(CH3)3/H (e)) give alkyne complexes [(6-C5H5)Re(NO)(PPh3)(RCCR)]+BF4 (4a-e; 7397%). A crystal structure of 4c-ClCH2CH2Cl (orthorhombic, Pbca; a = 15.724 (1) , b = 18.593 (2) , c = 22.128 (2) , Z = 8) shows a 3-hexyne ligand conformation that directs the ethyl substituents syn and anti to the PPh3 ligand. The CC bond is only slightly longer (1.24 (1) ) than in free alkynes, but the propargylic carbons are bent back from the CC axis (CC-C = 143.4 (8), 147 (1)). NMR data show high barriers to alkyne ligand rotation (4b,c: G(180 C) 22 kcal/mol). Difference 1H NOE and decoupling experiments enable detailed assignments of NMR resonances. Reactions of 4a,e and t-BuOK+ give acetylide complexes (5-C5Hs)Re(NO)(PPh3)(CCR) (5a,e; 7497%). Complex 4e rearranges (80 C, C6H6C1,1 h) to vinylidene complex [(5-C5H5)Re(NO)(PPh3)(=C=CHC(CH3)3)]+BF4, a sample of which can be independently prepared from 5e and HBF4O(CH2CH3)2 (92%). The physical and chemical properties of 4a-e are compared to those of other alkyne complexes. 1991, American Chemical Society. All rights reserved.
