Unusual Reductive Coupling of Alkynes and Ketones: Reactivity of Titanacycles Supported by Dimethylsilylcalix[4]arene (DMSC) Ligands

Reaction of [(DMSC)Ti{1,2,4-(Me 3 Si) 3 C 6 H 3 }] ( 1 ) with 1 equiv of bipyridine (bpy) or 4,4‘-dimethyl-2,2‘-dipyridyl (dmbpy) and a slight excess of RC⋮CH quantitatively produced [(DMSC)Ti(η 2 -RC⋮CH)(L 2 )] (L 2 = bpy or dmbpy) ( 5 − 16 ). (DMSC)TiPh 2 ( 17 ) reacted with ≥2 equiv of bpy or dmbpy to give (DMSC)Ti(bpy) 2 ( 18 ) and (DMSC)Ti(dmbpy) 2 ( 19 ), respectively. Both 1 H and 13 C NMR data, as well as X-ray crystallography in the case of [(DMSC)Ti(η 2 -HC⋮CBu t )(bpy)] ( 7 ), support exo-orientation of the alkyne's non-H substituent in 5 − 16 . Reaction of [(DMSC)Ti{1,2,4-(Me 3 Si) 3 C 6 H 3 }] ( 1 ) with a mixture of RC⋮CH and R 2 CO did not give the expected 5-oxa-1-titana-2-cyclopentene products but instead produced 2,7-dioxa-1-titana-4-cycloheptenes ( 20 − 25 ). The latter result may be understood in terms of the unique directing influence of the DMSC ligand.