Metal-Mediated Cycloaddition Reactions of the η⁵-Cyclohexadienyl Ligand. Synthesis and Reactions of [5 + 2] Single and [5 + 2],<i>homo</i>[5 + 2] Double Cyclohexadienyl−Alkyne Adducts of Manganese

Abstract

UV irradiation of the cyclohexadienyl complex [(η 5 -C 6 H 7 )Mn(CO) 3 ] ( 1 ) and 1 or 2 equiv of the alkynes PhC⋮CR (R = H, Me, Ph) in toluene gave both the [5 + 2] adducts [{η 2: 3 -C 8 H 7 Ph(R)}Mn(CO) 3 ] ( 2 ) and the tricyclic [5 + 2], homo [5 + 2] double adducts [(η 1:2:2 -C 10 H 7 Ph 2 R 2 )Mn(CO) 3 ] 3a − d . The reactions involve sequential metal-mediated alkyne cycloadditions, first across the η 5 -dienyl ligand and then across the resulting olefin−allyl manifolds. Reaction of the monoadducts 2 with an additional equivalent of 2-butyne or 1-phenyl-1-propyne gave the mixed double adducts 3e − j . The THF complex [(η 5 -C 6 H 7 )Mn(CO) 2 (THF)] ( 4 ) reacted thermally with 1-phenyl-1-propyne to form both types of adducts 2 and 3, indicating the role of UV light is to generate a coordinatively unsaturated metal center by CO ejection. Photolysis of 1 and dimethyl acetylenedicarboxylate gave the organic double adduct 1,2,3,10-tetrakis(methoxycarbonyl)tricyclo[5.2.1.0 4,9 ]deca-2,5-diene ( 5a ), whereas protonation of 3a, c using HBF 4 ·Et 2 O gave the analogous organic tricyclodienes 5b, c . Reaction of the adducts [(η 1:2:2 -C 10 H 7 R 4 )Mn(CO) 3 ] ( 3a, 3k R = Me; 3l R = Et) with carbon monoxide (200 psi; 100 °C) gave new CO insertion products 6a − c . The single addition product 2c, the double adducts 3a and 3j, and the CO insertion products 6b, c have been characterized by X-ray diffraction studies.