Activation of Terminal Alkynes by Frustrated Lewis Pairs

The reactions of frustrated Lewis pairs (FLPs) derived from B(C6F5)3 and the bulky Lewis bases 2,2,6,6-tetramethylpiperidine (TMP), tri-tert-butylphosphine, and lutidine (Lut) with terminal alkynes (acetylene, phenylacetylene, 3-ethynylthiophene) were investigated. The FLPs TMP center dot center dot center dot B(C6F5)(3), t-Bu3P center dot center dot center dot B(C6F5) and Lut center dot center dot center dot(C6F5) reacted with acetylene (HC CH) to yield the apparently thermodynamically more stable E isomers [TMPH][(C6F5)(2)B-C(C6F5)=C(H)B(C6F5)(3)] (1-E), t-Bu3PC(H)=C(H)B(C6F5)(3) (2-E; 90%), and [t-Bu3PH][(C6F5)(2)B-C(C6F5)=C(H)B(C6F5)(3)] (3-E; 10%),and LutC(H)=C(H)B(C6F5)(3) (4-E), respectively. A mechanistic pathway for the reaction of acetylene is suggested to start with the formation of a weak B(C6F5)(3)/acetylene adduct followed by it deprotonation of this species with any mentioned Lewis bases (LB), yielding the acetylicle salts [LBH][(C6F5)(3)BC CH]. Alternatively, nucleophilic addition of the LB to this adduct occurs to yield LBC(H)=C(H)B(C6F5)(3) compounds. Formation of 1 and 3-E is explained by the reactions of [LBH][B(C6F5)(3)C CH] salts with a second equivalent of B(C6F5)(3) to undergo clectrophilic addition, forming the vinylidene adduct (C6F5)(3)B-C+=C(H)B(C6F5)(3), which is subsequently stabilized by 1,2-migration Of a C6F5 group to form [(C6F5)(2)BC(C6F5)=C(H)B(C6F5)(3)]. The reaction between B(C6F5)(3) and phenylacetylene yielded a mixture of (Z)- and (E)-PhC(H)=C(C6F5)B(C6F5)(2) (11-Z and 11-E), confirming that the reaction proceeds via an acetylene/vinylidene rearrangement and subsequent 1,2-shift of a C6F5 group to the carbenic center. The FLPs TMP center dot center dot center dot B(C6F5)(3) and tBu(3)P center dot center dot center dot B(C6F5)(3) were converted with phenylacetylene or 3-ethynylthiophene to yield the acetylide products [TMPH][PhC CB(C6F5)3] (5), [TMPH][SC4H3C CB(C6F5)(3)] (6), [t-BU3PH][PhC CB(C6F5)(3)] (7), and [t-Bu3PH][SC4H3C=CB(C6F5)3] (8), where TMP and t-Bu3P acted as a base deprotonating the acetylenic proton. When the FLP Lut center dot center dot center dot B(C6F5)(3) was reacted with phenylacetylene or 3-ethynylthiophene, the deprotonated product [LutH][PhC CB(C6F5)(3)] (9; 47%) and the 1,2-addition compound LutC(SC4H3C)=C(H)B(C6F5)(3) (10; 55%) were obtained. Compounds 1-E, 2-E, 5, and 6 were characterized by X-ray diffraction studies.