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···B(C6F5)3, t-Bu3P···B(C6F5)3 and Lut···(C6F5)3 reacted with acetylene (HCCH) to yield the apparently thermodynamically more stable E isomers [TMPH][(C6F5)2B−C(C6F5)C(H)B(C6F5)3] (1-E), t-Bu3PC(H)C(H)B(C6F5)3 (2-E; 90%), and [t-Bu3PH][(C6F5)2B−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 a deprotonation of this species with any mentioned Lewis bases (LB), yielding the acetylide salts [LBH][(C6F5)3BC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)3CCH] salts with a second equivalent of B(C6F5)3 to undergo electrophilic addition, forming the vinylidene adduct (C6F5)3B−C+C(H)B(C6F5)3, which is subsequently stabilized by 1,2-migration of a C6F5 group to form [(C6F5)2BC(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···B(C6F5)3 and tBu3P···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···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.