pubs.acs.org/Organometallics Published on Web 12/11/2009 r 2009 American Chemical Society Organometallics 2010, 29, 125–133 125 DOI: 10.1021/om9008636 Activation of Terminal Alkynes by Frustrated Lewis Pairs Chunfang Jiang, Olivier Blacque, and Heinz Berke* Anorganisch-chemisches Institut, Universit € at Z€ urich, Winterthurerstrasse 190, CH-8057 Z€ urich, Switzerland Received October 5, 2009 The reactions of frustrated Lewis pairs (FLPs) derived from B(C 6 F 5 ) 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 333 B(C 6 F 5 ) 3, t-Bu 3 P 333 B(C 6 F 5 ) 3 and Lut 333 (C 6 F 5 ) 3 reacted with acetylene (HCtCH) to yield the apparently thermodynamically more stable E isomers [TMPH][(C 6 F 5 ) 2 B-C(C 6 F 5 )dC(H)B(C 6 F 5 ) 3 ] (1-E), t-Bu 3 PC(H)dC(H)B(C 6 F 5 ) 3 (2-E; 90%), and [t-Bu 3 PH][(C 6 F 5 ) 2 B-C(C 6 F 5 )dC(H)B(C 6 F 5 ) 3 ] (3-E; 10%), and LutC(H)dC(H)B(C 6 F 5 ) 3 (4-E), respectively. A mechanistic pathway for the reaction of acetylene is suggested to start with the formation of a weak B(C 6 F 5 ) 3 /acetylene adduct followed by a deprotonation of this species with any mentioned Lewis bases (LB), yielding the acetylide salts [LBH][(C 6 F 5 ) 3 BCtCH]. Alternatively, nucleophilic addition of the LB to this adduct occurs to yield LBC(H)dC(H)B(C 6 F 5 ) 3 compounds. Formation of 1 and 3-E is explained by the reactions of [LBH][B(C 6 F 5 ) 3 CtCH] salts with a second equivalent of B(C 6 F 5 ) 3 to undergo electrophilic addition, forming the vinylidene adduct (C 6 F 5 ) 3 B - C þ dC(H)B(C 6 F 5 ) 3 , which is subsequently stabilized by 1,2-migration of a C 6 F 5 group to form [(C 6 F 5 ) 2 BC(C 6 F 5 )dC(H)B(C 6 F 5 ) 3 ]. The reaction between B(C 6 F 5 ) 3 and phenylacetylene yielded a mixture of (Z)- and (E)-PhC(H)dC(C 6 F 5 )B(C 6 F 5 ) 2 (11-Z and 11-E), confirming that the reaction proceeds via an acetylene/vinylidene rearrangement and subsequent 1,2-shift of a C 6 F 5 group to the carbenic center. The FLPs TMP 333 B(C 6 F 5 ) 3 and tBu 3 P 333 B(C 6 F 5 ) 3 were converted with phenylacetylene or 3-ethynylthiophene to yield the acetylide products [TMPH][PhCtCB(C 6 F 5 ) 3 ] (5), [TMPH][SC 4 H 3 CtCB(C 6 F 5 ) 3 ] (6), [t-Bu 3 PH][PhCt CB(C 6 F 5 ) 3 ](7), and [t-Bu 3 PH][SC 4 H 3 CtCB(C 6 F 5 ) 3 ](8), where TMP and t-Bu 3 P acted as a base deprotonating the acetylenic proton. When the FLP Lut 333 B(C 6 F 5 ) 3 was reacted with phenylacety- lene or 3-ethynylthiophene, the deprotonated product [LutH][PhCtCB(C 6 F 5 ) 3 ](9; 47%) and the 1,2-addition compound LutC(SC 4 H 3 C)dC(H)B(C 6 F 5 ) 3 (10; 55%) were obtained. Compounds 1-E, 2-E, 5, and 6 were characterized by X-ray diffraction studies. I. Introduction The strong Lewis acid tris(pentafluorophenyl)boron is capable of strongly polarizing hydrogen and preferential sp and sp 2 carbon centers, shaping them for further reactivities. In the realm of metallocene Ziegler-type polymerization catalysis metal-carbon bonds were seen to be heterolytically cleaved by B(C 6 F 5 ) 3 , leaving vacant sites behind. In addition, electrophilic carbon centers could be opened up by B(C 6 F 5 ) 3 addition to coordinated or noncoordinated π systems. 1-5 Moreover, the Lewis acid B(C 6 F 5 ) 3 was shown to be capable of inducing novel transformations 6,7 or was employed as a stoichiometric reagent to generate new types of organo- metallic and organic compounds. 8-10 More recently, Stephan et al. discovered that H 2 can be activated reversibly with heterolytic splitting by the “metal-free” internal Lewis pair [(2,4,6-Me 3 C 6 H 2 ) 2 PC 6 F 4 B(C 6 F 5 ) 2 ]. 11 On the basis of this finding “metal-free” catalysts could be developed for the hydrogenation of bulky imines with the Lewis acid B(C 6 F 5 ) 3 . 12 This type of reactivity required the formation of encounter complexes called frustrated Lewis pairs (FLPs), consisting of sterically hindered Lewis bases in combina- tion with sterically hindered Lewis acids and providing “unquenched” reactivity and prevention of Lewis pair *To whom correspondence should be addressed. 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