Activation of Terminal Alkynes by Frustrated Lewis Pairs

Abstract

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.

Synthesis and Reactivity of Alkynyl‐Linked Phosphonium Borates

“…In a related fashion, the reaction of 13 with THF afforded the macrocycle [tBu 2 (17). In a related example, the reaction of Mes 2 PC CBA C H T U N G T R E N N U N G (C 6 F 5 ) 2 with PhC CH gave {[Mes 2 PC CB- (18).…”

“…[10] In addition, new synthetic routes to such zwitterionic derivatives have been developed from the reactions of sterically encumbered frustrated Lewis pairs (FLPs) with a variety of small molecules including olefins, [11][12][13][14] alkynes, [15][16][17][18][19] CO 2 , [20][21][22] and N 2 O. [23,24] In the case of the reactions of FLPs with alkynes, we have shown that a variety of Lewis bases, including phosphines, [15,17] polyphosphines, [16] amines, thioethers, [18] and pyrroles, [19] will effect addition reactions to give alkenyl-linked zwitterionic products.…”

A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes

“…It is known from previous publications that FLPs react with terminal alkynes via deprotonative borylation pathway, producing the respective onium alkynylborates 41,42,43,44,45,46 . Indeed, 6 reacts with hex-1-yne giving the respective adduct 31 (Scheme 6).…”

Design principles in frustrated Lewis pair catalysis for the functionalization of carbon dioxide and heterocycles

“…The same group also showed that some alkenes could undergo C-H bond cleavage using the combination of aluminum compounds and Lewis bases [91]. Terminal alkynes can also be activated by suitable FLPs in a process that is somewhat reminiscent of the copper-assisted deprotonation of alkynes that is at the core of the Sonogashira cross-coupling [25,[92][93]. However, none of these C-H bond cleavage processes could be used for catalytic transformations.…”