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.

No abstract

The frustrated Lewis pair (FLP) derived from ClB(C6F5)2 and the bulky Lewis bases 2,2,6,6-tetramethylpiperidine (TMP), tri-tert-butylphosphine, and tris(2,4,6-trimethylphenyl)phosphine cleaved H2 heterolytically to form the intermediate anion [HClB(C6F5)2]−, which quickly underwent hydride/chloride exchange with the remaining ClB(C6F5)2 to give the known compound [HB(C6F5)2] n (n = 1 or 2) and the anion [Cl2B(C6F5)2]− present in the products [TMPH][Cl2B(C6F5)2] (1a), [t-Bu3PH][Cl2B(C6F5)2] (2a), and [Mes3PH][Cl2B(C6F5)2] (3a). [HB(C6F5)2] n forms Lewis adducts with TMP and t-Bu3P: TMP-BH(C6F5)2 (1b) and t-Bu3P-BH(C6F5)2 (2b). The Lewis adduct t-Bu3P-BH(C6F5)2 was found capable of generating a FLP at elevated temperature and was reacted with H2, producing the splitting product [t-Bu3PH][H2B(C6F5)2] (2c). Mes3P forms no Lewis adduct with [HB(C6F5)2] n , but a FLP, which was also capable of splitting H2 to yield initially [Mes3PH][H2B(C6F5)2]. The [H2B(C6F5)2]− anion underwent disproportionation to form [Mes3PH][HB(C6F5)3] (3b), Mes3P, [H2B(C6F5)]2, and H2. Similarly, 2,4,6-tri-tert-butylpyridine (TTBP) and [HB(C6F5)2] n gave in the presence of H2 the final products [TTBPH][HB(C6F5)3] salt and [H2B(C6F5)]2. The contrasting reactivities of the t-Bu3P/[BH(C6F5)2] n , Mes3P/[HB(C6F5)2] n , and TTBP/[HB(C6F5)2] n pairs were explained on the basis of the different pK a’s of the [LBH]+ cations. After disproportionation of the [H2B(C6F5)2]− anion to give [Mes3PH][HB(C6F5)3] (3b) or [TTBPH][HB(C6F5)3] (4a), the also formed [H3B(C6F5)]− anion reacted with the more acidic cations ([Mes3PH]+, [TTBPH]+) to give H2 and syn- and anti-[H2B(C6F5)]2 (3c). 1a, 2a, 3a, and 4a were studied by single-crystal X-ray diffraction analysis.

Reactions of the perfluoroarylboranes RB(C(6)F(5))(2) (R = C(6)F(5), Ph, Cl, OC(6)F(5)) with Me(3)SiCH(N(2)), (C(6)F(5))CH(N(2)) or Ph(2)C(N(2)) yield (C(6)F(5))(2)B(Me(3)SiCH(C(6)F(5))) 1, (C(6)F(5))B(Me(3)SiCH(C(6)F(5)))(2) 2, (C(6)F(5))B(Me(3)SiCH(C(6)F(5)))(Me(3)SiCH(C(6)H(5))) 3, (C(6)F(5))(2)B(CH(C(6)F(5))(2)) 4, ClB(C(6)F(5))(Ph(2)C(C(6)F(5))) 5 and (C(6)F(5)O)B(C(6)F(5))(Me(3)SiCH(C(6)F(5))) 6 as a result of single or double insertion of a Me(3)SiCH, C(6)F(5)CH or Ph(2)C fragment into a B-C bond of the respective borane. Reactions of one or two equivalents of ethyl α-diazomethylacetate with B(C(6)F(5))(3) yielded (Me)(C(6)F(5))(C=C)(OC(2)H(5))(OB(C(6)F(5))(2)) 8 and [(Me)(C(6)F(5))(C=C)(OC(2)H(5))](2)(O(2)B(C(6)F(5))) 9, in addition to the corresponding pyridine adducts (Me)(C(6)F(5))(C=C)(OC(2)H(5))(OB(C(6)F(5))(2))(py) 10 and [(Me)(C(6)F(5))(C=C)(OC(2)H(5))](2)(O(2)B(C(6)F(5)))(py) 11. Similarly, reaction of α-diazomethylacetate with BPh(3) yielded analogous products of borane reorganization, (Me)(C(6)H(5))(C=C)(OC(2)H(5))(OBPh(2)) 12 and was isolated as a mixture of E and Z-isomers whereas BPh(3) reacts with Me(3)SiCH(N(2)) and pyridine yielding (py)B(Ph(2)(Me(3)SiCH(Ph)) 7. Reactions of Ph(2)C(N(2)) with RB(OH)(2) (R = C(6)F(5), p-F-C(6)H(4), C(6)H(5)) yielded cyclic boroxines of the form [Ph(2)C(R)BO](3) (R = C(6)F(5) 13, p-FC(6)H(4) 14, C(6)H(5) 15) while reactions of the boronate esters (C(6)H(4)O(2))BR (R = C(6)F(5), p-F-C(6)H(4)) with three or five equivalents of Me(3)SiCH(N(2)) yielded (C(6)H(4)O(2))B(Me(3)SiCH(Ar)) (Ar = C(6)F(5) 16, p-F-C(6)H(4) 17) and [(Py)B(C(6)H(4)O(2))(Me(3)SiCH(Ar))] (Ar = C(6)F(5) 18, p-F-C(6)H(4), 19) upon complexation with pyridine. Reaction of HBCat and ClBCat with Ph(2)C(N(2)) yielded the products of B-H and B-Cl bond derivatization (C(6)H(4)O(2))B(Ph(2)CR) (R = H 20, Cl 21), while the triethylphosphine oxide adduct (Et(3)PO)B(C(6)H(4)O(2))(CPh(2)Cl) 22, is readily isolable.

The concept of frustrated Lewis pairs (FLPs) was put forth by D. W. Stephan et al. after their remarkable discovery that H 2 can reversibly be activated by [(2,4,6-C 6 H 2 Me 3 ) 2 PC 6 F 4 B(C 6 F 5 ) 2 ], 1 which led to development of the first metal-free catalyst for hydrogenations of bulky imines as one of the fruitful applications of this concept. 2 The steric congestion of Lewis donors and acceptors precludes the formation of classical Lewis adducts, but provokes formation of FLPs with "unquenched" reactivity toward small molecules. 3À5 For instance, the mixture of appropriate phosphines and boranes can activate H 2 heterolytically under often mild conditions 6 and can undergo 1,2-addition reactions with olefins, as well. 7,8 After the pioneering work of Stephan et al., an increasing number of related FLP systems were found. The frustrated Lewis pairs were extended from the initial boron/phosphine species 9À20 to boron/carbene systems and borane/amine species. 21À24 Some of the resulting ionic products were shown to serve as active catalysts for the hydrogenation of imines, nitriles, and aziridines, as well as enamines and silyl enol ethers. 12,24,25 However, the mechanism of the H 2 activation by FLPs is still not fully understood. Theoretical studies proposed that the Lewis donor and acceptor initially form an "encounter complex" with long nonbonding distances between the Lewis centers frequently supported by multiple CH 3 3 3 F interactions. Such relatively weak specific forces, as well as the global electrostatic field of the Lewis pair, caused in sum too small interaction energies to allow proper identification by conventional analytical methodologies. H 2 can insert into this encounter complex, being thus activated by heterolytic HÀH splitting. 26,27 Recent sophisticated DFT studies by Grimme et al., which included dispersion forces, pointed out that the intermediate formed between H 2 and for instance a P/B FLP could show kinetic stabilization and would thus be spectroscopically detectable under the condition that the P 3 3 3 B nonbonding distance is over 4.5 Å. Otherwise, the H 2 heterolysis would be practically barrierless, once the H 2 molecule had "sneaked" into the FLP complex. 28 Herein, we explore several sterically hindered Lewis bases, mainly piperidine and pyridine derivatives, to modulate the B 3 3 3 N nonbonding distance of the FLP and to study the impact of varying the B 3 3 3 N distance on their ability to activate H 2 .