Xiaoxi Zhao

Olefin-borane "van der Waals complexes": intermediates in frustrated Lewis pair addition reactions.

The nature of the borane-olefin interactions that take place prior to frustrated Lewis pair addition reactions has been probed employing a Lewis acidic borane tethered to a vinyl group through an alkyl chain. (1)H{(19)F} HOESY spectral data obtained at -50 °C demonstrated the spatial proximity of the boryl and vinyl groups and computational data support the initial formation of a van der Waals borane-olefin complex. Such species serve as intermediates undergoing facile addition reactions with phosphine bases to afford cyclic zwitterionic products.

Frustrated Lewis pair olefin addition reactions: P-, N-, C- and H-based nucleophilic additions to an olefin-tethered borane

The electrophilic alkoxyborane, B(C6F5)2(OC(CF3)2CH2CHCH2) 2, was synthesized and the reactivity of the tethered olefinic fragment examined in frustrated Lewis pair (FLP) addition reactions. Treatment of 2 with tBu3P or Me3P afforded B(C6F5)2(OC(CF3)2CH2CHCH2)(PR3) (R = tBu 3, Me 4) with addition of the nucleophile to the internal carbon of the olefinic group. In contrast, reaction of 2 with bulky nitrogen-based nucleophiles, 2,6-lutidine or 2,2,6,6-tetramethylpiperidine, gave B(C6F5)2(OC(CF3)2CH2CHCH2)(NR′) (R′ = C5H3Me2N 5, NHC5H6Me46) where the nucleophile adds to the terminal carbon of the olefinic unit. Treatment with the carbon-based nucleophiles, 1,2,5-trimethylpyrrole, N-tert-butylpyrrole, 1,3-di-tert-butylimidazole-2-ylidene and benzylidene triphenylphosphorane, afforded the zwitterions B(C6F5)2(OC(CF3)2CH2CHCH2)(R′′) (R′′ = C4H2Me2NMe 7, C4H4NtBu 8, ItBu, 1,3-di-tert-butylimidazole-2-ylidene 9, P(CHPh)Ph310) which contained a new C–C bond at the internal carbon of the olefin. In the presence of 1,2,2,6,6-pentamethylpiperidine and a catalytic amount of B(C6F5)3, 2 reacts with H2 to convert to [HPMP][B(C6F5)2(OC(CF3)2CH2CH2CH2)] (PMP = 1,2,2,6,6-pentamethylpiperidine) 11. DFT calculations provide evidence that this latter reaction proceeds by heterolytic cleavage of H2 by B(C6F5)3 and 1,2,2,6,6-pentamethylpiperidine followed by transfer of the hydride from B(C6F5)3 to the internal carbon of the vinyl group of alkoxyborane 2.

Synthesis and reactivity of alkynyl-linked phosphonium borates.

The phosphine tBu(2 PC[triple bond]CH (1) was reacted with B(C(6)F(5)) to give the zwitterionic species tBu(2)P(H)C[triple bond]CB(C(6)F(5))(3) (2). The analogous species tBu(2)P(Me)C[triple bond]CB(C(6)F(5))(3) (3), tBu(2)P(H)C[triple bond]CB(Cl)(C(6)F(5))(2) (4), tBu(2)P(H)C[triple bond]CB(H)(C(6)F(5))(2) (5), and tBu(2)P(Me)C[triple bond]CB(H)(C(6)F(5))(2) (6) were also prepared. The salt [tBu(2)P(H)C[triple bond]CB(C(6)F(5))(2)(THF)][B(C(6)F(5))(4)] (7) was prepared through abstraction of hydride by [Ph(3)C][B(C(6)F(5))(4)]. Species 5 reacted with the imine tBuN=CHPh to give the borane-amine adduct tBu(2)PC[triple bond]CB[tBuN(H)CH(2)Ph](C(6)F(5))(2) (8). The related phosphine Mes(2)PC[triple bond]CH (9; Mes=C(6)H(2)Me(3)) was used to prepare [tBu(3)PH][Mes(2)PC[triple bond]CB(C(6)F(5))(3)] (10) and generate Mes(2)PC[triple bond]CB(C(6)F(5))(2). The adduct Mes(2)PC[triple bond]CB(NCMe)(C(6)F(5))(2) (11) was isolated. Reaction of Mes(2)PC[triple bond]CB(C(6)F(5))(2) with H(2) gave the zwitterionic product (C(6)F(5))(2)(H)BC(H)=C[P(H)Mes(2)][(C(6)F(5))(2)BC[triple bond]CP(H)Mes(2)] (12). Reaction of tBu(2)PC[triple bond]CB(C(6)F(5))(2), a phosphine-borane generated in situ from 5, with 1-hexene gave the species [tBu(2)PC[triple bond]CB(C(6)F(5))(2)](CH(2)CHnBu)[tBu(2)PC[triple bond]CB(C(6)F(5))(2)] (13) and subsequent reaction with methanol or hexene resulted in the formation of [tBu(2)P(H)C[triple bond]CB(C(6)F(5))(2)](CH(2)CHnBu)[tBu(2)PC[triple bond]CB(C(6)F(5))(2)](OMe) (14) or the macrocycle {[tBu(2)PC[triple bond]CB(C(6)F(5))(2)](CH(2)CH(2)nBu)}(2) (15), respectively. In a related fashion, the reaction of 13 with THF afforded the macrocycle [tBu(2)PC[triple bond]CB(C(6)F(5))(2)](CH(2)CHnBu)[tBu(2)PC[triple bond]CB(C(6)F(5))(2)][O(CH(2))(4)] (16), although treatment of tBu(2)PC[triple bond]CB(C(6)F(5))(2) with THF lead to the formation of {[tBu(2)[triple bond]CB(C(6)F(5))(2)][O(CH(2))(4)]}(2) (17). In a related example, the reaction of Mes(2)PC[triple bond]CB(C(6)F(5))(2) with PhC[triple bond]CH gave {[Mes(2)PC[triple bond]CB(C(6)F(5))(2)](CH[triple bond]CPh)}(2) (18). Compound 5 reacted with AlX(3) (X=Cl, Br) to give addition to the alkynyl unit, affording (C(6)F(5))(2)BC(H)=C[P(H)tBu(2)](AlX(3)) (X=Cl 19, Br 20). In a similar fashion, 5 reacted with [Zn(C(6)F(5))(2)]⋅C(7)H(8), [Al(C(6)F(5))(3)]⋅C(7)H(8), or HB(C(6)F(5))(2) to give (C(6)F(5))(3)BC(H)=C[P(H)tBu(2)][Zn(C(6)F(5))] (21), (C(6)F(5))(3)BC(H)=C[P(H)tBu(2)][Al(C(6)F(5))(2)] (22), or [(C(6)F(5))(2)B](2)HC=CH[P(H)tBu(2)] (23), respectively. The implications of this reactivity are discussed.

Ni→B Interactions in Nickel Phosphino-Alkynyl-Borane Complexes

The Ni complexes [{tBu2PC≡CB(C6F5)2}Ni(cod)] and [({tBu2PC≡CB(C6F5)2}Ni(NCMe))2] derived from the reaction between the phosphino-alkynyl-borane tBu2PC≡CB(C6F5)2 and [Ni(cod)2] exhibit an unprecedented metal–alkyne interaction in which the borane substituent bends towards the metal affording a Ni→B dative interaction.

CC Coupling by Thermolysis of Alkynyl Phosphonium Borates

Compounds containing both phosphine and borane fragments have attracted increasing attention over the past several years. Perhaps the most widely studied use of such molecules has been as ligands for transition metals. Indeed systems that incorporate both P and B centers have been shown to be quite versatile for the coordination chemistry of transition metals. Moreover the resulting complexes give rise to unusual reactivity and have furnished new organometallic catalysts. While the impact on the reactivity of such ancillary P/B ligands is interesting, materials containing P and B centers also exhibit interesting properties. For example, phosphorus ylide–borane adducts have been employed as latent catalysts for thermosetting resins. In addition, more elaborate molecules containing P and B centers have been prepared to exploit the combination of such electron-donor and -acceptor capabilities. Together with highly conjugated organic p systems, such materials exhibit intriguing electronic and optical properties and have found applications as fluorescent and nonlinear optical materials. In another application, Gabbai et al have pioneered combinations of the Lewis acidity of boranes with the electron-withdrawing abilities of phosphonium groups to effect enhanced Lewis acidity at the B center for use as fluoride sensors. In a somewhat more fundamental context, we have recently described “frustrated Lewis pairs” comprising P and B centers in which steric demands preclude classical Lewis acid–base adduct formation. Such systems retain Lewis acidity and basicity that can be exploited to activate a variety of small molecules including olefins, dienes, THF, BH bonds, terminal alkynes, CO2, [12] and N2O. [13] Most notably these systems effect the heterolytic activation of H2 and can be subsequently employed in metal-free hydrogenation catalysis of imines, aziridines, nitriles, enamines, and silylACHTUNGTRENNUNGenolethers.[10,14,19] More recently, Bercaw et al have used related phosphine-borane systems to effect H2 activation and reduce metal-bound carbonyl fragments, while Bourissou and co-workers have exploited donor/acceptor properties of phosphine-boranes to effectively trap reactive intermediates such as singlet O2. [16] The nature of the organic links between the P and B centers in the above systems are frequently arene-based although two-carbon alkyl or alkenyl chains have also been examined. Alkynyl-bridged systems bearing P and B centers were prepared some years ago by Marder and others. and shown to exhibit nonlinear optical properties. More recently, Yamaguchi and co-workers have described an ingenious synthetic pathway for phosphonium borate linked stilbene derivatives formed by intramolecular nucleophilic addition of phosphine and borane across a C C unit in efforts to develop new organic materials. Despite the interest in the electronic properties, little attention has focused on the reactivity of such alkynyl-bridged P/B systems. In a recent report, we described the unconventional binding mode of the alkynyl phosphine-borane, tBu2PC CBACHTUNGTRENNUNG(C6F5)2, to Ni. Herein, the thermal reactivity of alkynyl phosphonium borates is examined. In two such systems the strongly polarized alkynyl group is shown to react via group migration and C C coupling to give rise to unusual C4 olefinic and cumulenic fragments. The mechanism of these unusual rearrangements is probed computationally. Thermolysis of the phosphonium borate tBu2PHC CBHACHTUNGTRENNUNG(C6F5)2 (1) was performed in [D5]bromobenzene at 150 8C for 12 h. The solution became yellow and subsequently orange after about 5 min. Following completion of the thermolysis, the solution was allowed to cool and a product (2) crystallized from the solution (Scheme 1). The crystals were isolated and washed with benzene affording 2 in 40 % yield. The H NMR spectrum of 2 showed broad multiplets at d= 5.90 and 3.01 ppm as well as doublets at d=1.46 and [a] X. Zhao, Prof. Dr. D. W. Stephan Department of Chemistry, University of Toronto 80 St. George St., Toronto, ON, M5S 3H6 (Canada) E-mail : dstephan@chem.utoronto.ca Homepage: http://www.chem.utoronto.ca/staff/DSTEPHAN [b] Prof. Dr. T. M. Gilbert Department of Chemistry & Biochemistry Northern Illinois University DeKalb, IL 60115 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201001575.

Facile synthesis of electrophilic vinyl boranes: reactions of alkynyl-borates and diazonium salts

Reactions of alkynylborate salts, easily derived from reaction of frustrated Lewis pairs with terminal alkynes, with diazonium salts to induce 1,1-carboboration affording a facile and efficient route to substituted electrophilic vinyl boranes.