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1,1-Bis(N-methylimidazole)-2-(trimethylsilyl)-1-boracyclohexa-1,4diene Chloride: A Stable Intermediate or Tangent en Route to 1-(NMethylimidazole)borabenzene? Ian A. Cade and Anthony F. Hill* Research School of Chemistry, The Australian National University, Canberra, Australian Capital Territory 0200, Australia S Supporting Information *
ABSTRACT: The reaction of 1-chloro-2-(trimethylsilyl)-1-boracylohexa-2,5-diene (1) with N-methylimidazole (NMI) results not in the anticipated borabenzene adduct [(NMI)→BC5H5] (3) but rather the title imidazole-stabilized cyclic boronium salt [(NMI)2BC5H5SiMe3-2]Cl ([2]Cl), which represents a rare example of a trapped intermediate or possible tangent en route to a neutral borabenzene adduct (3) which may be obtained by heating [2]Cl under vacuum.
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interested in appending buttressing ligands that might support such putative M→B dative bonding. Of particular interest in this context was the possibility of using N-heterocyclic carbene ligands (NHC; Chart 1a), typically obtained from imidazolium salt precursors.9
erhaps the most versatile approaches to the synthesis of neutral Lewis base stabilized borabenzenes1 developed by Ashe,2 Paetzold,3 and Fu4 involve the reaction of the appropriate Lewis base with 1-X-2-(trimethylsilyl)-1-boracylohexa-2,5-diene (X = OMe (1a), Cl (1b); Scheme 1). Scheme 1. Synthesis of Lewis Base (L) Adducts of Borabenzene
Chart 1. Analogies among (a) 1,3-Dimethylimidazolylidene, (b) 1-Borane-3-methylimidazolylidene, and (c) (Hypothetical) 1-Borabenzene-3-methylimidazolylidene Coordination
In these reactions, the base apparently serves the dual purpose of facilitating the elimination of Me3SiX and trapping the nascent borabenzene. A useful variant4c involves the initial reaction of 1 with trimethylphosphine to generate Me3P→ BC5H5 followed by nucleophilic substitution of the phosphine by the (typically harder) base of choice. Despite the utility of this protocol, surprisingly little is known about the mechanistic details of this multistep process, beyond the observation by Paetzold that pyrolysis (500 °C) of 1b is not an effective route to free borabenzene when argon is employed as the carrier gas.3,5 In condensed phases there is, as yet, no evidence to suggest that elimination of Me3SiX occurs prior to adduct formation: i.e., it appears unlikely that free borabenzene is an intermediate. We have recently examined frameworks based upon basestabilized borabenzenes6 as potential precursors for reactions with low-valent metal centers to further expand on the growing field of metal−boron dative bonding.7,8 To this end, we were © 2012 American Chemical Society
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RESULTS AND DISCUSSION Siebert has explored the organometallic chemistry of anionic NHC isosteres in which one nitrogen carries a borane substituent (Chart 1b),10 which led us to consider the viability of the analogous borabenzene system (Chart 1c). We have therefore attempted what we anticipated would be a Received: January 2, 2012 Published: February 24, 2012 2112
dx.doi.org/10.1021/om300002y | Organometallics 2012, 31, 2112−2115
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straightforward synthesis of N(1)-borabenzene-3-methylimidazole, only to encounter an interesting problem. Pyridine−borabenzene (py→BC5H5) is readily obtained from the simple reaction of pyridine with 1b.4 Surprisingly, when 1b was treated with an excess of N-methylimidazole (NMI) a compound formulated as the remarkably stable boronium salt11 [(NMI)2BC5H5SiMe3-2] ([2]Cl) was obtained (Scheme 2) rather than the desired 1-(N-methylimidazole)borabenzene NMI→BC5H5 (3).
chloride counterion and one imidazolium proton (Cl(1)···H(171)C(17) = 3.366 Å), consistent with the characteristic acidity of this position (see Figure S1, Supporting Information). Remarkably, repeating this reaction with nondried N-methylimidazole also provided [2]Cl in high yield as the crystalline hydrate [2]Cl·H2O·0.5C6H6. X-ray analysis of this sample yielded a molecular structure very similar to, though more precisely modeled than, that for [2]Cl·0.5C6H6. This conspicuous insensitivity to water indicates an unexpectedly high stability of [2]+ toward hydrolysis and suggests that NMI competes effectively with adventitious water to prevent hydrolysis of any intermediates. This serendipitous result is noteworthy, as the cation [2]+ corresponds to a trapped intermediate species, not previously observed, en route to the borabenzene framework and invites mechanistic conjecture. The majority of known cyclic boronium cations typically involve one or two positively mesomeric heteroatom donors adjacent to boron, with carbocyclic examples being especially rare.10,12−14 Indeed, the closest analogues of [2]+ are obtained f rom rather than en route to borabenzene adducts. Thus, Piers has shown that the addition of pyridinium salts to pyridine−borabenzene results in 1,2- and 1,4-addition of the N−H bond to afford a 1:1 mixture of 2,4 (δB 4.82)- and 2,5-regioisomers (δB 0.36; cf. δ −4.5 for [2]+).13 An unusual polycylic cage compound has also been suggested to form via the Diels−Alder cycloaddition of a 7azaindole−borabenzene adduct.6b Less closely related are the spirocyclic boronium cations arising from the reactions of 9chloro-10-hydro-9-boraanthracene with bipyridyls (Scheme 3).14
Scheme 2. Synthesis of N-Methylimidazole−Borabenzene (3)
In a further attempt to obtain 3, the reaction of 1b with 1 equiv of N-methylimidazole (in benzene) was investigated and found to provide a mixture of products, which included a significant quantity of [2]Cl as a colorless precipitate. Optimally, compound [2]Cl could be obtained pure in high yield by treatment of 1b with 2.1 equiv of N-methylimidazole in benzene. An analytically pure sample of [2]Cl was obtained by washing the resulting precipitate with benzene and hexane, to remove traces of Me3SiCl and unreacted N-methylimidazole. Crystals of [2]Cl obtained directly from the reaction mixture were suitable for X-ray crystallographic analysis, the results of which are shown in Figure 1. In addition to the bond lengths
Scheme 3. Piers’ Syntheses of Carboyclic Boronium Ions: (i) Pyridinium Chloride;12 (ii) 5,5-Dimethyl-2,2-bipyridyl13
In yet a further attempt to obtain 3, the salt [2]Cl was treated with CsF in acetonitrile, the conditions typically employed to generate benzyne from 2-halophenylsilanes.15 However, under these conditions [2]Cl yielded a plethora of products. The largest single component of this mixture, on the basis of NMR integration (δB 5.52, d, 1JBF = 64 Hz) appeared to be the 1-fluoroboratabenzene anion.16,17 The conjugate acid 1fluoro-1-boracyclohexa-2,5-diene has been described previously19 and was not observed under these conditions (1H NMR). Each of the other observed products also exhibited 11B NMR resonances characteristic of three-coordinate boron environments. Formation of such species, following reaction with even 1 equiv of fluoride, suggests that fluoride-mediated
Figure 1. (a) Full molecular structure of [2]Cl and (b) simplified molecular structure of 2+ in a crystal of [2]Cl·0.5C6H5 (60% displacement ellipsoids). Selected bond lengths (Å) and angles (deg): B1−C1 = 1.619(3), B1−C5 = 1.590(3), C1−C2 = 1.339(4), C4−C5 = 1.325(4), C2−C3 = 1.500(4), C3−C4 = 1.479(5), B1−N1 = 1.592(3), B1−N3 = 1.605(3), H171···Cl1 = 2.529; N1−B1−N3 = 105.4(2), C1−B1−C5 = 112.7(2).
and angles summarized in Figure 1,12 the molecular structure of [2]Cl reveals a number of points of interest. The boracyclohexadiene ring is only slightly distorted from planarity, adopting a flattened boat conformation with torsional angles around the two double bonds averaging 177.9(3)°. Significant hydrogen bonding is also observed between the 2113
dx.doi.org/10.1021/om300002y | Organometallics 2012, 31, 2112−2115
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compounds 3,5-Me 2 C 5 H 3 B←C(NMeCMe) 2 22 and HC(C6H4)2B←C{NMesCH2)2 (Mes = C6H2Me3-2,4,6),23 while Braunschweig has recently reported an NHC adduct of a halo borole.24 The formation of the species suggested to be 4 and 5 only occurs under the thermolytic conditions used to generate 3: i.e., once isolated, 3 does not evolve to 4 and 5 under ambient conditions. At the B3LYP-6-31G* level of theory (see the Supporting Information) 3 would appear to be some 4.8 kJ mol−1 lower in energy than 4 such that, given a kinetically viable pathway for their interconversion, both 3 and 4 might be expected to coexist (K3→4 ≈ 0.14). The failure of isolated 3 to establish a 3/4 equilibrium mixture under ambient conditions therefore indicates a substantial kinetic barrier to their interconversion. We may not at this stage exclude the possibility that the strong base NMI liberated during thermolysis of [2]Cl mediates these proton transfers.
desilylation is unsuitable for inducing the transformation of [2]Cl to 3, due perhaps to the high B−F bond strength in comparison to that of Si−F (ca. 650 kJ mol−1 for threecoordinate boron, 580 kJ mol−1 for four-coordinate silicon). The desired transformation was, however, accomplished by heating [2]Cl under vacuum (110−130 °C, 0.2 mmHg) in a sublimation apparatus equipped with a cold finger cooled to −15 °C. The initial melting of the white microcrystalline [2]Cl was accompanied by significant effervescence (NMI, Me3SiCl) to provide a residual orange-red oil composed primarily of 3, which solidified upon standing. The mechanism of this reaction, which only ensues upon melting, is presumed to involve attack of the chloride at silicon, with elimination of Me3SiCl, a [4,2]-hydrogen shift and subsequent loss of N-methylimidazole. Notably, however, formation of compound 3 was accompanied by trace quantities (
