Chelate Restrained Boron Cations for Intermolecular Electrophilic

Dec 15, 2009 - X-ray diffraction analysis revealed a product derived from arene borylation and extensive ligand redistribution, comprised of a borylat...
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Organometallics 2010, 29, 241–249 DOI: 10.1021/om900893g

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Chelate Restrained Boron Cations for Intermolecular Electrophilic Arene Borylation Alessandro Del Grosso, Robin G. Pritchard, Chris A. Muryn, and Michael J. Ingleson* Department of Chemistry, University of Manchester, Manchester, M13 9PL U.K. Received October 13, 2009

Highly electrophilic boron species that borylate arenes are generated by halide abstraction from CatBX (Cat = catecholato, C6H4O22-, X = Cl or Br) by [Et3Si][CbBr6] (CbBr6 = [closo-1-HCB11H5Br6]-). A transient [CatB][CbBr6] related species reacts as a synthetic equivalent of [CatB]þ in intermolecular electrophilic borylation, with reactions proceeding rapidly at 25 °C. The [CatB]þ moiety was shown to be strongly Lewis acidic on the basis of 1H and 31P{1H} NMR spectroscopy of the crotonaldehyde and triethylphosphine oxide adducts, respectively. Catalytic quantities of [Et3Si][CbBr6] and CatBX were effective for the high-yielding borylation of arenes by CatBH in a highly atom efficient cycle with H2 the only byproduct. Successful catalysis was dependent on the robust [CbBr6]- anion and the use of electrophile-resistant borane sources.

The electrophilicity of boron Lewis acid reagents, such as BBr3, while strong, is limited. This is exemplified by the lack of reaction with unfunctionalized arenes by an electrophilic aromatic substitution (EAS) mechanism.1-5 Increasing cationic character by replacement of halide for a weakly interacting anion enhances electrophilic reactivity at boron.6-13 Gas phase studies, where anion coordination is precluded, with BH2þ confirmed the ability of an electrophilic boron cation to bind and activate unreactive molecules, including H2 and CH4.14,15 Recent elegant work by Vedejs et al. utilized the BH2þ cation stabilized by R3N coordination for intramolecular arene borylation.16-18 These three-coordinate *Corresponding author. E-mail: [email protected]. uk. (1) Yamamoto, H. Lewis Acids in Organic Synthesis; Wiley-VCH, 2000. (2) Muetterties, E. L. J. Am. Chem. Soc. 1959, 81, 2597. (3) Muetterties, E. L. J. Am. Chem. Soc. 1960, 82, 4163. (4) Muetterties, E. L.; Tebbe, F. N. Inorg. Chem. 1968, 7, 2663. (5) Erker, G. Dalton Trans. 2005, 1883. (6) Koelle, P.; N€ oth, H. Chem. Rev 1985, 85, 399. (7) Piers, W. E.; Bourke, S. C.; Conroy, K. D. Angew. Chem., Int. Ed. 2005, 44, 5016. (8) Vidovic, D.; Findlater, M.; Cowley, A. H. J. Am. Chem. Soc. 2007, 129, 8436. (9) Kato, T.; Tham, F. S.; Boyd, P. D. W.; Reed, C. A. Heteroat. Chem. 2006, 17, 209. (10) Wei, P.; Atwood, D. A. Inorg. Chem. 1998, 37, 4934. (11) Imamoto, T.; Asakura, K.; Tsuruta, H.; Kishikawa, K.; Yamaguchi, K. Tetrahedron Lett. 1996, 37, 503. (12) Ott, H.; Matthes, C.; Ringe, A.; Magull, J.; Stalke, D.; Klingebiel, U. Chem.;Eur. J. 2009, 15, 4602. (13) Uddin, M. K.; Nagano, Y.; Fujiyama, R.; Kiyooka, S.; Fujio, M.; Tsuno, Y. Tetrahedron Lett. 2005, 46, 627. (14) DePuy, C. H.; Gareyev, R.; Hankin, J.; Davico, G. E.; Krempp, M.; Damrauer, R. J. Am. Chem. Soc. 1998, 120, 5086. (15) Schneider, W. F.; Narula, C. K.; N€ oth, H.; Bursten, B. E. Inorg. Chem. 1991, 30, 3919. (16) Vedejs, E.; Nguyen, T.; Powell, D. R.; Schrimpf, M. R. Chem. Commun. 1996, 2721. (17) Vries, T. S. D.; Prokofjevs, A.; Harvey, J. N.; Vedejs, E. J. Am. Chem. Soc. 2009, 131, 14679. (18) Vries, T. S. D.; Vedejs, E. Organometallics 2007, 26, 3079. r 2009 American Chemical Society

boron cations, termed borenium cations, were calculated to be more electrophilic than Me3Cþ and remarkably appear to activate H2 analogously to transition metal superelectrophiles.19 Borenium species with weakly stabilizing substituents can be classed as superelectrophiles, combining a monocationic charge with a formally unoccupied p orbital.17,20,21 An alternative class of boron cations with the potential to be superelectrophiles are the two-coordinate boron cations, termed borinium cations.6,7 Borinium cations are invariably ligated by bulky, strongly π-donating substituents that effectively shield the boron cation from solvent and anion.6,7 Monodentate systems predominate due to the stabilization derived from a linear geometry at boron that maximizes electron donation from orthogonal π-donors (Figure 1), analogous to the isoelectronic allenes. While these factors enabled the synthesis of borinium cations in the condensed phase, they are undesirable from a reactivity perspective due to reduced electrophilicity and effective steric shielding at boron. Bidentate ligation of boron conceptually generates a “chelate restrained” borinium cation in which electrophilicity is enhanced by the nonlinear geometry at boron, resulting in an empty boron orbital that cannot be stabilized by ligand πdonation (Figure 1). Chelation also ensures a sterically more accessible boron cation, which is advantageous for substrate accessibility. In the condensed phase bona fide chelate restrained borinium cations are not feasible. The boron Lewis acid will readily interact with any Lewis base present in solution, generating borenium cations or anion-coordinated species. However, in a weakly nucleophilic environment chelate restrained boron species will be generated with electrophilicity approaching that of the superelectrophilic (19) Kubas, G. J. Metal Dihydrogen and Sigma Bond Complexes; Kluwer Academic/Plenum Publishers: New York, 2001. (20) Olah, G. A.; Klump, D. A. Superelectrophiles and Their Chemistry; Wiley: New York, 2008. (21) Chiu, C.-W.; Gabbai, F. P. Organometallics 2008, 27, 1657. Published on Web 12/15/2009

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Figure 1. Linear and chelate restrained borinium cations demonstrating the different degrees of boron stabilization by πdonation (potential anion/solvent coordination excluded for simplicity).

BX2þ gas phase cations (X = H, Me, OMe).14,22,23 An initial benchmark reaction of particular interest is the intermolecular electrophilic aromatic substitution of the C-H bonds of unactivated arenes. This requires an electrophilicity greater than BBr3, with only directed intramolecular arene C-H borylations proving successful using neutral borane Lewis acids.17,24-30 Herein we report attempts to synthesize and isolate chelate restrained boron cations based predominantly on the [CatB]þ (cat = catecholato, C6H4O22-) moiety partnered with a range of weakly coordinating anions. The phrase “chelate restrained boron cation” is used to signify a [CatB]þ group stabilized by interactions with weakly nucleophilic species and is represented throughout as [CatB][X] (where X is a weakly coordinating anion). Subsequent stoichiometric and catalytic (in superacid) reactivity studies are presented utilizing chelate restrained boron cations to generate aryl-boronic esters (ArB(OR)2). These findings are significant given the importance of aryl boronic esters and acids in transition metal catalyzed synthesis,31-33 combined with the scarcity of intermolecular reactivity studies between electrophilic boron cations and carbon nucleophiles.7

Results and Discussion [OTf]. N€ oth et al. previously reported a diazaborole system that utilized the relatively coordinating triflate anion (= [OTf]-) to stabilize the formally cationic chelate restrained boron species, A.34 The reduced degree of π-donation provided by chalcogens compared to pnicogens of the same period will afford a modest enhancement of electrophilicity at boron in the related compound [CatB][OTf], 1.35 Initial attempts to synthesize 1 from CatBCl using stoichiometric TMSOTf as the halide-abstracting reagent failed at (22) Ranatunga, T. D.; Kennady, J. M.; Kentt€amaa, H. I. J. Am. Chem. Soc. 1997, 119, 5200. (23) Ranatunga, T. D.; Kentt€amaa, H. I. J. Am. Chem. Soc. 1992, 114, 8600. (24) Allaoud, S.; Frange, B. Inorg. Chem. 1985, 24, 2520. (25) Zhou, Q. J.; Worm, K.; Dolle, R. E. J. Org. Chem. 2004, 69, 5147. (26) Davis, F. A.; Dewar, M. J. S. J. Am. Chem. Soc. 1968, 90, 3511. (27) Laaziri, H.; Bromm, L. O.; Lhermitte, F.; Gschwind, R. M.; Knochel, P. J. Am. Chem. Soc. 1999, 121, 6940. (28) Arcus, V. L.; Main, L.; Nicholson, B. K. J. Organomet. Chem. 1993, 460, 139. (29) Dewar, M. J. S.; Kubba, V. P.; Pettit, R. J. Chem. Soc. 1958, 3073. (30) Lee, G. T.; Prasad, K.; Repic, O. Tetrahedron Lett. 2002, 43, 3255. (31) Hall, D. Boronic Acids; Wiley-VCH: New York, 2005. (32) Nicolau, K. C.; Bulger, P. C.; Sarlah, D. Angew. Chem., Int. Ed. 2005, 44, 4442. (33) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457. (34) Narula, C. K.; N€ oth, H. Inorg. Chem. 1984, 23, 4147. (35) Rissler, J.; Hartmann, M.; Marchand, C. M.; Grutzmacher, H.; Frenking, G. Chem.;Eur. J. 2001, 7, 2834.

both 25 °C and at reflux in C6D6, with the reactants remaining unchanged. Use of a large excess (∼100 equiv) ofTMSOTf yielded 1 as the major product along with unreacted CatBCl. Subsequent removal of excess TMSOTf in vacuo regenerated CatBCl, indicating that this system was in an equilibrium favoring CatBCl.36 Access to [CatB][OTf] was achieved through silver salt metathesis with AgOTf (eq 1). The 11B chemical shift of 1 at 21.7 ppm was consistent with a three-coordinate B(OR)3 environment and close to that reported for compound A (23.3 ppm).34 A benzene solution of 1 was heated to 80 °C for 3 days; however there was no reaction, with 1 recovered unchanged. The triflate anion in 1 interacts strongly with the boron center, as indicated by the 11B chemical shift, limiting the electrophilicity at boron and preventing arene borylation.37 The synthesis of a [CatB]þ species partnered with a more weakly coordinating anion was sought to further enhance the Lewis acidity at boron. AlCl3 Reactivity. Halide abstraction is the prevalent synthetic route to linear borinium cations from (R2N)2BCl and strong MX3 Lewis acids (M = Al, Ga, B, X = halide).6,7,12 It is also noteworthy that AlCl3 enabled arene borylation with BCl3 (or BBr3) to yield aryldihaloboranes, presumably involving an unobserved boron species related to [BCl2][AlCl4].2,3,38 In contrast combination of stoichiometric CatBCl and AlCl3 in arene solvents resulted in effectively no reaction at 25 °C (by 11B and 27Al NMR spectroscopy). With similar B-Cl bond strengths expected for BCl3 and CatBCl (B-Cl bond lengths are 1.75 and 1.74 A˚, respectively),39,40 the lack of reactivity with CatBCl is attributed to the stabilization afforded by adopting a linear geometry at boron in the resultant borinium cation. Heating equimolar CatBCl/AlCl3 to 100 °C in toluene resulted in a slow reaction that could be followed by 11 B NMR (Figure 2). A new resonance observed at 46.4 ppm corresponds to BCl3, formed by a ligand redistribution reaction between CatBCl and AlCl3. Prolonged heating led to a further increase in BCl3 along with the appearance of two other minor products, one attributed to isomers of (C6H4CH3)BCl2 (55.0 ppm, generated by electrophilic borylation of the solvent toluene on activation of BCl3 by AlCl3)2,41 and the other consistent with an aryl boronic (36) See Supporting Information. (37) Reed, C. A. Chem. Commun. 2005, 1669. (38) Bujwid, Z. J.; Gerrard, W.; Lappert, M. F. Chem. Ind. 1959, 1091. (39) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Butterworth-Heinemann: Oxford, 1997. (40) Coapes, R. B.; Souza, F. E. S.; Fox, M. A.; Batsanov, A. S.; Goeta, A. E.; Yufit, D. S.; Leech, M. A.; Howard, J. A. K.; Scott, A. J.; Clegg, W.; Marder, T. B. Dalton Trans. 2001, 1201. (41) Haubold, W.; Herdtle, J.; Gollinger, W.; Einholz, W. J. Organomet. Chem. 1986, 315, 1.

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Figure 2. (Top) 11B NMR spectra of a 1:1 combination of AlCl3 and CatBCl in toluene. Resonance at 28.1 ppm corresponds to CatBCl. (Bottom) Molecular structure of 2 with thermal ellipsoids at 50% probability. Selected bond lengths (A˚): B1-C7 = 1.526(4), B1-O1 = 1.348(3), B1-O2 = 1.434(3), Al1-O2 = 1.866(2), Al1-O3 = 1.805(2), Al2-O3 = 1.913(2), Al2-O4 = 1.828(2), and Al3-O4 = 1.830(2).

ester (32.7 ppm). Heating beyond 48 h did not lead to the complete consumption of all CatBCl, signifying a nonstoichiometric reaction between CatBCl and AlCl3. This was confirmed by the use of excess AlCl3 in refluxing toluene, completely converting CatBCl to predominantly BCl3 and (tolyl)BCl2. The 27Al NMR spectra of this reaction consisted of two broad resonances, at 104 and 60 ppm, in the regions expected for four- and five-coordinate aluminum, respectively.42-44 Slow cooling of a toluene solution of CatBCl and AlCl3 (1:3 equiv, heated at 100 °C for two days) led to the deposition of a small quantity of yellow needles. X-ray diffraction analysis revealed a product derived from arene borylation and extensive ligand redistribution, comprised of a borylated catechol coordinated to two AlCl2 and one AlCl3 (compound 2, Figure 2 bottom). The two Al-Cl distances of the asymmetrically bridged Al-Cl-Al moiety (Al2-Cl5 = 2.681(1) A˚, Al3-Cl5 = 2.145(1) A˚) are elongated compared to the terminal Al-Cl bonds,44-46 with the distinct bridging Al-Cl bond lengths indicative of stronger Lewis acidity of the lower coordinate aluminum center. The structure of 2 (42) Atwood, D. A. Coord. Chem. Rev. 1998, 176, 407. (43) Moravec, Z.; Sluka, R.; Necas, M.; Jancik, V.; Pinkas, J. Inorg. Chem. 2009, 48, 8106. (44) Sharma, V.; Simard, M.; Wuest, J. D. Inorg. Chem. 1990, 30, 579. (45) Cowley, A. H.; Cushner, M. C.; Davis, R. E.; Riley, P. E. Inorg. Chem. 1981, 20, 1179. (46) Belanger-Gariepy, F.; Hoogsteen, K.; Sharma, V.; Wuest, J. D. Inorg. Chem. 1991, 30, 4140.

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supports the conclusions derived from NMR spectroscopy: B/Al ligand redistribution and arene borylation. The absence in the 11B NMR spectra of any significant boronic ester products from EAS with a “[CatB][AlCl4]” species is attributed to the competing ligand redistribution reaction. In a related attempt to achieve halide abstraction from CatBBr a stronger neutral Lewis acid, BBr3,47 was added in stoichiometric quantities to a toluene solution of CatBBr. This produced no arene borylation at both 25 and 100 °C after prolonged periods, indicating that neutral MX3 species are insufficiently Lewis acidic to abstract halide from CatBX. This is due to the respective M-Cl/CatB-Cl bond strengths combined with the lower stability of chelate restrained boron cations compared to the linear borinium analogues. Having observed the reaction of TMSOTf with CatBCl, halide abstraction with triethylsilylium ([Et3Si]þ) salts of the weakly coordinating anions [closo-1-H-CB11H5Br6] (termed CbBr6) and [B(C6F5)4]- was investigated.48,49 A chelate restrained boron cation partnered with these very weakly coordinating anions can be expected to react with arene solvents, thus driving the halide abstraction equilibrium to completion. [CbBr6]- Anion. [Et3Si][CbBr6], generated in situ from [Ph3C][CbBr6] and Et3SiH,48 reacted rapidly with CatB-X (X = Cl, Br) in benzene at 25 °C, with Et3Si-X observed as the expected byproduct (by 29Si{1H} NMR). The CatBcontaining products were CatB-Ph and CatBH, with no cationic boron species observed. CatB-Ph originated from the intermolecular reaction of a highly electrophilic boron species with the solvent benzene (Scheme 1), while CatBH was formed by a boron electrophile abstracting a hydride from Ph3CH, regenerating [Ph3C][CbBr6] (by 1H NMR), consistent with the absence of reactivity between Ph3C[CbBr6] and CatBH at 25 °C. However at elevated temperatures [Ph3C][CbBr6] and CatBH did react slowly in benzene to produce CatB-Ph.36 Removal of the Ph3CH byproduct (to eliminate the low-temperature side reaction between [CatB][CbBr6] and Ph3CH) by isolation of [Et3Si][CbBr6] was not possible in our hands due to the extreme sensitivity of cationic silylium species.50 Numerous unsuccessful attempts were made to detect any highly electrophilic boron intermediates. This included the use of the less nucleophilic arenes, o-C6H4Cl2 and C6H5F, that at 25 °C produced the respective arene borylation products (C6H3Cl2(BCat) and C6H4F(BCat)) and CatBH as the only new boron-containing species. Likewise the lowtemperature combination of reagents resulted in no detectable intermediates. At -10 °C in d8-toluene [Et3Si][CbBr6] and CatBCl reacted to give tolyl-BCat; below this temperature CatBCl and [CbBr6]- were the only species observed by 11 B NMR spectroscopy. Closely comparable results were obtained when the low-temperature reaction was performed in the more polar solvent C6H5F (with 10% C6D6) to improve the solubility of ionic species at lower temperatures.36 The mild conditions required for intermolecular electrophilic borylation implied the presence of a highly reactive [CatB][CbBr6] species, with previous arene borylations (47) Beckett, M. A.; Brassington, D. S.; Coles, S. J.; Hursthouse, M. B. Inorg. Chem. Commun. 2000, 3, 530. (48) Reed, C. A.; Xie, Z.; Bau, R.; Benesi, A. Science 1993, 262, 402. (49) Lambert, J. B.; Zhang, S.; Stern, C. L.; Huffman, J. C. Science 1993, 260, 1917. (50) Scott, V. J.; Celenligil-Cetin, R.; Ozerov, O. V. J. Am. Chem. Soc. 2005, 127, 2852.

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Scheme 1. Arene Borylation through Chelate Restrained Boron Cationsa

a

For clarity the anion [CbBr6] is omitted. Throughout formal net charges correspond to the molecule as a whole with assignment of specific charge at individual atoms avoided due to the complexity in locating the delocalized charge in such species.

Figure 3. (Inset) C-H insertion mechanism for the intramolecular arene borylation by a borenium cation.17 (Bottom) EAS and C-H insertion mechanisms applied to intermolecular arene borylation proceeding through [CatB][CbBr6]. The subsequent cationic species can be three- or four-coordinate at boron (by addition/loss of a weak nucleophile L).

requiring directing agents to precoordinate boron and/or aggressive reaction conditions.2-4,17,24-30 The absence of detectable [CatB]þ species, either anion coordinated or ligated with a weak nucleophile from the reaction mixture, also contrasts with the reactivity of closely related main group cations. Examples of particular relevance are the chelated borenium cation, compound B (Figure 3 inset), and [Et2Al][CbBr6], which are both indefinitely stable in arene solvents.17,51 Other highly electrophilic cationic species, E[1-H-CB11Me5Br6] (E = Meþ, R3Siþ, Hþ), react with arenes to give isolable arenium cations of the general formula [C6R6E]þ (R = H or Me or a combination thereof).37,48,52-56 The transient nature of any [CatB][CbBr6] compound is attributed to its highly reactive nature, which results in rapid arene borylation. Arenium cations ([C6R6(BCat)]þ) required for an EAS mechanism were also not detected. It is feasible that the Lewis acidic nature of the three-coordinate boron center in [C6R6(BCat)]þ destabilizes this cation, resulting in rapid Hþ loss and rearomatization to generate C6R5-BCat. However, an alternative mechanism can equally account for the absence of [C6R6(BCat)]þ species, with arenium intermediates not required for borylation reactions that proceed (51) Kim, K.-C.; Reed, C. A.; Long, G. S.; Sen, A. J. Am. Chem. Soc. 2002, 124, 7662. (52) Stasko, D.; Reed, C. A. J. Am. Chem. Soc. 2001, 124, 1148. (53) Kato, T.; Stoyanov, E.; Geier, J.; Gr€ utzmacher, H.; Reed, C. A. J. Am. Chem. Soc. 2004, 126, 12451. (54) Reed, C. A. Acc. Chem. Res. 1998, 31, 325. (55) Reed, C. A.; Kim, K.-C.; Stoyanov, E. S.; Stasko, D.; Tham, F. S.; Mueller, L. J.; Boyd, P. D. W. J. Am. Chem. Soc. 2003, 125, 1796. (56) Hubig, S. M.; Kochi, J. K. J. Org. Chem. 2000, 65, 6807.

by the insertion of a borenium electrophile into an aromatic C-H bond (Figure 3 inset).17 This mechanism can be applied to the intermolecular borylation by [CatB][CbBr6], where highly electron deficient borenium cations are equally feasible (C-H insertion mechanism, Figure 3). With no intermediates experimentally observed both EAS and C-H insertion mechanisms are viable, as are three-coordinate [CatB(arene)]þ and four-coordinate [CatB(arene)L]þ transition states (L = a weak nucleophile). An alternative route to cations that possess enhanced electrophilicity at boron is through coordination of a cationic Lewis acid (e.g., Et3Siþ) to an oxygen lone pair in CatBX. This is feasible, particularly in weakly nucleophilic arene environments, and is consistent with the reactivity observed between CatBCl and AlCl3. However, boron electrophiles generated by this route are unlikely to be the active borylating species due to the following observations: (i) the ability of the significantly weaker Lewis acid [Ph3C][CbBr6] to enable arene borylation with CatBH (the weakly basic oxygen donor in CatBH does not coordinate to the [Ph3C]þ cation, precluding enhancement of electrophilicity at boron); (ii) the lack of arene borylation products when equimolar amounts of [Et3Si][CbBr6] and CatBH were combined at 25 and 100 °C in ortho-dichlorobenzene. Coordination of Et3Siþ to oxygen in CatBH would produce a stronger boron electrophile than the respective Et3Siþ adduct of CatBX, due to the stabilizing effect of halide π-donation to boron in the latter. By this rationale arene borylation via a C-H insertion mechanism should also proceed using CatBH, with concomitant loss of H2, analogous to the production of B. The divergent

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reactivity actually seen for CatBX and CatBH is more consistent with the inability of [Et3Si][CbBr6] to abstract hydride from CatBH (due to the relative Si-H and B-H bond strengths),57 preventing formation of [CatB][CbBr6]. The only reaction observed between [Et3Si][CbBr6] and CatBH was slow ligand exchange (at 100 °C) to give CatB-Et as the new boron-containing product.36 [B(C6F5)4]- Anion. It was desirable, particularly from a cost perspective, to determine if arene borylation was possible with the perfluorinated tetraphenyl borate anion, [B(C6F5)4]-. Combination of CatBCl and [Et3Si][B(C6F5)4]49 in benzene at 25 °C produced PhBCat and [B(C6F5)4]- as the major boron-containing products. No low-temperature intermediates were observed, with NMR spectra closely comparable to the [CbBr6]- system obtained. The only notable difference between the [CbBr6]- and [B(C6F5)4]- borylation reactions was the observation of anion decomposition in the latter. In addition to PhBCat and [B(C6F5)4]- two minor boron-containing products were detected and identified as B(C6F5)3 and CatB-C6F5 (by 11B NMR and independent synthesis).36 CatB-C6F5 was generated from the attack on a B-C6F5 bond of [B(C6F5)4]- by a transient [CatB][CbBr6] electrophile analogous to that previously observed for related [R2Al]þ species, the borenium cation B (Figure 3), and protonated arenes.17,58,59

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Table 1. 31P{1H} Chemical Shifts of the Et3PdO Adducts of a Range of Lewis Acids Lewis acid B(C6F5)3 AlCl3 F2B(O3SCF3) CatB(O3SCF3) BBr3 Et3Si[CbBr6] Me3Si(O3SCF3) CatB[CbBr6] a

Et3PO adduct δ 31P{1H} (ppm)a 76.6b 80.3b 84 6c 85.4d 903b 91.2d 92.8b 106.9d

NMR spectra recorded in C6D6. b Ref 47. c Ref 65. d This work.

-Ph3 CH

CatBH þ Ph3 C½BðC6 F5 Þ4  sf CatBðC6 F5 Þ Δ

þ BðC6 F5 Þ3 ð2Þ The anion decomposition products, CatB-C6F5 and B(C6F5)3, were produced as the major products by the stoichiometric hydride abstraction from CatBH with Ph3C[B(C6F5)4] at 80 °C in benzene (eq 2, Ph3CH observed by 1H NMR). There was no anion decomposition in the corresponding reaction between CatBH and [Ph3C][CbBr6], consistent with the extreme resistance to electrophilic attack that halogenated carborane anions are well noted for.60,61 The observation of [B(C6F5)4]- decomposition, while severely limiting the usefulness of the anion in this system, does provide further indirect evidence for the exceptional Lewis acidity of [CatB]þ species in a weakly nucleophilic environment. Lewis Acidity. The Lewis acidity of [CatB]þ was directly assessed using two complementary NMR spectroscopy based methods; the first is based on the 31P{1H} chemical shift of OdPEt3 on adduct formation,47,62 and the second on the change in the 1H NMR chemical shift of the H3 proton of crotonaldehyde on binding of a Lewis acid to the carbonyl oxygen.63,64 [CatB(OdPEt3)][CbBr6], compound 3, was the major product on reaction of Ag[CbBr6] with CatBCl in the presence of 0.9 equiv of OdPEt3 and was purified by recrystallization from DCM/hexane. The 31P{1H} resonance of 3 at (57) Rablen, P. R.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 4648. (58) Bochmann, M.; Sarsfield, M. J. Organometallics 1998, 17, 5908. (59) Reed, C. A.; Fackler, N. L. P.; Kim, K.-C.; Stasko, D.; Evans, D. R. J. Am. Chem. Soc. 1999, 121, 6314. (60) Reed, C. A. Acc. Chem. Res. 1998, 31, 133. (61) Douvris, C.; Ozerov, O. V. Science 2008, 321, 1188. (62) Gutmann, V. Coord. Chem. Rev. 1976, 18, 225. (63) Childs, R. F.; Mulholland, D. L.; Nixon, A. Can. J. Chem. 1982, 60, 801. (64) Lazlo, P.; Teston, M. J. Am. Chem. Soc. 1990, 112, 8750.

Figure 4. (Left) Molecular structure of P 3, thermal ellipsoids at 50% probability, P1-O1 = 1.595(4) A˚, (O-B-O) = 359.6°. The closest B 3 3 3 Br contact (dashed line) is extremely long at 3.41 A˚. (Right) Phosphonium and borenium resonance structures.

106.9 ppm is shifted considerably downfield compared to the Et3PdO adducts of other strong Lewis acids (Table 1), suggesting exceptional Lewis acidity for the {CatB}þ moiety. In contrast the 31P{1H} chemical shift of the triflate analogue, [CatB(OPEt3)][OTf], at 85.4 ppm (comparable to BF2(OPEt3)(OTf)) is indicative of a significant B-OTf interaction.65 This was corroborated by the 11B chemical shift at 7.9 ppm, in the region expected for four-coordinate boron centers and significantly upfield from the 11B resonance of the [CbBr6] congener (21.9 ppm). The lack of arene borylation with CatB(OTf) is a result of the lower Lewis acidity at boron on strong anion coordination. For direct comparison of the Lewis acidity of {CatB}þ and {R3Si}þ moieties, [Et3Si(OPEt3)][CbBr6] was synthesized and found to have a 31P{1H} chemical shift of 91.2 ppm. The similarity to the 31P{1H} chemical shift of TMS(OPEt3)(OTf) (92.8 ppm) precludes any hypervalence by anion coordination.65 Thus [CatB]þ is more Lewis acidic than Et3Siþ toward Et3PdO, attributable to the additional formally empty orbital present in [CatB]þ. The solid-state structure of 3 was determined to confirm the absence of cation-anion interactions. The structure reveals a planar three-coordinate boron center with the nearest anion 3 3 3 boron interaction at 3.41 A˚ (Figure 4), considerably longer than compounds possessing significant (65) Myers, E. L.; Butts, C. P.; Aggarwal, V. K. Chem. Commun. 2006, 4434.

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Table 2. Magnitude of Downfield Shift of the H3 Proton of Crotonaldehyde on Binding to a Lewis Acid (in CD2Cl2 unless otherwise stated) Lewis acid

Δδ of the H3 proton of crotonaldehyde

Me3Si(O3SCF3) B(C6F5)3 AICI3 CatB[CbBr6] BF2(O3SCF3) BBr3

0.7d 1.05e 1.23a 1.28c 1.46b 1.49a

a

Ref 47. b Ref 65. c This work. d Ref 65, the lower than expected value was attributed to the presence of a hypervalent five-coordinate silicon center bonded to OTf. e Ref 66. The 1H NMR of [CatB(crotonaldehyde)][CbBr6] was recorded in C6D6.

anion 3 3 3 cation interactions (e.g., [CH3CHCH3][1-H-CB11Me5Br6], anion, and carbocation Cþ---Br distance = 2.104(3) A˚).53 The combination of planarity at boron with the long anion 3 3 3 cation distance indicates a very weak interaction at most. The NMR and structural data combined are more consistent with the phosphonium cation resonance structure dominating in 3.36,67 The P1-O1 distance (1.595(4) A˚) is elongated compared to free OdPEt3 and to the Lewis acid/base adduct Et3PdOfB(C6F5)3 (P-O = 1.4973(17) A˚, B-O = 1.533(3) A˚),47 while phosphonium P-O distances (e.g., [Ph2(Me)P-O-CH2tBu]þ, P-O = 1.57 A˚)68 are closely comparable to the P1-O1 distance observed in 3. The short bond distance for O1-B1 (1.380(7) A˚) is also consistent with a phosphonium cation formalism for 3 and comparable to the two B-Ocatecholato distances (1.374(2) and 1.372(2) A˚). Initial attempts to probe the Lewis acidity of [CatB]þ by formation of a crotonaldehyde adduct in CH2Cl2 were unsuccessful, with numerous intractable products formed. This was consistent with a cationic electrophile reacting with chlorinated solvent18,69 and distinct to neutral Lewis acidcrotonaldehyde complexes that are stable at 25 °C in CH2Cl2. [CatB(crotonaldehyde)][CbBr6] was synthesized in arene solvents and had an 11B NMR resonance at 23.8 ppm, consistent with a three-coordinate boron species. The 1H NMR spectrum (in C6D6) confirmed the deshielding of the H3 resonance, which is shifted 1.28 ppm downfield relative to free crotonaldehyde (in C6D6 at 25 °C). This shift is less than reported for the Br3B-crotonaldehyde adduct, albeit recorded in CD2Cl2 (Table 2). The discrepancy between the two Lewis acidity scales can be attributed to a greater hard-soft mismatch between the hard Lewis acidic boron center in [CatB]þ and the predominantly covalent doublebond character of the pπ-pπ double bond of the CdO group in crotonaldehyde. The reversal of relative Lewis acidities in the Et3PdO adducts is due to the more ionic OdP pπ-dπ bond, as noted previously66 and consistent with calculations on borenium cations.15 Catalysis. Aryl boronic esters (ArB(OR)2) are important synthetic intermediates widely utilized in metal-catalyzed C-C bond-forming reactions.31-33 Their production from arenes classically proceeds in an atom-inefficient, multistep (66) Britovsek, G. J. P.; Ugolotti, J.; White, A. J. P. Organometallics 2005, 24, 1685. (67) Dureen, M. A.; Lough, A.; Gilbert, T. M.; Stephan, D. W. Chem. Commun. 2008, 4303. (68) Henrick, K.; Hudson, H. R.; Kow, A. Chem. Commun. 1980, 226. (69) Gunnoe, T. B.; Caldarelli, J. L.; White, P. S.; Templeton, J. L. Angew. Chem., Int. Ed. 1998, 37, 2093.

Figure 5. Superelectrophile-catalyzed production of aryl boronic esters from CatBH.

process involving undesirable halogenated aromatics and stoichiometric organolithium or organomagnesium reagents.31,70 Recent advances have improved the synthesis of ArB(OR)2, particularly with the development of direct arene borylation using iridium catalysts.71-75 A catalytic (in Lewis acid) electrophilic borylation route to ArB(OR)2 from Ar-H is attractive, with the potential to provide complementary selectivity patterns to iridium catalysts. Stoichiometric (in cationic Lewis acid) arene borylation produces a strongly Broensted acidic byproduct, and these have been previously demonstrated to react with hydridic B-H bonds in neutral boranes, liberating H2 and generating a formally cationic boron electrophile.76,77 A combination of these two steps using CatBH and a cationic initiator will enable catalytic electrophilic arene borylation (Figure 5). The anion [CbBr6]- is essential for this cycle, having a nucleophilicity comparable to toluene37,60,61,78 and a conjugate Broensted superacidity,37,55 essential for protonation of the strong Bδþ-Hδ- bond in catecholborane. This cycle requires only catalytic quantities of the cationic Lewis acid, in sharp contrast to conventional EAS, which requires stoichiometric or excess Lewis acid reagents.1,3,4,79 This distinct behavior is enabled by the inherent polarity of the Bδþ-Hδ- bond, which reacts with superacidic “Hþ” species, in an entropically driven process irreversibly producing H2 and regenerating the active boron electrophile. Twenty equivalents of CatBH (with respect to starting [Ph3C][CbBr6]) was added to the reaction mixture resulting from the stoichiometric combination of [Et3Si][CbBr6] and CatBX in benzene (Scheme 1). No reaction was observed at 25 °C, but heating to reflux resulted in the complete conversion of all CatBH to CatB-Ph through the catalytic (in superacid) intermolecular electrophilic borylation. The observation of H2 (and HD when performed in C6D6, by 1H NMR) and the absence of catalysis when the hindered base 2,6-di-tert-butylpyridine is added to sequester Hþ is consistent with the protonation of CatBH by a Broensted acidic species during the catalytic cycle.36 Catalytic electrophilic borylation of alkyl-substituted arenes with CatBH (70) Constable, D. J. C.; Dunn, P. J.; Hayler, J. D.; Humphrey, G. R.; Leazer, J. L.; Linderman, R. J.; Lorenz, K.; Manley, J.; Pearlman, B. A.; Wells, A.; Zaks, A.; Zhang, T. Y. Green Chem. 2007, 9, 411. (71) Mkhalid, I. A. I.; Coventry, D. N.; Albesa-Jove, D.; Batsanov, A. S.; Howard, J. A. K.; Perutz, R. N.; Marder, T. B. Angew. Chem., Int. Ed. 2005, 45, 489. (72) Murphy, J. M.; Lao, X.; Hartwig, J. F. J. Am. Chem. Soc. 2007, 129, 15434. (73) Cho, J.-Y.; Tse, M. K.; Holmes, D.; Maleczka, R. E.; Smith, M. R., III Science 2002, 295, 305. (74) Ishiyama, T.; Miyaura, N. J. Organomet. Chem. 2003, 680, 3. (75) Miyaura, N. Bull. Chem. Soc. Jpn. 2008, 81, 1535. (76) Olah, G. A.; Aniszfeld, R.; Prakash, G. K. S.; Williams, R. E.; Lammertsma, K.; Guner, O. F. J. Am. Chem. Soc. 1988, 110, 7885. (77) Shapland, P.; Vedejs, E. J. Org. Chem. 2004, 69, 4094. (78) Moxham, G. L.; Douglas, T. M.; Brayshaw, S. K.; KociokKohn, G.; Lowe, J. P.; Weller, A. S. Dalton Trans. 2006, 5492. (79) Olah, G. A.; Molnar, A. Hydrocarbon Chemistry; Wiley Interscience: NJ, 2003.

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Table 3. Superacid-Catalyzed Aromatic Borylationj

entry 1 2 3 4 5 6 7 8 9 10 11 12 41

substrate

catalyst

temp. (°C)

time (h)

product

C6H6 C6H5Me m-C6H4Me2 m-C6H4Me2 p-C6H4Me2 C6H5Et o-C6H4Cl2 C6H5SiMe3g C6H5SiMe3g C6H5Meh C6H6 C6H6 C6H6

A A Ac A A A A A A A Et3Si[B(C6F5)4]i TMSOTfi AlCl3i

80 100 100 140 140 100 140 25 100 100 80 40 80

15 15 45 10 7 96 10 48 5 5 18 18 18

(C6H5)BCat (C6H4(Me))BCat (C6H3(Me)2)BCat (C6H3(Me)2)BCat (C6H3(Me)2)BCat (C6H4(Et))Bcat 1,2-Cl2,-4-Bcat-C6H3 (C6H5)BCat (C6H5)BCat (C6H4(Me))BCat (C6H5)BCat (C6H5)BCat (C6H5)BCat

yield (%)a 99 99(93)b 97d 92d 57d, e 35d, f 99 99 99 80