C À H Borylation DOI: 10.1002/ange.201006320 Iridium-Mediated Borylation of Benzylic C À H Bonds by Borohydride** Christina Y. Tang, William Smith, Amber L. Thompson, Dragoslav Vidovic, and Simon Aldridge* The transition-metal-mediated conversion of C À H to C À B bonds is an exciting recent development in the functionaliza- tion of both saturated and unsaturated hydrocarbons. [1] In part this reflects the fact that the resulting borylated compounds (boronic esters or acids) are attractive substrates for further chemistry through a range of established proto- cols. [2] C À H to C À B conversion in arenes/heteroarenes catalyzed by [{Ir(cod)X} 2 ]/4,4’-di-tert-butylbipyridine systems (X = Cl, OMe, indenyl; cod = cyclooctadiene) has been particularly well developed, [1, 3] in some cases achieving selectivity for substitution patterns which have proved difficult to access using classical synthetic methods. [4] Typi- cally these borylation protocols utilize HBpin or B 2 pin 2 as the boron reagent of choice (pin = pinacolato, OCMe 2 CMe 2 O), with Ir III –tris(Bpin) complexes thought to be key catalytic intermediates. [5] C ÀB bond formation proceeds through either MÀB/CÀH s bond metathesis or through distinct C À H oxidative addition/B À C reductive elimination steps in an Ir III /Ir V cycle. [1, 5, 6] An alternative mechanism implicating a mono(Bpin) complex and a Rh I /Rh III catalytic cycle has been proposed for benzylic borylation using HBpin. [7] In recent work we have examined the interaction of rhodium and iridium complexes containing bis(N-hetero- cyclic carbene) (NHC) ligand sets with boranes. [8, 9] In doing so we have discovered an unusual intramolecular C À H boryla- tion process mediated by [{Ir(coe) 2 Cl} 2 ] (coe = cyclooctene) which leads to the transfer of a BH 3 fragment from LiBH 4 to a benzylic carbon center. [7, 10, 11] Here, we investigate the funda- mental mechanistic steps which lead to this chemistry. The reaction of IMes [N,N’-bis(2,4,6-trimethylphenyl)- imidazol-2-ylidene; 1] with excess LiBH 4 in diethyl ether generates the known compound IMes . BH 3 (2) in 75% yield. [12] By contrast, the reaction of 1 with [{Ir(coe) 2 Cl} 2 ] (0.25 equiv of dimer)/excess LiBH 4 , leads to the formation of the lithium salt 3, in which one of the ortho-methyl substituents has undergone additional C ÀH activation, thereby generating an [ArCH 2 BH 3 ] À function (Scheme 1). The formation of 3 is suggested by 11 B NMR spectroscopy which reveals two quartet resonances (at d B À35.1, 1 J BH = 81 Hz and À27.4 ppm, 1 J BH = 78 Hz), the former being similar to that reported for 2 (d B À36.8 ppm, 1 J BH = 88 Hz), [12b] the latter consistent with other examples of [RBH 3 ] À species [e.g. d B À26.8 ppm, 1 J BH = 79 Hz for (2-naphthyl)BH 3 À ]. [13] These spectroscopic inferences were subsequently confirmed by crystallographic studies, with 3 being shown to exist as a centrosymmetric dimer in the solid state (Figure 1). Each lithium center interacts with six BH hydrogen atoms (with distances in the range 1.86–2.24 ), two of which originate from each of the carbene . BH 3 and [ArCH 2 BH 3 ] À units of one [(IMes’BH 3 )BH 3 ] À moiety, and the other two in the [ArCH 2 BH 3 ] À unit of the second. The C ÀB distances asso- ciated with the two different carbon donors are marginally different [1.587(3) and 1.634(3) ] with the shorter bond Scheme 1. Syntheses of 2 and 3 through borane complexation with or without additional C ÀH activation. Key reagents and conditions: a) LiBH 4 (10 equiv), diethyl ether, 20 8C, 6 h, 75 %; b) [{Ir(coe) 2 Cl} 2 ] (0.25 equiv), THF, then LiBH 4 (40 equiv), diethyl ether, 7 d, 42%. Figure 1. Molecular structure of dinuclear 3·C 6 H 5 F. Hydrogen atoms [except those attached to C(15), B(16) and B(30)] and fluorobenzene solvate omitted (and unactivated mesityl groups shown in wireframe format) for clarity; thermal ellipsoids set at the 40 % proability level. Key distances []: C(17)–B(30) 1.587(3), C(15)–B(16) 1.634(3). [*] Dr. C.Y. Tang, W. Smith, Dr. A.L. Thompson, Dr. D. Vidovic, Dr. S. Aldridge Inorganic Chemistry Laboratory, Department of Chemistry University of Oxford, South Parks Road, Oxford, OX1 3QR (UK) Fax: (+ 44) 1865-272-690 E-mail: simon.aldridge@chem.ox.ac.uk Homepage: http://users.ox.ac.uk/ ~ quee1989/ [**] We thank the EPSRC for funding and for access to the National Mass Spectrometry facility, Swansea University. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201006320. Angewandte Chemie 1395 Angew. Chem. 2011, 123, 1395 –1398  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim