Bü chner Reactions Catalyzed by a Silver(I) Pyridylpyrrolide: Understanding Arene CC Insertion Selectivity Nobuyuki Komine, Jaime A. Flores, Kuntal Pal, Kenneth G. Caulton,* and Daniel J. Mindiola* Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States * S Supporting Information ABSTRACT: The complex Ag 3 (μ 2 -3,5-(CF 3 ) 2 PyrPy) 3 (3,5-(CF 3 ) 2 PyrPy = 2,2′- pyridylpyrrolide(1-) ligand) catalytically promotes the insertion of the carbene of ethyl diazoacetate (EDA), at room temperature, into the CC bond of a series of arenes to ultimately ring-open them and form the corresponding cycloheptatrienes. In one case, the norcaradiene intermediate can be isolated, while regioselective CC insertion can be promoted with certain arene substrates. The mechanism of CC insertion, preference over C-H insertion, and origin of CC regioselectivity has been probed by a combination of experimental and theoretical studies. ■ INTRODUCTION Selectively adding a carbene unit, intermolecularly, across aromatic CC bonds as opposed to the more common C-H insertion pathway (eq 1) is an exceedingly rare occurrence, given the strength of that CC bond and also the loss of aromaticity of the molecule. Frequently, the primary norcar- adiene product of addition of carbene to one arene C/C bond (“cyclopropanation”) readily isomerizes to the cyclohepta- triene. Examples of catalytic systems capable of promoting some selectivity in intermolecular cyclopropanation and subsequent ring opening of aromatic CC bonds (the Bü chner reaction, discovered in 1885, or alternatively called the Bü chner-Curtius-Schlotterbeck reaction) 1-4 over C-H and C-X (X = halogen) insertion have been reported for Rh 2 (II,II) lantern type systems, 5 Au(I), 6-8 Ag(I), 9 and Cu(I) 6,10 and, more recently, in a noncatalytic Ru(II) system. 11 Interestingly, an intramolecular Bü chner type reaction has been implicated in the decomposition pathway of second-generation Grubbs olefin metathesis catalysts. 11-13 In general, the carbene precursor is typically a diazoacetate ester (e.g., EDA, N 2 CHCO 2 Et). In the context of silver catalysis, Lovely and Dias reported (THF)AgTp CF3 , where Tp CF3 = [HB(3,5- (CF 3 ) 2 Pz) 3 ] - , the only Ag(I) catalyst that can preferentially insert carbene into aromatic CC bonds rather than C-H bonds. 9 This report, with a highly fluorinated Tp ligand on Ag(I), still showed some C-H insertion and “ slow decomposition,” attributed to reduction to silver metal presumably by the B-H functionality in the Tp ligand. However, inserting the carbene fragment regioselectively into the CC bond of a substituted arene, to form a ring-expanded product, is an issue that has not been clearly addressed, in part because of the competing C-H insertion pathway or challenges associated with minimizing carbene dimer formation, which wastes the EDA carbene source. To date, no study has examined the origin and regioselectivity of CC insertion versus C-H insertion. We have reported separately 14 that the product of reaction of Ag 2 O with HL, where L is the bidentate 3,5-(CF 3 ) 2 PyrPy - (2,2′-pyridylpyrrolide) ligand, gives not a simple two- coordinate, mononuclear LAg species but instead the trimer Ag 3 (μ 2 -3,5-(CF 3 ) 2 PyrPy) 3 (1), where each ligand bridges adjacent metals (Scheme 1). The reaction of this molecule with ethyl diazoacetate occurs quickly (minutes) at 25 °C to produce the monomeric adduct AgL(EDA) containing intact EDA: in spite of the idea that N 2 loss might be facile, the observed adduct is formed with intact EDA, and adduct formation can even be reversed simply by vacuum at 25 °C. This adduct has a structure where the carbenoid carbon, carbanionic in one Lewis structure, binds to silver. The trimer, interesting as it might be, is simply illustrative of how silver, in the ligand-deficient environment of its synthesis, increases its number of donor neighbors; when offered the substrate EDA, it coordinates to form a three-coordinate monomer. This adduct was shown to have a significant barrier to lose N 2 to form a transient carbene, where the carbene carbon is established to be Received: December 20, 2012 Article pubs.acs.org/Organometallics © XXXX American Chemical Society A dx.doi.org/10.1021/om301240d | Organometallics XXXX, XXX, XXX-XXX