Ru(II) Catalysts Supported by Hydridotris(pyrazolyl)borate for the Hydroarylation of Olefins: Reaction Scope, Mechanistic Studies, and Guides for the Development of Improved Catalysts

Abstract

Carbon-carbon bond formation is the central method by which synthetic chemists add complexity, which often represents value, to molecules. Uniting a carbon chain with an aromatic substrate to yield an alkyl arene product is thus a molecular means of creating value-added materials. A traditional method for generating alkyl arenes is Friedel-Crafts catalysis, in which an alkyl halide or olefin is activated to react with an aromatic substrate. Unfortunately, despite the development of new generations of solid-state catalysts, the reaction often requires relatively harsh conditions and frequently gives poor to moderate selectivity. Conversely, a halide can first be incorporated into the aromatic ring, and the aryl halide can subsequently be joined by a variety of catalytic coupling techniques. But generating the aryl halide itself can be problematic, and such methods typically are not atom-economical. The addition of aromatic C-H bonds across the C-C double bonds of olefins (olefin hydroarylation) is therefore an attractive alternative in the preparation of alkyl arenes. Despite the dominance and practical advantages of heterogeneous catalysts in industrial synthesis, homogeneous systems can offer an enhanced ability to fine-tune catalyst activity. As such, well-defined homogeneous catalysts for the hydroarylation of olefins provide a potentially promising avenue to address issues of selectivity, including the production of monoalkylated arene products and the control of linear-to-branched ratios for synthesis of long-chain alkyl arenes, and provide access to more ambient reaction conditions. However, examples of homogeneous catalysts that are active for the conversion of unactivated aromatic and olefin substrates to alkyl arene products that function via metal-mediated C-H activation pathways are limited. In this Account, we present results from research aimed at the development of Ru(II) catalysts supported by the hydridotris(pyrazolyl)borate (Tp) ligand for the addition of aromatic C-H bonds across olefins. On the basis of detailed mechanistic studies with TpRu(L)(NCMe)R catalysts, in which the neutral ancillary ligand L is varied, we have arrived at guidelines for the development of improved catalysts that are based on the octahedral-d6 motif.

Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes

“…18,19,23À26,31 The primary focus has been to elucidate the effect of the donor and steric properties of the ligand L on the key steps of the catalytic cycle (i.e., olefin insertion and aromatic/ olefin CÀH activation). 19 In an effort to understand more broadly transition metal catalyzed olefin hydroarylation, we have begun to probe structure/activity relationships for catalysts outside the octahedral/d 6 motif. Platinum(II) seemed well suited for this transformation as it is known to readily activate CÀH bonds as well as undergo olefin insertion.…”

“…These data are consistent with the following possibility: during catalysis, β-hydride elimination occurs but is reversible; however, eventually styrene dissociation occurs (regardless of the exact pathway), and under catalytic conditions, the resulting PtÀhydride decomposes. β-Hydride elimination from 8 19 For 7 0 , the large kinetic preference for CdC insertion over ethylene CÀH activation suggests substantial latitude with respect to potential strategies for catalyst refinement before vinylic CÀH activation becomes problematic for this family of Pt(II) catalysts, thus providing one major advantage over reported Ru(II) catalysts (Scheme 14).…”

“…All 1 H spectra are referenced against residual proton signals of the deuterated solvents. 19 F NMR (282 MHz operating frequency) spectra were obtained on a Varian 300 MHz spectrometer and referenced against an external standard of hexafluorobenzene (δ = À164.9 ppm). 2 H NMR (77 MHz operating frequency) spectra were obtained on a Varian Mercury 500 MHz spectrometer.…”

Electrophilic RhI catalysts for arene H/D exchange in acidic media: Evidence for an electrophilic aromatic substitution mechanism

“…Efficient and selective functionalization of hydrocarbon C−H bonds has been an area of intense study [1][2][3][4][5][6][7][8][9][10][11][12][13][14], but many examples of transition metal mediated C−H bond functionalization rely on directing groups to promote selectivity and activity [15][16][17][18]. Thus, developing catalysts that functionalize unactivated hydrocarbons (e.g., arenes and alkanes) remains challenging [2,[19][20][21][22][23].…”

The Development of PtII Complexes as Catalyst Precursors for Transition Metal Mediated Olefin Hydroarylation

“…The transition state features a concerted four-membered transition state in which the metal center directly interacts with the proton being transferred from the arene to the alkyl substituent (TS2 in Scheme 1.9). [78][79] As the transition state is oxidative in character with respect to the metal, the C−H activation process could be argued to possess transient Ru IV is primarily attributed to ligand effects rather than the metal identity and oxidation state of the metal center. Stated another way, the TpRu(CO)(NCMe)(Ph) catalyst achieves greater harmony between activation barriers for olefin insertion and benzene C-H activation than does the bis-acac ligand motif.…”