Intermolecular Hydroarylation of Unactivated Olefins Catalyzed by Homogeneous Platinum Complexes

Abstract

The hydroarylation of olefins is a valuable C À C bond forming reaction used to produce alkyl arenes.[1] Olefin hydroarylation can be catalyzed by Lewis acids, but such reactions proceed through a Friedel-Crafts type mechanism involving an intermediary carbocation. Thus, these reactions give predominantly Markovnikov products, and ortho, meta, and para selectivity is determined by the substituents on the aromatic ring. In contrast, the use of transition-metal catalysts can afford different regioselectivities acting via a mechanism of arene CÀH bond activation and olefin insertion.[1] While in the past transition-metal-catalyzed olefin hydroarylation reactions were primarily limited to activated arenes wherein a chelating functionality on the arene was available to assist and direct the C À H bond activation step, [1] recently Ir III and Ru II catalysts have demonstrated hydroarylation with unactivated arenes and olefins. [2,3] Mechanistic and computational studies on these Ir III and Ru II catalysts suggest that the hydroarylation does not proceed through a Friedel-Crafts type activation but through olefin insertion followed by oxidative hydrogen migration. In addition, selectivity for antiMarkovnikov over Markovnikov products (ca. 60:40) was observed. However, significantly higher selectivities and turnover numbers (TONs) are needed to make these processes economical, so a broadly tunable system that can be modified both sterically and electronically is likely needed.One promising metal for olefin hydroarylation is platinum. There is considerable precedent for both arene C À H bond activation and olefin insertion at Pt II ; [4,5] however, attempts at olefin hydroarylation with Pt II have been disappointing.[6] Selectivities consistent with an electrophilic Friedel-Crafts type pathway were observed using a mixed Ag-Pt catalyst system. [7] With a related Pt II catalyst, the hydroarylation of norbornene was reported, but other olefins were found to be unreactive.[8] Finally, tridentate chelation of a tris(pyrazolyl)borate ligand stabilized a potential Pt IV intermediate preventing catalytic turnover.[5] Herein, we describe the rational development of an effective Pt II system for intermolecular hydroarylation with unactivated arenes and olefins and present mechanistic evidence consistent with a pathway involving aryl-olefin insertion and C À H bond oxidative addition at Pt II . While Markovnikov products are favored, anti-Markovnikov products are observed, and the mechanistic insight gained is promising for rational design of more selective and productive Pt II catalysts for these reactions.We recently reported that thermolysis of the five-coordinate Pt IV complexes [(LX)PtMe 3 ] {LX = dtbpp [3,5-di-tertbutyl-2-(2-pyridyl)pyrrolide] (1 a) or dppp [3,5-diphenyl-2-(2-pyridyl)pyrrolide] (1 b)} at 85-100 8C in C 6 D 6 in the presence of C 2 H 4 (9-60 equivalents) led to the release of ethane and methane and formation of Pt II complexes 2 a or 2 b, which contain a cyclometalated substituted pyrrolide group and C 2 H 4 ...

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

“…87,88 Also relevant here are data consistent with alkane intermediates involved in Pt(II) systems, 53,89 and implications of barriers for hydrocarbon dissociation which facilitate the polyfunctionalization of methane, best studied in cases of H/D exchange. 54 While previous studies of H/D exchange have implicated coordinated hydrocarbons as intermediates, 46,90 to our knowledge this is a rare example of such an effect impacting the selectivity of a catalytic reaction, and it suggests an important 21 This working hypothesis further hints at improved design strategies for olefin hydroarylation catalysts, e.g. via attenuation of the electrophilicity of the metal center with overall neutral catalysts.…”

“…It is also feasible that diethylbenzene formation begins with an initial cyclometalation of the phenethyl ligand, as reported by Goldberg et al for a related Pt system. 21 However, in this pathway (Scheme 16) the formation of 1,3-diethylbenzene and 1,4-diethylbenzene would likely require that the Pt(II) complex coordinate ethylbenzene with sufficient lifetime to allow a second CÀH activation (and ethylene insertion) to compete with net dissociation of ethylbenzene; hence, complex A is still implicated as an intermediate.…”

Exploratory Studies on the Reaction Between Iodoarenes and Acetylenes: One-pot, Pd-[Bmim][BF4] Catalyzed Preparation of Trianisylethylene

“…Current approaches to these substituted ethylenes include the hydroarylation reaction mediated by transition metals like copper [10], cobalt [11], iron [12], gold [13,14], rhodium [15,16], rhenium [17], platinum [18][19][20] or palladium [20][21][22][23][24][25][26][27], and metal-free protocols [28].…”

Synthesis and Characterization of Ru(II) Complexes as Potential Catalysts in Olefin Hydroarylation

“…Arene coordination is followed by C-H activation, and alkyl arene dissociation regenerates the starting catalyst. [67][68][69][70][71] Catalysts based on ruthenium, iridium and platinum complexes have been utilized to promote olefin hydroarylation using simple hydrocarbons. [42][43][44][45]56,62,[67][68][69][70][72][73][74][75][76] There are multiple potential benefits of olefin hydroarylation that proceeds by the cycle shown in Scheme 1.9.…”

“…(Figure 1.3). 68 The Friedel-Crafts reactions generally are not selective for dialkylation. The proposed catalytic cycle is shown in Scheme 1.12.…”