Addition of Aromatic C−H Bonds to η3-Allenyl/Propargyl Complexes. Organoplatinum-Induced Electrophilic Aromatic Substitution Reactions

One of the fundamental strategies to achieve C -C coupling between an aromatic ring and an unsaturated carbon -carbon bond is via Friedel -Crafts reactions in which a Lewis acid is often essential. 1 To use organometallic electrophiles for such a purpose is attractive because the metal may activate the ligated organic moiety intrinsically and facilitate the carbon -carbon bond formation in new manners. 2,3 A recently discovered cationic η3-allenyl/ -propargyl complex of platinum, [Pt(PPh 3)2(η-C3H3)](BF4) (1), is found to be a potent electrophile and is prone to regioselective nucleophilic addition at the central carbon of the novel η-C3H3 moiety.4 The remarkable reactivity of 1 toward a wide variety of hard nucleophiles prompted us to explore the reaction of 1 with soft aromatic species. We report here the unprecedented regioselective addition of aromatic C -H bonds across the C tC bond of the C3H3 fragment. These reactions are found to undergo an organoplatinum-induced electrophilic substitution and lead to aryl vinylation. Hydropyrrolylation of [Pt(PPh 3)2(η-C3H3)](BF4) (1) is accomplished by directly reacting 1with pyrrole orN-methylpyrrole at 25°C in a dry nitrogen atmosphere. As the electrophilic attack on pyrrole normally occurs at the 2-position, the C coupling takes place exclusively between the central carbon of C 3H3 and the 2-pyrrolyl carbon, thereby generating the central-carbonsubstitutedη3-2-pyrrolylallyl complexes{Pt(PPh3)2[η-CH2C(2C4H3NR)CH2]}(BF4) (R ) H (2a), Me (2b)) with excellent yields. In the reaction of 1 with indole, the insertion of C3H3 into the 3-indolyl C-H bond leads to the formation of {Pt(PPh3)2[η-CH2C(3-indolyl)CH2]}(BF4) (3a). The addition of 3-methylindole to 1 gives rise to the formation of{Pt(PPh3)2[ηCH2C(2-(3-methyl)indolyl)CH2]}(BF4) (3b) at a relatively slow rate. As to the aryl system, complex 1 can undergo selective addition of the para aryl C-H bond of PhNMe2 to form an arylallyl complex{Pt(PPh3)2[η-CH2C(p-Me2NC6H4)CH2]}(BF4) (4). Hydroarylation reactions of other electron-riched arenes such as dimethoxyor trimethoxybenzene to 1 with regioselectivity of electrophilic aromatic substitution are also successful and yield {Pt(PPh3)2[η-CH2C(Ar)CH2]}(BF4) (Ar ) 2,4-(MeO)2C6H3 (5), 2,4,6-(MeO) 3C6H2 (6)). In contrast, less electron rich aromatics such as benzene, toluene, xylene, and anisole do not react with 1 under the same conditions. Nucleophilic addition to 6 with nBu4NBH4, NaSPh, or Na[CH(SO 2Ph)2] results in CH2C[2,4,6(MeO)3C6H2]CH2Nu (Nu) H (7a), SPh (7b), CH(SO2Ph)2 (7c), respectively) (Scheme 1). With respect to the arenes, the overall transformation accomplishes vinylation of arenes, which provides the products as resulting from the Heck or Stille vinylic arylation.5 The regiochemistry of the C -H activation in the aromatics appears to be in agreement with the reactivity of electrophilic aromatic substitution. To look for further evidence for the mechanism of electrophilic substitution, a crossover labeling experiment has been carried out. A sample of 1,3,5(MeO)3C6D3-nHn (n ) 0, 1) with d2:d3 ) 22:78 was mixed with an equimolar amount of 1,3,5-(MeO) 3C6H3. The resulting 1,3,5trimethoxybenzene with a labeling distribution of d 0:d2:d3 ) 50:11:39 was allowed to first react with 1 and followed by treatment with NaSPh. The NMR spectra showed the formation of 7b with deuterium appearing at the aryl ring and the allyl termini but without stereospecificity. The mass spectroscopy provides the deuterium distribution of d 0:d1:d2:d3 ) 28: 31:34:7 in the products. Such results strongly support a process with intermolecular hydrogen scrambling during the reaction courses. A mechanism that involves electrophilic aromatic substitution may elucidate these reactions (Scheme 2). The electrophilic 1 replaces an aryl proton to form a metallacyclobutene intermediate. Ensuing protonation of metallacyclobutene then generates the arylallyl complexes. Casey’s and our findings concerning synthesis of metallacyclobutene complexes by adding nucleophiles such as Et 3N, Ph3P, or pyridine to η3-allenyl/propargyl complexes provide the evidence for such a mechanism. 6