Indium(III) Bromide-Catalyzed Chemioselective Dimerization of Vinylarenes

Abstract

I n d i u m ( I I I ) B r o m i d e -C a t a l y z e d C h e m i o s e l e c t i v e D i m e r i z a t i o n o f V i n y l a r e n e sAbstract: Indium(III) bromide catalyzes the dimerization of a-substituted vinylarenes. Chemioselectivity towards open chain or cyclic dimers depends on the nature of the substituent at the aryl group of the vinylarene.Acid-catalyzed dimerization of vinyl arenes has been known for long time. 1 Since this pioneer work, several acid catalysts have been used. 2 Aminium salts acting as one electron oxidants also promote the dimerization. 3 The products which were obtained from these reactions (Scheme 1) are all derived from a common intermediate 2.Scheme 1 Acid-catalyzed of a-methylstyrene derivatives Open chain unsaturated dimers 3 and 4 are generated by proton elimination, while production of indan cyclodimers 5 is the result of an intramolecular FriedelCrafts reaction within intermediate 2. Frequently, the formation of higher oligomers (trimers, tetramers) 6 as the result of reaction between intermediate 2 with the vinyl arene starting material 1 is observed. A common feature of these reactions is that, in most cases, a mixture of products is obtained. Therefore, the search for new chemoselective catalysts is still of importance.Recently, a protocol using palladium(II)-indium triflate catalyst was reported to transform a-methylstyrene into a mixture of the open chain unsaturated dimers 3 and 4. 4As a part of our interest to apply indium compounds in organic synthesis, we describe here the results of our investigation on such dimerization reactions catalyzed by indium(III) bromide.We first examined the catalytic activity of InBr 3 (10 mol%) on a-methylstyrene 1a (1 mmol) in CH 2 Cl 2 (2 mL) at 0°C (Scheme 2). After 2 hours of continuous stirring under nitrogen atmosphere, we observed (TLC) the complete consumption of 1a. The reaction was quenched with H 2 O, and the aqueous phase extracted with CH 2 Cl 2 . The organic phase was dried in vacuo and the product 2,4-diphenyl-4-methyl-1-pentene (4a) was purified by column chromatography on silica gel with hexanes. This procedure afforded 90% of yield of an analytically pure compound. When the reaction mixture was allowed to stir at 0°C for longer periods (>5 h), heavier oligomers 6a were obtained in 30% of yield together with 65% of the cyclic dimer 5a. At room temperature, 1a is directly transformed into 5a, when all other experimental conditions were kept constant. We further observed the isomerization of the open chain unsaturated dimer 4a into the corresponding indan derivative 5a. These results demonstrate that chemoselectivity towards dimers 4 is determined by kinetically controlled conditions; while thermodynamic control produces dimers 5. Further, the results suggest that oligomerization is achieved under kinetic conditions.

Study on Selective Dimerization of α-Methylstyrenes Promoted by Ionic Liquids

“…When MDP-1 was stirred at 120°C in [HexMIm]BF 4 /HBF 4 system for 24 h, it was completely converted into TPI. In addition, Peppe and co-workers have presumed that chemoselectivity towards MDP-1 was determined by kinetically controlled conditions, while thermodynamic control produced TPI [10]. Our experimental results were consistent with observations of Chaudhuri and Peppe.…”

“…The dimerization reaction of a-methylstyrene to produce TPI has been developed in the presence of cation-exchanged montmorillonite, a 75% yield of TPI was achieved [8]. Recently, metallic catalysts such as RuCl 3 Á H 2 O, InBr 3 and Pd(OAc) 2 /PPh 3 /In(OTf) 3 , have been used in the dimerization reaction of a-methylstyrene, and moderate to good product selectivities were obtained [9][10][11].…”

An Efficient and Selective Hydroarylation of Styrenes with Electron‐Rich Arenes, Catalyzed by Bismuth(III) Chloride and Affording Markovnikov Adducts

“…The structures of 4 were confirmed by 1 H-and 13 C NMR spectroscopy, MS data, and compared with the reported data. [7] It should be noted that no results were obtained when α,β-trisubstituted styrenes 2h and 2i were employed (Scheme 1). The reaction of 2h led to a complicated mixture of dimers, which was confirmed by GC and GC-MS.…”

Cationic Cyclizations of Fluoro Alkenes: Fluorine as a Controller and an Activator

“…4,4-Difluorohomoallylbenzene 23a (n = 1, R = Ph) afforded the six-membered cyclic ketone 24a, whereas the fluorine-free counterpart 27 was reported to give the flve-membered carbocycle 28 (Scheme 17) (22). These results suggest that the Friedel-Crafts-type ring closure is regioselectively controlled by the vinylic fluorines to give the difluorocycloalkanes via the a-fluorocarbocations 25.…”