Simple and Efficient 1,3‐Isomerization of Allylic Alcohols using a Supported Monomeric Vanadium‐Oxide Catalyst

The catalytic isomerization of allylic alcohols by the 1,3-transposition of a hydroxy group is an important reaction in organic synthesis. 2] This isomerization has been applied to the industrial production of terpene alcohols, such as geraniol and nerol from linalool. In early studies, the isomerization of allylic alcohols was accomplished by using metal-oxo catalysts, such as trialkyl vanadates and tungsten-oxo complexes, operating at high temperatures (>130 8C). Since then, some improvements in the catalytic efficiency have been reported. However, these catalytic systems have a variety of associated problems, including the decomposition of the catalyst, the formation of byproducts, 12] the requirement for the addition of activators, 6, 10] or use of precious metals 11, 12] and their difficulty in recycling the catalysts. Heterogeneous catalysts have numerous advantages over homogeneous catalysts in terms of their durability and their facile separation from the reaction mixture and subsequent reusability, as well as their applicability in packed column reactors and multi-step flow reactors. However, despite these advantages, few efficient heterogeneous catalysts have been developed for the 1,3-isomerization of allylic alcohols. The attempted heterogenization of a formerly homogeneous active tungsten species by using polyvinyl pyridine as a support has been reported, but was unsuccessful owing to decomposition of the polymer and leaching of the active metal. Thus far, there has only been one report of an effective heterogeneous catalytic system, which was based on the use of polymerbound vanadyl phosphate (PhosphonicS POVO). Herein, we report the use of silica-supported monomeric vanadium-oxo species (V/SiO2) to effectively promote the isomerization of various allylic alcohols under mild reaction conditions, without the addition of any activators. The V/SiO2 catalyst can be easily prepared and is highly durable, thus exhibiting excellent reusability without any loss of efficiency. The V/SiO2 catalyst was prepared according to an impregnation method. SiO2 (3.0 g) was added to an aqueous solution of NH4VO3 (8.4 mm). The mixture was stirred at 80 8C for 2 h, followed by evaporation of the water. The obtained slurry was dried overnight at 110 8C and subsequently calcined under a flow of dry air at 500 8C for 5 h to afford V/SiO2 (V: 1.4 wt. %). V/SiO2 was characterized by X-ray absorption fine structure (XAFS) spectroscopy. The V K-edge X-ray absorption near-edge structure (XANES) spectrum of V/SiO2 exhibited an absorption edge at 5480 eV, which was assigned to V species (see the Supporting Information, Figure S1). The spectrum also showed a strong pre-edge peak, which was more intense than that of V2O5 yet weaker than that of Na3VO4, thus suggesting that the V species in V/SiO2 adopted a distorted tetrahedral structure. 20] The diffuse-reflectance UV/Vis spectrum of V/ SiO2 showed a charge-transfer band at about 250–300 nm, which was assigned to monomeric tetrahedral VO4 species (see the Supporting Information, Figure S2). The Raman spectrum showed a peak at 1040 cm , which was attributed to the V=O stretching vibration (see the Supporting Information, Figure S3). These results indicate that vanadium is present on the SiO2 surface as a monomeric vanadium-oxo species with a distorted tetrahedral structure, which is in good agreement with previous results reported by Tanaka and co-workers. Initially, we examined the catalytic activity of V/SiO2 for the 1,3-isomerization of a-vinylbenzyl alcohol (1) in MeCN at room temperature. Interestingly, V/SiO2 exhibited pronounced catalytic activity, thereby affording cinnamyl alcohol (2) in 73 % yield with no byproducts (Table 1, entry 1). A longer reaction time further increased the yield of 2 to 95 % (Table 1, entry 2). In sharp contrast to these results, the use of supported vanadium species V/Al2O3, V/TiO2, and V/hydrotalcite [24] hardly promoted the isomerization (Table 1, entries 8–10). Substituting V/ SiO2 with the previously reported and commercially available heterogeneous V catalyst PhosphonicS POVO resulted in a very low yield of 2 (Table 1, entry 11). In addition, the use of V compounds V2O5 and VO(acac)2, as well as the NH4VO3 precursor compound and the SiO2 catalyst support, all led to poor yields of 2 (Table 1, entries 12–15). These results clearly demonstrate that V/SiO2 has outstanding catalytic activity for the isomerization reaction. Hence, the immobilization of V on the SiO2 support clearly resulted in the creation of highly active vanadium species for promoting the isomerization reaction. Subsequently, the V/SiO2 catalyst was removed from the reaction solution by filtration once 40 % conversion of 1 had been achieved and the stirring of the filtrate was resumed with periodic analysis of the conversion. No further conversion was found after the removal of the catalyst, thus demonstrat[a] Dr. T. Mitsudome, S. Sueoka, S. Ikeda, Dr. T. Mizugaki, Prof. Dr. K. Jitsukawa, Prof. Dr. K. Kaneda Department of Materials Engineering Science Graduate School of Engineering Science Osaka University 1-3 Machikaneyama, Toyonaka, Osaka 560-8531 (Japan) Fax: (+ 81) 6-6850-6260 E-mail : kaneda@cheng.es.osaka-u.ac.jp