Metallocene bis(perfluoroalkanesulfonate)s as air-stable cationic Lewis acids Renhua Qiu a , Guoping Zhang a , Xinhua Xu a, * , Kangbin Zou a , Lingling Shao a , Dawei Fang a , Yinhui Li a , Akihiro Orita b, * , Ryosuke Saijo b , Hidetaka Mineyama b , Tomoyoshi Suenobu c , Shunichi Fukuzumi c, * , Delie An a, * , Junzo Otera b, * a Department of Chemistry, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China b Department of Applied Chemistry, Okayama University of Science, Ridai-cho, Okayama 700-0005, Japan c Department of Material and Life Science, Graduate School of Engineering, Osaka University, SORST, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan article info Article history: Received 19 November 2008 Received in revised form 24 December 2008 Accepted 24 December 2008 Available online 3 January 2009 Keywords: Metallocenes Perfluorooctanesulfonates Lewis acids abstract Zirconocene and titanocene bis(perfluorooctanesulfonate)s were synthesized. In contrast to the corre- sponding triflates and perchlorates, these compounds are air- and water-stable. They were proved to be ionic on the basis of conductivity measurements and X-ray analysis, allowing these complexes to be stored for months. The strong Lewis acidity of these cationic metallocene species, which was proved by ESR study, enabled catalytic glycosylation. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The Lewis acid chemistry has played a major role in modern or- ganic synthesis [1], yet the conflict between the following issues remains still unsettled. The Lewis acid is desired to be as strongly acidic as possible to acquire higher activity, while it becomes more susceptible to hydrolysis upon increasing the acidity. Cationic group 4 metallocene compounds, which have attracted increasing attention recently [2], represent a typical example. The metallo- cene bis(triflate) complexes of zirconium and titanium Cp 2 M(OTf) 2 (Cp = C 5 H 5 , Tf = CF 3 SO 2 ) were initially obtained by treatment of Cp 2 MCl 2 with AgOTf [3] and later from Cp 2 ZrMe 2 and TfOH [4]. These complexes were successfully employed as catalysts for Diels–Alder reaction [5], Mukaiyama–Aldol reaction [6], Hosomi– Sakurai reaction [6a,7], [3+2] nitrone–olefin cycloaddition reaction [8], and glycosylation [9]. Unfortunately, however, these metallo- cene bis(triflate)s are not stable in open air [10], suffering from fac- ile hydrolysis. Thus, these complexes must be handled under strictly anhydrous conditions. Another notable species are putative metallocene perchlorates Cp 2 M(ClO 4 ) 2 (M = Hf and Zr), which have found extremely versatile application to glycosylation technology [11]. These species also cannot be isolated due to their hydrolytic instability, and so they are usually generated in situ prior to reac- tion by treating Cp 2 MCl 2 with potentially explosive AgClO 4 . Accordingly, improvement of the hygroscopic character of the cat- ionic metallocene derivatives is highly demanding from the stand- point of practical utilization as catalysts. Recently, we disclosed that the perfluorooctanesulfonate group worked as an effective counter-anion to provide cationic organotin species [12], which were found air-stable and water-tolerant in sharp contrast to the corresponding highly hygroscopic organotin triflates [13]. This finding has led us to postulate that longer per- fluoroalkanesulfonate groups could serve for overcoming the hydrolytic instability of the cationic organometallic species in a general sense. We report herein successful isolation of air-stable cationic metallocene (M = Zr and Ti) bis(perfluoroalkanesulfo- nate)s, which enabled facile assessment of Lewis acidity and cata- lytic activity. 2. Results and discussion The synthesis of zirconocene and titanocene bis(perfluoroal- kanesulfonate)s is straightforward (Scheme 1). Treatment of metal- locene dichlorides, Cp 2 MCl 2 [M = Zr (1a), Ti (1b)] with silver perfluorooctanesulfonate (AgOSO 2 C 8 F 17 = AgOPFOS) (2 equiv.) in THF or silver perfluorobutanesulfonate (AgOSO 2 C 4 F 9 = AgOPFBS) (2 equiv.) in Et 2 O or THF allowed us to isolate the corresponding perfluoroalkanesulfonate derivatives Cp 2 M(OPFOS) 2 Á nH 2 O Á THF 0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2008.12.057 * Corresponding authors. Address: Department of Applied Chemistry, Okayama University of Science, Ridai-cho, Okayama 700-0005, Japan. Tel./fax: +81 86 256 4292 (A. Orita). E-mail addresses: xhx1581@yahoo.com.cn (X. Xu), orita@high.ous.ac.jp (A. Orita), fukuzumi@chem.eng.osaka-u.ac.jp (S. Fukuzumi), deliean@sina.com (D. An), otera@high.ous.ac.jp (J. Otera). Journal of Organometallic Chemistry 694 (2009) 1524–1528 Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem