Titanium superoxide: a heterogeneous catalyst for anti-Markovnikov aminobromination of olefins Tanveer Mahamadali Shaikh, Pratibha U. Karabal, Gurunath Suryavanshi, Arumugam Sudalai * Chemical Engineering and Process Development Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune, 411008 Maharashtra, India article info Article history: Received 3 March 2009 Revised 20 March 2009 Accepted 26 March 2009 Available online 28 March 2009 Keywords: Titanium superoxide N-Bromosuccinimide p-Toluenesulfonamide Bromoamination anti-Markovnikov addition abstract A new facile procedure for the aminobromination of olefins in high yields has been described using p-tol- uene sulfonamide (p-TsNH 2 ) and N-bromosuccinimide (NBS) as nitrogen and bromine sources, respec- tively, and titanium superoxide as a truly heterogeneous catalyst. The formation of anti-Markovnikov product exclusively in all the cases studied possibly proceeding through a free radical reaction pathway is remarkable. Ó 2009 Elsevier Ltd. All rights reserved. The vicinal haloamine functionality represents a useful struc- tural moiety as well as a versatile building block in organic and medicinal chemistry. 1 In the area of synthetic organic chemistry, these can be readily converted into a variety of useful functional derivatives on replacement of halogen with nucleophiles in both intramolecular and intermolecular fashion. 2 Literature search reveals that vicinal haloaminations are carried out using several catalysts such as V 2 O 5 , 3 MnSO 4 , 3 Mn(II) salen, 3 Cu-salts, 4 InBr 3 , 5 Fe-salt, 6 BF 3 , 7 SnCl 4 , 8 Pd-complex, 9 PhI(OAc) 2 , 10 N-N-dihalosulf- onamides, 11a N-N-dihalocarbamates, 11b–g N-halocarbamates 11h cyanamide-NBS 11i as well as by noncatalytic routes which include use of Bronsted acids 12 such as H 2 SO 4 or ionic liquid media [Bmim][BF 4 ]. 13 But most of these methods suffer from drawbacks such as lack of product selectivity and use of non-economic, unsta- ble, and toxic metals as catalysts. Hence, a search for readily avail- able non-toxic, inexpensive and reusable catalyst is highly desirable. In this Letter, we report that titanium superoxide 14a cat- alyzes aminobromination of olefins, which proceeds regiospecifi- cally in an anti-Markovnikov fashion and under truly heterogeneous conditions. Some time ago, we have reported a new method for the prepa- ration of titanium superoxide, which was found to exist as a remarkably stable radical at ambient conditions. 15 This heteroge- neous catalyst has been subsequently found to catalyze the selec- tive oxidation of aromatic primary amines and phenols to the corresponding nitro aromatics and p-quinones, respectively. 15 Dur- ing the course of our study on further application of titanium superoxide in organic synthesis, we have now found that olefins can be regiospecifically aminobrominated using p-TsNH 2 and NBS as nitrogen and bromine sources under ambient conditions. For in- stance, when styrene was subjected to bromoamination, the corre- sponding anti-Markovnikov product, 1, was formed in 81% yield; whereas the commercially available TiO 2 , under similar conditions, gave the expected Markovnikov product, 2 in 30% yield (Scheme 1). Encouraged by this result, it was of interest to screen several other titanium salts such as titanium silicalite (a zeolite), TiCl 4 , and titanium isopropoxide under similar reaction conditions; the results of which are presented in Table 1. Remarkably, titanium superoxide gave the anti-Markovnikov product 1 in 81% yield whereas all other titanium salts furnished the expected Markovni- kov product, 2 with low yields. Among several solvents screened, CH 2 Cl 2 was found to be more suitable for titanium superoxide-cat- alyzed aminobromination of olefins. Thus, the optimal condition 0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2009.03.169 * Corresponding author. Tel.: +91 20 25902174; fax: +91 20 25902676. E-mail address: a.sudalai@ncl.res.in (A. Sudalai). i Br NHTs NHTs Br 1 2 TiO 2 titanium superoxide -- -- 81% 30% Scheme 1. Reagents and conditions: Titanium catalyst (10 wt %), p-TsNH 2 (1.1 equiv), NBS (1 equiv), CH 2 Cl 2 , 25 °C, 14 h. Tetrahedron Letters 50 (2009) 2815–2817 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet