Microwave-assisted SuzukieMiyaura cross-coupling of 2-alkyl and 2-alkenyl-benzo-1,3,2-diazaborolanes Siphamandla W. Hadebe y , Siphamandla Sithebe, Ross S. Robinson * Warren Research Laboratory, School of Chemistry, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa article info Article history: Received 14 July 2010 Received in revised form 6 March 2011 Accepted 28 March 2011 Available online 19 April 2011 Keywords: Hydroboration 2-Alkyl-1,3,2-diazaborolanes Coupling reactions Microwave abstract Nitrogen-based boronate esters, such as 2-octyl-benzo-1,3,2-diazaborolane, 2-phenethyl-benzo- 1,3,2-diazaborolane, and 2-{(1E)-hexenyl}-benzo-1,3,2-diazaborolane have been shown to be suitable coupling partners with arylhalides in microwave accelerated Suzuki cross-coupling reactions. Reaction yields of up to 89% were achieved. The use of a silicon group attached to the nitrogen atom, proved to enhance the reactivity of 2-octyl-benzo-1,3,2-diazaborolane. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction For two decades there has been intense research into the palladium-catalyzed cross-coupling reaction also known as the SuzukieMiyaura cross-coupling reaction since its discovery by Miyaura, Yanagi, and Suzuki. 1 The Suzuki-coupling methodology has proven to be extremely powerful and versatile in the formation of carbonecarbon bonds, 2aed and for the production of building blocks of pharmaceutical importance. 2eeh Recently, a number of research groups have focused on the de- velopment of new palladium catalyst systems, 3e5 more effective ligands 6 and bases 7 with the aim of enhancing the applicability of the Suzuki type chemistry. Much emphasis has been on in- vestigating the effects of different solvents, additives, ligand 8,9,10a,b and more recently the use of microwave irradiation. 10c However, literature precedent accumulated in this area has been based almost exclusively on the utility of boronic acids and boronate esters. 1,2 To date, only a few research groups have directed their attention to- ward expanding the scope of other potential Suzuki-coupling type organoboranes. 8,11,12 Specifically, nitrogen-based organoboranes have not been investigated in SuzukieMiyaura chemistry. To the best of our knowledge, only a few publications have been reported on the synthesis of nitrogen-based organoboranes. 13 We have recently published a new route to the synthesis of nitrogen-based organoboranes via rhodium-catalyzed hydro- boration to afford organoboranes in high yields. 14 The success of our new approach prompted our research into the synthesis of a range of nitrogen-based organoboranes (Table 1) and to explore their respective Pd-mediated coupling reactions with a range of arylhalides as shown in Scheme 1 . 2. Results and discussion Nitrogen-based organoboranes, namely 2-octyl-benzo-1,3,2- diazaborolane 2, 2-phenethyl-benzo-1,3,2-diazaborolane 3, and 2-[2-(4-methoxyphenyl)-ethyl]benzo-1,3,2-diazaborolane 4 were synthesized from benzo-1,3,2-diazaborolane 1 , which is readily prepared from commercially available boraneemethyl sulfide complex and inexpensive o-phenylenediamine, as shown in Table 1 . It was interesting to note that 1 was very robust to air and moisture when compared to the widely utilized oxygen analogues benzo- 1,3,2-dioxaborolane (catecholborane) and 4,4,6,6-tretramethyl- 1,3,2-dioxaborolane (pinacolborane). Benzo-1,3,2-diazaborlane 1 could be handled in an open vessel for several hours without no- table oxidation to boric acid. 14 We were delighted at the ease with which we were able to prepare the novel organoboranes 2, 3, and 4 in excellent yields, via Rh(I) catalyzed hydroboration of the corresponding alkenes. Rh(I) catalyzed reactions of similar alkenes with catecholborane have previously been intensively investigated and have been found to lead to a mixture of both internal and terminal products, which is * Corresponding author. Tel.: þ27 33 260 6363; fax: þ27 33 260 5009; e-mail address: RobinsonR@ukzn.ac.za (R.S. Robinson). y Present address: Hydroprocessing and Catalysis, Sasol Technology Research and Development, P.O. Box 1, Sasolburg 1947, South Africa. Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2011.03.095 Tetrahedron 67 (2011) 4277e4282