Rhodium-catalyzed hydroformylation of olefins: Effect of [bis(2,4-di-tert-butyl) pentaerythritol] diphosphite (alkanox P-24) on the regioselectivity of the reaction Jimoh Tijani, Bassam El Ali * Chemistry Department, KFUPM, 31261 Dhahran, Saudi Arabia Received 2 March 2007; received in revised form 14 April 2007 Available online 24 April 2007 Abstract Rhodium (I) associated with [bis(2,4-di-tert-butyl) pentaerythritol] diphosphite (I) as a ligand represents an active catalyst system for highly regioselective hydroformylation of various alkenes. The commercially available bis(2,4-di-tert-butyl)pentaerythritol diphosphite (alkanox P-24) (I), which has been used so far as an antioxidant in the stabilization of polymers, was used as a diphosphite ligand for the selective hydroformylation reaction of olefins. Excellent selectivity towards linear aldehydes and excellent conversions were achieved in the hydroformylation of alkenes. The hydroformylation reaction was applied to various olefinic substrates including the internal alkenes. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Hydroformylation; Rhodium; Alkanox P-24; 1-Octene; Olefins; Phosphite ligands; Syngas 1. Introduction The rhodium-catalyzed low-pressure hydroformylation of olefins is, in terms of production volume, one of the most important homogeneous catalysis processes [1]. Since Wilkinson’s discovery through addition of phosphine ligands to rhodium-catalysts, the hydroformylation can be now performed at lower temperature and pressure [2]. Phosphite ligands and especially bulky phosphites are very useful in rhodium catalyzed hydroformylation because of the higher reaction rates obtained compared to phosphine ligands [3,4]. An important drawback however is the loss of selectivity, caused by isomerization [5]. One way to improve the selectivity was by changing to a diphosphite system. It was only after the first report from Bryant and coworkers that diphosphites were recognized as a new gen- eration of promising ligands in rhodium catalyzed hydro- formylation of alkenes [6]. High regioselectivity with diphosphite ligands (with 2,2 0 -biphenol backbone) in the rhodium catalyzed hydroformylation of functionalized alk- enes was claimed [7,8]. The hydroformylation of 1-octene catalyzed by Rh- (CO) 2 (acac) with chelated bisphosphites (calix[4]arene backbone) gave 99.5% nonanal at 80–100 °C and 5–20 bars [1]. The naturally occurring cinchona alkanoids, chiconi- dine, quinine and quinidine were hydroformylated to ter- minal aldehyde derivatives in yields of 87%, 81% and 85%, respectively, using Rh(CO) 2 (acac)/tetraphosphite cat- alyst at 90 °C and 20 bars CO/H 2 in toluene [9]. Sterically constrained diphosphonite ligands with xan- thene backbone were used in the rhodium-catalyzed hydro- formylation of 1-octene and 2-butene. High activities were obtained for 1-octene, with good selectivity to the linear aldehyde and for the 2-butene the selectivity for the n-pent- anal was 62% [10]. Bis(2,4-di-tert-butyl)pentaerythritol diphosphite (alka- nox P-24) (I), which was used mainly as an antioxidant in the stabilization of polymers [11], was used in the hydro- formylation of internal alkenes or mixture of alkenes [12,13]. 0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2007.04.022 * Corresponding author. Tel.: +966 3 8604491; fax: +966 3 8604277. E-mail address: belali@kfupm.edu.sa (B. El Ali). www.elsevier.com/locate/jorganchem Journal of Organometallic Chemistry 692 (2007) 3492–3497