Rate Enhancement with a Bowl‐Shaped Phosphane in the Rhodium‐Catalyzed Hydrosilylation of Ketones

Abstract

Since the nature of P ligands is very important in transitionmetal-catalyzed reactions, a wide variety of these ligands has been designed to realize high catalytic activity and selectivity. [1] So far, most P ligands are rather small, and their design and modification have hitherto been performed within close proximity of the P atom. Recently, several large (nanosized) phosphorus ligands were developed for transition-metalcatalyzed reactions. [2,3] In the course of our studies, [3] we found that a bowl-shaped [4] phosphane ligand markedly enhances the rate of rhodium-catalyzed hydrosilylation of ketones. [5] The two triarylphosphanes tris(2,2'',6,6''-tetramethyl-mterphenyl-5'-yl)phosphane [6] (denoted as P(tm-tp) 3 ) and tris(m-terphenyl-5'-yl)phosphane (denoted as P(tp) 3 ) were prepared and compared with common phosphanes in the rhodium-catalyzed hydrosilylation of cyclohexanone with a trisubstituted silane (Table 1). P(tm-tp) 3 was first prepared in 2001 [6a] and its Pd 0 complex [Pd{P(tm-tp) 3 } 2 ] was reported in 2002. [6b] In the presence of catalytic amounts of P(tm-tp) 3 and [{RhCl(C 2 H 4 ) 2 } 2 ] (P/Rh = 2), the reaction proceeded smoothly in benzene at room temperature over 3 h, and cyclohexanol was obtained in 97 % yield after desilylation (Table 1, entry 1). In contrast, the same reaction with P(tp) 3 afforded the product in only 25 % yield (entry 2). Furthermore, other representative triarylphosphanes (entries 3-6) and trialkylphosphanes (entries 7-9) were also much less effective than P(tm-tp) 3 . With these ligands (entries 2-9), the reactions were sluggish at room temperature, and much longer reaction times (40-500 h) were required to obtain the products in good yields (70-95 %). A kinetic study indicated that the P(tm-tp) 3 catalyst system (entry 1) realized 154, 31, and 28 times faster reactions than PPh 3 (entry 3), P(tp) 3 (entry 2), and P(o-tol) 3 (entry 5), respectively. [7] Benzene is a better solvent than CH 2 Cl 2 in the reactions of entries 1-3.The rate enhancement with P(tm-tp) 3 was further confirmed with various silanes and ketones, and compared with P(tp) 3 and PPh 3 (Table 2). With HSiEt 3 (Table 2, entries 1-3) or HSiMePh 2 (entries 4-6), the hydrosilylation of cyclohexanone proceeded much faster with P(tm-tp) 3 (entries 1 and 4) than with P(tp) 3 (entries 2 and 5) and PPh 3 (entries 3 and 6). Furthermore, in the hydrosilylation of various ketones such as acetophenone (entries 7-9), 2-octanone (entries 10-12), and (À)-menthone (entries 13-15), rate enhancement with P(tmtp) 3 was also evident. As catalyst precursor, the cationic rhodium complex [Rh(cod) 2 ]BF 4 (cod = cyclooctadiene) showed a similar rate enhancement with P(tm-tp) 3 (entries 16-18). P(tm-tp) 3 is a much more efficient than P(tp) 3 , although the two ligands strongly resemble each other. The structures of P(tm-tp) 3 and P(tp) 3 were optimized by HF/6-31G(d) calculations [8a] on initial structures generated by CON-P R R R R A B B 3 R= Me: P(tm-tp) 3 R= H: P(tp) 3 Table 1: Effects of ligands in the hydrosilylat...

Monophosphanylcalix[6]arene Ligands: Synthesis Characterization, Complexation, and Their Use in Catalysis

“…The X-ray analysis of [PtCl 2 (3) 2 ] shows that 3 adopts a cone conformation upon complexation. Combination of 3 and 5 with [Rh(COD) 2 ]BF 4 provides an active catalyst for the hydroformylation of a variety of terminal alkenes.…”

“…(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) ene, [6b] and an Ru II complex of a tetrakis(diphenylphosphanylmethyl)calix [4]arene.…”

“…Phosphane ligands in general play an important role in transition-metal-catalyzed reactions, and a wide variety of phosphanes have been prepared to realize high catalytic activity and selectivity. [3,4] The phosphanylcalixarenes are attractive ligands since they integrate the strong coordination ability of phosphanes and the unique cavity of the calixarenes to create a spatially confined environment upon complexation with transitional metals. [2] As far as the size of the cavity is concerned, major attention has been paid to phosphanylcalix [4]arenes [5] due to their rigid conformations.…”

“…[3,4] The phosphanylcalixarenes are attractive ligands since they integrate the strong coordination ability of phosphanes and the unique cavity of the calixarenes to create a spatially confined environment upon complexation with transitional metals. [2] As far as the size of the cavity is concerned, major attention has been paid to phosphanylcalix [4]arenes [5] due to their rigid conformations. As for the phosphanylcalix [4]arenes, we have reported Pt II and Pd II complexes of a bis(diphenylphosphanyl)calix [4]arene, [6a] solid-state and solution structures of a tetrakis(diphenylphosphanyl)calix [4] 4 , respectively.…”

“…[2] As far as the size of the cavity is concerned, major attention has been paid to phosphanylcalix [4]arenes [5] due to their rigid conformations. As for the phosphanylcalix [4]arenes, we have reported Pt II and Pd II complexes of a bis(diphenylphosphanyl)calix [4]arene, [6a] solid-state and solution structures of a tetrakis(diphenylphosphanyl)calix [4] 4 , respectively. The X-ray analysis of [PtCl 2 (3) 2 ] shows that 3 adopts a cone conformation upon complexation.…”