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 Pd0 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(C2H4)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 PPh3 (entry 3), P(tp)3 (entry 2), and P(o-tol)3 (entry 5), respectively.[7] Benzene is a better solvent than CH2Cl2 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 PPh3 (Table 2). With HSiEt3 (Table 2, entries 1–3) or HSiMePh2 (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 PPh3 (entries 3 and 6). Furthermore, in the hydrosilylation of various ketones such as acetophenone (entries 7–9), 2-octanone (entries 10–12), and ( )-menthone (entries 13–15), rate enhancement with P(tmtp)3 was also evident. As catalyst precursor, the cationic rhodium complex [Rh(cod)2]BF4 (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 CONP R