A new and convenient one-pot method for a catalytic addition-elimination reaction using selenium electrophiles has been developed. In the presence of 5 mol % diphenyl diselenide, [bis(trifluoroacetoxy)iodo]benzene in acetonitrile converted a range of (E)-3-butenoic acids into the corresponding butenolides in good yields.
Thiocarboxylic acids, such as selenocarboxylic acids, exist predominantly in the thioxo form (RCSOH, thion acid) in polar solvents such as tetrahydrofuran (THF) at temperatures below −50 °C. Tellurocarboxylic acids (5) were observed for the first time by acidolysis of the corresponding cesium tellurocarboxylates with hydrogen chloride. The telluroic acids (6) exist predominantly in the telluroxo form (RCTeOH, telluron acid) in THF at temperatures below −70 °C. Telluron acids were reddish to blue violet for the aliphatics (R = alkyl) and dark green for the aromatics (R = aryl) and reacted with aryl isocyanates at −70 °C to give crystalline acyl carbamoyl tellurides in good yields.
Bowl-shaped phosphine (BSP) ligands markedly accelerated the rhodium-catalyzed hydrosilylation of ketones and ketimines as compared with the effect of conventional phosphine ligands such as PPh 3 and P(t-Bu) 3 . 31 P NMR study of a mixture of [RhCl(C 2 H 4 ) 2 ] 2 and phosphines at various P/Rh ratios revealed that coordination of BSP to the rhodium metal was successfully regulated, and the resultant rhodium complex bearing only one phosphine ligand (a mono(phosphine) rhodium species) was responsible for the acceleration. Structural comparison between BSP and the conventional phosphines was carried out using HF/6-31G(d)-CONFLEX/MM3-optimized structures. The mono(phosphine) rhodium complex having 1,5-cyclooctadiene as a ligand was isolated, and its X-ray molecular structure was determined.
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...
Dithiocarboxylic acids and their trimethylsilyl esters were found to readily react with potassium, rubidium, and cesium fluorides to give the corresponding alkali metal dithiocarboxylates 3-5 in moderate to good yields. A series of tetramethylammonium dithiocarboxylates 8 have been prepared in good yields by the reaction of sodium dithiocarboxylates 7 with tetramethylammonium chloride. The structures of potassium (3b), rubidium (4g), cesium (5f), and tetramethylammonium 4-methylbenzenecarbodithioates (8e) and tetramethylammonium 2-methoxybenzenecarbodithioate (8f) were characterized crystallographically. These heavier alkali metal salts (3b, 4g, 5f) have a dimeric structure [(RCSSM)(2), M = K, Rb, Cs] in which the two dithiocarboxylate groups are chelated to the metal cations which are located on the upper and lower sides of the plane involving the two opposing dithiocarboxylate groups. The K(+) cations interact with the tolyl fragment of a neighboring molecule, while the Rb(+) and Cs(+) cations interact with two neighboring tolyl fragments, in which the ipso and ortho carbons are positioned close to the metals. The interaction number of the metals with surrounding atoms is 8 for K(+) and Rb(+) and 12 for Cs(+). The C-S distances of the dithiocarboxylate group are different for the potassium salt 3b. In contrast, those of the rubidium salt 4g and cesium salt 5f are equal. Similarly, the chelating sulfur-metal bond distances of 3b are different, while those of 4g and 5f are almost equal. The dihedral angles of the phenyl ring and dithiocarboxylate plane increase in the order of the K, Rb, and Cs salts. The structural analysis of sodium 4-methylbenzenecarbodithioate (7g) revealed the presence of 4-CH(3)C(6)H(4)CS(2)Na(0.36). In contrast, the tetramethylammonium salts 8 are monomeric where the cation moieties are located out of the dithiocarboxylate plane. The potassium 3, rubidium 4, and cesium dithiocarboxylates 5 readily reacted with methyl iodide or triorganotin chlorides at room temperature to give the corresponding methyl 9 and triorganotin dithioesters 10 in good yields. At 0 degrees C, the reactivity of the rubidium 4 and cesium salts 5 to methyl iodides decreases dramatically compared with those of the sodium salts 7 and potassium salts 3.
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