Iridium-catalyzed CÀC bond-forming reactions have recently attracted much attention. [1] However, the reaction with main group organometallic reagents containing elements such as boron, tin, and silicon has not been demonstrated so far. Although carbon±carbon bond formation with such main group reagents with an a,b-unsaturated carbonyl compound has been reported recently to undergo a Mizoroki±Heck-type or conjugate addition reaction with palladium, [2] rhodium, [3] and ruthenium [4] catalysts, the specific nature of the metal catalysts largely influences the reaction mechanism as well as the reaction course. Herein, we report that an iridium catalyst effects the Mizoroki±Heck-type addition/elimination reaction of a,b-unsaturated carbonyl compounds with several organosilicon reagents, and constitutes the first C À C bond formation with a main group reagent. [5] The reaction of PhSi(OMe) 3 (1) with butyl acrylate (2 a) in the presence of 5 mol % of [{IrCl(cod)} 2 ] (cod ¼ 1,5-cyclooctadiene) and tetrabutylammonium fluoride (TBAF) in toluene/H 2 O (6/1) at 120 8C for 24 h afforded the addition/ elimination product 3 a in 71 % yield while the conjugate addition product 4 a was not obtained at all. This result sharply contrasts that of the related reaction with the rhodium catalyst [{RhCl(cod)} 2 ] at 608C in THF/H 2 O (6/1), which affords the conjugate adduct 4 a as the major product with high selectivity, 3 a/4 a ¼ 3/97 (Scheme 1). [6] In contrast to the combination of silicon reagent 1 and TBAF under anhydrous conditions, which is highly effective for the palladiumcatalyzed cross-coupling reaction with organic halides, [7] the rhodium-or iridium-catalyzed reaction of this combination with 2 a proceeded only very slightly without the addition of water. Table 1 summarizes the iridium-catalyzed Mizoroki± Heck-type addition/elimination reactions of several organosilicon reagents with a,b-unsaturated carbonyl compounds. Both iridium chloride and methoxide exhibited similar reactivities in the reactions with TBAF. The reaction also proceeded in THF/H 2 O at lower temperature (70 8C), although a small amount of conjugate addition product was formed (70/5). Several aryl silanes bearing a substituent on the aromatic ring also effected the reaction. Although the reaction of an ortho-substituted aryl silane was found to be slightly slower, the Mizoroki±Heck-type product was selectively obtained over the 1,4-addition product. Alkenylsilanes, which were prepared by hydrosilylation of alkynes, could also effect the reaction to afford a diene in 67 % yield.Aryl silanediols (7±9) were also found to participate in the addition/elimination reaction. By contrast to alkoxysilanes, silanediols effected the reactions without addition of TBAF and water. However, no reaction occurred when [{IrCl(cod)} 2 ] was used as the catalyst. Only [{Ir(OMe)(cod)} 2 ] was found to be an effective catalyst. Worthy of note is that the reaction with silanediols can be a halogen-free process, which is in contrast to the palladium-catalyzed Mizoroki±Heck ...

Rhodium-catalyzed hydrosilylation of internal alkynes furnished (E)-1,2-disubstituted alkenylsilanes. The obtained alkenylsilane was subjected to reaction with a,b-unsaturated carbonyl compounds in the presence of a rhodium catalyst to undergo conjugate addition. One-pot hydrosilylationconjugate addition with a rhodium catalyst was also performed.Keywords: conjugate addition; 1,2-disubstituted alkenylsilanes; hydrosilylation; internal alkynes; rhodium Hydrosilylation of alkynes serves as an important tool for the synthesis of alkenylsilanes, which can be transformed into a variety of organic molecules by transition metal-catalyzed carbon-carbon bond-forming reactions with organic electrophiles.[1] We have been studying the use of rhodium complexes as a catalyst for the hydrosilylation of alkynes and found that a rhodium catalyst was effective for the regio-and stereoselective hydrosilylation of terminal alkynes.[2] Our further interest has turned to investigation of the reaction of internal alkynes, which form 1,2-disubstituted alkenylsilanes. [3] We herein report that with the use of a rhodium complex as a catalyst the hydrosilylation of internal alkynes took place at room temperature. Further reactions of the thus formed 1,2-disubstituted alkenylsilanes with several a,b-unsaturated carbonyl compounds in the presence of a rhodium complex were also studied.The reaction of diphenylethyne (1a) with triethoxysilane (2) was carried out in the presence of 0.5 mol % of [RhCl(cod)] 2 (cod ¼ 1,5-cyclooctadiene) at room temperature. The corresponding alkenylsilane (3a) was obtained after stirring for 3 h [Eq. (1)]. Hydrosilylation of 3-hexyne (1b) with 2 also proceeded stereoselectively to give (E)-3-triethoxysilyl-hex-3-ene (3b) in a quantitative yield. The reaction proceeded efficiently without solvent. Worthy of note is that the reaction took place at room temperature with a small amount of the rhodium catalyst (0.025 -0.5 mol %). By contrast, the similar reaction with a platinum catalyst, (n-Bu 4 N) 2 PtCl 6 , did not proceed at room temperature. ð1ÞThe stereochemistry of the product was found to be the E-form, which was confirmed by treatment of 3a with tetra-n-butylammonium fluoride (TBAF) in the presence of CuI to give stilbene (4a) (Z/E ¼ 9 : 1), suggesting that cis-addition of H À Si took place [Eq. (2)].(4) [4] To confirm the stereochemistry of 3b, whose desilylation led to the rather volatile 3-hexene, hydrosilylation of dodecyne (1c) was carried out and the desilylation of 3c with TBAF afforded (Z)-6-dodecene (4c) in 81% yield. The formation of 4c was also confirmed by comparison with the authentic sample, which was synthesized by a Wittig reaction of hexanal and the phosphonium salt of 1-bromohexane. The stereochemical outcome of hydrosilylation contrasts to that with a ruthenium catalyst reported by Trost to induce the trans-addition. [5,6] ð2Þ COMMUNICATIONS

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Iridium-catalyzed CÀC bond-forming reactions have recently attracted much attention. [1] However, the reaction with main group organometallic reagents containing elements such as boron, tin, and silicon has not been demonstrated so far. Although carbon±carbon bond formation with such main group reagents with an a,b-unsaturated carbonyl compound has been reported recently to undergo a Mizoroki±Heck-type or conjugate addition reaction with palladium, [2] rhodium, [3] and ruthenium [4] catalysts, the specific nature of the metal catalysts largely influences the reaction mechanism as well as the reaction course. Herein, we report that an iridium catalyst effects the Mizoroki±Heck-type addition/elimination reaction of a,b-unsaturated carbonyl compounds with several organosilicon reagents, and constitutes the first C À C bond formation with a main group reagent. [5] The reaction of PhSi(OMe) 3 (1) with butyl acrylate (2 a) in the presence of 5 mol % of [{IrCl(cod)} 2 ] (cod ¼ 1,5-cyclooctadiene) and tetrabutylammonium fluoride (TBAF) in toluene/H 2 O (6/1) at 120 8C for 24 h afforded the addition/ elimination product 3 a in 71 % yield while the conjugate addition product 4 a was not obtained at all. This result sharply contrasts that of the related reaction with the rhodium catalyst [{RhCl(cod)} 2 ] at 608C in THF/H 2 O (6/1), which affords the conjugate adduct 4 a as the major product with high selectivity, 3 a/4 a ¼ 3/97 (Scheme 1). [6] In contrast to the combination of silicon reagent 1 and TBAF under anhydrous conditions, which is highly effective for the palladiumcatalyzed cross-coupling reaction with organic halides, [7] the rhodium-or iridium-catalyzed reaction of this combination with 2 a proceeded only very slightly without the addition of water. Table 1 summarizes the iridium-catalyzed Mizoroki± Heck-type addition/elimination reactions of several organosilicon reagents with a,b-unsaturated carbonyl compounds. Both iridium chloride and methoxide exhibited similar reactivities in the reactions with TBAF. The reaction also proceeded in THF/H 2 O at lower temperature (70 8C), although a small amount of conjugate addition product was formed (70/5). Several aryl silanes bearing a substituent on the aromatic ring also effected the reaction. Although the reaction of an ortho-substituted aryl silane was found to be slightly slower, the Mizoroki±Heck-type product was selectively obtained over the 1,4-addition product. Alkenylsilanes, which were prepared by hydrosilylation of alkynes, could also effect the reaction to afford a diene in 67 % yield.Aryl silanediols (7±9) were also found to participate in the addition/elimination reaction. By contrast to alkoxysilanes, silanediols effected the reactions without addition of TBAF and water. However, no reaction occurred when [{IrCl(cod)} 2 ] was used as the catalyst. Only [{Ir(OMe)(cod)} 2 ] was found to be an effective catalyst. Worthy of note is that the reaction with silanediols can be a halogen-free process, which is in contrast to the palladium-catalyzed Mizoroki±Heck ...