Iridium‐Catalyzed Mizoroki–Heck‐Type Reaction of Organosilicon Reagents

Abstract

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‐ and Palladium‐Catalyzed Asymmetric Conjugate Additions

“…On the basis of the high reactivity towards the diene moiety and the high cis selectivity in the 1,6-addition product, the catalytic cycle that was surmised is depicted in Scheme 1. 75 from the boron to iridium-hydroxide A forms an aryl-iridium species B [229]; the coordination of the dienone to the phenyl-iridium complex with a cisoid diene moiety results in the formation of a (η 4 -diene)-iridium complex C. Insertion of the diene into the phenyl-iridium bond then occurs, forming the π-allyl-iridium moiety D, and this is followed by a selective hydrolysis of D at the α position to the carbonyl function with the assistance of phenylboronic acid or boric acid to give the 1,6-addition product cis-196 and regenerate the hydroxo-iridium species A.…”

Addition‐Elimination of Aryldimethylaluminum Reagents to Vinylarenes Promoted by the Addition of a Ketone

“…[2,3] Analogous reactions with several main-group organometallic reagents insteado fa ryl halidesh ave also been shown to proceed with several transition-metal catalysts. [4,5] We have recently reported, as ap reliminary communication, that rhodium-catalyzed oxidative Mizoroki-Heck-type reactions of arylaluminumr eagents with styrene derivatives affords 1,2diarylethenes. [6] The reactiond emonstrated the facile synthesis of stilbene derivatives, which can potentially be applied to the preparation of p-conjugated advanced organic materials.…”

Recent Developments in the Chemistry of Vinylsiloxanes

“…Treatment of alkenylsiloxane 112 with ethyl acrylate (113) in the presence of 5 mol% [{IrCl(cod)} 2 ] and TBAF under aqueous conditions resulted in a cross-coupling reaction to furnish ethyl (2E,4E)-5-phenylpenta-2,4dienoate (114, Scheme 30). 34 It should be noted that the use of triethoxy(vinyl)silane gave comparatively higher yields than diethoxy(vinyl)silane.…”