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 ...

Highly Selective 1,6‐Addition of Aryl Boronic Acids to α,β,γ,δ‐Unsaturated Carbonyl Compounds Catalyzed by an Iridium Complex

“…Although rhodium catalysts have been often used for the addition of aryl metal reagents to electron‐deficient alkenes,4 the iridium catalyst system employed in this addition has not been well demonstrated so far. As a related reaction, Mori and co‐workers reported the first example of the iridium‐catalyzed addition/elimination reaction of aryl silicon reagents with acrylates to give Mizoroki–Heck‐type products 9. To the best of our knowledge, the present 1,6‐addition is the first example of an iridium‐catalyzed addition of aryl boronic acids to electron‐deficient alkenes or dienes 10.…”

“…On the basis of the high reactivity toward the diene moiety and the high Z selectivity in the 1,6‐addition product, the catalytic cycle of the present iridium‐catalyzed reaction is speculated as shown in Scheme . Transmetalation of a phenyl group from the boron to iridium center forms a phenyl–iridium species II 9. 12 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 III 13.…”

A Decade of Exploration of Transition-Metal-Catalyzed Cross-Coupling Reactions: An Overview

“…[38][39][40] The importance of these palladium-catalyzed crosscouplings was finally recognized when the Nobel Prize in Chemistry in 2010 was jointly awarded to A. Suzuki, R. Heck, and E. Negishi. [41][42][43] Since the early developments in the dynamic area of cross-coupling reactions nearly fifty years ago, the diversity, scope, reactivity, value effectiveness, toxicity, required synthetic skill, and number of workable applications and limitations of TMs such as palladium, [44][45][46][47][48] iron, 38,[49][50][51][52] cobalt, [53][54][55][56] nickel, [57][58][59][60][61] copper, [62][63][64] rhodium, [65][66][67][68][69] ruthenium, 70,71 and iridium, 72 has led to thousands of publications in this field, and many reviews and books have cataloged the advancements. With all this, bottlenecks in cross-coupling reactions have encouraged scientists and researchers to formulate novel catalysts primarily based on naturally abundant and environmentally benign elements.…”

Highly Selective 1,6‐Addition of Aryl Boronic Acids to α,β,γ,δ‐Unsaturated Carbonyl Compounds Catalyzed by an Iridium Complex

“…As a related reaction, Mori and co-workers reported the first example of the iridiumcatalyzed addition/elimination reaction of aryl silicon reagents with acrylates to give Mizoroki-Heck-type products. [9] To the best of our knowledge, the present 1,6-addition is the first example of an iridium-catalyzed addition of aryl boronic acids to electron-deficient alkenes or dienes. [10] The unique 1,6-selectivity observed in the iridium-catalyzed addition is made clear by comparison with the result obtained with [{Rh(OH)(cod)} 2 ]…”

“…Transmetalation of a phenyl group from the boron to iridium center forms a phenyliridium species II. [9,12] The coordination of the dienone to the phenyl-iridium complex with a cisoid diene moiety results in the formation of a (h 4 -diene)-iridium complex III. [13] Insertion of the diene into the phenyl-iridium bond, thus forming p-allyl-iridium moiety IV, [14] followed by subsequent hydrolysis with the assistance of phenylboronic acid or boric acid gives the 1,6-addition product 2 a with Z configuration and the hydroxo-iridium species I.…”