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Organosilicon Reagents: Synthesis and Application to Palladium-Catalyzed Cross-Coupling Reactions

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Organosilicon Reagents: Synthesis and Application to Palladium-Catalyzed Cross-Coupling Reactions Organosilicon Reagents: Synthesis and Application to Palladium-Catalyzed Cross-Coupling Reactions Scott E. Denmark* and Michael H. Ober Roger Adams Laboratory Department of Chemistry University of Illinois Urbana, IL 61801, USA Email: [email protected] Outline 1. Introduction 2. Synthesis of Organosilicon Reagents 2.1. Reaction of Organometallic Reagents with Halosilanes or Cyclosiloxanes 2.2. Hydrosilylation of Alkynes and Alkenes 2.3. Transition-Metal-Catalyzed Coupling of Aryl Halides with Silyl Hydrides 3. Palladium-Catalyzed Cross-Coupling 3.1. Alkynylsilanes 3.2. Alkenylsilanes 3.2.1. Fluoride Activation 3.2.1.1. Alkenyl(trialkyl)silanes 3.2.1.2. Alkenylsilanol Surrogates Professor Scott E. Denmark (left) receiving the 2003 ACS Michael H. Ober 3.2.1.2.1. Alkenylsiletanes Award for Creative Work in Synthetic Organic Chemistry from Dr. Chris D. Hewitt, Aldrich Vice President of 3.2.1.2.2. Alkenyl(thienyl)- and Marketing and R&D. Photo © James Tkatch. Alkenyl(pyridyl)- silanes 3.2.1.2.3. Alkenyl(benzyl)- 4.1. Intermolecular Hydrosilylation/ reactions, and have found wide application in silanes Cross-Coupling synthetic organic chemistry. But, due to a 3.2.1.2.4. Alkenylsilyl Hydrides 4.2. Intramolecular Hydrosilylation/ number of drawbacks inherent to these 3.2.1.3. Heteroatom-Substituted Cross-Coupling substrates, organosilicon reagents have Alkenylsilanes 4.3. Silylformylation/Cross-Coupling emerged as competitive alternatives. The 3.2.1.3.1. Alkenylsilyl Halides 4.4. Mizoroki–Heck Reaction/ lack of toxicity, high chemical stability, and 3.2.1.3.2. Alkenyl(alkoxy)- Cross-Coupling low molecular weight of organosilanes make silanes 4.5. Ring-Closing Metathesis/ them ideal for use as nucleophilic partners in 3.2.1.3.3. Alkenyldisiloxanes Cross-Coupling cross-coupling with organic halides and and Poly(alkenyl- 4.6. Alder-Ene/Cross-Coupling pseudohalides. Silicon-based reagents were siloxanes) 5. Summary and Outlook originally considered to be insufficiently 3.2.1.3.4. Alkenylsilanols 6. Acknowledgements active toward palladium-catalyzed cross- 3.2.2. Fluoride-Free Activation of 7. References coupling, but early work by Hiyama et al. Alkenylsilanols, Diols, and showed that organosilanes could be activated Triols 1. Introduction by a nucleophilic promoter.4 Following this 3.3. Arylsilanes discovery, a multitude of organosilanes 3.3.1. Arylsiletanes Transition-metal-catalyzed cross-coupling bearing a wide variety of substituents about 3.3.2. Arylsilyl Halides of organometallic reagents with organic the silicon center as well as an assortment of 3.3.3. Aryl(alkoxy)silanes halides has become a powerful method for transferable groups have been identified.5 3.3.4. Arylsilanols, Diols, and Triols carbon–carbon-bond formation. Organotin,1 This review will concentrate on recent 3.4. Alkylsilanes organoboron,2 and organozinc3 reagents are advances in the use of organosilanes in 4. Tandem Reactions of Organosilicon well established as competent precursors palladium-catalyzed cross-coupling reac- Compounds for palladium-catalyzed cross-coupling tions, with an emphasis on the preparative VOL.36, NO. 3 • 2003 75 vinylmagnesium bromide with a dialkyl- chlorosilane6 readily provides the corre- sponding aryl- or vinylsilyl hydride. The newly formed silyl hydride can then be subjected to further manipulations to gain access to silyl ethers, silyl halides, and silanols.6,7 Direct formation of organosilanols can be similarly accomplished by addition of the organometallic reagent to a number of readily available and inexpensive cyclo- siloxanes.8 Aqueous hydrolysis of the substituted polysiloxane formed in situ provides the organosilanol, which can then be easily purified by column chroma- Figure 1. Preparation of Organosilanes by Reaction tography or distillation. Although this of Organometallic Reagents with Halosilanes or Cyclosiloxanes. method of synthesis is both straightforward and efficient, it is limited by substrate compatibility with the organometallic agent employed. This problem can be avoided by protection of the sensitive functional group or, better still, by use of one of the alternative methods described below. 2.2. Hydrosilylation of Alkynes and Alkenes The metal-catalyzed hydrosilylation of Figure 2. Preparation of alkynes and alkenes to provide substituted Organosilanes by Hydrosilylation of Alkynes. alkenylsilanes and alkylsilanes avoids the use of highly reactive organometallic reagents and the limitations associated with them.9 The insertion of a platinum,8e,10 rhodium,11 or ruthenium12 catalyst into an Si–H bond, provides an active metal hydride that undergoes addition across an unsaturated organic precursor generating an organosilyl halide or organosilyl ether (Figure 2). As with the previous method, the organosilicon compounds thus generated can be further manipulated to obtain organosilanols or other silicon species. The stereo- and regioselectivity of the hydro- Figure 3. Preparation of Organosilanes by silylation are dependent on the catalyst Transition-Metal-Catalyzed Coupling of Aryl Halides with Silyl Hydrides. employed and the silicon precursor involved in the reaction, but a judicious choice of the aspects of this chemistry and on the synthesis in large measure, to the relatively low cost two agents can lead to high levels of control and applications of these substrates. and availability of the silicon-containing in most cases.9 Obviously, this method starting materials. Once synthesized, organo- is well suited for the preparation of 2. Synthesis of Organosilicon silanes are stable compounds that can be alkenylsilanes and alkylsilanes. Other Reagents purified by chromatography or distillation methods are needed to install the silicon and can be easily handled in most cases. functionality into aryl or heteroaryl subunits. In response to the growing interest in the palladium-catalyzed cross-coupling of 2.1. Reaction of Organometallic 2.3. Transition-Metal-Catalyzed organosilicon compounds, a number of Reagents with Halosilanes Coupling of Aryl Halides methods to prepare a wide range of coupling or Cyclosiloxanes with Silyl Hydrides precursors are now available. The prepara- tions of these compounds are diverse, and are The use of organolithium and organo- Recently, investigations into the trans- based on the type of organic moiety being magnesium reagents for the nucleophilic ition-metal-catalyzed coupling of organic employed as well as the substituents about displacement of a leaving group at the silicon halides with silyl hydrides have provided a the silicon atom. Of the methods described, center is one of the simplest methods to powerful method for the synthesis of most can be preformed on a large scale to introduce silicon into an organic molecule organosilanes. Base-activated insertion of a provide useful quantities of material thanks, (Figure 1). The reaction of phenyllithium or transition-metal complex into an Si–H bond 76 VOL.36, NO. 3 • 2003 followed by transfer of the silyl group to an aryl or vinyl halide successfully provide the desired organosilane (Figure 3).13 This reaction is limited to the coupling of alkoxysilanes, alkylsilanes, and halosilanes with electron-rich aryl iodides and bromides. eq 1 13b,c The process is catalyzed by Pd(dba)2 or 13a [Rh(cod)(MeCN)2]BF4 in the presence of a stoichiometric amount of base, and is thought to proceed through Si–H/C–I bond exchange by oxidative addition of the hydrosilane followed by σ-bond metathesis [π-allylPdCl2]2 (APC) in THF at ambient all of these precursors react via a common of the Si–Pd/C–I bonds.13c temperature, to provide substituted alkynes intermediate, hypothesized to be a hydrogen- in good yields (eq 1).15a Catalytic amounts of bonded silanol–fluoride adduct.17 Because of 3. Palladium-Catalyzed CuCl or CuCl–Pd(PPh3)4 can also be used at this mechanistic commonality, a wide range Cross-Coupling elevated temperatures in DMF to effect the of organosilanes, that display a similar coupling of alkynyl(trialkyl)silanes with reactivity yet provide a variety of precursor The coupling of organosilicon reagents aryl chlorides or triflates.15b Alternatively, options, are competent reagents for the with organic halides and pseudohalides has dimethyl(phenylethynyl)silanol undergoes fluoride-activated cross-coupling reaction. evolved to be comparable in scope to other facile coupling with aryl iodides in THF at Three classes of alkenylsilanes undergo palladium-catalyzed coupling methods.4,5 60 ˚C in the presence of a stoichiometric facile fluoride-promoted cross-coupling: (1) Generally, the conditions for the palladium- amount of TBAF or Ag2O and 5 mol % of alkenylsilanes bearing alkyl groups on the 15c catalyzed cross-coupling are mild, but do Pd(PPh3)4. silicon, (2) alkenylsilanes bearing fluoride- require a promoter [tetrabutylammonium cleavable organic groups (alkenylsilanol fluoride (TBAF), tetramethylammonium 3.2. Alkenylsilanes surrogates), and (3) heteroatom-substituted fluoride (TMAF), tris(diethylamino)sulfonium alkenylsilanes. In general, alkyl-substituted trimethyldifluorosilicate (TASF), potassium The palladium-catalyzed cross-coupling alkenylsilanes are extremely stable to trimethylsilanolate (TMSOK), Ag2O, Cs2CO3, of alkenylsilanes with organic halides and chemical manipulations prior to fluoride etc.] to provide high yields of the desired pseudohalides has gained much attention activation, but do not readily undergo cross-coupling products. The byproducts of because of the mild reaction conditions productive cross-coupling. Alkenylsilanol
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