Synthesis and cross-coupling reaction of alkenyl[(2-hydroxymethyl)phenyl]dimethylsilanes

Copper-Catalyzed Asymmetric Synthesis of Silicon-Stereogenic Benzoxasiloles

A Cu-catalyzed asymmetric synthesis of silicon-stereogenic benzoxasiloles has been realized via intramolecular Si-O coupling of [2-(hydroxymethyl)phenyl]silanes. Cu(I)/difluorphos is found to be an efficient catalytic system for enantioselective Si-C bond cleavage and Si-O bond formation. In addition, kinetic resolution of racemic substituted [2-(hydroxymethyl)phenyl]silanes using Cu(I)/ PyrOx (pyridine-oxazoline ligands) as the catalytic system is developed to afford carbon- and silicon-stereogenic benzoxasiloles. Ring-opening reactions of chiral benzoxasiloles with organolithiums and Grignard reagents yield various enantioenriched functionalized tetraorganosilanes.

Palladium- and nickel-catalyzed cascade enantioselective ring-opening/coupling reactions of cyclobutanones

The chemistry of small ring compounds is an intriguing subject in organic chemistry. As the smallest stable cyclic aliphatic ketones, cyclobutanones have garnered tremendous attention owing to their intrinsic high reactivity such as transition-metal catalyzed C-C bond cleavage. In this context, transition-metal catalyzed formal cycloaddition of cyclobutanones via a "cut and sew" strategy has gained marvelous advances. In contrast, an alternative reaction paradigm, i.e., transition-metal catalyzed ring-opening reactions of cyclobutanones, is still underdeveloped. This feature article aims to summarize our efforts in developing enantioselective palladium-catalyzed ring-opening/coupling reactions and recently emerging nickel-catalyzed ring-opening/reductive coupling reactions of cyclobutanones with a tethered aryl halide. The possible mechanisms are briefly showcased and the advantages and limitations of each strategy as well as their synthetic applications in the synthesis of natural products or bioactive compounds are presented.

Palladium-catalyzed enantioselective domino ring-opening/Hiyama coupling of cyclobutanones: development and application to the synthesis of (+)-herbertene-1,14-diol

A palladium-catalyzed enantioselective domino ring-opening/Hiyama coupling of cyclobutanones is reported, providing chiral 1-indanones bearing a versatile alkenyl group.

Designing Cross-Coupling Reactions using Aryl(trialkyl)silanes

Organo(trialkyl)silanes have several advantages, including high stability, low toxicity, good solubility, easy handling, and ready availability compared with heteroatom-substituted silanes. However, methods for the cross-coupling of organo(trialkyl)silanes are limited, most probably because of their exceeding robustness. Thus, a practical method for the cross-coupling of organo(trialkyl)silanes has been a long-standing challenging research target. This article discusses how aryl(trialkyl)silanes can be used in cross-coupling reactions. A pioneering example is Cu^II catalytic conditions with the use of electron-accepting aryl- or heteroaryl(triethyl)silanes and aryl iodides. The reaction forms biaryls or teraryls. This design concept can be extended to Pd/Cu^II -catalyzed cross-coupling polymerization reactions between such silanes and aryl bromides or chlorides and to Cu^I -catalyzed alkylation using alkyl halides.

Carboallylation of electron-deficient alkenes by palladium/copper catalysis

The carboallylation of electron-deficient alkenes with tetraorganosilicon reagents and allylic carbonates has been developed by palladium/copper catalysis.

Pincer cobalt complex-catalyzed Z-selective hydrosilylation of terminal alkynes

Co-Catalyzed Z-selective hydrosilylation of terminal alkynes produces (Z)-β-vinylsilanes, which are further converted to (Z)-disubstituted alkenes by Pd-catalyzed cross couplings.

Ruthenium-catalysed hydrosilylation of carbon–carbon multiple bonds

Recent advances in ruthenium-catalysed hydrosilylation of C–C multiple bonds and its application to organic synthesis are highlighted.

Diastereoselective and enantioselective conjugate addition reactions utilizing α,β-unsaturated amides and lactams

The conjugate addition reaction has been a useful tool in the formation of carbon-carbon bonds. The utility of this reaction has been demonstrated in the synthesis of many natural products, materials, and pharmacological agents. In the last three decades, there has been a significant increase in the development of asymmetric variants of this reaction. Unfortunately, conjugate addition reactions using α,β-unsaturated amides and lactams remain underdeveloped due to their inherently low reactivity. This review highlights the work that has been done on both diastereoselective and enantioselective conjugate addition reactions utilizing α,β-unsaturated amides and lactams.

Synthesis of cyclic alkenylsiloxanes by semihydrogenation: a stereospecific route to (Z)-alkenyl polyenes

Cyclic alkenylsiloxanes were synthesized by semihydrogenation of alkynylsilanes-a reaction previously plagued by poor stereoselectivity. The silanes, which can be synthesized on multigram scale, undergo Hiyama-Denmark coupling to give (Z)-alkenyl polyene motifs found in bioactive natural products. The ring size of the silane is crucial: five-membered cyclic siloxanes also couple under fluoride-free conditions, whilst their six-membered homologues do not, enabling orthogonality within this structural motif.

The design, synthesis and validation of recoverable and readily reusable siloxane transfer agents for Pd-catalyzed cross-coupling reactions

The development of competent, recoverable and reusable 1-oxa-2-silacyclopentene (siloxane) transfer agents for Pd-catalyzed cross-coupling reactions (CCRs) of organolithium reagents with aryl and alkenyl iodides has been achieved. Drawbacks of the first-generation siloxane-transfer agent (1), relating to facile recovery for potential recycling, have been addressed.

Palladium-catalysed cross-coupling of organosilicon reagents

The palladium catalysed cross-coupling of organosilicon reagents with organo halides and pseudo-halides has developed over the past 30 years into an efficient and attractive carbon-carbon bond forming strategy. Extensive research within this field to expand and diversify on the scope of the organosilicon coupling reaction will continue to promote its use in the synthesis of biologically and pharmaceutically important organic molecules. The recent advances made within this area are explored in this critical review (199 references).

Solvent-Free Synthesis of Siloxanes and their Usage as Potential Cross-Coupling Reagents

Hexaarylcyclotrisiloxanes were selectively prepared by solvent-free synthesis from diarylsilanediol. These well-known stable compounds were found to undergo palladium-catalyzed cross-coupling reaction with aryl halides in good yields. All of attached aryl groups on silicon atom could be utilized. Furthermore, base-sensitive functionality such as acetyl and nitro groups survived the reaction.

Iridium-catalyzed arene ortho-silylation by formal hydroxyl-directed C-H activation

A strategy for the ortho-silylation of aryl ketone, benzaldehyde, and benzyl alcohol derivatives has been developed in which a hydroxyl group formally serves as the directing element for Ir-catalyzed arene C-H bond activation. One-pot generation of a (hydrido)silyl ether from the carbonyl compound or alcohol is followed by dehydrogenative cyclization at 80-100 °C in the presence of norbornene as a hydrogen acceptor and the combination of 1 mol % [Ir(cod)OMe]2 and 1,10-phenanthroline as a catalyst to form benzoxasiloles. The synthetic utility of the benzoxasilole products is demonstrated by conversion to phenol or biaryl derivatives by Tamao-Fleming oxidation or Hiyama cross-coupling. Both of these transformations of the C-H silylation products exploit the Si-O bond in the system and proceed by activation of the silyl moiety with hydroxide, rather than fluoride.

Silicon-based cross-coupling reactions in the total synthesis of natural products

Unlike other variants of transition-metal-catalyzed cross-coupling reactions, those based on organosilicon donors have not been used extensively in natural product synthesis. However, recent advances such as: 1) the development of mild reaction conditions, 2) the expansion of substrate scope, 3) the development of methods to stereoselectively and efficiently introduce the silicon-containing moiety, 4) the development of a large number of sequential processes, and 5) the advent of bifunctional bis(silyl) linchpin reagents, signify the coming of age of silicon-based cross-coupling reactions. The following case studies illustrate how silicon-based cross-coupling reactions play a strategic role in constructing carbon-carbon bonds in selected target molecules.

Palladium(II)/cationic 2,2'-bipyridyl system as a highly efficient and reusable catalyst for the Mizoroki-Heck reaction in water

A water-soluble and air-stable Pd(NH₃)₂Cl₂/cationic 2,2’-bipyridyl system was found to be a highly-efficient and reusable catalyst for the coupling of aryl iodides and alkenes in neat water using Bu₃N as a base. The reaction was conducted at 140 °C in a sealed tube in air with a catalyst loading as low as 0.0001 mol % for the coupling of activated aryl iodides with butyl and ethyl acrylates, providing the corresponding products in good to excellent yields with very high turnover numbers. In the case of styrene, Mizoroki-Heck coupling products were obtained in good to high yields by using a greater catalyst loading (1 mol %) and TBAB as a phase-transfer agent. After extraction, the residual aqueous solution could be reused several times with only a slight decrease in its activity, making the Mizoroki-Heck reaction “greener”.

Palladium-catalyzed cross-coupling reactions of potassium alkenyltrifluoroborates with organic halides in aqueous media

Potassium vinyl and alkenyltrifluoroborates are cross-coupled with aryl and heteroaryl bromides using 1 mol % Pd loading of 4-hydroxyacetophenone oxime derived palladacycle or Pd(OAc)2 as precatalysts, K2CO3 as base, and TBAB as additive and water reflux under conventional or microwave heating to afford styrenes, stilbenoids, and alkenylbenzenes. These borates can be cross-coupled diastereoselectively with allyl and benzyl chlorides using KOH as base in acetone-water (3:2) at 50 degrees C and 0.1 mol % Pd loading, giving the corresponding 1,4-dienes and allylarenes, respectively. These simple phosphine-free reaction conditions allow the palladium recycling from the aqueous phase during up to five runs by extractive separation of the products, which contain 58-105 ppm of Pd.