James P. Stambuli

Transition Metal Catalyzed Synthesis of Aryl Sulfides

The presence of aryl sulfides in biologically active compounds has resulted in the development of new methods to form carbon-sulfur bonds. The synthesis of aryl sulfides via metal catalysis has significantly increased in recent years. Historically, thiolates and sulfides have been thought to plague catalyst activity in the presence of transition metals. Indeed, strong coordination of thiolates and thioethers to transition metals can often hinder catalytic activity; however, various catalysts are able to withstand catalyst deactivation and form aryl carbon-sulfur bonds in high-yielding transformations. This review discusses the metal-catalyzed arylation of thiols and the use of disulfides as metal-thiolate precursors for the formation of C-S bonds.

Zinc-Mediated Palladium-Catalyzed Formation of Carbon−Sulfur Bonds

A catalytic amount of zinc chloride in combination with a palladium catalyst ligated by a monodentate phosphine allows the coupling of aryl and alkyl thiols with aryl bromides in high yields. The addition of zinc chloride to a palladium catalyst system that reportedly failed to promote sulfide formation allows this once ineffective catalyst system to provide the sulfide product in good yield. This paper describes a high-yielding and general monodentate phosphine-ligated palladium catalyst for biaryl and alkyl aryl sulfide formation.

Allylic oxidations of terminal olefins using a palladium thioether catalyst.

A palladium catalyst that converts terminal olefins to linear allylic acetates at lower catalyst loadings and faster reaction times than current systems is reported. This reaction can be conducted using benzoquinone as the oxidizing agent or catalytic amounts of copper and hydroquinone under one atmosphere of oxygen. Preliminary reactivity studies of pi-allylpalladium complexes under our reaction conditions do not provide results similar to those obtained in the catalytic reaction, which may suggest an alternative reaction pathway. The palladium catalyst is ligated by an aryloxyalkyl aryl sulfide, which is identified as a new ligand for homogeneous catalysis.

Nickel-Catalyzed Synthesis of Oxazoles via C−S Activation

The synthesis of 2-substituted oxazoles is achieved via nickel-catalyzed cross-coupling reaction of 2-methylthio-oxazole and various organozinc reagents. An extension of this method is demonstrated with a chemoselective, one-pot synthesis of unsymmetrical 2,5-disubstituted oxazoles. This synthesis of 2- and 2,5-substituted oxazoles using this method provides great advantages over previous methods for these compounds and is highly complementary to current cyclodehydration strategies.

Synthesis of N-Arylindazoles and Benzimidazoles from a Common Intermediate

A variety of N-aryl-1H-indazoles and benzimidazoles were synthesized from common arylamino oximes in good to excellent yields. The product selectivity depends upon the base used in the reaction, as triethylamine promoted the formation of benzimidazoles, whereas 2-aminopyridine promoted the formation of N-arylindazoles. This method is valuable to the synthetic community because both indazoles and benzimidazoles are prevalent in pharmaceuticals.

A Practical, Metal-Free Synthesis of 1H-Indazoles

The synthesis of 1H-indazoles is achieved from o-aminobenzoximes by the selective activation of the oxime in the presence of the amino group. The reaction occurs with a variety of substituted o-aminobenzoximes using a slight excess of methanesulfonyl chloride and triethylamine at 0-23 degrees C and is amenable to scale-up. The synthesis of 1H-indazoles under these conditions is extremely mild compared with previous synthetic approaches and affords the desired compounds in good to excellent yields.

Development of an Asymmetric Trimethylenemethane Cycloaddition Reaction: Application in the Enantioselective Synthesis of Highly Substituted Carbocycles

A protocol for the enantioselective [3+2] cycloaddition of trimethylenemethane (TMM) with electron-deficient olefins has been developed. The synthesis of novel phosphoramidite ligands was critical in this effort, and the preparation and reactivity of these ligands is detailed. The evolution of the ligand design, commencing with acyclic amine-derived phosphoramidites and leading to cyclic pyrrolidine and azetidine structures, is discussed. The conditions developed to effect an asymmetric TMM reaction using 2-trimethylsilylmethyl allyl acetate were shown to be tolerant of a wide variety of alkene acceptors, providing the desired methylenecyclopentanes with high levels of enantioselectivity. The donor scope was also explored, and substituted systems were tolerated, including one bearing a nitrile moiety. These donors were reactive with unsaturated acylpyrroles, giving the product cyclopentane rings bearing three stereocenters in high enantioselectivity and complete diastereoselectivity.

Recent advances in the discovery of organometallic catalysts using high-throughput screening assays.

Combinatorial techniques were developed initially to accelerate the identification of molecules with biological activity. The successes of these techniques inspired the design of high-throughput methods to assist in the discovery of new catalysts. Over the past year, many groups in academia and industry have utilized high-throughput screening assays to reduce the time required to identify catalysts for asymmetric processes, cross-coupling reactions and other metal-catalyzed transformations. The continued success of combinatorial techniques in organometallic chemistry should propagate the development of new and improved methods to facilitate catalyst discovery.

Oxidant-controlled stereoselectivity in the Pd-catalyzed allylic oxidation of cis-vinylsilanes.

The allylic oxidation of cis-vinylsilanes is reported. The reaction requires a low catalyst loading of Pd(OAc)(2) without the need for an external ligand. Interestingly, trans-vinylsilanes are unreactive, whereas allylic oxidations of cis-vinylsilanes proceed in good yields giving a single diastereo- and regioisomer of the branched allylic acetate trans-vinylsilane when benzoquinone is employed. The use of PhI(OAc)(2) as oxidant in place of benzoquinone provides the branched, cis-vinylsilane as the major product. Additionally, the first intramolecular allylic C-H etherifications of cis-vinylsilanes to give oxygen heterocycles are also described.

Calcium-Catalyzed Pictet−Spengler Reactions

Pictet-Spengler reactions of m-tyramine and aldehydes produced tetrahydroisoquinolines in the presence of a catalytic amount of Ca[OCH(CF3)2]2. This reaction occurs with a variety of aryl, heteroaryl, and alkyl aldehydes, producing tetrahydroisoquinolines in high yield and with high regioselectivity. This calcium-promoted Pictet-Spengler reaction provides a mild alternative to the traditional Brønsted acids typically employed in these reactions.