Fumitoshi Shibahara

Diene Hydroacylation from the Alcohol or Aldehyde Oxidation Level via Ruthenium-Catalyzed C−C Bond-Forming Transfer Hydrogenation: Synthesis of β,γ-Unsaturated Ketones

Under the conditions of ruthenium-catalyzed transfer hydrogenation, isoprene couples to benzylic and aliphatic alcohols 1a-g to deliver beta,gamma-unsaturated ketones 3a-g in good to excellent isolated yields. Under identical conditions, aldehydes 2a-g couple to isoprene to provide an identical set of beta,gamma-unsaturated ketones 3a-g in good to excellent isolated yields. As demonstrated by the coupling of butadiene, myrcene, and 1,2-dimethylbutadiene to representative alcohols 1b, 1c, and 1e, diverse acyclic dienes participate in transfer hydrogenative coupling to form beta,gamma-unsaturated ketones. In all cases, complete branch regioselectivity is observed, and, with the exception of adduct 3j, isomerization to the conjugated enone is not detected. Thus, formal intermolecular diene hydroacylation is achieved from the alcohol or aldehyde oxidation level. In earlier studies employing a related ruthenium catalyst, acyclic dienes were coupled to carbonyl partners from the alcohol or aldehyde oxidation level to furnish branched homoallylic alcohols. Thus, under transfer hydrogenative coupling conditions, all oxidation levels of substrate (alcohol or aldehyde) and product (homoallyl alcohol or beta,gamma-unsaturated ketone) are accessible.

Ruthenium-Catalyzed C−C Bond Forming Transfer Hydrogenation: Carbonyl Allylation from the Alcohol or Aldehyde Oxidation Level Employing Acyclic 1,3-Dienes as Surrogates to Preformed Allyl Metal Reagents

Under the conditions of ruthenium-catalyzed transfer hydrogenation, commercially available acyclic 1,3-dienes, butadiene, isoprene, and 2,3-dimethylbutadiene, couple to benzylic alcohols 1a-6a to furnish products of carbonyl crotylation 1b-6b, carbonyl isoprenylation 1c-6c, and carbonyl reverse 2-methyl prenylation 1d-6d. Under related transfer hydrogenation conditions employing isopropanol as terminal reductant, isoprene couples to aldehydes 7a-9a to furnish identical products of carbonyl isoprenylation 1c-3c. Thus, carbonyl allylation is achieved from the alcohol or the aldehyde oxidation level in the absence of preformed allyl metal reagents. Coupling to aliphatic alcohols (isoprene to 1-nonanol, 65% isolated yield) and allylic alcohols (isoprene to geraniol, 75% isolated yield) also is demonstrated. Isotopic labeling studies corroborate a mechanism involving hydrogen donation from the reactant alcohol or sacrificial alcohol (i-PrOH).

Direct Arylation of Simple Azoles Catalyzed by 1,10-Phenanthroline Containing Palladium Complexes: An Investigation of C4 Arylation of Azoles and the Synthesis of Triarylated Azoles by Sequential Arylation

Direct triarylation and sequential triarylation reactions of simple azoles catalyzed by [Pd(phen)(2)]PF(6) are described. Simple azoles, such as N-methylimidazole, thiazole, and oxazole, were observed to undergo triaryaltion reactions even at their C4 positions when treated with aryl iodides in the presence of [Pd(phen)(2)]PF(6) as a catalyst and a stoichiometric amount of Cs(2)CO(3) in DMA at 150 °C. Using excess amounts of azoles, selective C5 monoarylation was achieved by using the same catalytic system. Subsequent efforts demonstrated that C5 arylated azoles undergo exclusive C2 arylation using [Pd(phen)(2)]PF(6) as the catalyst with galvinoxyl as an additive. Finally, unprecedented C4 arylation reactions of 2,5-diaryl-azoles occur by using the new catalytic system to give the corresponding triarylated products in good to excellent yields. The results of mechanistic studies suggest that the C2 arylation process takes place by way of an electrophilic aromatic substitution (S(E)Ar) palladation pathway, while arylation reactions at the C4 position occur via a S(E)Ar palladation and/or radical mechanism. Finally, a concise, three-step synthesis of the Tie-2 Tyrosine Kinase Inhibitor has been executed starting with commercially available N-methylimidazole by a route that employs the new sequential arylation process.

Synthesis of Fluorescent 1,3-Diarylated Imidazo[1,5-a]pyridines: Oxidative Condensation−Cyclization of Aryl-2-Pyridylmethylamines and Aldehydes with Elemental Sulfur as an Oxidant

Oxidative condensation-cyclization of aldehydes and aryl-2-pyridylmethylamines proceeded in the presence of a stoichiometric amount of elemental sulfur as an oxidant in the absence of catalyst. The reaction gave a variety of 1,3-diarylated imidazo[1,5-a]pyridines in good to high yields. The products showed fluorescence emission in a wavelength range of 454-524 nm. The quantum yields of 1,3-diarylated imidazopyridines were greatly improved compared to those of the parent 3-monosubstituted compounds.

Palladium-catalyzed C-H bond direct alkynylation of 5-membered heteroarenes: a well-defined synthetic route to azole derivatives containing two different alkynyl groups.

A widely applicable oxidative coupling of 5-membered heteroarenes and terminal alkynes that uses a combination of palladium and silver salts was developed. Under suitable conditions, imidazole and benzimidazole, which are sluggish under similar previously reported oxidative coupling conditions, as well as imidazo[1,5-a]pyridines, oxazole, benzoxazole, thiazole, and benzothiazole could be alkynylated. In addition, the bromine atom on the substrates was intact under the reaction conditions, and conventional Sonogashira coupling did not occur at all. With these reactivities in hand, a well-defined synthetic route to imidazo[1,5-a]pyridines and thiazole containing two different alkynyl groups was achieved in a simple manner. In addition, linear correlations were observed between the fluorescence wavelength and the Hammett substituent constants of aryl groups, not only on the C1- but also on the C3-alkynyl group of the obtained 1,3-bis(arylethynyl)imidazo [1,5-a]pyridines.

One-pot Sequential Direct C–H Bond Arylation of Azoles Catalyzed by [Pd(phen)2](PF6)2: Synthetic Methods for Triarylated Azoles

Synthetic methods for triarylated azoles containing three different aryl groups via one-pot sequential multiple C-H bond arylations are described. The one-pot sequential diarylation of C5-monoarylated azoles was achieved by the simple sequential addition of two different aryl iodides with a [Pd(phen)(2)]PF(6) catalytic system. The one-pot triarylation of N-methylimidazole was achieved by the combination of a previously reported Pd(OAc)(2)-P(2-furyl)(3) system and the present [Pd(phen)(2)]PF(6) system. In this case, portionwise addition of aryl halide, base and the catalyst in the final step significantly improved the overall yield of the desired triarylated product. These protocols led to triarylated azoles without a loss of efficiency compared to the corresponding previously reported stepwise syntheses via direct C-H bond arylation.

Copper-Catalyzed CH Bond Direct Chalcogenation of Aromatic Compounds Leading to Diaryl Sulfides, Selenides, and Diselenides by Using Elemental Sulfur and Selenium as Chalcogen Sources Under Oxidative Conditions

The reactions of aromatic compounds and elemental chalcogens catalyzed by a copper salt with molecular oxygen as an oxidant were carried out. The reaction of 3-substituted imidazo[1,5-a]pyridines and elemental sulfur in the presence of CuTC (copper(I) thiophenecarboxylate) gave the corresponding bisimidazopyridyl sulfides in good to quantitative yields. The reaction proceeded even under aerobic oxidation conditions. The use of a polar solvent was crucial for the reaction, and DMSO (dimethyl sulfoxide) in particular stimulated the reaction. The reaction could be applied to common aromatic compounds, such as N-methyl indole and dialkyl anilines. The reaction of indole proceeded at the nucleophilic C3 position rather than at the acidic C2 position. In addition, the reaction of dialkyl anilines proceeded with an ortho, para orientation. The reactions of imidazopyridines and elemental selenium under similar conditions gave the corresponding bisimidazopyridyl diselenides along with bisimidazopyridyl monoselenides. The resulting diselenides were readily converted to the corresponding monoselenides with unreacted imidazopyridines under the same conditions. The reaction could be applied to the copolymerization of bifunctional bisimidazopyridines and elemental sulfur to give oligomeric copolymers in quantitative yield.

Direct thionation and selenation of amides using elemental sulfur and selenium and hydrochlorosilanes in the presence of amines.

Reactions of amides with elemental sulfur in the presence of hydrochlorosilanes and amines give the corresponding thioamides in good to high yields. The process takes place via reduction of elemental sulfur by the hydrochlorosilane in the presence of a suitable amine. The methodology can be applied to the selenation of amides by using elemental selenium. Thionation and selenation of an acetyl-protected sialic acid derivative are found to take place selectively at the amide group.

1-Alkynyl- and 1-Alkenyl-3-arylimidazo[1,5-a]pyridines: Synthesis, Photophysical Properties, and Observation of a Linear Correlation between the Fluorescent Wavelength and Hammett Substituent Constants

1-Alkynyl- and 1-alkenyl-3-arylimidazo[1,5-a]pyridines were synthesized. The Sonogashira coupling of 3-aryl-1-iodoimidazo[1,5-a]pyridines and various terminal alkynes with Pd(PPh(3))(2)Cl(2) (10 mol %) and CuI (10 mol %) in triethylamine at 80 °C for 12 h afforded the corresponding 1-alkenyl-3-arylimidazo[1,5-a]pyridines in good to excellent yields. The Mizoroki-Heck reaction of 3-aryl-1-iodoimidazo[1,5-a]pyridines and various styrene derivatives proceeded smoothly with Pd(OAc)(2) (5 mol %), IMes·HCl (10 mol %), and Cs(2)CO(3) (2 equiv) in DMA at 130 °C for 20 h to give the alkenylated imidazo[1,5-a]pyridines in moderate to high yields. The fluorescence maxima and fluorescence quantum yields of the alkynylated products were 458-560 nm and Φ(F) = 0.08-0.26 in chloroform solution, and those of the alkenylated imidazopyridines were 479-537 nm and Φ(F) = 0.03-0.13. The absorption behaviors of the obtained alkynylated and alkenylated imidazo[1,5-a]pyridines showed a good fit to the values predicted by TDDFT calculations at the B3LYP/6-311++G(d,p) level. In addition, the alkynylated imidazo[1,5-a]pyridines obtained showed linear correlations between the Hammett substituent constants of the substituents on the arylalkynyl group and their fluorescence wavelengths.

Copper-catalyzed oxidative desulfurization–oxygenation of thiocarbonyl compounds using molecular oxygen: an efficient method for the preparation of oxygen isotopically labeled carbonyl compounds

A novel copper-catalyzed oxidative desulfurization reaction of thiocarbonyl compounds, using molecular oxygen as an oxidant and leading to formation of carbonyl compounds, has been developed, and the utility of the process is demonstrated by its application to the preparation of a carbonyl-18O labeled sialic acid derivative.