[RuHCl(CO)(PPh3)3] was found to be an efficient catalyst for the coupling reaction of arylboronic acids with aryl aldehydes. The reaction proceeded smoothly in the presence of K2CO3 and H2O to give diaryl ketones in good yields. Diaryl ketones were obtained from primarily formed alcohols via Ru-catalyzed transfer hydrogenation reaction.
A practical route to 6-bromo-5,7-dihydroxyphthalide 5-methyl ether, a versatile intermediate in the synthesis of hericenones and related bioactive polyphenols, was developed. The synthesis features a combination of tandem Michael addition-Claisen condensation and CuBr2-mediated multi-step reactions. With this product in hand, total syntheses of hericenone J and 5′-deoxohericenone C (hericene A) were achieved.
The potential of the oxy-Favorskii rearrangement to form branched cis-fused bicyclic ethers was explored. Both tertiary and quaternary centers were constructed in highly stereospecific manners. Methanol and primary amines were effective nucleophiles for the rearrangement. The total synthesis of (±)-communiol E was achieved based on this method.
The Koch–Haaf reaction of adamantanols was successfully carried out in a microflow system at room temperature. By combining an acid-tolerant hastelloy-made micromixer, a PTFE tube, and a hastelloy-made microextraction unit, a packaged reaction-to-workup system was developed. By means of the present system, the multigram scale synthesis of 1-adamantanecarboxylic acid was achieved in ca. one hour operation.
Phosphonium amino acid ionic liquids were found to be useful for the copper-free and amine-free Sonogashira coupling reaction. A hydrophobic and lipophobic ionic liquid permitted easy separation of the product from ionic liquid and catalyst, and the recovered ionic liquid containing Pd catalyst could be reused.
Photocycloaddition of cyclohexenones with vinyl acetates or vinyl ethers, and the Paterno–Büchi reaction were carried out using photomicroreactors in combination with compact light sources such as low-power black lights and UV LEDs. The observed high efficiency holds promise as an energy-saving protocol for photoinduced [2 + 2] cycloaddition reactions.
The regioselective radical bromoallylation of allenes proceeded efficiently in the presence of AIBN as a radical initiator to give 2-bromo-substituted 1,5-dienes in excellent yields. The addition of a bromine radical took place regioselectively onto the central carbon of allenes generating a stable allyl radical, which underwent addition/β-fragmentation reactions with allylbromides. The products could be further functionalized by Pd-catalyzed coupling reactions.
The reaction of arylboronic acids with 2-acyl-2,3-dihydro-4H-pyrans proceeded smoothly in the presence of a catalytic amount of Et2MeN˙HI to give acyl-substituted 2-aryltetrahydrocyclopenta[1,3,2]dioxaboroles in good yields. The reaction is likely to involve the acid-catalyzed ring-opening reaction of pyrans, an intramolecular aldol reaction, and condensation with boronic acids.
Under photoirradiation conditions using a xenon light, and in the presence of PdCl2(PPh3)2 as a catalyst, four-component coupling reactions comprising of α-substituted iodoalkanes, alkenes, carbon monoxide, and alcohols proceeded smoothly to give functionalized esters in good yields. When alkenyl alcohols were used as acceptor alkenes, three-component coupling reactions accompanied by intramolecular esterification proceeded to give lactones in good yields. The present reaction system represents the vicinal C-functionalization of alkenes.
Tin hydride mediated radical carbonylation and cyclization reaction was investigated using a variety of ω-alkynyl amines as substrates. In this reaction α-methylene and α-stannylmethylene lactams having five to eight membered rings were obtained as principal products. In cases where the nitrogen has a substituent capable of giving stable radicals, such as an α-phenethyl group, the lactam ring formation again took place with extrusion of an α-phenethyl radical. Coupled with the subsequent protodestannylation procedure (TMSCl plus MeOH), these reactions provide a useful entry to α-methylene lactams with incorporation of CO as a lactam carbonyl group. In cases where the amines do not have a substituent acting as a radical leaving group, a reaction course involving a 1,4-H shift is chosen so as to liberate tin radicals ultimately. Thus the proposed mechanism involves (i) nucleophilic attack of amine nitrogen onto a carbonyl group of α,β-unsaturated acyl radicals/α-ketenyl radicals via lone pair–π* interaction, which leads to zwitterionic radical species, (ii) the subsequent proton shift from N to O to give hydroxyallyl radicals, (iii) 1,4-hydrogen shift from O to C, and (iv) β-scission to give lactams with liberation of tin radicals. DFT calculations reveal that the 1,4-hydrogen shifts, the key step of the reaction mechanism, can proceed under usual reaction conditions. On the other hand, an SHi type reaction to give lactams may be the result of the β-scission of the similar zwitterionic radical intermediates. DFT calculations also predict that an SHi type reaction would result when the intermediate has a good (radical) leaving group such as a phenethyl group.
Vaska’s complex, IrCl(CO)(PPh3)2, when combined with KI as an additive, served as an excellent catalyst for the decarbonylation of long-chain aliphatic carboxylic acids to give internal alkenes with high selectivity. On combination with KI and Ac2O as additives under controlled temperatures, decarbonylation proceeded to give terminal alkenes with high selectivity.
A three-component coupling between alkanes, CO, and electron-deficient alkenes in the presence of a catalytic amount of (nBu4N)4W10O32 (TBADT) has resulted in the efficient formation of unsymmetrical ketones. This process is based on the carbonylation of alkyl radicals photocatalytically generated by CH activation of alkanes and the subsequent addition to alkenes (see scheme; EWG=electron-withdrawing group).
Photo-initiated radical chlorination of cycloalkanes was investigated using microflow reactors. Under natural light, microflow chlorination of cyclohexane with molecular chlorine proceeded well to give chlorocyclohexane with high selectivity for single-chlorination. Single-chlorination reaction of cycloalkanes with sulfuryl chloride can also be successfully carried out using a microflow reactor under irradiation using a 15 W black light.