Nobuharu Iwasawa

Hydrocarboxylation of Allenes with CO2 Catalyzed by Silyl Pincer-Type Palladium Complex

Tridentate PSiP pincer-type palladium complex-catalyzed hydrocarboxylation of allenes under carbon dioxide to give synthetically useful beta,gamma-unsaturated carboxylic acids was developed. This novel CO2-fixation reaction is thought to proceed through the catalytic generation of sigma-allyl palladium species via hydropalladation of allenes, followed by its regioselective nucleophilic addition to CO2 in the presence of an appropriate reducing agent. The reaction is successfully applied to various allenes bearing functional groups such as ester, carbamate, ketone, and alkene, showing high synthetic utility of this protocol.

Rhodium(I)-Catalyzed Direct Carboxylation of Arenes with CO2 via Chelation-Assisted C−H Bond Activation

Rh-catalyzed direct carboxylation of unactivated aryl C-H bond under atmospheric pressure of carbon dioxide was realized via chelation-assisted C-H activation for the first time. Variously substituted and functionalized 2-arylpyridines and 1-arylpyrazoles underwent the carboxylation in the presence of the rhodium catalyst and a stoichiometric methylating reagent, AlMe(2)(OMe), to give carboxylated products in good yields. The catalysis is proposed to consist of methylrhodium(I) species as the key intermediate, which undergoes C-H activation to afford rhodium(III), followed by reductive elimination of methane to give nucleophilic arylrhodium(I). This approach demonstrates promising application of C-H bond activation strategy in the field of carbon dioxide fixation.

Copper(I)-Catalyzed Carboxylation of Aryl- and Alkenylboronic Esters

The copper(I)-catalyzed carboxylation reaction of aryl- and alkenylboronic esters proceeded smoothly under CO(2) to give the corresponding carboxylic acid in good yield. This reaction showed wide generality with higher functional group tolerance compared to the corresponding Rh(I)-catalyzed reaction.

Palladium(II)-Catalyzed Direct Carboxylation of Alkenyl C–H Bonds with CO2

Pd-catalyzed direct carboxylation of alkenyl C-H bonds with carbon dioxide was realized for the first time. Treatment of 2-hydroxystyrenes and a catalytic amount of Pd(OAc)2 with Cs2CO3 under atmospheric pressure of CO2 afforded corresponding coumarins in good yield. Furthermore, isolation of the key alkenylpalladium intermediate via C-H bond cleavage was achieved. The reaction was proposed to undergo reversible nucleophilic addition of the alkenylpalladium intermediate to CO2.

Efficient one-to-one coupling of easily available 1,3-dienes with carbon dioxide.

An efficient one-to-one coupling reaction of atmospheric pressure carbon dioxide with 1,3-dienes is realized for the first time through PSiP-pincer type palladium-catalyzed hydrocarboxylation. The reaction is applicable to various 1,3-dienes including easily available chemical feedstock such as 1,3-butadiene and isoprene. This protocol affords a highly useful method for the synthesis of β,γ-unsaturated carboxylic acid derivatives from CO(2).

Efficient Synthesis of Diborylalkenes from Alkenes and Diboron by a New PSiP-Pincer Palladium-Catalyzed Dehydrogenative Borylation

The efficient synthesis of various diborylalkenes such as 1,1-, trans-1,2-, and cyclic 1,2-diborylalkenes from alkenes and diboron was achieved for the first time. Selective preparation of di- and monoborylalkenes was also realized by the appropriate choice of reaction conditions. The reaction was found to proceed via a new mechanism of dehydrogenative borylation through a monoborylpalladium complex bearing an anionic PSiP-pincer ligand as a key intermediate, which realized the efficient borylation without sacrificial hydroboration or hydrogenation of the alkene.

Platinum(II)-Catalyzed Generation and [3+2] Cycloaddition Reaction of α,β-Unsaturated Carbene Complex Intermediates for the Preparation of Polycyclic Compounds

Pt(II)-catalyzed generation of unsaturated carbene complex intermediates from various propargyl ether derivatives based on electrophilic activation of alkynes was realized. These in situ generated unsaturated carbene complexes undergo [3+2] cycloaddition reaction with various vinyl ethers, leading to efficient formation of indoles, naphthols, and benzofuran fused with a five-membered ring in high yields.

Rhodium(I)-Catalyzed [4+1] Cycloaddition Reactions of α,β-Unsaturated Imines with Terminal Alkynes for the Preparation of Pyrrole Derivatives†

Vinylidene complexes have attracted much attention for being unique reactive intermediate and synthetically useful, and characteristic reactions have been developed using various kinds of transition-metal complexes. One of the main reactions involves the addition of heteroatom nucleophiles such as alcohols, carboxylates, and carbamates to give anti-Markovnikov addition products. However, the addition of the heteroatom of C=X bonds to the vinylidene carbon atom has rarely been achieved in spite of the high synthetic potential of the zwitterionic intermediates that are produced. Herein, we report a rhodium(I)-catalyzed [4+1] cycloaddition reaction between a,b-unsaturated imines and terminal alkynes for the preparation of synthetically useful, substituted pyrrole derivatives through the addition of the imine nitrogen atom to rhodium vinylidene intermediates. When a mixture of b-TMS-substituted a,b-unsaturated imine 1a and 1-octyne (2 a) was treated with a catalytic amount of [{RhCl(coe)2}2] and PCy3 in toluene at 110 8C for nine hours, pyrrole 3a (a formal [4+1] cycloaddition product) was obtained in good yield (Scheme 1). In this reaction [{RhCl(coe)2}2], as well as other phosphine ligands such as PPh3, P(iPr)3, 1,4-bis(diphenylphosphanyl)butane (dppb), and 1,1’-bis(diphenylphosphanyl)ferrocene (dppf) showed almost no activity. Also, [{RhOH(cod)}2] (cod = 1,5-cyclooctadiene), [Rh(cod)2]BF4, and [{IrCl(coe)2}2] in the presence of PCy3 did not show any activity either. Meanwhile, [{RhCl(cod)2}2]/PCy3 showed somewhat lower activity compared to [{RhCl(coe)2}2]/PCy3. The reaction is thought to proceed by the nucleophilic addition of the nitrogen atom of the imine 1a to the carbene carbon atom of the rhodium vinylidene complex A, which is generated in situ from the terminal alkyne, to afford a zwitterionic intermediate B (Scheme 2). This intermediate

Enantioselective preparation of 8-oxabicyclo[3.2.1]octane derivatives via asymmetric [3+2]-cycloaddition of platinum-containing carbonyl ylides with vinyl ethers.

A catalytic asymmetric synthesis of 8-oxabicyclo[3.2.1]octane derivatives was achieved through the [3+2]-cycloaddition of the platinum-containing carbonyl ylides generated from acyclic gamma,delta-ynones on treatment with 10 mol % of PtCl(2)-Walphos and AgSbF(6). Synthetically useful 8-oxabicyclo[3.2.1]octane derivatives were obtained in good yields and mostly in over 90% ees.

Gold‐Catalyzed Tandem Cyclization of Dienol Silyl Ethers for the Preparation of Bicyclo[4.3.0]nonane Derivatives

The reaction was thought to proceed through nucleophilic attack of the silyl enol ether moiety to the electrophilically activated alkyne followed by attack of the generated alkenyl metallic species A to the a,b-unsaturated silyloxonium moiety at the position b to the metal to give bicyclic carbene complex intermediate B, which gave the product through 1,2-hydrogen shift with regeneration of the catalyst. We then thought of the possibility of carrying out a similar geminal carbofunctionalization reaction by employing one-carbon-atom elongated substrates (3-siloxy-1,3-dien-8-ynes 3) with the expectation that 5-exo attack of the silyl enol ether moiety followed by ring closure (geminal carbo-functionalization) would give bicyclo[3.3.0]octane derivative D having a carbene complex moiety as a substituent on the bridgehead carbon atom. As there is no a-hydrogen atom at the carbene complex moiety, 1,2-hydrogen shift is not possible and the behavior of this intermediate is intriguing (Scheme 1). We now report that 1,2-alkyl migration occurred at the generated carbene moiety to give ring-expanded products by employing 3siloxy-1,3-dien-8-yne derivatives as substrates, to stereoselectively obtain synthetically useful bicyclo[4.3.0]nonane derivatives, whose configuration was different from that of the Diels–Alder adduct. We first examined the reaction of 3-siloxy-1,3-dien-8-yne 3a with the geometry of the silyl enol ether moiety as (Z) by using various electrophilic transition-metal catalysts. Although [W(CO)5(thf)] or [ReCl(CO)5] gave the products in reasonable yield (45 % and 78 %, respectively), the cationic gold catalyst gave the best result. Thus, treatment of 3a with 10 mol % of [AuClPPh3]/AgSbF6 [6] in the presence of molecular sieves (4 ) gave bicyclo[4.3.0]nonane derivative 4a, a formal [4+2] cycloadduct, as a single stereoisomer in 92% yield (Scheme 2). Interestingly, the configuration of the product was different from that of the thermal Diels– Alder adduct 5 obtained stereoselectively by heating a solution of (Z)-3 a in toluene for 1.5 h. Thus, two diastereoisomers of the same bicyclo[4.3.0]nonane derivative were obtained selectively by choosing Au-catalyzed or thermal conditions. Next, we examined the reaction of 3-siloxy-1,3-dien-8-yne derivative 3a with the geometry of the silyl enol ether moiety as (E) under thermal and Au-catalyzed conditions (Scheme 2). In this case, (E)-3a did not undergo thermal Scheme 1. Tandem cyclization of dienol silyl ethers.