Intermolecular Hydroacylation by Transition‐Metal Complexes

Synthesis of naphtho[1,8-bc]pyran derivatives and related compounds through hydroxy group directed C-H bond cleavage under rhodium catalysis

The straightforward and efficient synthesis of naphtho[1,8-bc]pyran derivatives and related polycyclic compounds is achieved by the rhodium-catalyzed oxidative coupling of 1-naphthols or other phenolic and alcoholic substrates with alkynes. In these annulation reactions, the hydroxy groups effectively act as the key function for the regioselective C-H bond cleavage.

Iridium-catalyzed coupling reaction of primary alcohols with 2-alkynes leading to hydroacylation products

A novel iridium-catalyzed intermolecular coupling reaction of primary alcohols or aldehydes with 2-alkynes was successfully achieved with high regioselectivity to give hydroacylation products such as alpha,beta-unsaturated ketones in good yields. The mechanistic investigation of the reaction strongly indicated that the coupling proceeds through the initial formation of homoallylic alcohols followed by dehydrogenation to beta,gamma-unsaturated ketones and then isomerisation, which leads to the hydroacylation products.

An olefination via ruthenium-catalyzed decarbonylative addition of aldehydes to terminal alkynes

A ruthenium-catalyzed olefination via decarbonylative addition of aldehydes and alkynes has been developed. A strong electronic effect and high chemoselectivity between aromatic and aliphatic aldehydes were observed in this reaction.

Waste-free synthesis of condensed heterocyclic compounds by rhodium-catalyzed oxidative coupling of substituted arene or heteroarene carboxylic acids with alkynes

The direct oxidative coupling of 2-amino- and 2-hydroxybenzoic acids with internal alkynes proceeds efficiently in the presence of a rhodium/copper catalyst system under air to afford the corresponding 8-substituted isocoumarin derivatives, some of which exhibit solid-state fluorescence. Depending on conditions, 4-ethenylcarbazoles can be synthesized selectively from 2-(arylamino)benzoic acids. The oxidative coupling reactions of heteroarene carboxylic acids as well as aromatic diacids with an alkyne are also described.

Hydroacylation of 2-Butyne from the Alcohol or Aldehyde Oxidation Level via Ruthenium Catalyzed C-C Bond Forming Transfer Hydrogenation

Under the conditions of ruthenium catalyzed transfer hydrogenation, 2-butyne couples to alcohols 1a-1j to deliver alpha,beta-unsaturated ketones 3a-3j in good to excellent isolated yields with complete E-stereoselectivity. Under identical conditions, aldehydes 2a-2j couple to 2-butyne to provide an identical set of alpha,beta-unsaturated ketones 3a-3j in good to excellent isolated yields with complete E-stereoselectivity. Nonsymmetric alkyne 4a couples to alcohol 1d or aldehyde 2d in good yield to deliver enone 3k as a 5:1 mixture of regioisomers. Thus, intermolecular alkyne hydroacylation is achieved from the alcohol or aldehyde oxidation level. In earlier studies employing the same ruthenium catalyst under slightly different conditions, alkynes were coupled to carbonyl partners from the alcohol or aldehyde oxidation level to furnish allylic alcohols. Therefore, under the conditions of C-C bond forming transfer hydrogenation, all oxidation levels of substrate (alcohol or aldehyde) and product (allylic alcohol or alpha,beta-unsaturated ketone) are accessible.

Hydrocarbamoylation of unsaturated bonds by nickel/Lewis-acid catalysis

Formamides are found to undergo addition reactions across alkynes and 1,3-dienes by nickel/Lewis acid catalysis to give a variety of alpha,beta- and beta,gamma-unsaturated amides with stereo- and regioselectivity. Intramolecular insertion reactions of olefins into the C-H bonds of formamides also proceed under similar conditions. The presence of Lewis acid cocatalysts is crucial, and formamide coordination to the Lewis acid is considered to be responsible for the activation of their formyl C-H bonds probably through oxidative addition to nickel(0).

Intermolecular alkene and alkyne hydroacylation with beta-S-substituted aldehydes: mechanistic insight into the role of a hemilabile P-O-P ligand

A straightforward to assemble catalytic system for the intermolecular hydroacylation reaction of beta-S-substituted aldehydes with activated and unactivated alkenes and alkynes is reported. These catalysts promote the hydroacylation reaction between beta-S-substituted aldehydes and challenging substrates, such as internal alkynes and 1-octene. The catalysts are based upon [Rh(cod)(DPEphos)][ClO(4)] (DPEphos=bis(2-diphenylphosphinophenyl)ether, cod=cyclooctadiene) and were designed to make use of the hemilabile capabilities of the DPEphos ligand to stabilise key acyl-hydrido intermediates against reductive decarbonylation, which results in catalyst death. Studies on the stoichiometric addition of aldehyde (either ortho-HCOCH(2)CH(2)SMe or ortho-HCOC(6)H(4)SMe) and methylacrylate to precursor acetone complexes [Rh(acetone)(2)(DPEphos)][X] [X=closo-CB(11)H(6)Cl(6) or [BAr(F) (4)] (Ar(F)=3,5-(CF(3))(2)C(6)H(3))] reveal the role of the hemilabile DPEphos ligand. The crystal structure of [Rh(acetone)(2)(DPEphos)][X] shows a cis-coordinated diphosphine ligand with the oxygen atom of the DPEphos distal from the rhodium. Addition of aldehyde forms the acyl hydride complexes [Rh(DPEphos)(COCH(2)CH(2)SMe)H][X] or [Rh(DPEphos)(COC(6)H(4)SMe)H][X], which have a trans-spanning DPEphos ligand and a coordinated ether group. Compared to analogous complexes prepared with dppe (dppe=1,2-bis(diphenylphosphino)ethane), these DPEphos complexes show significantly increased resistance towards reductive decarbonylation. The crystal structure of the reductive decarbonylation product [Rh(CO)(DPEphos)(EtSMe)][closo-CB(11)H(6)I(6)] is reported. Addition of alkene (methylacrylate) to the acyl-hydrido complexes forms the final complexes [Rh(DPEphos)(eta(1)-MeSC(2)H(4)-eta(1)-COC(2)H(4)CO(2)Me)][X] and [Rh(DPEphos)(eta(1)-MeSC(6)H(4)-eta(1)-COC(2)H(4)CO(2)Me)][X], which have been identified spectroscopically and by ESIMS/MS. Intermediate species in this transformation have been observed and tentatively characterised as the alkyl-acyl complexes [Rh(CH(2)CH(2)CO(2)Me)(COC(2)H(4)SMe)(DPEphos)][X] and [Rh(CH(2)CH(2)CO(2)Me)(COC(6)H(4)SMe)(DPEphos)][X]. In these complexes, the DPEphos ligand is now cis chelating. A model for the (unobserved) transient alkene complex that would result from addition of alkene to the acyl-hydrido complexes comes from formation of the MeCN adducts [Rh(DPEphos)(MeSC(2)H(4)CO)H(MeCN)][X] and [Rh(DPEphos)(MeSC(6)H(4)CO)H(MeCN)][X]. Changing the ligand from DPEphos to one with a CH(2) linkage, [Ph(2)P(C(6)H(4))](2)CH(2), gave only decomposition on addition of aldehyde to the acetone precursor, which demonstrated the importance of the hemiabile ether group in DPEphos. With [Ph(2)P(C(6)H(4))](2)S, the sulfur atom has the opposite effect and binds too strongly to the metal centre to allow access to productive acetone intermediates.

Rhodium-catalysed hydroacylation or reductive aldol reactions: a ligand dependent switch of reactivity

The pathway for the combination of enones and beta-S-substituted aldehydes using Rh-catalysis can be switched between a hydroacylation reaction or a reductive aldol reaction by simple choice of the phosphine ligand; this catalyst controlled switch allows access to new ketone hydroacylation products; useful 1,4-diketone intermediates for the synthesis of N-, S- and O-heterocycles.

Dual functionalities of hydrogen-bonding self-assembled catalysts in chelation-assisted hydroacylation

A recyclable catalyst for chelation-assisted hydroacylation of an olefin with primary alcohol was developed using hydrogen-bonding self-assembled catalysts consisting of 2,6-diaminopyridine and barbiturate phosphine-rhodium(I) complex. Upon heating, these two catalysts act as homogeneous catalysts due to cleavage of the hydrogen bond, and these associate to form supramolecular assemblies via hydrogen bonding that can be separated from immiscible product phase upon cooling after the reaction.

Metal-organic cooperative catalysis in C-H and C-C bond activation and its concurrent recovery

The development of an efficient catalytic activation (cleavage) system for C-H and C-C bonds is an important challenge in organic synthesis, because these bonds comprise a variety of organic molecules such as natural products, petroleum oils, and polymers on the earth. Among many elegant approaches utilizing transition metals to activate C-H and C-C bonds facilely, chelation-assisted protocols based on the coordinating ability of an organic moiety have attracted great attention, though they have often suffered from the need for an intact coordinating group in a substrate. In this Account, we describe our entire efforts to activate C-H or C-C bonds adjacent to carbonyl groups by employing a new concept of metal-organic cooperative catalysis (MOCC), which enables the temporal installation of a 2-aminopyridyl group into common aldehydes or ketones in a catalytic way. Consequently, a series of new catalytic reactions such as alcohol hydroacylation, oxo-ester synthesis, C-C triple bond cleavage, hydrative dimerization of alkynes, and skeletal rearrangements of cyclic ketones was realized through MOCC. In particular, in the quest for an optimized MOCC system composed of a Wilkinson's catalyst (Ph 3P) 3RhCl and an organic catalyst (2-amino-3-picoline), surprising efficiency enhancements could be achieved when benzoic acid and aniline were introduced as promoters for the aldimine formation process. Furthermore, a notable accomplishment of C-C bond activation has been made using 2-amino-3-picoline as a temporary chelating auxiliary in the reactions of unstrained ketones with various terminal olefins and Wilkinson's catalyst. In the case of seven-membered cyclic ketones, an interesting ring contraction to five- or six-membered ones takes place through skeletal rearrangements initiated by the C-C bond activation of MOCC. On the other hand, the fundamental advances of these catalytic systems into recyclable processes could be achieved by immobilizing both metal and organic components using a hydrogen-bonded self-assembled system as a catalyst support. This catalyst-recovery system provides a homogeneous phase at high temperature during the reaction and a heterogeneous phase at room temperature after the reaction. The product could be separated conveniently from the self-assembly support system by decanting the upper layer. The immobilized catalysts of both 2-aminopyridine and rhodium metal species sustained high catalytic activity for up to the eight catalytic reactions. In conclusion, the successful incorporation of an organocatalytic cycle into a transition metal catalyzed reaction led us to find MOCC for C-H and C-C bond activation. In addition, the hydrogen-bonded self-assembled support has been developed for an efficient and effective recovery system of homogeneous catalysts and could be successful in immobilizing both metal and organic catalysts.