The catalytic mechanism of the hydrogen-mediated coupling of acetylene to carbonyl compounds and imines has been examined using three techniques: (a) ESI-MS and ESI-CAD-MS analyses, (b) computational modeling, and (c) experiments wherein putative reactive intermediates are diverted to alternative reaction products. ESI-MS analysis of reaction mixtures from the hydrogen-mediated reductive coupling of acetylene to α-ketoesters or N-benzenesulfonyl aldimines corroborate a catalytic mechanism involving C=X (X = O, NSO 2 Ph) insertion into a cationic rhodacyclopentadiene obtained by way of acetylene oxidative dimerization with subsequent Brønsted acid cocatalyzed hydrogenolysis of the resulting oxa-or azarhodacycloheptadiene. Hydrogenation of 1,6-diynes in the presence of α-ketoesters provides analogous coupling products. ESI mass spectrometric analysis again corroborates a catalytic mechanism involving carbonyl insertion into a cationic rhodacyclopentadiene. For all ESI-MS experiments, the structural assignments of ions are supported by multi-stage collisional activated dissociation (CAD) ESI-MS analyses. Further support for the proposed catalytic mechanism derives from experiments aimed at the interception of putative reactive intermediates and their diversion to alternate reaction products. For example, rhodium catalyzed coupling of acetylene to aldehyde in the absence of hydrogen or Brønsted acid cocatalyst provides the corresponding (Z)-butadienyl ketone, which arises from β-hydride elimination of the proposed oxarhodacycloheptadiene intermediate, as corroborated by isotopic labeling. Additionally, the putative rhodacyclopentadiene intermediate obtained from the oxidative coupling of acetylene is diverted to the product of reductive [2+2+2] cycloaddition when N-p-toluenesulfonyldehydroalanine ethyl ester is used as the coupling partner. The mechanism of this transformation also is corroborated by isotopic labeling. Computer model studies based on density functional theory (DFT) support the proposed mechanism and identify Brønsted acid cocatalyst assisted hydrogenolysis to be the most difficult step. The collective studies provide new insight into the reactivity of cationic rhodacyclopentadienes, which should facilitate the design of related rhodium catalyzed C-C couplings.*
Under the conditions of ruthenium catalyzed transfer hydrogenation, 2-butyne couples to benzylic and aliphatic alcohols 1a–1i to furnish allylic alcohols 2a–2i, constituting a direct C-H vinylation of alcohols employing alkynes as vinyl donors. Under related transfer hydrogenation conditions employing formic acid as terminal reductant, 2-butyne couples to aldehydes 4a, 4b, and 4e to furnish identical products of carbonyl vinylation 2a, 2b, and 2e. Thus, carbonyl vinylation is achieved from the alcohol or the aldehyde oxidation level in the absence of any stoichiometric metallic reagents. Nonsymmetric alkynes 6a–6c couple efficiently to aldehyde 4b to provide allylic alcohols 2m–2o as single regioisomers. Acetylenic aldehyde 7a engages in efficient intramolecular coupling to deliver cyclic allylic alcohol 8a.
Over the past half century, numerous protocols for carbonyl propargylation using allenylmetal reagents have been developed.[1] Allenic Grignard reagents were used by Prévost et al.[2a] in carbonyl additions to furnish mixtures of β-acetylenic and α-allenic carbinols, which led to them to coin the term "propargylic transposition." [2a,b] Subsequent studies by Chodkiewicz and co-workers[2c] demonstrated relative stereocontrol in such additions. Shortly thereafter, Lequam and Guillerm[2d] reported that isolable allenic stannanes provide products of carbonyl propargylation upon exposure to chloral. Later, Mukaiyama and Harada[2e] demonstrated that stannanes generated in situ from propargyl iodides and stannous chloride reacted with aldehydes to provide mixtures of β-acetylenic and α-allenic carbinols. Related propargylations employing allenylboron reagents were first reported by Favre and Gaudemar,[2f] and propargylations employing allenylsilicon reagents were first reported by Danheiser and Carini.[2g] Asymmetric variants followed (Scheme 1). Allenylboron reagents chirally modified at the boron center engage in asymmetric propargylation, as was first reported by Yamamoto and coworkers[2h] and Corey et al.[2i] Allenylstannanes chirally modified at the tin center also induce asymmetric carbonyl propargylation, as was first reported by Minowa and Mukaiyama.[2j] Axially chiral allenylstannanes, allenylsilanes, and allenylboron reagents propargylate aldehydes enantiospecifically, as was first described by Marshall et al.,[2k,l] and Hayashi and coworkers,[2m] respectively. Finally, asymmetric aldehyde propargylation using allenylmetal reagents may be catalyzed by chiral Lewis acids or chiral Lewis bases, as was first reported by Keck et al.,[2n] and Denmark and Wynn,[2o] respectively.Here, we report a new approach to carbonyl propargylation based on ruthenium-catalyzed C-C bond-forming transfer hydrogenation. [3][4][5] Specifically, upon exposure of 1,3-enynes 1a-1g to alcohols 2a-2o in the presence of [RuHCl(CO)(PPh 3 ) 3 ]/dppf (dppf =1,1′-bis (diphenylphosphino)ferrocene), hydrogen shuffling between reactants occurs to generate nucleophile-electrophile pairs that regioselectively combine to furnish products of carbonyl
Under the conditions of ruthenium catalyzed transfer hydrogenation, 2-butyne couples to alcohols 1a–1j to deliver α,β-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 α,β-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 α,β-unsaturated ketone) are accessible.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.