Three-Component Reactions of Diazoesters, Aldehydes, and Imines Using a Dual Catalytic System Consisting of a Rhodium(II) Complex and a Lewis Acid

A dual catalytic system, dirhodium tetrapivalate/ytterbium(III) triflate, enables the three-component reactions of α-alkyl-α-diazoesters, aromatic aldehydes, and N-benzylidenebenzylamine derivatives to afford the corresponding β-amino alcohols in good yields after hydrolysis of the oxazolidine cycloadducts, whereas no β-amino alcohols are obtained in the absence of ytterbium(III) triflate. A similar dual catalytic system, dirhodium tetraacetate/ytterbium(III) triflate, is found to be effective in accelerating the reactions of α-aryl-α-diazoesters in high yields. Furthermore, the reactions using dimethyl diazomalonate are described.

Reaction of electron-deficient N=N, N=O double bonds, singlet oxygen, and CC triple bonds with acyloxyketenes or mesoionic 1,3-dioxolium-4-olates: generation of unstable mesoionic 1,3-dioxolium-4-olates from acyloxyketenes

Reactions of azodicarboxylates and nitrosobenzene derivatives with acyloxyketenes generated from dehydrochlorination of alpha-acyloxyarylacetyl chlorides were carried out to give triacylbenzamidine and N-arylimide derivatives, respectively, in good yields. The same compounds were obtained from the reaction with mesoionic 1,3-dioxolium-4-olates generated by Rh2(OAc)4-catalyzed decomposition of aryldiazoacetic anhydride derivatives. Formation of the same compounds from the different starting materials indicates that their reactions involve the same intermediates. The formation of triacylbenzamidine and N-arylimide derivatives is explained by 1,3-dipolar cycloaddition between electron-deficient N=N or N=O bonds and mesoionic 1,3-dioxolium-4-olates following by decarboxylation, ring opening of the resultant carbonyl ylides, and subsequent Mumm rearrangement of the corresponding imidates. The reaction with singlet oxygen composed of more electronegative atoms than N=N and N=O bonds also gave products arising from the singlet oxygen adducts with 1,3-dioxolium-4-olates. The generation of less stable mesoionic 1,3-dioxolium-4-olates from acyloxyketenes was also confirmed by isolation of furandicarboxylates on generation of acyloxyketenes from alpha-acyloxyarylacetyl chlorides in the presence of reactive dipolarophile dimethyl acetylenedicarboxylate.

Generation of Acyloxyketenes from Unstable Mesoionic 1,3-Dioxolium-4-olates and Their Reaction with Ketenophiles To Give [2 + 2] Cycloadducts

Fast ring opening of mesoionic 1,3-dioxolium-4-olates, generated by Rh(2)(OAc)(4)-catalyzed decomposition of phenyldiazoacetic anhydride derivatives, to acyloxyphenylketenes was demonstrated by trapping the ketenes with several ketenophiles. Reactions of phenyldiazoacetic anhydride derivatives with several ketenophiles such as dihydrofuran, carbodiimides, and imines were carried out. No 1,3-dipolar cycloadducts of the latter with 1,3-dioxolium-4-olates were observed. Instead, only their [2 + 2]-cycloadducts with the acyloxyketenes generated by ring-opening of the initially formed 1,3-dioxolium-4-olates were isolated. In the reaction with cyclopentadiene, 1,3-dipolar cycloadducts with 1,3-dioxolium-4-olates were formed as main products along with the [2 + 2]-ketene adduct. PM3 calculation of heats of formation of 2,5-diphenyl-1,3-dioxolium-4-olate and the corresponding benzoyloxyphenylketene indicates that the ring-opened acyloxyketenes are ca. 9 kcal/mol more stable than the corresponding 1,3-dioxolium-4-olates.

Cyclopropenes in the 1,3-Dipolar Cycloaddition with Carbonyl Ylides: Experimental and Theoretical Evidence for the Enhancement of σ-Withdrawal in 3-Substituted-Cyclopropenes

The carbonyl ylide dipoles generated by the dirhodium tetra-acetate-catalyzed decomposition of diazocarbonyl precursors 1, 5, and 8 cycloadd to 3-substituted 1,2-diphenylcyclopropenes 3a-e and 3,3-disubstituted cyclopropenes 13, 14, 19, and 20 to give polycyclic compounds with 8-oxatricyclo[3.2.1.0(2,4)]octane and 9-oxatricyclo[3.3.1.0(2,4)]nonane frameworks. Generally, reactions proceed stereoselectively to give adducts of exo stereochemistry with the approach of the carbonyl ylide dipoles from the less-hindered face of cyclopropenes. The electronic properties of the substituent at the C3 position of cyclopropenes play an important role in governing the reactivity of cyclopropenes: when the C3 position is substituted by electron-acceptors such as the methoxycarbonyl or cyano groups, the yields of adducts are decreased significantly or no adducts can be detected at all. Relative reactivities of cyclopropenes were quantified by competition experiments to give the best correlation with sigmaF-Taft constants. Both measured photoelectron spectra and ground-state calculations of a series of 1,2-diphenylcyclopropenes indicate considerable lowering of cyclopropene pi-HOMO energies by substitution with an acceptor group. Such changes in electronic structures of cyclopropenes may cause the inversion of frontier molecular orbital (FMO) interactions from HOMO(cyclopropene)-LUMO(ylide) to LUMO(cyclopropene)-HOMO(ylide) type. In terms of philicity, nucleophilic properties of acceptor-substituted cyclopropenes are diminished to such an extent that these species are no longer good nucleophiles in the reaction with carbonyl ylides, and neither are they good electrophiles, being unreactive. This was shown by the B3LYP calculations of addends.

Chiral 2,6-Bis(oxazolinyl)pyridine−Rare Earth Metal Complexes as Catalysts for Highly Enantioselective 1,3-Dipolar Cycloaddition Reactions of 2-Benzopyrylium-4-olates

Significant levels of enantioselectivity were obtained in 1,3-dipolar cycloadditions of 2-benzopyrylium-4-olate generated from the Rh(2)(OAc)(4)-catalyzed decomposition of o-methoxycarbonyl-alpha-diazoacetophenone. This reaction utilized chiral 2,6-bis(oxazolinyl)pyridine (Pybox)--rare earth metal triflate complexes as chiral Lewis acid catalysts. The reactions with several benzyloxyacetaldehyde derivatives catalyzed by a Sc(III)--Pybox-i-Pr complex (10 mol %) proceeded smoothly to yield endo-adducts selectively with high enantioselectivity (up to 93% ee). For the reaction with benzyl pyruvate, the Sc(III)-Pybox-i-Pr complex (10 mol %) catalyzed the reaction effectively in the presence of trifluoroacetic acid (10 mol %) to yield an exo-adduct with both high diastereo- and enantioselectivity (94% ee). This catalytic system was efficiently applied to the reactions with several other alpha-keto esters with high exo- and enantioselectivities (up to 95% ee). In contrast to the reaction with carbonyl compounds, Yb(III)--Pybox-Ph complex (10 mol %) was found to be effective to obtain high enantioselectivity (96% ee) of diastereoselectively produced exo-cycloadduct in the reaction with 3-acryloyl-2-oxazolidinone.

Catalytic enantioselective [3 + 2]-cycloadditions of diazoketone-derived aryl-substituted carbonyl ylides

An evaluation of alpha-aryl-alpha-diazodiones in tandem carbonyl ylide formation-enantioselective [3 + 2]-cycloaddition reactions is described. Such substrates were designed to allow investigation of the electronic characteristics of the dipole upon asymmetric induction. Intramolecular cycloadditions (with a tethered alkene dipolarophile) were found to occur in good to quantitative yields, with a difference in ee exhibited by the two electronically different diazodiones 8 and 9. Intermolecular cycloadditions using diazodiones 12 and 13 with DMAD and arylacetylenes 16-18 again demonstrated that electronics play a key role in determining the outcome of the cycloaddition reactions. Enantioselectivities of up to 76% were observed.

Stereocontrol in rare earth metal triflate-catalyzed 1,3-dipolar cycloaddition reaction of 2-benzopyrylium-4-olate with aldehydes

Exo-cycloadduct was obtained with high stereoselectivity by the addition of ytterbium triflate (Yb(OTf)(3)) (10 mol %) in the rhodium(II) acetate (Rh(2)(OAc)(4))-catalyzed decomposition of o-(methoxycarbonyl)-alpha-diazoacetophenone in the presence of benzaldehyde. In the reaction in the absence of Yb(OTf)(3), almost no selectivity was obtained. Stereoselectivity in the reactions with p-nitrobenzaldehyde, p-anisaldehyde, and benzyloxyacetaldehyde could be also controlled to high exo preference by the addition of Yb(OTf)(3).