Vinyl-aziridines and cyclopropanes in Pd-catalyzed (3+2)-cycloaddition reactions with cyclic N-sulfonyl imines

Efficient palladium-catalyzed (3 + 2)-cycloaddition reactions of cyclic N-sulfonyl imines and vinyl-aziridines (or cyclopropanes) have been achieved. The reactions, with either vinylic substrate, proceed with excellent yields affording highly functionalized imidazolidine and pyrrolidine derivatives. The cycloadditions take place via the reaction of zwitterionic π-allyl palladium intermediates with cyclic N-sulfonyl imines through i) the formation of two NC bonds in the presence of vinylaziridines (synthesis of imidazolidines) and ii) one CC bond and one NC bond in the presence of vinylcyclopropanes (synthesis of pyrrolidines). Following on preliminary works on the diastereoselective synthesis of imidazolidines, herein we wish to give a broader view on the subject by describing derivatization reactions and attempts towards an enantioselective version. Moreover, we describe and discuss the behavior of each vinylic substrate (aziridine or cyclopropane) on the (3 + 2)-cycloaddition reactions. Mechanistic and (intriguing) selectivity outcomes are also going to be discussed.     

Lewis Acid Catalyzed Stereoselective Dearomative Coupling of Indolylboron Ate Complexes with Donor–Acceptor Cyclopropanes and Alkyl Halides

Indolylboron ate complexes readily generated from 2‐lithioindoles and boronic esters underwent multicomponent dearomative coupling with D‐A cyclopropanes and alkyl halides in the presence of Sc(OTf)₃ as a catalyst. The reactions proceeded with complete diastereoselectivity and excellent stereospecificity to provide indolines containing three contiguous stereocenters. The valuable boronic ester moiety remains in the product and allows for subsequent functionalization.     

Synthesis and structure of 4′,4′-dimethyl[16α,17α]spiropentanopregn-4-ene-3,20-dione

4′,4′-Dimethyl[16α,17α]spiropentanopregn-4-ene-3,20-dione was synthesized. The addition of diazo-2,2-dimethylcyclopropane generated from N-(2,2-dimethylcyclopropyl)-N-nitrosourea to 16,17-didehydropregnenolone acetate occurs regio-and stereospecifically to give 3β-acetoxy-1′,1′-dimethyl-20-oxopregn-5-ene-[16α,17α;7′,6′]-4′, 5′-diazaspiro[2.4]-hept-4′-ene in high yield. Its thermolysis affords a spiropentane-containing steroid, which is transformed into the target diketone. The anti position of the gem-dimethyl group in the fused spiropentane fragment is evident from the X-ray diffraction study of the final product. Dedicated to Academician O. M. Nefedov on the occasion of his 75th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2040–2042, November, 2006.     

Titanium-mediated synthesis of bicyclic cyclopropylamines from unsaturated nitriles

Here we report an easy synthesis of bicyclic primary cyclopropylamines directly from unsaturated nitriles. The described reaction involves Ti(II)-mediated intramolecular coupling of alkene and nitrile moieties.     

Photoelectron Spectra and Electronic Structures of Some Acceptor‐Substituted Cyclopropanes: Linear Correlation of Substituent Effects on MO Energies with Molecular Structures

The relationship between electronic and geometrical structures in acceptor‐substituted cyclopropanes has been investigated by B3LYP DFT calculations and photoelectron (PE) spectroscopy. The spectra of cyclopropanecarbaldehyde ( 2 ), cyclopropanecarboxylic acid ( 3 ), cyclopropanecarboxylic acid methyl ester ( 4 ), nitrocyclopropane ( 5 ), isothiocyanatocyclopropane ( 6 ), cyanocyclopropane ( 7 ), and 1,1‐dicyanocyclopropane ( 8 ) have been analyzed. The first ionization potential (IP₁) of compounds 2 – 5 was found to be 0.1–0.4 eV higher than that of the analogous isopropyl derivatives indicating—contrary to expectation—that in these compounds the cyclopropyl group acts as a weaker electron donor than an isopropyl group. In the other compounds, IP₁ values are 0.4–1.1 eV lower than in the open‐chain congeners. The Walsh orbitals ω_S and ω_A of the three‐membered ring are substantially stabilized to different extents by interactions with substituent orbitals, and this is reflected in shortened distal and elongated vicinal C? C bonds. Although the nitro group in compound 5 causes large stabilizations of both ω_S and ω_A, their energy difference Δω remains rather small; this is in agreement with a relatively small difference Δr of the C? C bond lengths. For the investigated monosubstituted cyclopropanes 2 – 7 , the largest effects with respect to Δω and Δr are caused by the formyl group in carboxaldehyde 2 . Comparison of the results for nitriles 7 and 8 indicates that the effects of the cyano groups are additive. A linear relationship between Δω and Δr was established by B3LYP DFT calculations on geometrically distorted cyclopropane ( 1 ) and from the PE data of 2 – 8 .     

Free-radical functionalisation of vinylcyclopropanes

Free-radical mediated cyclopropane ring opening of 2-silylbicyclo[3.1.0]hexane 1 has been carried out leading to the corresponding trisubstituted cyclopentenes 5 and 6 in good yield with complete 1,2-stereocontrol. [3+2]-Annulation has also been performed by trapping the resulting radical with electron-rich and electron-poor olefins, leading to the corresponding polycyclic compounds. Further studies on the functionalisation of dihydropyridines and pyrroles using this methodology is also described.     

Carbon-13 NMR of fluorocyclopropanes

Carbon-13 and hydrogen NMR data are presented for several cyclopropanes with one to four fluorine atoms attached to the ring. Substituents directly attached to a ring carbon are found to unshield that carbon to an extent increasing with their electronegativity, but remote substituents may have an opposite effect. Relative orientation of the substituent groups, especially if they are bulky, also has a strong influence on the ring-carbon shifts. Chemical shifts of carbons in methyl groups attached to the ring change to an extent which is additive in the effects of gem and cis substituents. One-bond coupling of ring carbons with fluorines is affected both by geminal and vicinal substituents, in a way generally related to their electronegativity. It is tentatively concluded that two-bond coupling between ring carbons and ring-attached fluorines depends in magnitude primarily on what groups are cis to the fluorine. Two-bond coupling between a carbon atom in a ring-attached methyl and the hydrogen gem to the methyl appears to be vanishingly small, but there is appreciable three-bond coupling of the methyl carbons to vicinal hydrogens.     

Azo-coupling and reduction of cyclopropanediazonium ions in the reactions with polyhydroxyarenes and aminophenols

A reaction of cyclopropanediazonium ion, generated by decomposition of N-cyclopropyl-N-nitrosourea upon treatment with potassium or cesium carbonates, with various poly-hydroxyarenes and aminophenols has been studied. The reaction of azo-coupling proceeds with phloroglucinol, resorcinol, 3-methoxy- and 3-aminophenol giving rise to mono-, bis-, and tris(cyclopropylazo)arenes as the major products. Oxidizable phenols such as hydro-quinone, 2-methoxy-, 4-amino-, and 2-aminophenol give products of radical transformations with participation of cyclopropyl radical. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1671–1679, August, 2008.     

Gallium trichloride-mediated reactions of ‘double’ donor–acceptor cyclopropanes with alkenes and dienes

The reactions of ‘double’ donor–acceptor cyclopropanes containing a p- or m-phenylene moiety with alkenes or dienes in the presence of GaCl₃ comprise formation of gallium 1,2-zwitterionic intermediates, the structure of final products being substrate dependent. In contrast to the para-or meta-isomers, reaction of 2,2'-(1,2-phenylene)bis(cyclopropane-1,1-dicarboxylate) does not involve alkene and affords isomeric tricyclo[6.2.2.0^2,7]dodeca-2,4,6-triene-9,9,11,11-tetra-carboxylate, a product of intramolecular rearrangement.     

Iron(III) chloride promoted Ritter amidation of saturated cyclopropane-containing polycyclic hydrocarbons

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