Center for Magnetic Resonance

I.A. Smetanin, M.S. Novikov, N.V. Rostovskii, A.F. Khlebnikov, G.L. Starova, D.S. Yufit
“4-Halo-2-azabuta-1,3-dienes as intermediates in the rhodium carbenoid-initiated transformation of 2-halo-2H-azirines into 2,3-dihydroazetes and 2,5-dihydrooxazoles”

Tetrahedron, 2015, 71, 4616-4628
DOI:10.1016/j.tet.2015.05.022

A wide range of electron-poor 4-bromo-/4-chloro-2-azabuta-1,3-dienes were synthesized by the Rh2(OAc)4-catalyzed reaction of diazo esters and diazo ketones with methyl 2-halo-2H-azirine-2-carboxylates. The E stereoselectivity with respect to the configuration of the Cdouble bond; length as m-dashC bond of the 2-azadiene moiety is in good agreement with the results of DFT (M06-2X/6-31+G(d,p)) calculations of the reaction pathway. The reaction proceeds via the formation of an azirinium ylide intermediate followed by ring opening with outward rotation of the halogen atom. Depending on the substitution pattern at C1 electron-poor 4-halo-2-azabutadienes can undergo two types of cyclization at elevated temperatures: reversible 1,4-electrocyclization to give 2,3-dihydroazetes or 1,5-exo-trig cyclization to give 5-methylene-2,5-dihydrooxazoles, both in good yields. The dihydrooxazole derivatives can be also obtained at ambient temperature under DBU catalysis.

E.E. Galenko, O.A. Tomashenko, A.F. Khlebnikov, M.S. Novikov
“Metal/organo relay catalysis in a one-pot synthesis of methyl 4-aminopyrrole-2-carboxylates from 5-methoxyisoxazoles and pyridinium ylides”

Org. Biomolec. Chem., 2015, 13, 9825-9833
DOI:10.1039/c5ob01537e

Methyl 4-aminopyrrole-2-carboxylates were synthesized in one-pot mode by the relay catalytic cascade reaction of 5-methoxyisoxazoles with pyridinium ylides by the use of a FeCl₂/Et₃N binary catalytic system leading to 1-(5-methoxycarbonyl-1H-pyrrol-3-yl)pyridinium salts followed by hydrazinolysis. The approach permits the introduction of a substituent at the pyrrole nitrogen via a nucleophilic reaction of the pyrrolylpyridinium ylide derived from the salt. Catalytic reduction of the ylides gives methyl 4-piperidinopyrrole-2-carboxylates.

N.V. Rostovskii, P.A. Sakharo, M.S. Novikov, A.F. Khlebnikov, G.L. Starova

“Cu(I)-NHC-Catalyzed (2+3)-Annulation of Tetramic Acids with 2H-Azirines: Stereoselective Synthesis of Functionalized Hexahydropyrrolo[3,4-b]pyrroles”

Org. Lett., 2015, 17, 4148-4151
DOI:10.1021/acs.orglett.5b01883

A stereoselective and high-yield synthesis of hexahydropyrrolo[3,4-b]pyrroles from tetramic acids and 2H-azirines under Cu(I)–NHC catalysis is developed. An unusual N–C2 azirine bond cleavage, initiated by a copper enolate, was rationalized in terms of a free radical reaction mechanism.

N.V. Rostovskii, M.S. Novikov, A.F. Khlebnikov, G.L. Starova, M.S. Avdontseva

“Azirinium ylides from ?-diazoketones and 2H-azirines on the route to 2H-1,4-oxazines: three-membered ring opening vs 1,5-cyclization”

Beilstein J. Org. Chem., 2015, 11, 302-312
DOI:10.3762/bjoc.11.35

Strained azirinium ylides derived from 2H-azirines and α-diazoketones under Rh(II)-catalysis can undergo either irreversible ring opening across the N–C2 bond to 2-azabuta-1,3-dienes that further cyclize to 2H-1,4-oxazines or reversibly undergo a 1,5-cyclization to dihydroazireno[2,1-b]oxazoles. Dihydroazireno[2,1-b]oxazoles derived from 3-aryl-2H-azirines and 3-diazoacetylacetone or ethyl diazoacetoacetate are able to cycloadd to acetyl(methyl)ketene generated from 3-diazoacetylacetone under Rh(II) catalysis to give 4,6-dioxa-1-azabicyclo[3.2.1]oct-2-ene and/or 5,7-dioxa-1-azabicyclo[4.3.1]deca-3,8-diene-2-one derivatives. According to DFT calculations (B3LYP/6-31+G(d,p)), the cycloaddition can involve two modes of nucleophilic attack of the dihydroazireno[2,1-b]oxazole intermediate on acetyl(methyl)ketene followed by aziridine ring opening into atropoisomeric oxazolium betaines and cyclization. Azirinium ylides generated from 2,3-di- and 2,2,3-triaryl-substituted azirines give rise to only 2-azabuta-1,3-dienes and/or 2H-1,4-oxazines.

A.N. Shestakov, М.А. Кuznetsov

“Synthesis of Di-, Tri-, and Tetracyclopropylhydrazines”

Chem.Comm, 2016, 52, 2398-2400
DOI:10.1039/c5cc07477k

Previously unknown 1,1-dicyclopropylhydrazine was obtained in two steps starting from dicyclopropylamine. It serves as a convenient starting material to tri- and tetracyclopropylhydrazines, which have not been described in the literature either. Tricyclopropylhydrazine was prepared in an overall four-step sequence featuring the de Meijere–Chaplinski modification of the Kulinkovich reaction as a key step. Tetracyclopropylhydrazine was obtained by the reductive amination of the cyclopropanone ethyl trimethylsilyl acetal with 1,1-dicyclopropylhydrazine or with the parent hydrazine. Synthetic utility of these cyclopropylhydrazine building blocks is presented as well.

E. Abakumov, E. Lodygin, V. Tomashunas

“13C NMR and ESR Chacterization of Humic Substances Isolated Soils of Two Siberian Arctic Islands”

Int. J. Ecology, 2015, 7, 1-10
DOI:10.1155/2015/390591

Humic acids (HAs) and fulvic acids (FAs) of two Polar soils were investigated by 13C NMR and ESR spectroscopies, investigating the degree of humification and the molecular structure. One soil, from Bolshoi Lyakhovsky Island, contains two humus horizons: modern and buried. The other soil, from Wrangel Island, had only one modern humus horizon. The HAs and FAs of the two soils investigated show essential differences. The HAs show fewer oxygen-containing groups in comparison with the FAs, whereas the degree of aromaticity is two or three times higher in the HAs. The 13C NMR data also show that HAs are very different from FAs in terms of their molecular composition and hydrophobicity. Humification in the Arctic is limited by the very low content of lignin-derived compounds, due to the restricted vascular flora. As a result, the HAs, isolated from Polar soils, are more similar to the corresponding FAs than to the typical HAs of temperate soils. This was confirmed by ESR data, which show similar levels of free radical concentration for HAs and FAs and are related to the low level of aromaticity of both materials investigated. Apparently, the humification process in the soils of Polar Arctic deserts is in an initial stage.

M.Ya. Demakova, D.S. Bolotin, N.A. Bokach, R.M. Islamova, G.L. Starova, V.Yu. Kukushkin

“Click-Type PtII-Mediated Hydroxyguanidine–Nitrile Coupling Provides Useful Catalysts for Hydrosilylation Cross-Linking”

ChemPlusChem, 2015, 80, 1607-1614
DOI:10.1002/cplu.201500327

The nitrile complexes trans-[PtCl₂(RCN)₂] (R=Et (NC1), tBu (NC2), Ph (NC3), p-BrC₆H₄ (NC4)) and cis-[PtCl₂(RCN)₂] (R=Et (NC5), tBu (NC6), Ph (NC7)) react with 1 equiv of the hydroxyguanidine OC4H8NC(=NOH)NH2 (HG) furnishing the mono-addition products trans- and cis-[PtCl₂(RCN){NH=C(R)ON=C(NH₂)NC₄H₈O}] (1–4 and 9–11; 7 examples; 54–74 % yield). Treatment of any of the nitrile complexes NC1–NC7 with HG in a 1:2 molar ratio generated the bis-addition products trans- and cis-[PtCl₂{NH=C(R)ON=C(NH₂)NC₄H₈O}₂] (5–8 and 12–14; 7 examples; 69–89 % yield). The PtII-mediated coupling between nitrile ligands and HG proceeds substantially faster than the corresponding reactions involving amid- and ketoximes and gives redox stable products under normal conditions. Complexes 1, 6⋅4 CH₂Cl₂, 7⋅4 CH₂Cl₂, 8⋅2 CH₂Cl₂, and NC4 were studied by X-ray crystallography. Platinum(II) species 1–3, 10, 11, and especially 9, efficiently catalyze the hydrosilylation cross-linking of vinyl-terminated poly(dimethylsiloxane) and trimethylsilyl-terminated poly(dimethylsiloxane-co-ethylhydrosiloxane) giving high-quality thermally stable silicon resins with no structural defects. The usage of these platinum species as the catalysts does not require any inhibitors and, moreover, the complexes and their mixtures with vinyl- and trimethylsilyl-terminated polysiloxanes are shelf-stable in air.