Dmitry E. Arkhipov

Difluoromethylene Phosphabetaine as an Equivalent of Difluoromethyl Carbanion

A method for nucleophilic difluoromethylation of reactive Michael acceptors, aldehydes, and azomethines is described. The reaction is performed using the readily available and air-stable reagent difluoromethylene phosphabetaine. The process involves interaction of an electrophilic substrate with in situ generated difluorinated phosphonium ylide followed by hydrolysis of the carbon-phosphorus bond under mild conditions.

Geminal Silicon/Zinc Reagent as an Equivalent of Difluoromethylene Bis-carbanion

A new difluorinated reagent, [difluoro(trimethylsilyl)methyl]zinc bromide, bearing C-Zn and C-Si bonds is described. The reagent is conveniently prepared by cobalt-catalyzed halogen/zinc exchange. It can be coupled with two different C-electrophiles in a stepwise manner (with allylic halides for C-Zn bond and aldehydes for C-Si bond) affording products containing a difluoromethylene fragment.

Trifluoromethylation of N-Benzoylhydrazones

A method for the nucleophilic trifluoromethylation of N-benzoylhydrazones using Me3SiCF3/AcONa has been described. The CN bond of the hydrazones is activated by difluoroboron group, which is introduced by means of boron trifluoride and allylsilane.

Pentacoordinate silicon complexes with dynamic motion resembling a pendulum on the SN2 reaction pathway

A series of glutarimide derivatives which has two carbonyl coordination sites for intramolecular pentacoordination at silicon with a X(1+n)SiC(3-n)O moiety have been synthesised and characterized. The substituent (leaving group) effects on the Si-O bond exchange between the two coordination sites (resembling a pendulum) have been studied by comparison of the differently substituted (X = F, Cl, OTf, Br and I) structures. The activation parameters for the Si-O bond exchange process were measured by NMR and separately computed and are consistent with the strength of Si-O bond coordination and the nature of the leaving group, X. The temperature-dependent (29)Si NMR spectroscopy is supported by X-ray crystallography and shows that the tetrahedral reactant is converted into pentacoordinate intermediates by intramolecular O-Si association followed by reversion to a tetrahedral geometry by Si-X dissociation. The two association/dissociation patterns offer a model for nucleophilic substitution at a silicon atom. A continuum of structures on the S(N)2 reaction profile from the glutarimide derivatives correlates reasonably well with the structural data obtained from derivatives of lactams, diketopiperazines and quinolones.

Fluorocyanation of enamines.

A method for the fluorocyanation of enamines has been described. The reaction involves fluorination of the electron rich double bond with N-F reagent (Selectfluor or NFSI) accompanied by trapping of beta-fluoroiminium cationic intermediate with cyanide nucleophile.

Four independent structures of a pentacoordinate silicon species at different points on the Berry pseudorotation pathway

A new pentacoordinate silicon species containing two chelating ligands has been synthesized. The structures of four independent cations of the same compound correspond to different points on the Berry pseudorotation pathway. The percentage of square planar character varies between 19% and 40%.

Synthesis of gem-difluorinated nitroso compounds.

A method for the synthesis of gem-difluorinated nitroso compounds is described. The reaction involves interaction of organozinc reagents with (bromodifluoromethyl)trimethylsilane followed by nitrosation of difluorinated organozinc species with an n-butyl nitrite/chlorotrimethylsilane system.

Pentacoordinated chlorosilanes with C,O-chelate ligands derived from N-methyl-N'-organosulfonyl-prolinamides*

The reaction of amides RSO2-Pro-NHMe with ClCH2SiMe2Cl in the presence of (Me3Si)2NH gave pentacoordinated chlorosilanes RSO2-Pro-N(Me)CH2SiMe2Cl with an organosulfonyl group (R = Me, Ph, 4-ClC6H4, 4-BrC6H4, 4-MeC6H4, and 4-O2NC6H4) attached to the proline nitrogen atom. An alternative method for the preparation of these compounds comprises the cyclosilylmethylation of proline methylamide by dimethylchloromethylchlorosilane to give the previously unreported heterocyclic 2-sila-5-piperazinone system in the first step. The bicyclic silacyclane synthesized is 2-sila5-piperazinone condensed with a proline residue. The action of sulfonyl chlorides RSO2Cl leads to cleavage of the sila ring Si–N bond to give the desired chlorosilanes. The hydrolysis of these products, depending on the reaction conditions, gives either silyloxonium chlorides [RSO2-ProN(Me)CH2SiMe2OH2]Cl or disiloxanes [RSO2-Pro-N(Me)CH2SiMe2]2O. X-ray diffraction structural analysis showed that the silicon atom in the chlorides and silyloxonium chlorides is pentacoordinated due to an intramolecular O → Si bond and has distorted trigonal-bipyrimidal configuration. 29Si NMR spectroscopy showed that the disiloxanes and bicyclic sila-5-piperazinone have a tetracoordinated silicon atom.

Intra- and intermolecular coordination reactions in solution involving pentacoordinated fluorosilanes based on proline

NMR spectroscopy was used to study a series of C,O-monochelate N′-(fluorodimethyl-silylmethyl)-N′-methyl-N-(organosulfonyl)proline amides in the solid state and in solutions in chloroform, pyridine, and acetone. The structure of one of the complexes was established by X-ray crystallography. Quantum chemical calculations were performed to interpret the spectral data obtained. The molecular conformation observed in crystal was found to be retained in solutions. The coordination bonds of silicon atom with acetone and pyridine are very weak and cannot stabilize the acyclic form (without the Si-O coordination bond). The 29Si chemical shift was found to be directly dependent on the Si-O bond distance. The mutual influence of organosulfonyl group and chelate ring was described in terms of polarization effects.

Donor-stabilized five-coordinate cationic chelate silicon compounds with two (O→Si)-coordinating ligands

Cationic O→Si-coordinated bis-C,O-chelate silicon complexes [(LCH2)2Si(F)]BF4, containing monoanionic AcN(Me)CH2, 2-oxoperhydroazepinomethyl, 2,2-dimethyl-4-oxobenzo[1,3]oxazin-3-ylmethyl, and 4-methyl-2-oxoquinolinomethyl C,O-coordinating ligands were synthesized by the reaction of trimethylsilyl derivatives of amides, lactams, and related compounds with (ClCH2)2SiCl2 in a 2:1 ratio. The synthesized complexes were reacted with KF to obtain six-coordinate bis-C,O-chelates [(LCH2)2SiF2] which were then converted into the starting tetrafluoroborates by treatment with BF3·Et2O. First representatives of cationic bis-O,O′-chelate silicon complexes with a 2-hydroxyacid amide fragment {XSi[OCH(R)C(O)NMe2]2}Y (X = Cl, Me, t-Bu, Ph, BrCH2; R = H, Me; Y - Cl−, ClHCl−, HgBr3−) were synthesized by the reaction of XSiCl3 with O→SiMe3 derivatives of dimethylamides of (S)-lactic and glycolic acids in a 1:2 ratio or by transesterification of XSi(OMe)3 with glycolic acid dimethylamide followed by addition of acetyl bromide (ratio 1:3:1). The structure of the resulting chelates was proved by X-ray diffraction analysis.