Reaction of tetracyanoethylene with aldehydes. Synthesis of 6-imino-2,7-dioxabicyclo[3.2.1]octane-4,4,5-tricarbonitriles

It was discovered by means of dynamic NMR that the 1-(cis-1-methylprop-1-en-1-yl)-1,2-dimethyl-acenaphthylenonium ion generated under conditions of “long life” for carbocations underwent fast (ΔG^#35.8 kJ mol^?1 at ?103°C) degenerate 1,2-shift of the cis-dimethylvinyl group. Quantum-chemical calculations by DFT method predict lower rate of 1,2-shift for the trans-dimethylvinyl group compared to cis-dimethylvinyl group and dependence on the cations conformation of the rates of these processes and of the rearrangement mechanism into the ions of phenalenyl type.     

Degenerate Rearrangement of Long-Lived 9,10-Dimethyl-9-(cis-1-methyl-1-propenyl)phenanthrenium Ion: 1,2-Shift of the Dimethylvinyl Group

Dynamic NMR study showed that 9,10-dimethyl-9-(cis-1-methyl-1-propenyl)phenanthrenium ion generated by reaction of 1,2,2a,10b-tetramethyl-2a,10b-dihydrocyclobuta[l]phenanthrene with HSO3F below -100°C undergoes very fast (G = 22 kJ/mol at -120°C ) degenerate 1,2-shift of the dimethylvinyl group.     

Darstellung von PdCl2-komplexen substituierter cis,cis-cycloocta-1,5-diene aussachtringen und den entsprechenden substituierten cis-1,2-divinylcyclobutanen. cis,trans-zuordnung 3,4-,3,7- und 3,8-disubstituierter cis,cis-cycloocta-1,5-diene

cis-1,2-Divinylcyclobutanes are transformed with dibenzonitrilepalladium(II) chloride into the corresponding cis,cis-cycloocta-1,5-diene-PdCl₂ complexes. When e.g. the 3-methyl-cis,cis-cycloocta-1,5-diene-PdCl₂ complex is prepared using trans- or cis-3-methyl-cis-1,2-divinylcyclobutane or the corresponding eight-membered ring. two PdCl₂ complexes with the methyl group in the equatorial or axial position are formed in different percentages. With the aid of ¹H NMR spectroscopy the cis- or trans-configurations of 3,4-, 3,7- or 3,8-disubstituted cis,cis-cycloocta-1,5-dienes can be determined unambiguously in PdCl₂ complexes.     

Synthesis of 1-(pyridylalkyl)-1-aza-3,6-diphosphacycloheptanes

Reactions of 1,2-bis[(hydroxymethyl)(phenyl)phosphino]ethane with primary pyridylalkylamines gave earlier unknown 1-aza-3,6-diphosphacycloheptanes containing the pyridyl group in the exocyclic substituent. The reactions were found to be stereoselective, preferentially yielding the racemate with the RR/SS-configuration of the P atoms. A mixture of diastereomers of 3,6-diphenyl-1-[2-(2-pyridyl)ethyl]-1-aza-3,6-diphosphacycloheptane reacted with dichloro(cycloocta-1,5-diene)platinum(II) to give cis-P,P-chelate complexes from the meso-isomer and bridged oligomeric complexes from the racemate.     

Synthesis of enantiopure ethyl (1S,9aS)- and (1S,9aR)-1-phenyl-4,9-dioxohexahydropyrrolo[1,2-d][1,4]oxazepine-9a(7H)-carboxylate by carbenoid/ylide/Stevens-[1,2]-shift with ring enlargement

Enantiomerically pure ethyl (1S,9aS)- and (1S,9aR)-1-phenyl-4,9-dioxohexahydropyrrolo[1,2-d][1,4]oxazepine-9a(7H)-carboxylate were obtained by Cu(II)-catalyzed decomposition of an α-diazo carbonyl tethered to a chiral morpholinone. The reaction occurred with moderate diastereoselectivity but with complete enantioselectivity through the carbenoid/spiro-[5,6]-ammonium ylide/Stevens-[1,2]-shift with ring enlargement sequence.     

Polarity and structure of 2-(1-methylbenzimidazol-2-yl)-1-phenyl- and -1,2-diphenyl-1-nitroethenes

Polarity of 2-(1-methylbenzimidazol-2-yl)-1-phenyl- and -1,2-diphenyl-1-nitroethenes was determined and their structure was studied using electronic and ¹H, ¹³C NMR spectroscopy, dipole moments measuring, XRD analysis, and quantum-chemical calculations. It was shown that the 2-(1-methylbenzimidazol-2-yl)-1-nitro-1-phenylethene has Z-configuration both in crystal and solution. The nitro group and benzimidazole substituent in its molecule are removed from the plane of the double bond. For 1,2-diphenyl-1-nitroethene E-structure is typical.     

The vibrational analysis and torsional study of trans-1,2-dimethylcyclopropane and cis-1,2-dimethylcyclopropane

The infrared spectra of cis-1,2-dimethylcyclopropane and trans-1,2-dimethylcyclopropane have been recorded between 4000 and 200 cm^?1 in the polycrystalline solid phase, and 4000 to 80 cm^?1 in the gas phase. The Raman spectra of these two compounds in the gaseous and liquid phases were also recorded between 3100 and 10 cm^?1. An assignment of the thirty-nine fundamental vibrations for both cis- and trans-1,2-dimethylcyclopropane is proposed, and comparisons are made with the vibrations of other similar molecules. Additionally, ten torsional transitions were observed in the far infrared and Raman spectra of cis-1,2-dimethylcyclopropane, and four transitions were observed in the spectra of the trans compound. From these spectral data, torsional barriers were determined. The effective barriers to methyl torsion are 2.92 kcal mol^?1 (12.20 kJ mol^?1) for cis-1,2-dimethylcyclopropane and 2.61 kcal mol^?1 (11.14 kJ mol^?1) for trans-1,2-dimethylcyclopronane.     

Degenerate rearrangement of 9-isopropenyl-9,10-dimethyl-phenanthren-9-yl and 9,10-dimethyl-9-(trans-1-methylprop-1-en-1-yl)phenanthren-9-yl cations

¹H NMR study has shown that long-lived 9-R-9,10-dimethylphenanthren-9-yl cations (R = isopropenyl, trans-1-methylprop-1-en-1-yl) generated in the system HSO₃F-SO₂ClF-CD₂Cl₂ at ?130°C undergo degenerate rearrangement via 1,2-vinyl shifts (ΔG’ = 37 and 39 kJ/mol, respectively, at ?88°C). Analysis of the geometric parameters of the initial structures and transition states calculated by the DFT method indicates that unfavorable steric factors are responsible for the sharp deceleration of 1,2-shifts of the isopropenyl and trans-1-methylprop-1-en-1-yl groups as compared to vinyl and cis-1-methylprop-1-en-1-yl groups, respectively.     

Hochdruckversuche—IX: Die thermische isomerisierung von 1,2-diphenyl-3-methyl-cyclobuten-(1)-cis-3,4-dicarbonsäuredimethylester und 3,4-dimethyl-1,2-diphenyl-cyclobuten-(1)-cis-3,4-dicarbonsäuredimethylester

Dimethyl-1,2-diphenyl-3-methyl-cyclobutene-(1)-cis-3,4-dicarboxylate 2 leads in a thermal reaction to an equilibrium with (E, Z)-dimethyl-3,4-diphenyl-5-methyl-muconate (4). The equilibrium is shifted to the cyclic compound by pressure. Dimethyl-3,4-diphenyl-cyclobutene-(1,2-diphenyl-cyclobutene-(1)-cis-3,4-dicarboxylate (3) isomerizes thermally to (E, Z)-dimethyl-2,5-dimethyl-3,4-diphenylmuconate (6). Both reactions are accelerated by pressure. The activation volumes ΔV₀⁺ are given for each ringopening reaction.     

Protophilic isotopic hydrogen exchange of 1-ferrocenyl-2-(nitrophenyl)ethylenes

cis-1-Ferrocenyl-2-(4-nitrophenyl)ethylene enters into the protium/deuterium exchange in basic medium at the expense of hydrogens of the phenyl ring, at ortho positions in respect of the nitro group. The homoaromatic analogue, 4-nitrostilbene, under the same conditions, undergoes isotopic exchange occurring exclusively at the vinylic CH fragment attached to the nitrophenyl group. The difference is eliminated as a result of the shift of the nitro group from position 4 into position 2 of the phenyl ring: cis-1-ferrocenyl-2-(2-nitrophenyl)ethylene enters into H⁺/D⁺ exchange in the same manner as 4-nitrostilbene. Correspondence to: Professor Z.V. Todres.     

Stability of boronic esters - Structural effects on the relative rates of transesterification of 2-(phenyl)-1,3,2-dioxaborolane

Relative rates of reaction of the achiral cyclic phenylboronic ester 2-(phenyl)-1,3,2-dioxaborolane with a wide variety of structurally modified diols, have been studied to understand the factors influencing the relative stabilities of boronic esters. It is found that the alkyl substituents on the α-carbons of diols slow down the transesterification, but produce thermodynamically more stable boronic ester. Six-membered boronic esters are thermodynamically more stable than their corresponding five-membered analogs. Amongst cyclic 1,2-diols, cis-1,2-cyclopentanediol displaces ethylene glycol instantaneously whereas trans-1,2-cyclopentanediol is totally unreactive, which suggests that the cis-stereochemistry of the 1,2-diol is a prerequisite for transesterification. Among the 1,5-diols, diethanolamine displaces ethylene glycol quite rapidly forming a more stable bicyclic chelate in which nitrogen is attached to boron by a coordinating bond (as evident by ¹¹B NMR spectroscopy). The oxygen atom of di(ethylene glycol) and the sulfur atom of 2,2′-thiodiethanol do not assist in displacing the ethylene glycol from their boronic esters.     

Benchmarking the Fluxional Processes of Organometallic Piano-Stool Complexes

The correlation consistent Composite Approach for transition metals (ccCA-TM) and density functional theory (DFT) computations have been applied to investigate the fluxional mechanisms of cyclooctatetraene tricarbonyl chromium ((COT)Cr(CO)₃) and 1,3,5,7-tetramethylcyclooctatetraene tricarbonyl chromium, molybdenum, and tungsten ((TMCOT)M(CO)₃ (M = Cr, Mo, and W)) complexes. The geometries of (COT)Cr(CO)₃ were fully characterized with the PBEPBE, PBE0, B3LYP, and B97-1 functionals with various basis set/ECP combinations, while all investigated (TMCOT)M(CO)₃ complexes were fully characterized with the PBEPBE, PBE0, and B3LYP methods. The energetics of the fluxional dynamics of (COT)Cr(CO)₃ were examined using the correlation consistent Composite Approach for transition metals (ccCA-TM) to provide reliable energy benchmarks for corresponding DFT results. The PBE0/BS1 results are in semiquantitative agreement with the ccCA-TM results. Various transition states were identified for the fluxional processes of (COT)Cr(CO)₃. The PBEPBE/BS1 energetics indicate that the 1,2-shift is the lowest energy fluxional process, while the B3LYP/BS1 energetics (where BS1 = H, C, O: 6-31G(d′); M: mod-LANL2DZ(f)-ECP) indicate the 1,3-shift having a lower electronic energy of activation than the 1,2-shift by 2.9 kcal mol⁻¹. Notably, PBE0/BS1 describes the (CO)₃ rotation to be the lowest energy process, followed by the 1,3-shift. Six transition states have been identified in the fluxional processes of each of the (TMCOT)M(CO)₃ complexes (except for (TMCOT)W(CO)₃), two of which are 1,2-shift transition states. The lowest-energy fluxional process of each (TMCOT)M(CO)₃ complex (computed with the PBE0 functional) has a ΔG^‡ of 12.6, 12.8, and 13.2 kcal mol⁻¹ for Cr, Mo, and W complexes, respectively. Good agreement was observed between the experimental and computed ¹H-NMR and ¹³C-NMR chemical shifts for (TMCOT)Cr(CO)₃ and (TMCOT)Mo(CO)₃ at three different temperature regimes, with coalescence of chemically equivalent groups at higher temperatures.     

Cross-conjugated biradicals: Kinetics and kinetic isotope effects in the thermal isomerizations of cis- and trans-2,3-dimethylemethylnecyclopropane,cis- and trans-3,4-dimethyl-1,2-dimethylenecyclobutane,and of 1,2,8,9-decatetraene

cis- and trans - 2,3 - Dimethylenemethylenecyclopropane ($\text{C}$ and $\text{T}$) interconvert at 160.0° with a small normal kinetic isotope effect (KIE) when the exo-methylene is deuterated, but the 1,3-shift products, 2-methylethylidenecyclopropane, show a large normal KIE, 1.35 and 1.31, when formed from $\text{C}$ and $\text{T}$, respectively. This data can be interpreted in terms of either parallel reactions or a common trimethylenemethane diradical intermediate formed with a normal KIE of 1.11 and closing to 1,3-shift product with a normal KIE of 1.29 due to the effect of deuterium in the required 90° rotation of the exo-methylene carbon.The kinetics of the thermal 1,3- and 3,3-shifts of cis- and rans-3,4-dimethyl-1,2-dimethylenecyclobutane ($\text{CB}$ and $\text{TB}$) were determined in a flow reactor. The first order rate constants are log k_CB (sec^?1) = 13.7 ? 42,200/2.3 RT and log k_TB (sec^?1) = 13.6 ? 41,900/2.3 RT (E_a in kcal/m) which compare favorably to that from the parent hydrocarbon. 1,2-dimethylenecyclobutane, after reasonable correction for dimethyl substitution.Rearrangement of $\text{TB}$ and its bis(dideuteriomethylene) derivative at 230.0° revealed a normal KIE of 1.08. This KIE could be interpreted in terms of either a methylene rotational isotope effect in a concerted reaction or formation of a bisallyl diradical with the expected normal rotational IE on closure to the 1,3-shift product of 1.12 with no IE in the ring opening when the result is corrected for return of the biradical to starting material.The kinetics of intramolecular 2 + 2 cycloaddition of 1,2,8,9-decatetraene were determined in a flow reactor. The first order rate constant is log k(sec^?1) = 9.4 ? 30,800/2.3 RT (E_a in kcal/m). These energetics are compared with those of other 2 + 2 cycloadditions. The major product is 3,4-dimethylenecyclooctene ($\text{DC}$) which is also found from the minor product, cis-7,8-dimethylenebicvyclo[4.2.0]octane ($\text{CO}$), at higher temperatures. The trans isomer, $\text{TO}$, also gives $\text{DC}$ at about the same rate as $\text{CO}$.     

Resolution and conformational analysis of diastereoisomeric esters of cis- and trans-2-(aminomethyl)-1-carboxycyclopropanes

(1R,2S)-, (1S,2R)-, (1R,2R)- and (1S,2S)-2-(Aminomethyl)-1-carboxycyclopropanes, conformationally restricted analogues of the neurotransmitter γ-aminobutyric acid (GABA), have been resolved by chromatographic separation of the corresponding diastereoisomeric esters which were formed between the cis- and trans-2-(acetamidomethyl)-1-carboxycyclopropanes with (R)-(−)-pantolactone. ¹H NMR, semi-empirical conformational analysis, ab initio (DFT) structure and NMR shielding tensor calculations of the cis-diastereoisomers allowed the absolute configuration assignments of the cis-amino acids.     

Fast and reversible migrations of N,S-centered groups around the perimeter of cyclopropene and cycloheptatriene rings

The kinetics and mechanism of circumambulatory rearrangements of N-centered (NCS) and S-centered (SPh, SC₃Ph₃, SC(OEt)=S) groups in corresponding derivatives of 1,2,3-triphenylcyclopropene and cycloheptatriene were studied by dynamic¹H and¹³C NMR spectroscopy. Migrations of the isothiocyanate group occur by the dissociation-recombination mechanism with intermediate formation of a tight ionic pair. Migrations of the phenylthio group around the perimeter of cyclopropene and cycloheptatriene rings occur by the 1,2-shift mechanism. It was found that rearrangements of theO-ethyl dithiocarbonate group inS-(1,2,3-triphenylcyclopropen-3-yl)-O-ethyl dithiocarbonate occur by the 3,3-sigmatropic shift mechanism. The molecular and crystal structure ofO-ethylS-(1,2,3-triphenylcyclopropen-3-yl) dithiocarbonate was studied by X-ray analysis.     

Stereochemical effects in the chemical ionization mass spectra of cyclic diols

The chemical ionization mass spectra of configurational isomers of many cyclic diols give substantial differences which are characteristic of their stereochemistry. For the cis-isomer of 1,3- and 1,4-cyclohexanediols, formation of a stable intramolecular proton bridge involving the OH groups gives rise to dominant MH⁺ peaks, suppressing the (M-H)⁺ peaks found in the spectra of the trans-isomers and monoalcohols. The stability of the proton bridge in cis-1,3-cyclohexanediol structures is decreased by a sterically interfering cis-5-methyl substituent, but increased by a cis-5-OH group due to additional proton solvation. cis-Stereochemistry also gives increased formation of the dimeric M₂H⁺ ions, but decreased formation of trimers, at higher diol concentrations, for the 1,3- and 1,4-diols. The similarity of the CI spectra of cis- and trans-1,2-cyclohexanediol are explicable in terms of the similarities of the most stable proton-bridged conformers; the reduced ring flexibility in cis- and trans-1,2-cyclopentanediols makes such similar structures unfavorable, as shown by the substantial differences between their CI spectra. The substantial, but expected, variations in behavior with temperature and reagent gas are useful for maximizing stereochemical effects on CI spectra; conditions of lowest energy are usually, but not always, the most useful.     

Chromium-mediated fluoroalkenylation reactions of 1,1-dibromo-1-fluoroalkane and 1-bromo-1-fluoroalkene derivatives

CrCl₂- and NiCl₂-mediated coupling reactions of E/Z mixture of 1-bromo-1-fluoroalkenes with aldehydes proceeded in a high stereoselectivity to give the corresponding (Z)-2-fluoroallylic alcohol derivatives. On the other hand, in the reaction of 1,1-dibromo-1-fluoroalkane with CrCl₂, (Z)-fluoroalkene derivative was formed via α-elimination reaction of the chromium carbenoid intermediate accompanying the concomitant 1,2-shift of β-hydrogen.     

Co(II), Ni(II) and Cu(II) complexes of new [1+1] and [2+2] macrocyclic ligands derived from 1,4-bis(2′-formylphenyl)-1,4-dioxabutane and cis-1,2-diaminocyclohexane

Two new macrocyclic ligands, L¹ (14-membered N₂O₂) and L² (28-membered N₄O₄) from [1+1] and [2+2] condensation, respectively, have been obtained in a one-pot synthesis starting from 1,4-bis(2′-formylphenyl)-1,4-dioxabutane and cis-1,2-diaminocyclohexane.     

Studies towards the synthesis of (1R,2S)- and (1S,2S)-1,2-epoxy-3-hydroxypropylphosphonates and (1S,2S)- and (1R,2S)-2,3-epoxy-1-hydroxypropylphosphonates

The trans-configured fosfomycin analogue, diethyl (1S,2S)-1,2-epoxy-3-hydroxypropylphosphonate, was synthesised by the intramolecular Williamson reaction of diethyl (1S,2R)-1,3-dihydroxy-2-mesyloxypropylphosphonate. The cis-analogue was obtained as O-ethyl or O,O-diethyl (1R,2S)-1,2-epoxy-3-hydroxypropylphosphonates, when (1R,2R)-1,3-dihydroxy-2-mesyloxypropylphosphonate or its 3-O-trityl derivative were used as starting materials, respectively. The intramolecular Williamson cyclisations of diethyl (1S,2R)- and (1R,2S)-1-benzyloxy-3-hydroxy-2-mesyloxypropylphosphonates led to diethyl (1S,2S)- and (1R,2S)-2,3-epoxy-1-benzyloxypropylphosphonates, respectively, with the concomitant formation of diethyl (E)-1-benzyloxy-3-hydroxyprop-1-en-1-phosphonate. From diethyl (1S,2S)- and (1R,2S)-2,3-epoxy-1-benzyloxypropylphosphonates, enantiomerically pure diethyl (1S,2S)- and (1R,2S)-1,2-dihydroxypropylphosphonates were obtained by catalytic hydrogenation, while diethyl (1S,2S)- and (1R,2S)-3-acetamido-1,2-dihydroxypropylphosphonates were produced after epoxide ring opening with dibenzylamine, acetylation and hydrogenolysis.     

Rearrangement reactions in the fluorination of 3-deoxy-3-C-methyl-3-nitro-hexopyranosides (and hexo-1-thiopyranosides) of the d- and l-series by the DAST reagent

Fluorination of diverse 3-deoxy-3-C-methyl-3-nitro-hexopyranosides and hexo-1-thiopyranosides of the d- and l-series by the DAST reagent was studied in order to establish, on this 3-branched-chain sugar domain, the influence of the stereochemical relationship of the substituents at positions 1 and 2, as well as the protection of the HO-4, on the kinds of rearrangement reaction promoted by this fluorinating agent. Three classes of pyranosidic substrate were employed: (a) 1,2-trans configured, irrespective of whether HO-4 is protected or not; (b) 4-O-protected 1,2-cis configured; and (c) 4-O-unprotected 1,2-cis configured. They were prepared starting from simple glycosides through routes involving a Baer reaction and subsequent transformations by known methodology. All the substrates of class a essayed underwent, on treatment with DAST at room or higher temperatures, a rearrangement involving a 1,2-shift to give both the anomeric 2-inverted pyranosyl fluorides (or, for the 1-thioglycosides, only the α fluoride). Substrates of class b led, through a mechanism similar to that proposed for transformations of related substrates, to ring-contracted 2,5-anhydro-1-fluoro-1-O-methyl- (or 1-deoxy-1-phenylthio-)hexitol derivatives (sometimes as 1-epimers), which are precursors of 2,5-anhydro-aldehydo-sugars. Substrates of class c led to 4,5-anhydro-1-fluoro-1-O-methylalditol derivatives in all cases, together with a ring-contracted 5-fluoro-hexofuranoside in only one case; a rationalisation for their formation is proposed.