Radical-type reactivity of the 1,3-dibora-2,4-diphosphoniocyclobutane-1,3-diyl

Despite through-space and through-bond B-B interactions, the stable 1,3-bora-2,4-diphosphoniocyclobutane-1,3-diyl displays some radical-type behavior, as illustrated by the spontaneous formation of the trans 1,3-adduct with trimethyltin hydride and a B-spiro derivative with bromotrichloromethane.     

Boat conformation in 2,5-substituted 1,3-dioxanes

The calculation of the energy equilibrium according to Pitzer between the chair and boat conformations in 2,5-substituted 1,3-dioxanes is presented, as well as the energies of the electrostatic dipole interactions. It is shown that the unsymmetrical boat conformation is stabilized in 2,5-dialkyl- and in 2,2-dimethyl-5-alkyl-5--alkoxyalkyl-1,3-dioxanes because of the presence of hetero atoms in the ring, because of the introduction of substituents in the 2 and 5 positions, and because of the interaction between the hybridized, unshared electron pairs of the oxygen atom at the apex of the boat with the hydrogen atom of the CH₂ group.     

α-epithioketosteroids: conformation and reactivity

In order to correlate reactivity with geometry, the conformational analysis of 16,17α-epithiopregnenolone has been carried out using X-ray analysis, low-temperature circular dichroism and quantum chemical calculation. Lack of conformational homogeneity of the acetyl side-chain is revealed; however the conformational equilibrium is restricted to the conformers with relative trans-orientation of epithio- and keto-groups.     

Intramolecular hydrogen bonding and conformation of 1,3-propanediol

Conformational differences in molecules of 1,3-propanediol with variations of medium, temperature, and phase composition appear in their IR and Raman spectra. In the gas phase and in dilute solutions of 1,3-propanediol in CCl₄ forms with intramolecular hydrogen bonding (IHB) and without it exist in comparable amounts. The difference in the weighted mean energies of the corresponding groups of conformers was found from the temperature dependence of the intensities of the bands for free _OH and bounded _OH. By calculating the conformational term of E^* and by using the energy values of the 25 conformers of the diol molecules, calculated by molecular mechanics methods, the IHB energy was determined for two of them.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2277–2284, October, 1991.     

Synthesis, structure and conformation of 1,3-difluorotetraphenyl benzene

The compound trifluoroiodomethane can be activated to react with the electron donor,1,4-dilithiotetraphenyl butadiene,by means of electron transfer catalysis (ETC).The structure of the product,1,3-difluorotetraphenyl,benzene (1),was determined by MS,NMR,and X-ray single crystal diffraction.The conformations of compound 1 and its derivatives were analysized by molecular mechanics.     

Dependence of the reactivity of allophanates on the conformation of the main chain

The CNDO/2 method was used to investigate the conformation and structure of alkylsubstituted allophanates. The possibility of the existence of isomers both with coplanar and with noncoplanar main chains NCNC was studied. It was shown that the formation of conformers with a cis-structure of the ester group is more probable. On the basis of an analysis of the dependence of the reactivity on the structure of the main chain, hypotheses were advanced on the role of the more stable conformers in various channels of polyaddition of isocyanates.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 3, pp. 360–363, May–June, 1986.     

Tetra-CMPO-derivatives of calix[4]arenes fixed in the 1,3-alternate conformation

Calix[4]arene derivatives fixed in the 1,3-alternate conformation and substituted at one side by four carbamoylmethylphosphine oxide (CMPO) residues were synthesised. Two CMPO groups are directly attached to the wide rim, while the second pair is bound to the narrow rim via a tri- or tetramethylene spacer. Similar compounds, in which two CMPO groups at the wide rim are combined with two picolinamide groups or two ionisable carboxylic groups at the narrow rim, were also prepared. Some of these calixarene derivatives were studied as extractants for lanthanides (La³⁺, Eu³⁺, Yb³⁺) and thorium (Th⁴⁺) from acidic solution into methylene chloride. For selected samples, stability constants in methanol were determined by spectrophotometric titrations. Three compounds (1b′, 13, 17) in the 1,3-alternate conformation and one intermediate in the cone conformation (18) were confirmed by a crystal structure.     

On 1,3-phosphaazaallenes and their diverse reactivity

1,3-Phosphaazaallenes are heteroallenes of the type RP C NR′ and little is known about their reactivity. In here we describe the straightforward synthesis of ArPCNR (Ar = Mes*, 2,4,6-tBu-C₆H₂; ^MesTer, 2.6-(2,4,6-Me₃C₆H₂)–C₆H₃; ^DipTer, 2.6-(2,6-iPr₂C₆H₂)–C₆H₃; R = tBu; Xyl, 2,6-Me₂C₆H₃) starting from phospha-Wittig reagents ArPPMe₃ and isonitriles CNR. It is further shown that ArPCNtBu are thermally labile with respect to the loss of iso-butene and it is shown that the cyanophosphines ArP(H)CN are synthetically feasible and form the corresponding phosphanitrilium borates with B(C₆F₅)₃, whereas deprotonation of ^DipTerP(H)CN was shown to give an isolable cyanidophosphide. Lastly, the reactivity of ArPCNR towards Pier''s borane was investigated, showing hydroboration of the C N bond in Mes*PCNtBu to give a hetero-butadiene, while with ^DipTerPCNXyl the formation of the Lewis acid–base adduct with a B–P linkage was observed.

The combination of phospha-Wittig reagents with isonitriles affords 1,3-phosphaazaallenes and their diverse reactivity has been studied in detail.     

Synthesis and conformation of 2-silylethynyl-substituted 2,4,4,6-tetramethyl-1,3-dioxanes

Reaction of 2,4,4,6-tetramethyl-1,3-dioxanium perchlorate with lotsich reagents led to the synthesis of new silylethynyl-substituted 2,4,4,6-tetramethyl-1,3-dioxanes. According to the data of¹H and¹³C NMR and x-ray diffraction analysis, it was established that the 1,3-dioxane ring of the products obtained has the chair conformation.Kuban' State Technological University, Krasnodar 350072, Russia. Ufa State Oil Technical University, Ufa 450062, Russia. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 487–493, April, 1998.     

Diels-alder reactivity of 1-alkoxyphenyl-3-trialkylsiloxy-1,3-dienes

The synthesis of 1-(3,4,5-trimethoxyphenyl)-3-trialkylsiloxy-1,3-butadienes and their Diels-Alder reaction with selected dienophiles at room temperature is described. These aryldienes are useful building blocks for the synthesis of natural and pharmacological active products, as has been shown by the preparation of a tetracyclic ketone needed for the synthesis of new anthracycline analogues.     

Molecular structure and conformation of the bisdimethylamide of 2-methyl-1,3-butadienethionophosphonic acid

An x-ray diffraction investigation of C₉H₁₉N₂PS has been carried out ( Mo, R = 0.039 from 2656 reflections). The crystals are triclinic: a = 6.567, b = 7.169, c = 14.020 Å, a = 85.73, = 80.88, = 68.51°, Z = 2, space group P¯1. The molecule includes a planar six-atom fragment (five atoms of the isoprene residue and the P atom), from which the N(1), N(2), and S atoms deviate by 1.356, 0.426, and –1.609 Å. The lengths of the R-N bonds at the planar trigonal N atom are appreciably smaller than those at the pyramidal N atom [1.644 and 1.681(2) Å], The variation of the conformational energy of the molecule as a function of rotation around the C-C bond in the isoprene residue has been analyzed with the aid of the CNDO/2 and INDO methods.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 5, pp. 543–549, September–October, 1985.     

Symmetry-enforced conformational control of photochemical reactivity in 2-vinyl-1,3-terphenyl

The photocyclization of o-vinylbiphenyl to 9,10-dihydrophenanthrene occurs with very low quantum yield because of the low ground-state population of the reactive syn rotamer. 2-Vinyl-1,3-terphenyl exists as a single rotamer which undergoes highly efficient photocyclization. Irradiation at 77 K in a rigid glass permits observation of the 8a,9-dihydrophenanthrene which rearranges to the 9,10-dihydrophenanthrene upon warming of the glass. The barriers for photocyclization and the thermal sigmatropic hydrogen shift are both remarkably small.     

Structure and conformation of 1,3-dithlane: an electron diffraction study

A gas-phase electron diffraction study of 1,3-dithiane, carried out at 100° C, has found no statistically significant evidence for the presence of any conformer in the vapor other than the chair, within an estimated uncertainty of 10%. An index of the degree of ring puckering in 1,3-dithiane is the average torsional angle which was found to be 61.3°, appreciably greater than that in cyclohexane, but somewhat less than that in 1,4-dithiane and 1,3,5-trithianc. The C-C-C, C-C-S and S-C-S valency angles, 113.6(33)°, 114.9(4)° and 115.0(3)° respectively, were all larger than the C-C-C valency angles in cyclohexane. The C-S-C valency angle, 98.1(7)°, was slightly smaller than that of dimethyl sulfide. Observed bond lengths were r_g(C-H) = 1.116(10) Å, r_g(C-H) = 1.533(5)Å, and r_g(C-S) = 1. 812(3)Å and mean amplitudes of vibration were l_g(C-H) = 0.081(12)Å, l_g(C-C) = 0.052(6)Å and l_g(C-S) = 0.052(4) Å (parenthesized quantities correspond to 3σ). Curiously, nonbonded distances between the axial hydrogen atoms in 1,3-dithiane are virtually identical to those in cyclohexane, even though these molecules have greatly different bond lengths, valency angles, and torsional angles.     

On the reactivity of 2-alkyl-1,3-thiazolium-4-olates toward electrophiles

On the basis of our synthetic methodologies employing mesoionic synthons, the nucleophilic character of 2-alkyl-1,3-thiazolium-4-olates (2-alkylthioisomünchnones) has been envisaged and developed, at the expenses of their common role as masked 1,3-dipoles. Reactions with aliphatic acid chlorides lead to monoketones derived from thiazolidin-4-ones, whose structure can be rationalized in terms of orbital interactions by computational studies. Aromatic acid chlorides invariably produce 1,3-dicarbonyl compounds, yet maintaining the mesoionic core. Unlike [3+2]-cycloadditions reported previously for thioisomünchnones with isocyanates and isothiocyanates, these heterocumulenes react with 2-alkylthioisomünchnones affording conjugated amides or thioamides.     

Synthesis and reactivity of ethoxyvinyl derivatives of 1,3-dioxolanium salts

2-(-Ethoxyvinyl)dioxolanium salts were obtained by reaction of ortho esters with 2,4,4,5,5-pentamethyl-1,3-dioxolanium perchlorate in acetic anhydride. Alkaline hydrolysis of 2-(-ethoxyvinyl)-4,4,5,5-tetramethyl-1,3-dioxolanium perchlorate leads to the corresponding ,-unsaturated aldehyde. Substituted 2-(-aminovinyl)-1,3-dioxolanium perchlorates are formed by reaction of this salt with aromatic amines, amino acids, and urea. These perchlorates readily split out perchloric acid in alkaline media to give Schiff bases. The charges and electron densities were calculated for the 2-(-methoxyvinyl)-1,3-dioxolanium cation.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1614–1617, December, 1973.