Cyclodimerisation reactions of conjugated dienes with aldehydes—I: Formation of tetrahydropyrans and their 300 MHz 1H NMR spectra

The palladium catalysed reaction of butadiene with aldehydes yields 2-substituted 3,6-divinyl-tetrahydropyrans.^1–4 The reaction has been extended to isoprene and myrcene. With formaldehyde only 2,5-substituted pyrans have been isolated. The isomeric tetrahydropyrans obtained were fully characterised by ¹H NMR, and representative 300 MHz spectra are given. Attempts to react aldehydes with 2-methoxybutadiene were unsuccessful.     

The ultraviolet spectra of silocyclobutanes

The UV spectra of Si-substituted silocyclobutanes in vacuum were studied. The bathochromic shift of the absorption band, disclosed by comparison with bands of the corresponding acyclic compounds, can be explained by the highly strained state of the four-membered ring. This same principle in disilocyclobutanes leads to the reduction of the barrier effect of the methylene group, disrupting the interaction of the silicon atoms in the unstrained molecule.     

The effect of electronegative substituents on the electronic spectra of porphyrins

The Hückel approximation MO method is used to calculate -electron levels of energy and forces of dipole transitions, giving rise to a Soret absorption band in the spectra of porphyrins with carbethoxyl substituents. Comparison with experimental data shows that the theory correctly represents changes in electronic absorption spectra with changes in structure.     

Photolysis of organopolysilanes. The reaction of photochemically generated methylphenylsilylene with conjugated dienes

Photolysis of 2-phenylheptamethyltrisilane (I) in the presence of acyclic and cyclic conjugated dienes has been investigated using both a high-pressure mercury lamp with a quartz filter and a low-pressure mercury lamp with a Vycor filter. Irradiation of I in the presence of 1,3-butadiene, isoprene or 2,3-dimethylbutadiene with a high-pressure mercury lamp gave a product arising from photochemical isomerization of a silacyclopropane derivative and a compound apparently formed by 1,4-silylene addition, along with a 1/1 “ene” adduct of the diene to a photo-rearranged intermediate containing the silicon—carbon double bond. Irradiation of I in the presence of the conjugated diene with a low-pressure mercury lamp, followed by treatment of the product with methanol, afforded a methoxysilane arising from methanolysis of the corresponding silacyclopropane, together with the isomerization product, silacyclopentene and rearranged addition product. Irradiation of I in the presence of cyclopentadiene with a high-pressure mercury lamp produced methylphenylsilylcyclopentadiene, while irradiation of a similar mixture with a low-pressure mercury lamp followed by treatment with methanol gave 4-(methoxymethylphenylsilyl)-1-cyclopentene. With 1,3-cyclooctadiene, the photochemically generated methylphenylsilylene afforded many types of addition product. Photolysis of I in the presence of 1,3-cyclohexadiene, however, afforded none of the silylene addition products.     

Vinyl-copper derivatives XIII: Synthesis of conjugated dienes of very high stereoisomeric purity

Conjugated dienes are obtained by coupling of alkenyl cuprates and alkenyl halides in the presence of ZnBr₂ and a catalytic amount of Pd^oL₄.     

Electrochemical reduction of aliphatic conjugated dienes in the presence of carbon dioxide

A voltammetric and electrolytic study involved in the electroreductive carboxylation of multi-substituted aliphatic coujugated dienes has been successfully conducted. With methyl sorbate as the modal compound, acceptable yields of carboxylation and dimerization were achieved, which were influenced by various reaction conditions such as the supporting electrolyte, cathode nature, current density, charge passed and temperature. A correlation was first established between distinct electronic effects of the dienes and the electrochemical characteristics of their reduction and the distribution law of target products.     

The vacuum ultraviolet spectra of methylated silanes

The vacuum ultraviolet spectra of monomethylsilane, monomethylsilane-d₃, dimethylsilane, dimethylsilane-d₂, trimethylsilane, trimethylsilane-d₁, trimethylmonofluorosilane and tetramethylsilane have been recorded in the range 135 nm to 190 nm.     

Electronic absorption spectra of charge-transfer complexes and effects of substituents in radical cations

The effects of X substituents on the energies of charge-transfer bandshv _CT in electronic absorption spectra of charge-transfer complexes of π-, n-, or σ-donors (DX) with π- or σ-acceptors (A) as well as on the ionization potentialsI _D of individual DX molecules are described by the equationhv _CT(l _D)=a +bσ₁ +cσ _R ⁺ +dσ_α. When DX and A are fixed, the inductive (bσ₁), resonance (cσ _R ⁺ ), and polarization (dσ_α) contributions tohv _CT andI _D are virtually identical. The electronic structure of the D^.+X donor component of the compact [A^.−, D^.+X] radical-ionic pair in a solution is similar to that of the radical cation generated upon photoionization of the individual DX molecule in the gaseous phase. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1002–1006, June, 2000.     

The reactions of trifluoromethyl and trichloromethyl radicals with conjugated and methylene-interrupted dienes and enynes

The photochemical addition of CF₃I to buta-1,3-diene and vinylacetylene in the gas phase gave 1,4-adducts as the main products. In gas phase reactions only products derived from hydrogen abstraction were detected in the CF₃I/penta-1,3-diene system, but the 1,4-adducts again predominated in the liquid phase. CCl₃Br similarly gave a mixture of 1,2- and 1,4-adducts, the latter predominating, on photochemical reaction with penta-1,3-diene. The 1,2-adduct was the main product from the CCl₃Br/pent-1-en-4-yn reaction, but some hydrogen abstraction was observed. The preference shown by conjugated dienes to give 1,4-adducts in the $\text{trans}$- configuration is interpreted in terms of steric factors and the propensity of dienes to adopt the $\text{s}$-$\text{trans}$ conformation.     

Migration of dimethylsilyl substituents upon electron-impact: The fragmentation of methyl 12-dimethylsilyloxyoctadecanoate

Fragmentation of the dimethylsilyl (DMS) derivative of methyl 12-hydroxyoctadecanoate is found to be closely analogous to that of the previously studied trimethylsilyl (TMS) derivative, including migration of the silyl substituent from C-12 to the ester carboxyl function. besides establishing comparable migratory aptitudes of the TMS and DMS substituents, details of several other processes were revealed by use of deuterium labelling of selected positions and compared with related fragmentation of simple long-chain alcanoic esters.