Radical alternating copolymerization of vinyl ethers

New alternating equimolar copolymers of electrophilic trisubstituted ethylenes, methyl 3-phenyl-2-cyanopropenoate and 2-phenyl-1,1-dicyanoethene, with ethyl, n-butyl, i-butyl, t-butyl, 2-chloroethyl, and phenyl vinyl ethers were prepared by free radical initiation. Chemical compositions of the copolymers are 1 : 1 in broad ranges of monomer ratios. The copolymerization rate of both electrophilic monomers with the vinyl ethers increase in the series 2-chloroethyl > ethyl > phenyl > n-butyl > i-butyl > t-butyl. These variations in the reactivity of the vinyl ethers are discussed in terms of their preferred conformations in donor-acceptor complexes with electrophilic trisubstituted ethylenes. © 1993 John Wiley & Sons, Inc.     

Reaction of N-[(α-acetoxy)-4-pyridylmethyl]-3,5-dimethylbenzamide with alkyl isocyanates

Reaction of N-(α-acetoxy)4-pyridylmethyl]-3,5-dimethylbenzamide 3 with methyl and ethyl isocyanates afforded 1,3-dimethyl and 1,3-diethyl-4-(3,5-dimethylbenzoylamino)-2-oxoimidazolidine-5-spiro-4′-[1′,4′-dihydro-1′-acetyl]pyridine 6a,b , respectively. However, the reaction of 3 with isopropyl, t-butyl and phenyl isocyanates gave the corresponding N,N′-diurea and the dimerization compound 8 . The structure of 6a was confirmed by crystal X-ray diffraction analysis.     

Electric dipole moment studies of carboxylic ester conformations: alkyl acetates,benzoates, 2,4,6-trimethyl-benzoates and 2,3,5,6-tetramethylbenzoates

Electric dipole moments μ in benzene at 30 °C have been determined (Table 3) on methyl, ethyl, isopropyl and t-butyl esters of the title compounds to determine steric effects on conformation in solution. Experimental moments were compared with those calculated for various possible conformations by a 3-dimensional vectorial addition method using bond moments and bond angles. The experimental moments for the alkyl acetates were best interpreted in terms of an out-of-plane deviation of the alkyl group from an s-trans conformation caused by steric interference between the alkyl group and the carbonyl oxygen and increasing in the series from methyl to t-butyl. The dihedral angles 0 (deviations) were calculated using a vector addition method. An increase in the moments of the benzoate series over the acetates was interpreted in terms of conjugative interaction between phenyl and carbonyl groups. Angles of twist φ for the benzoates and trimethylbenzoates were calculated using the Braude-Sondheimer equation. A decrease in the moments of the methyl, ethyl, and isopropyl trimethyl-benzoates as compared with the benzoates was interpreted in terms of steric interference between ortho methyls and both oxygens. The decrease in the angles of twist from methyl to t-butyl for the trimethylbenzoates was tentatively explained by greater steric interaction of the alkyl group with both carbonyl oxygen and ortho methyls, which forces adoption of a more coplanar arrangement between the ring and the carbonyl group than for the other alkyl derivatives, this interaction increasing with the size of the alkyl group. Dipole moments for 2,3,5,6-tetramethylbenzoates were nearly the same as for corresponding trimethyl-benzoates, thus showing no conclusive evidence for operation of a “buttressing” effect.     

Studies on [PtCl2]‐ or [AuCl]‐Catalyzed Cyclization of 1‐(Indol‐2‐yl)‐2,3‐Allenols: The Effects of Water/Steric Hindrance and 1,2‐Migration Selectivity

The [PtCl₂]‐ or [AuCl]‐catalyzed reaction of 1‐(indol‐2‐yl)‐2,3‐allenols occurred smoothly at room temperature to afford a series of poly‐substituted carbazoles efficiently. Compared with the [PtCl₂]‐catalyzed process, the [AuCl]‐catalyzed reaction represents a significant advance in terms of the scope and the selectivity. Selective 1,2‐alkyl or aryl migration of the gold carbene intermediate was observed: compared with the methyl group, the isopropyl, cyclopropyl, cyclobutyl, and cyclohexyl groups migrate exclusively; the cyclopropyl group shifts selectively over the ethyl group; the 1,2‐migration of a non‐methyl linear alkyl is faster than methyl group; the phenyl group migrates exclusively over methyl or ethyl group. DFT calculations show that water makes the elimination of H₂O facile requiring a much lower energy and validates the migratory preferences of different alkyl or phenyl groups observed.     

A facile synthesis of pyrrolo[1,2-a]pyrimidines and pyrrolo[1,2-a]imidazoles

The one-step synthesis of 8-t-butoxycarbonyl-6,7-dimethyl-2-oxo-1,2,3,4-tetrahydropyrrolo[1,2-a]pyrimidine from acetyl methyl carbinol, 3-aminopropionic acid, and t-butyl cyanoacetate is reported. Utilizing a similar technology under basic conditions, 7-substituted 5,6-dimethyl-2-oxo-2,3-dihydro-(1H)-pyrrolo[1,2-a]imidazole is synthesized from acetyl methyl carbinol, ethyl glycinate, and the appropriate acetonitrile.     

1,2-Chlorine atom migration in 3-chloro-2-butyl radicals: a computational study

The structure as well as several unimolecular reaction pathways of the 3-chloro-2-butyl radical 1 have been studied at several different theoretical levels (B3LYP/aug-cc-pVDZ, BHLYP/aug-cc-pVDZ, G3(ROMP2)B3). The symmetrically chlorine-bridged structure is a transition state at all levels of theory and the most favorable ground state structure is the unbridged beta-chloroalkyl radical. Reaction barriers for the 1,2-chlorine migration process are higher than those for rotation around the central C-C bond. 1,2-migration of the chlorine atom is accompanied by an increase in chlorine negative charge as well as chlorine spin density. This hybrid homo-/heterolytic process is well known from rearrangement reactions in beta-(dialkoxyphosphoryloxy)alkyl radicals and suggests that chlorine migration can be influenced by polar substituent effects.     

Substituted 2-norbornyl carbenes

A series of endo-6-substituted bicyclo[2.2.1]hept-2-yl carbenes have been generated by pyrolysis of the sodium salt of the tosylhydrazone precursors. The endo-6-trimethylsilyl and endo-6-thiomethoxy substituted carbenes give 1,3-migration of these substituents to the carbene center. However groups such as methyl, CH₂SiMe₃, phenyl, and methoxy are ineffective 1,3-migrating groups. The facile 1,3-migration of trimethylsilyl and thiomethoxy has been rationalized in terms of stabilized transition states where the migrating group interacts effectively with the carbene vacant orbital. In the cases of endo-6-thiomethoxy and methoxy derivatives, the carbene also inserts effectively into the methyl group of the substituent. Heteroatom stabilization of the transition state for C-H insertion facilitates these processes. Certain exo-6-substituted bicyclo[2.2.1]hept-2-yl carbenes have also been generated. While the exo-6-thiomethoxy and carbomethoxy systems give exclusive 1,3-hydrogen migration, the exo-6-sulfonyl derivative gives, in addition to 1,3-hydrogen migration, an alkene product derived from 1,2-hydrogen migration. The lowered propensity for 1,3-hydrogen migration is attributed to the strong electron-withdrawing sulfonyl group, which decreases the hydridic 1,3-interaction with the carbene vacant orbital.     

Structural effects on the acidity of E-α-phenyl-β-arylacrylic acids

The dissociation equilibrium constants of some is-α-phenyl-β-arylacrylic acids (Ar = 2-pyridyl, 3-pyridyl, 4-pyridyl, 1-naphthyl, 2-naphthyl, anthracen-9-yl) have been measured in 80% aqueous 2-methoxyethanol at 25°. The pKa values of these acids, together with those of p-substituted phenyl, 2-furyl, 2-thienyl and selenophen-2-yl derivatives, have been rationalized by an equation involving separate contributions of polar, conjugative and steric effects of heterocycles. The pKa values of some E-α-phenyl-β-alkylacrylic acids (alk = methyl; ethyl; n-propyl; i-propyl; n-butyl; i-butyl) are also reported.     

Kinetics and mechanism of unimolecular heterolysis of cage-like compounds: XXI. Solvent effect on the realtive rate of heterolysis of 2-methyland 2-phenyl-2-haloadamantanes. Role of activation parameters

The kinetics of heterolysis of 2-chloro-2-methyladamantane, 2-bromo-2-methyladamantane, 2-chloro-2-phenyladamantane, and 2-bromo-2-phenyladamantane in isopropyl alcohol, tert-butyl alcohol, acetonitrile, nitromethane, cyclohexanone, and γ-butyrolactone were studied using the verdazyl technique. The rate constant ratio k _Ph/k _Me decreases from three orders of magnitude to unity in the solvent series BuOH > i-PrOH > t-BuOH > MeCN > PhNO₂ > cyclohexanone > γ-butyrolactone > sulfolane, which results from weakening of conjugation between the phenyl group and emerging carbocationic center. The effect of solvent on the entropy and enthalpy of heterolysis in going from 2-methyl-substituted 2-haloadamantanes to their 2-phenyl analogs is discussed.     

The kinetics and mechanism of thermal decomposition of alkyl isocyanates

The kinetics of the gas-phase decomposition of ethyl, isopropyl, and t-butyl isocyanates have been studied in the temperature range of 380–530°C. t-Butyl isocyanate decomposes almost exclusively by a unimolecular route to isobutene and HNCO, but in EtNCO and i-PrNCO this route competes with a free-radical chain which produces CO, CH₄, and HCN or CH₃CN. In i-PrNCO, however, the chain process is very rapidly inhibited by the propene formed in the parallel unimolecular route. A minor heterogeneous bimolecular decomposition in each case gives rise to carbon dioxide and a carbodiimide. Mechanisms and trends in the alkyl isocyanates from methyl through t-butyl are discussed.