Oxovanadium(IV) and -(V) complexes of dithiocarbazate-based tridentate Schiff base ligands: syntheses,structure, and photochemical reactivity of compounds involving imidazole derivatives as coligands

The tridentate dithiocarbazate-based Schiff base ligands H(2)L (S-methyl-3-((5-R-2-hydroxyphenyl)methyl)dithiocarbazate, R = NO(2), L = L(2); R = Br, L = L(3)) react with [VO(acac)(2)] in the presence of imidazole derivatives as coligands to form oxovanadium(IV) and cis-dioxovanadium(V) complexes. With benzimidazole and N-methylimidazole, the products are oxovanadium(IV) complexes, viz. [VOL(3)(BzIm)].0.5CH(3)CN (1a) and [VOL(N-MeIm)(2)] (L = L(3), 1b; L = L(2), 1c), respectively. In both 1a,b, the O and S donor atoms of the tridentate ligand are cis to the terminal oxo group (in the "equatorial" plane) and mutually trans, but the N donor atom is respectively cis and trans to the oxo atom, as revealed from X-ray crystallography. When imidazole or 4-methylimidazole is used as the ancillary ligand, the products obtained are water-soluble cis-dioxovanadium(V) complexes [VO(2)L(R'-ImH)] (L = L(3) and L(2), R' = H and Me, 2a-d). These compounds have zigzag chain structures in the solid state as confirmed by X-ray crystallographic investigations of 2a,d, involving an alternating array of LVO(2)(-) species and the imidazolium counterions held together by Coulombic interactions and strong hydrogen bonding. Complexes 2a-d are stable in water or methanol. In aprotic solvents, viz. CH(3)CN, DMF, or DMSO, however, they undergo photochemical transformation when exposed to visible light. The putative product is a mixed-oxidation divanadium(IV/V) species obtained by photoinduced reduction as established by EPR, electronic spectroscopy, and dynamic (1)H NMR experiments.     

Isomerization polymerization of β-propiolactones carrying side-chain ester groups

Based on our recent discovery of the isomerization polymerization of β-(2-acetoxyethyl)-β-propiolactone into poly-δ-ester,^1,2 we examined the generality of this phenomenon by using two related monomers. The catalysts were (EtAlO)_n and Et(ZnO)₂ZnEt. The side-chains in the monomers selected were the (CH₃)₂CHCOO? CH₂CH₂? (2) and (CH₃)CICHCOO? CH₂CH₂? (3) groups in which steric effects are almost identical but electronic effects are in opposition. The monomers yielded isomerized poly-δ-ester units, depending on the terminal substituent groups in the side-chain. These observations can be interpreted with the bicyclic intermediate proposed in the earlier work. Monomer (2) was reactive and produced a poly-δ-ester structure most readily, probably because of the higher electron density at the side-chain ester group which coordinated with the catalyst. In contrast, monomer (3) was less reactive, and the probability of isomerization was the lowest, i.e., the electron deficient side-chain ester group apparently interfered with the formation of the intermediate, especially in the Zn-catalysis. Equibinary random copolymers were prepared from (2) and (3) according to the catalyst and polymerization conditions chosen.     

Cationic copolymerization of indene with styrene derivatives: Synthesis of random copolymers of indene with high molecular weight

Random copolymers with high molecular weights of indene and p‐methylstyrene (pMeSt) were synthesized by cationic polymerization with trichloroacetic acid/tin tetrachloride in CH₂Cl₂ at low temperatures. When indene and pMeSt (1:1 v/v), for example, were polymerized at ?40 °C, both monomers were consumed at very similar rates to give a copolymer with high molecular weight [number‐average molecular weight (M_n): 8–9 × 10⁴]. This is indeed quite unexpected behavior for the combination of these two monomers because pMeSt polymerized over 1000 times faster than indene in the homopolymerization under the reaction conditions previously described. The product copolymer of indene and pMeSt had a random monomer sequence in it that was confirmed by NMR analyses and thermal‐property measurements. In sharp contrast with pMeSt, styrene and p‐chlorostyrene, which have no electron‐donating groups on the phenyl ring, led to low molecular weight polymers (M_n < 10,000) in the copolymerization with indene (1:1 v/v). © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2449–2457, 2002     

Rate constants for the reactions of a series of alkylperoxy radicals with NO

The rate constants for the gas-phase reactions of isopropyl- and tert-butylperoxy radicals with nitric oxide (NO) have been studied at 298 +/- 2 K and a total pressure of 3-4 Torr (He buffer) using a laser flash photolysis technique coupled with a time-resolved negative-ionization mass spectrometry. The alkyl peroxy radicals were generated by the reaction of alkyl radicals with excess O(2), where alkyl radicals were prepared by laser photolysis of several precursor molecules. The rate constants were determined to be k(i-C(3)H(7)O(2) + NO) = (8.0 +/- 1.5) x 10(-12) and k(t-C(4)H(9)O(2) + NO) = (8.6 +/- 1.4) x 10(-12) cm(3) molecule(-1) s(-1). The results in combination with our previous studies are discussed in terms of the systematic reactivity of alkyl peroxy radicals toward NO.     

Synthesis of pyrimidine derivatives by the reaction of ketene dithioacetals with amides

Reactions of methyl 2-cyano-3,3-bis(methylthio)acrylate ( 1a ) with carboxamides 2a-g in the presence of sodium hydride in a mixture of benzene and N,N-dimethylacetamide took place displacement with the methylthio group to give the corresponding methyl 3-N-acylamino-2-cyano-3-(methylthio)acrylates 3a-g which were readily converted to the corresponding pyrimidine derivatives at reflux in methanol in good yields. Reactions of 2-cyano-3,3-bis(methylthio)acrylonitrile ( 1b ) with the carboxamides 2a-f gave directly pyrimidine-5-carbonitrile derivatives 7a-f . Ketene dithioacetals 1a,b smoothly reacted with thioamide 2g or urea 2h,i to give the expected pyrimidine derivatives 9,10a,b . Polyfunctionalized pyrimidines, thus obtained, were also used for the synthesis of fused pyrimidine derivatives.     

Conjugate addition of grignard reagents to N-(α,β-unsaturated)acylpyrazoles. Diastereoselective β-alkylation using 3-phenyl-l-menthopyrazole

The conjugate additions of N-(α,β-unsaturated)acylpyrazoles were carried out by the treatment with Grignard reagents in the presence of cuprous halides. The reaction of 2-(α,β-unsaturated)acyl-3-phenyl-l-menthopyrazoles 3a-h occurred in higher chemical yields and with asymmetric inductions on β-position, where the addition of magnesium bromide as a Lewis acid influenced to the yields and the diastereoselectivities. In the case of α-methylated 2-(α,β-unsaturated)acyl-3-phenyl-l-menthopyrazoles 3i-n , the excellent asymmetric induction on the α-position was also observed through the diastereofacial protonation.     

Synthesis of directly linked zinc(II) porphyrin-imide dyads and energy gap dependence of intramolecular electron transfer reactions

A series of zinc(II) porphyrin-imide dyads (ZP-Im), in which an electron donating ZP moiety is directly connected to an electron accepting imide moiety in the meso position, have been prepared for the examination of energy gap dependence of intramolecular electron transfer reactions with large electronic coupling. The nearly perpendicular conformation of the imide moiety towards the porphyrin plane has been revealed by Xray crystal structures. The energy gap for charge separation, 1ZP* - Im --> ZP+ - Im-, is varied by changing the electron accepting imide moiety to cover a range of about 0.8 eV in DMF. Definitive evidence for electron transfer has been obtained in three solvents (toluene, THF, and DMF) through picosecond-femtosecond transient absorption studies, which have allowed us to determine the rates of photoinduced charge separation, 1ZP* - Im --> ZP+ - Im-, and subsequent thermal charge recombination ZP+ - Im- --> ZP - Im. The free-energy gap dependence (energy gap law) has been probed from the normal to the nearly top region for the charge separation rate alone, and only the inverted region for the charge recombination rate. Although both of the energy gap dependencies can be approximately reproduced by means of the simplified semiclassical equation, when we take into consideration the effect of the high frequency vibrations replaced by one mode of averaged frequency, many features, including the effects of solvent polarity and the electron tunneling matrix element on the energy gap law, differ considerably from those of the previously studied porphyrin-quinone systems, which have weaker interchromophore electronic interactions.     

Diacetoxylation of conjugated dienes with thallium(III) acetate in acetic acid

The reaction of conjugated dienes such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 2,5-dimethyl-2,4-hexadiene, 1,3-cyclopentadiene, and 1,3-cyclohexadiene, with thallium(III)acetate in acetic acid at 10–65° for 0.5–15 hr affords an isomeric mixture of the corresponding diacetoxyalkenes (1,2- and 1,4-addition products) in 10–92% yields. The 1,2-addition products are predominantly formed in all cases examined except the case of 1,3-cyclopentadiene. The reaction is assumed to proceed through acetoxythallation and dethallation steps, the latter step being accompanied and/or followed by an attack of acetoxyl group. An initial attack of thallium moiety is proposed to occur mainly at C-1 and C-2 carbons in the cases of linear terminal dienes and cyclic dienes, respectively.     

Tetrahedranyllithium: synthesis, characterization, and reactivity

The first representative of stable tetrahedranyl anion, tris(trimethylsilyl)tetrahedranyllithium (3), has been synthesized by the reaction of tetrakis(trimethylsilyl)tetrahedrane (2) with methyllithium in tetrahydrofuran. The structural characterization of the tetrahedranyllithium has been achieved by X-ray crystallography, showing that the structure of 3.(TMEDA)1.5 represents a stretched tetrahedron. The endocyclic C(Li)-C(SiMe3) bond lengths range from 1.5408(15) to 1.5441(15) A (av 1.5425(15) A), and are longer than the endocyclic C(SiMe3)-C(SiMe3) bond lengths, which range from 1.4961(15) to 1.5009(15) A (av 1.4986(15) A). Methyl- and hydrogen-substituted tetrahedranes have also been prepared by the reaction of 3 with dimethyl sulfate and cyclopentadiene, respectively.