Circumambulatory Rearrangements of 5-Halo-1,2,3,4,5-pentaphenylcyclopentadienes

Russian Journal of Organic Chemistry - DFT quantum chemical calculations at the CAM-B3LYP/Def2TZVP level of theory showed that intramolecular migrations of halogens in...     

The Photoisomerization of 1,2,3,4,5-Pentamethyl-5-vinyl-1,3-cyclopentadiene

The title compound 1 is shown to give, both upon direct irradiation at 254 nm and upon acetophenone-sensitized photolysis at 300 nm, the syn-vinyl-pentamethylhousene 5 , which spontaneously rearranges in a [3,3]0sigmatropic process to give the bicyclo[3.2.0]heptadiene skeleton 2 . Based on the photochemical behaviour of selectively deuterated starting material, the suggestion is made that the direct photolysis produces the vinylhousene skeleton by a classic electrocyclization, whereas the sensitized reaction reaches the same target via a di-π-methane rearrangement. The bicyclo[3.2.0]heptadiene derivative 2 give pentamethylhomoprismane 3 upon prolonged irradiation at 254 nm.     

五甲基环戊二烯基硅烷的合成

五甲基环戊二烯是最重要的配位体,它通过电子效应和空间位阻能够稳定高价氧化态的金属,本文研究了五甲基环戊二烯锂与卤代硅烷反应,合成了6种五甲基环戊二烯基硅烷(2～7),其化学反应方程式如下:     

5-卤代1,2,3,-三唑互变异构的密度泛函理论研究

用密度泛函B3LYP/6-311 G**方法,对气相和水相中的1,2,3三,-唑及5-卤(-F、C l和-B r)代1,2,3,-三唑互变异构体进行了几何构型全自由度优化,获得了它们在气相和水相中的几何结构和电子结构。计算结果显示,在气相和水相中1,2,3三-唑和5-卤代1,2,3三-唑的N2-H型要比对应的N1-H型和N3-H型稳定。讨论了不同的取代基团和溶剂化效应对互变异构体的几何结构、能量和电荷分布以及互变异构反应活化能的影响带。并进一步研究了N1-H、N2-H和N3-H型三唑之间的互变异构机理:(a)分子内质子转移;(b)水助质子转移。计算结果表明,途径(b)所需要的活化能较小,为120.06KJ/mol,途径(a)为204.12KJ/mol。     

Sigmatropic rearrangements in 5-allyloxytetrazoles

Mechanisms of thermal isomerization of allyl tetrazolyl ethers derived from the carbocyclic allylic alcohols cyclohex-2-enol and 3-methylcyclohex-2-enol and from the natural terpene alcohol nerol were investigated. In the process of the syntheses of the three 1-aryl-5-allyloxytetrazoles, their rapid isomerization to the corresponding 1-aryl-4-allyltetrazol-5-ones occurred. The experiments showed that the imidates rearrange exclusively through a [3,3']-sigmatropic migration of the allylic system from O to N, with inversion. Mechanistic proposals are based on product analysis and extensive quantum chemical calculations at the DFT(B3LYP) and MP2 levels, on O-allyl and N-allyl isomers and on putative transition state structures for [1,3']- and [3,3']-sigmatropic migrations. The experimental observations could be only explained on the basis of the MP2/6-31G(d,p) calculations that favoured the [3,3']-sigmatropic migrations, yielding lower energies both for the transition states and for the final isomerization products.     

1,2,3,4,5-Pentamethyl-5-acetyl-cyclopentadien-1,3, ein ungewöhnliches Keton

It has been shown that the reaction of the title ketone with phosphorous pentachloride leads to regiospecific chlorine transfer to the diene moiety of the molecule, leaving the carbonyl function intact. Attempted acetalization with ethylene glycol did not give the dioxolane but rather 1,2,3,4,5-pentamethylcyclopenta-1, 3-diene in quantitative yield. An independent synthesis of this hydrocarbon, previously available only by lengthy multistep synthesis, has also been described. The only normal carbonyl reaction of the permethyl-acetyl-cyclopentadiene observed was the reduction by metal hydrides.     

Synthesis of 2-(5-halogeno-2-f uryl)-2,3-dihydroperimidines

By boiling naphthylene-1,8-diamine with 5-halogeno- and 5-nitrofurfural in benzene or xylene, 90–95% yields of the corresponding 2-(5-halogeno-2-furyl)- and 2-(5-nitro-2-furyl)-2, 3-dihydroperimidines have been obtained. The dehydrogenation of 2-(5-bromo-2-furyl)-2, 3-dihydroperimidine with palladium on carbon has given 2-(5-bromo-2-furyl)perimidine. The compounds obtained have been characterized by their UV and IR spectra.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 1001–1003, July, 1973.     

Reactivity of 2-fluoro-5-nitrothiophene towards sodium thiophenoxide and piperidine. Comparison with other 2-halogeno-5-nitrothiophenes

2-Fluoro-5-nitrothiophene reacts with sodium thiophenoxide and piperidine much faster than other 2-halogeno-5-nitrothiophenes. In methanol the reactions with both nucleophiles follow overall second order kinetics, while in benzene the observed second order rate constants of the reaction with piperidine show a linear dependence by the piperidine concentration. Such a dependence, which is mild for the chloro, bromo and iodo derivative, becomes strong for the fluoro compound. Moreover, the reaction of 2-fluoro-5-nitrothiophene with [1-²H]piperidine shows the absence of a primary isotope effect. The results are interpreted within the framework of the two-stage, intermediate-complex mechanism, the first stage (attack of the nucleophile on the substrate) being rate determing for the reactions of 2-fluoro-, -chloro-, -bromo- and -iodo-5-nitrothiophene with thiophenoxide in methanol and of 2-chloro-, -bromo- and -iodo-5-nitrothiophene with piperidine in benzene. In the case of the reaction of 2-fluoro-5-nitrothiophene with piperidine in benzene the data are in agreement with a mechanism in which the rate determining step is the decomposition of the tetrahedral intermediate into products. The intervention of a second amine molecule in the transition state of the rate determining step can be rationalized in terms of bifunctional catalysis. A comparison of reactivity of thiophenoxide and piperidine towards 2-halogeno-5-nitrothiophenes (Hal = F, Cl, Br, I) indicates a greater sensitivity of the reaction with piperidine than that with thiophenoxide to the change of the leaving group.     

Direct synthesis of fused 1,2,3,4,5-pentathiepins

Treatment of nucleophilic heterocycles like pyrroles and thiophenes, and their tetrahydro derivatives, with S2Cl2 and DABCO in chloroform at room temperature provides a simple one-pot synthesis of fused mono and bispentathiepins. N-Methylpyrrole and its 2-chloro and 2,5-dichloro derivatives and N-methylpyrrolidine all give the same dichloropentathiepin 1a. N-Ethyl, isopropyl and tert-butylpyrrolidine behave similarly; the isopropylpyrrolidine also gives the bispentathiepin 6which undergoes an intriguing rearrangement to the symmetrical monopentathiepin 1c. N-Methyl and ethyl indole give either 2,3-dichloro derivatives 8 or the pentathiepinoindoles 9, depending upon the reaction conditions. Thiophene and tetrahydrothiophene give the pentathiepin 10. X-Ray crystal structures are provided for the pentathiepins 1a and 1d, and possible reaction pathways are suggested for the extensive cascade reactions reported.     

Cannabinoid rearrangements: Synthesis of Δ5-tetrahydrocannabinol

1β,6β-Epoxy-hexahydrocannabinol acetate (1) in the presence of borontrifluoride rearranged to 6-oxo-hexahydrocannabinol acetate (3b) and to the aldehyde (4). Hydroboration of Δ⁶-THC gave the 6-hydroxy hexahydrocannabinols 5a and 8a. The latter was converted into Δ⁵-THC (11). This THC isomer shows no cannabis-type activity in rhesus monkeys.