1,720,975 research outputs found
Addition reactions of a methyleneaziridine: Part 2: Conformational aspects of cyclotriveratrylene derivatives
No full textStudies of Some Strained organic Molecules: Alkylidenecycloproparenes
No full textThe alkylidenecycloproparenes, e.g. 54, have surprising stability when the double bond carries aryl substituents. Theoretical calculations reveal that these compounds have an inherent ability to minimize strain and charge separation is the likely reason. In the present study the alkylidenecycloproparenes 50a-50d, and 73a-73d were synthesized and systematic investigations of their physical and spectroscopic properties made.
Experimental evidence has been collected from changes in the characteristic infrared stretching band in the range 1740-1795 cm-1, unusual solvent effects upon the ultraviolet absorption maxima' and correlations between 13C ,n.m.r. chemical shifts and the appropriate Hammett substituent constant σ p +. The results suggest that the cycloproparene component of these molecules is ambiphilic and will sustain both positive and negative character depending upon the specific nature of the substituents. Several of the alkylidene derivatives are found to fluoresce strongly with quantum efficiency close to unity. Twisted internal charge transfer states are proposed to account for this behaviour. Furthermore, these compounds were found to absorb and lase with exceptional large "Stokes shifts" and are classified as unusual tunable laser dyes.
The measurement of dipole moments and of crystallographic parameters have also been employed to unambiguously establish charge separation in the alkylidenecycloproparenes. The dipole measurements show that the alkylidene compounds are moderately polar molecules (0.4-3.2 Debye) with mesomerism dominating. Interesting geometric predictions made from the dipole results have been confirmed by x-ray crystallography.
1H, 4H-Dicyclopropalb, glnaphthalene 96 is one of the most highly strained organic compounds thus far isolated. The synthesis of this compound has been improved upon and its methylenation studied in an attempt to extend the charge separation character of the alkylidenecycloproparenes into the long range “push-pull” system, e.g. 144
The formation and decomposition of Δ2-triazolines derived from bicyclo [2,2,1] hept-2-enes.
No full textThe 1,3-dipolar cycloadditions of phenyl azide with a series of substituted bicyclo [2,2,1] hept-2-enes, were observed to afford the exclusive formation of the exo-Δ2 -triazolines in accord with the Alder "exo-addition rule". As expected, photochemical decomposition of all the exo-Δ2 -triazolines examined, was shown to lead to the stereospecific formation of the corresponding exo-aziridines. Pyrolysis of the 8exo, 9endo -dichloro-(72), the 8endo, 9exo -dichloro-(73) and the unsubstituted (2), Δ -triazolines led to the formation of at least five products. Thermal decomposition of cis, endo-dichloro-(74), and tetrachloro-(75), Δ2 -triazolines resulted in the loss of nitrogen and the formation of at least two products, while pyrolysis of tetracyano-Δ2-triazoline (76) gave predominantly the corresponding exo-aziridine
Studies of Some Strained Organic Molecules
No full textThe studies described in this thesis comprise three distinct aspects of cycloproparene chemistry. The first part of the work addresses the preparation of alkylidene-1H-cyclopropa[b]naphthalene-3,6-diones (132) from the corresponding 3,6-dimethoxy homologues in order to form "push-pull" electronic systems. This study required the synthesis of 1H-cyclopropa[b]naphthalene-3,6-dione (144). This cyclopropaquinone is stable and is the first to be characterised; its reactions are discussed. The compound exhibits typical quinine-like properties and under mild conditions reacts at the quinone C4-C5 double bond rather than the strained "cyclopropene” ring. Efforts to prepare the oxygen containing 3,6-dimethoxy-1 H-cyclopropa[b]anthracene (134), and its dicyclopropa-homologue (135) have also been examined in order to test the suitability of gem-dihalobicyclo[4. 1.0]heptenes as precursors to such molecules.
The second component of the work addresses Diels-Alder cycloaddition reactions of diarylmethylidenecyclopropabenzenes (92a,b) and compares the results with those obtained using the naphthalene analogues. The regioselectivity found for the benzene derivatives is addition to the endocyclic bond to give stereoisomer (189) only. In contrast, the naphthalene homologues react at the exocyclic bond; the results are in accord with a theoretical analysis. The use of a hydrophilic solvent (ethylene glycol) leads to a two to three fold increase in reaction rate over reactions in toluene.
The third section describes attempts to prepare C2-substituted alkylidenecyclopropa[b]naphthalenes (138) with the ultimate aim of introducing polar or conjugating substituents at this position. Three possible routes have been explored using disilyl (108), the methylidene derivatives (93a,b) and the 1H-cyclopropa[b]naphthalene precursor (214), but none was successful
Aspects of Strain in Organic Chemistry
No full textApproximately 100 methylidene-1H-cyclopropa[b]naphthalenes (25) have been prepared from disilanes (45) and (72) by use of five methods devised to provide otherwise scarcely accessible derivatives. In many cases, the procedures now give increased product yield, e.g. olefin (141) from (45) and acetophenone is now available in 85% yield, cf. 39% previously, and nitrile (129l) is obtained in 67% yield rather than 22%. However, parent methylidenecyclopropanaphthalene (132) was not attained by any procedure.
The ketocyanine dyes (166) give rise to novel alkylidenecycloproparenes (167) with extended conjugation. The cis,cis to cis,trans rotameric ratios of the products (167b, e, g) have been determined by comparison of their measured dipole moments, relative to those of their ring-locked analogues (170b, e, g).
Attempts to prepare p-trifluoromethylphenylmethylidene derivative (129k) fail and instead a Tishchenko-like mechanism is operative in which intervention by, and subsequent ejection of, the 3,6-dimethoxy-e-silyl anion (73) is proposed in the formation of p-trifluoromethylbenzyl p-trifluoromethylbenzoate (183). The structure of (183) has been confirmed by X-ray analysis and base-hydrolysis to acid (186) and alcohol (187). An analogous product, ester (198) is obtained from 2-thiophenecarboxaldehyde under the same conditions. With tetracyclone (206), disilane (45) provides silanol (207) from protonation of the Peterson oxyanion (208) and not the exocyclic olefin (204) as steric factors dictate the product outcome.
Use of Ag(I) to effect dimerisation of diphenylmethylidenecycloproparene (127a) is unsuccessful and provides ethoxystyrene (218) or ethanone (219) from solvent-dependent reactions, instead. With the monophenyl derivative (126a), alkyne (223) and ethanone (224) are attained in yields of 62 and 21%, respectively. These results negate the use of this reagent to give cross-conjugated pentacenes by analogy with cyclopropanaphthalene (5). Disilane (45) also does not dimerise under analogous conditions. Attempted oxidative demethylation of aryl 1,4-diethers (129)-(131) with cerium(IV) fails to provide the alkylidenecyclopropanaphthoquinones (76).
A comprehensive dipole moment study employing data collected from c. 100 alkylidenecycloproparenes has been performed, for which the electron-donating character of the cycloproparenyl moiety is unequivocally established. Crystal structure data for cycloheptatrienylidene diether (163) show the seven-membered ring double bond to be bent out of the plane of the cycloproparene even more (45°) than that of its less polar benzo analogue (249) (28°). The molecular polarisation, predicted by theory to be in the direction of the cycloheptatrienylidene is confirmed as such and the relative twisting is proposed as a function of resistance to an 8π7C antiaromatic subunit. The polarity of aryl-substituted alkylidenecycloproparenes is a function of exocyclic substitution pattern, molecular planarity, and the relative inductive vs. mesomeric properties of remote substituent(s) R. Use of Hammett σp+ constants with µ indicates when the ambiphilic cycloproparenyl nucleus changes from electron sink to electron source.
The n.m.r. spectroscopic features of the alkylidenecycloproparenes have been examined and the impact of the remote substituent evaluated. Plots of σp+ against δc for each range of p-aryl-substituted derivatives (126)-(131) and (167) show a systematic shift in δc for each of the cycloproparenyl carbon atoms in accord with changes in R. Conversely, these molecules show no linear shift of σp+ with δH for H8 and H2/7.
Finally, the first olefin derivative of cyclobuta[a]cyclopropa[d]benzene "rocketene" (14) has been prepared. The recently published synthesis of stannylcyclobutabenzene (279) fails under the conditions specified. Detailed studies have led to a now reproducible synthesis of this compound. The cyclobutabenzene (279) is transformed into rocketene in accord with the prescribed preparation, that itself, has been converted into the 4,4-disilane (292). When treated with benzophenone and base, (292) provides the first methylidenerocketene (295)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A Study of some alkylidenephthalimidine derivatives
No full textThe formation of trimethyl-1-ethylpyrrole-2,3,4-tricarboxylate (1) from 1-ethyl-2-methyleneaziridine (2) and dimethylacetylenedicarboxylate (DMAD) has been reported by Cookson et al. R.C. COOKON, B. HALTON, I.D.R. STEVENS and C.T. WATTS. J. (c) 928 (1967). The reaction is believed to proceed by addition of the acetylenic ester to the exocyclic double bond of the aziridine, followed by ring opening and then participation of the nitrogen non-bonded pair to produce the Δ3 -pyrroline derivative (3) as shown in Scheme 1. The pyrrole (4) could be formed by aromatisation of (3) but it was shown that the pyrroletricarboxylate (1) does not result from treatment of (4) with DMAD under conditions identical to those used for (2). Plausible routes for the formation of (1) from (3) have been written and involve addition of DMAD to the exocyclic double bond of (3) as shown, in part, in Scheme 2. One objective of this work was to investigate further the possibility of (3) as intermediate in this reaction scheme
Studies of Some Strained Organic Molecules: Cycloproparenes
No full textCyclopropabenzene (2) is the parent and most highly strained member of the ortho-fused cycloalkabenzene series. The present study has been directed towards the imposition of additional strain upon this molecule in an attempt to establish the limits to which the benzenoid framework can be distorted. Two distinct approaches have been investigated. The first part of the study involves 2- and 3-halocyclopropabenzene synthesis and subsequent cyclopropabenzyne formation whereas the second section of the work involves alkylidenecycloproparene synthesis.
3-Halocyclopropabenzenes have been known for some time and are readily prepared by tris-dehydrohalogenation of 3,4,7,7-tetrahalobicyclo[4.l.0]heptanes, e.g, (41). In this present study the dehydrohalogenation of 2,3,7,7- tetrahalobicyclo [4.1.0]heptanes (83) and (82) has provided2-chloro- and 2-bromo-cyclopropabenzene (80) and (47) respectively, by a pathway which is shown by labeling studies to be much more complex than that pertaining to the 3-isomers; 3,4-dibromo-7,7-dichlorobicyclo [4.I.0]heptane (45) furnished a c. 2:3 mixture of cyclopropabenzenes (47) and (80.
Dehydrobromination of 2- and 3-bromocyclopropabenzene (47) and (42) in the presence of furan provides 2,5-epoxY-2,5-dihydro-1H/-cyclopropa [a]naphthalene (105) and 3,6-epoxy-3.6-dihydro-1H-cyclopropa [b]naphthalene (111) respectively. The formation of these compounds provide compelling evidence for the intermediacy of cyclopropa[1,2-d]benzyne (54) and cyclopropa[l,2-c]benzyne (55) and these molecules are taken to be the most highly strained dehydroaromatics yet produced.
The incorporation of, an exocyclic carbon-carbon double bond at C-1 of cyclopropabenzene (2) makes the cyclopropabenzene system wholly planar. To this end, alkylidene cycloproparenes have been synthesised by the Peterson reaction of α-silyl cycloproparenyl anions (76) and (117) with suitable aldehydes and ketones- Although highly strained, these derivatives are surprisingly stable and this is indicative of some resonance stabilisation
Gem-Dihalogenobenzocyclopropenes
No full textThe attempted dehydrochlorination of 1,6,7,7-tetrachlorobicyclo-[4,1,0]hept-3-ene to 7,7-dichlorobenzocyclopropene by the method used for the synthesis of the fluorine analogue was unsuccessful. Of a series of 2,5-disubstituted-1,6,7,7-tetrahalogenobicyclo[4,1,0]hept-3-enes synthesised with the intent of facilitating dehydrohalogenation, only 1,6,7,7-tetrachloro-2,5-diphenylbicyclo[4,1,0]hept-3-ene was successfully dehydrochlorinated - with hydroxide and alkoxide in alcohol solvents to 2,5-diphenylorthobenzoates, and with potassium t-butoxide in tetrahydrofuran to 7,7-dichloro-2,5-diphenylbenzocylcopropene which was isolated as a stable, crystalline solid in high yield. 1,6,7,7-Tetrabromo-2,5-diphenylbicyclo[4,1,0]hept-3-ene was similarly dehydrobrominated in protic solvents but 7,7-dibromo-2,5-diphenylbenzocyclopropene could not be synthesised under the same aprotic conditions. The gem-dichlorobenzocyclopropene rapidly solvolysed in the presence of water or alcohols to 2,5-diphenylbenzoic acid or its esters respectively, the latter reaction proceeding via intermediate 2,5-diphenylorthobenzoates. These latter species were isolated when the solvolysis was effected in the presence of base. Whilst heating 7,7-dichloro-2,5-diphenylbenzocyclopropene resulted in dimerisation to 9,9,10,10-tetrachloro-9,10-dihydro-1,4,5,8-tetraphenylanthracene and 9,9,10,10-tetrachloro-9,10-dihydro-1,4,5,8-tetraphenylphenanthrene, dissolution in polar aprotic solvent gave rise only to the former species. Reaction of the gem-dichlorobenzocyclopropene with phenyl magnesium bromide afforded 2,5,7,7-tetraphenylbenzocyclopropene, but this could not be isolated in a pure state due to its rapid rearrangement to 1,4,9-triphenylfluorene. Attempts to synthesise acetals of 2,5-diphenylbenzocyclopropenone and to generate and stabilise a benzocyclopropenyl cation from the gem-dichlorobenzocyclopropene were both unsuccessful. The existence of the cation could only be inferred from the mass spectrum of the proposed precursor
- …
