Π-acceptor Substituents Research Articles

Kinetics of Proton Transfer from 2-Nitro-4-X-phenylacetonitriles to Piperidine and Morpholine in Aqueous Me2SO. Solvent and Substituent Effects on Intrinsic Rate Constants. Transition State Imbalances

Rate constants ( ) for the deprotonation of 2-nitro-4-X-phenylacetonitrile, 2-X (X = NO2, SO2CH3, CN, CF3, Br, and Cl) by piperidine and morpholine and for the reverse reaction ( ) have been determined in 90% Me2SO−10% water, 50% Me2SO−50% water, and water (X = NO2, SO2CH3, CN only). Brønsted βB values (dlog /dp ), Brønsted αCH values (dlog /dlog ), and intrinsic rate constants (log ko = log(k1/q) for p − p + log(p/q) = 0) were calculated from these data. αCH is smaller than βB, implying an imbalance which is consistent with a transition state in which delocalization of the negative charge into the 2-nitrophenyl moiety lags behind proton transfer. A consequence of this imbalance is that the intrinsic rate constant decreases with increasing electron withdrawing strength of X. For π-acceptor substituents (NO2, SO2CH3, CN) there is a further decrease in ko due to a lag in the delocalization of the charge into X. The intrinsic rate constants depend very little on the Me2SO content of the solvent which is shown to be the result of compensation of mainly two competing factors. One is the stabilization of the polarizable transition state by the polarizable Me2SO which increases ko; the other is attributed to a lag in the solvation of the developing carbanion behind proton transfer at the transition state which leads to a decrease in ko.

Structural and substituent effects on the ketene infrared stretching frequency

The frequencies and intensities of the ketene asymmetric stretching vibration near 2100 cm−1 for a series of substituted ketenes and fulvenones have been calculated using ab initio molecular orbital methods. Comparisons of the calculated values with experimentally determined results show reasonably good agreement in most cases, with a typical deviation of ± 10 cm−1 between the measured and calculated values. A test of a correlation proposed by Gano and Jacob (Spectrochim. Acta, 43A, 1023 (1987)) of the frequencies with substituent field and resonance parameters F and R shows a modest correlation coefficient of 0.86, with no significant dependence on R. These results are consistent with enhancements of ketene infrared absorption frequencies by interaction of substituents with the negative charge density at Cβ of ketenes, as indicated by the resonance structure [Formula: see text] The calculated intensities I (km/mol) for non-π acceptor substituents are correlated with the group electronegativities X by the relationship I = 1841.6 − 305.7X (r = 0.94). The π-acceptor substituents show exaltations of the intensities, and the results are interpreted as showing an enhancement of intensities by electropositive substituents increasing the normal ketene dipole directed towards oxygen, and an opposite effect by π-acceptor substituents delocalizing the negative charge at Cβ.

Photochemistry of 2-amino-2H-benzochromenes

Diabatic photoinduced ring opening of the chromene ring of 2-amino-5,6-benzo-2H-chromenes and 2-amino-6-methyl-7,8-benzo-2H-chromenes leads, in the same way as a thermally induced reaction, to the establishment of ring-chain tautomer equilibrium in the ground electronic state, the position of this equilibrium depending on the polarity of the solvent, the temperature, and structural factors. o-Quinoid tautomers exist as several stable isomeric forms: an acoplanar cis-S-cis-trans-form absorbing in the shortwave region, and S-trans-isomers absorbing in the longwave region of the spectrum. Photoexcitation of the o-quinoid forms in the temperature range 125–190 K initiates mutual conversions of these and the initial 2H-chromene structure. The relative stability of the conformers of the o-quinoid form depends on steric and electronic factors: benzanellation in positions 5,6 leads to stabilization of the cis-S-cis-trans-isomer and in positions 7,8 the S-trans-isomers are more preferred: π-acceptor substituents on the amine component increase the stability of the cis-S-cis-trans isomer and electron-donor substituents stabilize the S-trans-isomer.

The geometry of small rings—V : Geometric variations and hybridization in three-membered heretocycles C 2X (X=N, O, Si, P, S) and CXY (X, Y=N, O)

Structural data relating to 369 organic derivatives of C 2X(X=N, O, Si, P, S) and CXY(X, Y=N, O) heterocycles have been retrieved from the Cambridge Crystallogrphic Database and analysed in conjuction with pertinent gas-phase results. Heterocycle geometries are compared with each other, and with those for the ‘parent’ carbocycles cyclopropane and cyclopropene. For saturated C 2X rings the previously observed (gas phase) decrease in CC bond length ( d cc) and bent-back angle (γ) with increasing heteroatom electronegativity (χ x) are confirmed as linear relationships using mean solid state geometry for X=C, N, O, S. The CX bonds shows an effective increase in length with increasing χ x, in line with their facile cleavage in ring opening reactions. A model for hybridization changes at C in saturated C 2X rings is derived empirically and is in broad agreement with theoretical studies. There is no evidence for geometric variations in the heterocyclic rings induced by π-acceptor substituents, but π-donor substituent effects are directly comparable to those occurring in cyclopropane and cyclopropene. Geometric variations in unsaturated heterocycles are analogous to those in cyclopropene derivatives; CC double bond lengths in available C 2X systems appear to indicate a π x dependence. Heteroatom-heteroatom bonds in CYX systems are weak, with N weaker than NN, in agreement with thermochemical reasoning.

The geometry of small rings—IV: Molecular geometry of cyclopropene and its derivatives

Numeric structural data for 34 derivatives of cyclopropene and cyclopropenium ion have been retrieved from the Cambridge Crystallographic Database and analysed in conjunction with available gas-phase results. Geometric data indicate that the vinylic C atoms in cyclopropene use sp 1.19 hybrids in bond formation to substituents and contribute sp 2.68 hybridges to the ring σ-framework. The D 3 h -symmetric cyclopropenium ion has a bond length of 1.373(3)Å, which can be related to distances in unstrained systems. Comparison of data for cyclopropenylidenes and 3,3-difluorocyclopropene with analogous cyclopropanes shows that π-donor effects (distal bond lengthening, vicinal bond shortening) are apparent in cyclopropene. Rehybridization and π-donation are largely responsible for cyclopropenylidene geometry, rather than significant contributions from dipolar and pseudo-aromatic resonance forms. Insufficient data exist to quantify the effect of π-acceptor substituents on cyclopropene, but some lengthening of vicinal bonds is apparent. Three major bonding patterns are exhibited by organomental derivatives of cyclopropenium and cyclopropene.

ChemInform Abstract: DI‐Π‐METHANE REARRANGEMENTS OF SIMPLE ALLYLBENZENE DERIVATIVES WITH Π‐ACCEPTOR SUBSTITUENTS

Chemischer InformationsdienstVolume 12, Issue 26 Preparative Organic Chemistry ChemInform Abstract: DI-Π-METHANE REARRANGEMENTS OF SIMPLE ALLYLBENZENE DERIVATIVES WITH Π-ACCEPTOR SUBSTITUENTS K. DIETLIKER, K. DIETLIKERSearch for more papers by this authorH.-J. HANSEN, H.-J. HANSENSearch for more papers by this author K. DIETLIKER, K. DIETLIKERSearch for more papers by this authorH.-J. HANSEN, H.-J. HANSENSearch for more papers by this author First published: June 30, 1981 https://doi.org/10.1002/chin.198126148Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume12, Issue26June 30, 1981 RelatedInformation

Experimental and theoretical investigation of the unusual substituent effect of the vinyl group

σ+ Substituent constants for the ortho- and para-vinyl group have been determined by the application of the linear free-energy relationship to the nitration of the β-substituted styrene derivatives.Energy changes (relative to benzene system) for the proton and hydride ion transfer to individual positions in the styrene molecule have been calculated. Both approaches indicate that the vinyl group is capable of stabilizing both positively and negatively charged transition states. The interactions of the vinyl group with other substituents in the phenyl ring are also determined. Again, stabilizing effects with respect to both π-donor and π-acceptor substituents have been demonstrated for the vinyl group.