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Substituent X (substituent + x)
Selected AbstractsSubstituent effects on conformational preference in , -substituted , -fluorophenylacetic acid methyl ester model systems for chiral derivatizing agentsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2009Riadh Sahnoun Abstract In connection with study of chiral derivatizing agents (CDAs) for NMR determination of absolute configuration of organic compounds, factors controlling the conformational preference between syn - and anti -forms in , -substituted , -fluorophenylacetic acid methyl ester (FC(X)(Ph)COOMe) model systems were theoretically investigated. Substituents X at the stereogenic carbon atom were X,=,H, CCH and CH3, the electronic and steric properties of which were significantly different from each other. The model system with X,=,CCH and that with X,=,CH3 were found to be possible candidates for fluorine-containing CDAs. The syn conformation is stable compared with the anti one by 0.7,kcal,mol,1 for the ester with X,=,CCH. On the other hand, the anti conformation is stable compared with the syn one by 0.5,kcal,mol,1 for the ester with X,=,CH3. Both natural bond orbital (NBO) analysis and deletion of selected orbitals based on the donor,acceptor NBO scheme were adopted for semi-quantitative estimation of factors responsible for the conformational preference as well as a qualitative inspection of occupied canonical molecular orbitals (MOs). It was shown that [,,(,*,+,,*)(CO)] and [,,,*(Ph) and ,(Ph),,*] hyperconjugations are the main factors controlling the conformational preferences between the syn and anti conformations. Other types of effects such as electrostatic effects were also investigated. The role of the fluorine atom was also clarified. Copyright © 2009 John Wiley & Sons, Ltd. [source] Long-range polar and steric effects in propionate-SG1-type alkoxyamines (SG1-CHMeCOOX): a multiparameter analysis,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2006Gennady Ananchenko Abstract The effects of the substituent X on the homolysis rate constants (kd) of SG1-propionate type alkoxyamines (SG1-CHMeCOOX) are analyzed by a multiparametric equation with ,, the steric constant and ,I, the polar inductive/field Hammett constant of X. An influence of long-range polar and steric effects on kd was observed, that is, decrease in kd with increasing size of the X group and increase in kd with increasing polarity of the X group. Copyright © 2006 John Wiley & Sons, Ltd. [source] 29Si,13C spin,spin couplings over Si,O,Carom linkMAGNETIC RESONANCE IN CHEMISTRY, Issue 7 2006Jan Sýkora Abstract 29Si,13C couplings were measured in para substituted silylated phenols, XC6H4OSiR1R2R3(X 2, CF3, Cl, F, H, CH3, CH3O). The SiR1R2R3 silyl groups included trimethylsilyl (Si(CH3)3, TMS), tert- butyldimethylsilyl (Si(CH3)2C(CH3)3, TBDMS), dimethylsilyl (SiH(CH3)2, DMS), and tert- butyldiphenylsilyl (Si(C6H5)2C(CH3)3, TBDPS). Previously developed (Si,C,Si)gHMQC methods and narrow 29Si lines allowed the determination of coupling constants over up to five bonds. Besides the number of intervening bonds between the silicon and carbon atoms, all the measurable couplings depend also on the nature of the substituents on the silicon. The two- and three-bond couplings are not affected by ring substitution in the para position. These properties render the 29Si,13C couplings suitable for line assignment in the spectra of silylated polyphenols. The experimental results are in reasonable agreement with theoretical calculations. The calculations show, in agreement with the data reported in the literature for couplings between other nuclei, that the two-bond and three-bond couplings, which are of similar magnitudes, are of opposite signs. If the signs of these geminal and vicinal couplings could be determined experimentally, they would greatly facilitate the line assignment. The four- and five-bond couplings are affected by the substituent X in a nontrivial manner. Copyright © 2006 John Wiley & Sons, Ltd. [source] Analysis of Linear Free-Energy Relationships Combined with Activation Parameters Assigns a Concerted Mechanism to Alkaline Hydrolysis of X-Substituted Phenyl DiphenylphosphinatesCHEMISTRY - A EUROPEAN JOURNAL, Issue 24 2008Ik-Hwan Um Prof. Abstract A kinetic study is reported for alkaline hydrolysis of X-substituted phenyl diphenylphosphinates (1,a,i). The Brřnsted-type plot for the reactions of 1,a,i is linear over 4.5 pKa units with ,lg=,0.49, a typical ,lg value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with ,o and ,, constants are linear but exhibit many scattered points, while the corresponding Yukawa,Tsuno plot results in excellent linear correlation with ,=1.42 and r=0.35. The r value of 0.35 implies that leaving-group departure is partially advanced at the rate-determining step (RDS). A stepwise mechanism, in which departure of the leaving group from an addition intermediate occurs in the RDS, is excluded since the incoming HO, ion is much more basic and a poorer nucleofuge than the leaving aryloxide. A dissociative (DN + AN) mechanism is also ruled out on the basis of the small ,lg value. As the substituent X in the leaving group changes from H to 4-NO2 and 3,4-(NO2)2, ,H,, decreases from 11.3,kcal,mol,1 to 9.7 and 8.7,kcal,mol,1, respectively, while ,S,, varies from ,22.6,cal,mol,1,K,1 to ,21.4 and ,20.2,cal,mol,1,K,1, respectively. Analysis of LFERs combined with the activation parameters assigns a concerted mechanism to the current alkaline hydrolysis of 1,a,i. [source] Synthesis of 2,4-Diaryl-3,4-dihydro-2H - naphth[2,1- e][1,3]oxazines and Study of the Effects of the Substituents on Their Ring - Chain TautomerismEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 10 2004István Szatmári Abstract A number of 2-(,-amino-Y-substituted-benzyl)-1-naphthol hydrochlorides were prepared by a convenient Mannich-type aminoalkylation. 2,4-Diaryl-3,4-dihydro-2H -naphth[2,1- e][1,3]oxazines were prepared through the ring-closure reactions of the starting aminonaphthols with aromatic aldehydes, which proved to furnish three-component (ring1,open,ring2) tautomeric mixtures in CDCl3 at 300 K. The electronic effects of the 2-aryl groups on the ratios of the ring - chain tautomeric forms at equilibrium could be described by Equation (1). Study of the effects of substituents X and Y on the tautomeric equilibria [by the aid of the multiple linear regression analysis of Equations (2) and (3)] revealed that the trans -chain equilibrium constants are significantly influenced by the inductive effect (,F) of substituent Y on the 4-phenyl ring. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Structural systematics of 4,4,-disubstituted benzenesulfonamidobenzenes.ACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2007One hundred 4,4,-disubstituted benzenesulfonamidobenzenes, X,C6H5,SO2,NH,C6H5,Y, where X, Y = NO2, CN, CF3, I, Br, Cl, F, H, Me, OMe, have been synthesized and their crystal structures determined. The resulting set of 133 structures, which includes polymorphic forms, is used to make a comparative study of the molecular packing and the nature of the intermolecular interactions, including the formation of hydrogen-bonding motifs and the influence of the two substituents X and Y on these features. Nine distinct supramolecular connectivity motifs of hydrogen bonding are encountered. There are 74% of all the structures investigated which exhibit one of two motifs based on N,H...O=S interactions, a dimer or a chain. There are three other, infrequent motifs, also employing N,H...O=S links, which exhibit more complexity. Four different chain motifs result from either N,H...O=N, N,H...C[triple-bond]N or N,H...OMe interactions, arising from the presence of a nitro (position Y), nitrile (X or Y) or methoxy (Y) substituent. The program XPac [Gelbrich & Hursthouse (2005). CrystEngComm, 7, 324,336] was used to systematically analyse the packing relationships between crystal structures. Similar discrete (zero-dimensional) and extended (one-dimensional and two-dimensional) structure components, as well as cases of isostructurality were identified. A hierarchy for the classification of the 56 distinct structure types of this set is presented. The most common type, a series of 22 isostructures containing the simple centrosymmetric N,H...O=S-bonded dimer, is discussed in detail. [source] | ||||||||||