The Electronegativity of Multiply Bonded Groups

. Can. J. Chem. 66, 2860 (1988).The stereochemical course of the reduction of ketones adjacent to a chiral center normally shows some diastereoselectivity (described by Cram's rule), the degree of which is dependent on the structure of the ketone and on the reaction conditions; the selectivity in acyclic species is often not very great. In this paper, we describe the sodium borohydride reduction of four acyclic P-ketosulfones, containing a chiral center at the a position, in which the products are formed with high diastereoselectivity. Reasons for this selectivity became apparent when we were able to show that these ketosulfones apparently exist predominantly in the same conformation in solution as in the solid state. This conformation requires that the preferred trajectory for nucleophilic attack on the carbonyl group leads to reaction on the re face, to yield the threo diastereomer of the P-hydroxysulfone. ' The results that led to these conclusions were obtained from structural studies by X-ray crystallography, as well as by detailed 'H and 13C nuclear magnetic resonance spectroscopy. In some cases, the latter spectra were run both in solution and in the solid state. 66, 2860 (1988).La degrC de diastCrCosClectivitC (dCcrit par la rtgle de Cram) de la rCduction de cCtones adjacentes d'un centre chiral dCpend de la structure de la cCtone ainsi que des conditions de la rCaction; la sClectivitC des esptces acycliques est souvent faible. Dans ce travail, on dCcrit la rkduction par le borohydrure de sodium de quatre P-~Ctosulfones acycliques, contenant un centre chiral en position a, qui donnent lieu i une haute diastCrCosClectivitC. Les raisons de cette sClectivit6 sont devenues apparentes lorsqu'on a pu montrer que la conformation principale de ces ~Ctosulfones est la mCme i 1'Ctat solide qu'en solution. Cette conformation implique que l'attaque nuclCophile sur le groupement carbonyle se produit prCfCrentiellement selon une trajectoire d'attaque sur la face re qui conduit au diastCrCoisomtre thrio de la P-hydroxysulfone. Les rksultats qui ont permis de proposercette conclusion ont Ct C obtenus i l'aide d'Ctudes de structures par diffraction des rayons-X ainsi que par resonance magnCtique nuclCaire du 'H et du 13C. Dans quelques cas, on a dCterminC ces derniers spectres tant en solution qu'i 1'Ctat solide.[Traduit par la revue]

Section: Formula Weightsupporting

confidence: 69%

Steric effects in the diastereoselective reduction of β-ketosulfones

Grossert¹,

Dharmaratne²,

Cameron³

et al. 1988

“…An upper limit to an assumed twofold barrier for the sulfoxide and the sulfone follows as 20 kJ/mol because the lower limit of ( sin2 8 ) is -0.32,' -1.12 = 0.286. The electronegativity of S 0 2 C H 3 is given as 3.1 (36), implying a 6~9 0 of -1.05 HZ. This means that (sin2 8 ) becomes 0.31(1), corresponding to an apparent V2 of 12.6 2 2.5 kJ/mol.…”

Section: Computed Barrier In Benzyl Thiolmentioning

confidence: 99%

The benzylic anomeric effect. Internal rotational potentials of ArCH₂X compounds (X = F, Cl, SH, SCH₃, S(O)CH₃, SO₂CH₃)

Penner¹,

Schaefer²,

Sebastian³

et al. 1987

. Can. J . Chem. 65, 1845Chem. 65, (1987. The apparent twofold barriers to rotation about the exocyclic C-C bonds in ArCH2X (X = SH, SMe, S(O)Me, S02Me) compounds have been determined in CS, and in benzene-d6, from the magnitudes of long-range proton-proton coupling constants. The work extends earlier studies of the systems X = F, C1. Except for X = F, all of the title compounds have a low energy conformation in which the C-X bond is perpendicular to the plane of the benzene ring. The introduction of two meta-chlorine substituents leads to a 2 kJ/mol relative destabilization of this structure for X = F, C1, SH. The trend in the rotational barriers associated with the sulfur substituents is SH, SMe < < S(O)Me, S02Me. Ab initio calculations, with geometry optimization at the STO-3G level for X = F, C1, SH, reproduce the experimental results, and also the magnitudes of the rotational barriers. Quantitative PMO analyses of these a b initio wavefunctions have been employed to distinguish between "steric" and "other" interpretations, and reveal that the preferred conformation of X = F is determined by a first order perturbational effect, manifested in an electrostatic attraction between the heteroatom and a syn-periplanar ortho-hydrogen. For X = C1, the PMO analysis suggests that a T T~, -u & interaction, analogous to the n -~:~ interaction postulated as the origin of the anomeric effect in heterocyclohexanes, plays a dominant role. For X = SH, both T T~~-~:~ and ucx-n:, interactions are found; the latter is not possible in the anomeric effect itself. It is concluded that a benzylic anomeric effect exists, and that its magnitude, as a function Chem. 65, 1845Chem. 65, (1987.Utilisant les amplitudes des constantes de couplage proton-proton i longue distance, on a dCterminC les barrikres binaires apparentes 21 la rotation autour des liaisons exocycliques C-C des composCs ArCH2X (X = SH, SMe, S(0)Me et S0,Me) dans le CS2 et dans le benzene-d6. Ces Ctudes sont une extension d'etudes anttrieures sur des systkmes dans lesquels X = F et C1. Except6 pour le cas oB X = F, tous les composCs mentionnCs dans le titre prtsentent une conformation de basse Cnergie dans laquelle la liaison C-X est perpendiculaire au plan du noyau benzCnique. L'introduction de deux chlores en positions me'ta conduit a une distabilisation relative de 2 kJ/mol de cette structure pour X = F, C1 et SH. Cette tendance dans les barrikres rotationnelles associCes avec les substituants contenant du soufre est la suivante : SH. SMe << S(O)Me, S02Me. Des calculs a b initio impliquant des optimisations de la gComCtrie au niveau STO-3G ont CtC rCalisCs sur les composCs contenant X = F, C1 et SH et ils permettent de reproduire les resultats expkrimentaux ainsi que les amplitudes des barrikres a la rotation. On a utilisC des analyses quantitatives PMO de ces fonctions d'onde a b initio pour distinguer entre les interpretations ste'riques et antres; ces analyses rCvklent que la conformation privilCgiCe pour X = F est dCterminCe par un effet de perturbation du pre...

“…Predictions may be improved by utilizing group electronegativities (4,5). The shift of electron density in one a bond is supposed to trigger a parallel but smaller shift in adjacent bonds with the result that each donor atom will lose electrons in greater proportion than it receives them (6).…”

Section: Introductionmentioning

confidence: 99%

An MNDO study of substituent effects in carbocations: the unimportance of inductive effects

Aissani¹,

Baum²,

Langler³

et al. 1986

. 64, 532 (1986).Stabilization effects for both saturated and unsaturated substituents were examined using MNDO calculations. (By "saturated substituents" we mean to imply that the substituent is attached to C+ by sp3 carbon, whether or not the substituent has a site of unsaturation at some point remote from the carbocationic center.) Relative gas phase stabilities of regioisomeric L carbocations have been calculated. For simple groups, directly bound to C, a donation, a donation, and polarization effects are found to be important. Saturated-substituent stabilization effects are examined by means of reaction enthalpies for 64, 532 (1986).On utilise des calculs MNDO sur des carbocations choisis pour dCterminer les effets de stabilisation de substituants tant saturCs que non saturks. (Par "substituant saturC", on entend un substituant qui est attache au C+ par un carbone sp3 mCme si le substituant porte un site d'insaturation en un autre point CloignC du centre carbocationique.) On a calculC les stabilitCs relatives + de carbocations rCgioisom&res. Pour des groupements simples qui sont directement liCs au C , on a trouve que les effets de donation TI ou a ainsi que de polarisation sont importants. On a CtudiC les effets de stabilisation des substituants saturCs a l'aide + des enthalpies de rCactions isodesmiques entre les molCcules neutres appropriCes et le CH3. Les analyses traditionnelles des effets de substituants qui sont uniquement bakes sur les effets inductifs des groupements saturCs n'ont que peu ou pas de signification. Les effets inductifs n'ont pas d'importance spCciale dans la dktermination des effets de stabilisation des substituants saturCs des carbocations.[Traduit par le journal] Introduction As part of our work in organosulfur chemistry, we have recently initiated a program of theoretical studies. Our initial study (1) has raised two questions: (i) how does sulfur compare to saturated carbon as a carbocation stabilizer? and (ii) how useful is the concept of inductive stabilization in anticipating substituent-stabilization effects in carbocations? Recent studies authored by Topsom et al. (2, 3) have further stimulated our interest in problems associated with inductive effects.Inductive effects accompany a shift of electron density in a u bond toward the more electronegative atom involved in the bond. Predictions may be improved by utilizing group electro-