An X-ray and conformational study of the α-chloro derivatives of 5,7-dihydro-1,11-dimethyl-6H-dibenzo[a,c]cyclohepten-6-one

Abstract

. The crystal structures for four a-chloro derivatives 2-5 of the titled ketone 1 have been determined, confirming their earlier assignments of stereochemistry. Variations in the conformations of 2-5 suggested some flexibility in the central seven-membered ring. Molecular mechanics calculations of the changes in energy as a function of the internal torsional angles at the carbonyl of these chloro derivatives indicated the ring flexibility to be comparable to that of cyclohexanone. In the three compounds containing an axial chlorine the distance between the chlorine atom and two of the carbon atoms of the more remote benzene ring was 3.2 + 0.1 A, suggesting the value for the thickness of a benzene ring may be significantly smaller than the accepted value when it is interacting with a single atom.ROBERT R. FRASER, CORINNE BENSIMON, NEIL C. FAIBISH et FANZUO KONG. Can. J. Chem. 72, 148 1 (1994). On a dCterminC les structures cristallines des quatre dCrivCs a-chlorks 2-5 de la cCtone 1 mentionnCe dans le titre et on a ainsi confirm6 les attributions de stCrCochimie faites antCrieurement. Les variations dans les structures des composCs 2-5 suggkrent l'existence d'une certaine flexibilitk dans le cycle central B sept chainons. Des calculs de mCcanique molCculaire des changements d'Cnergie en fonctions des angles de torsion internes au niveau du carbonyle de ces dCrivCs chlorCs indiquent que la flexibilitC du cycle est comparable B celle de la cyclohexanone. Dans les trois composCs contenant un atome de chlode axial, la distance entre l'atome de chlore et deux des atomes de carbone du noyau benzCnique le plus Cloignt est de 3,2 + 0, I A; ceci suggbre que la valeur pour 1'Cpaisseur d'un noyau benzCnique interagissant avec un seul atome est peut-&tre beaucoup plus faible que la valeur gCnCralement acceptCe.[Traduit par la RCdaction]The bridged biaryl ketone, 5.7-dihydro-1, 11-dimethyl-6H- through its central ring giving rise to two possible orientations a to the carbonyl, either "axial" or "equatorial" on the basis of their similarity to the geometry at the a positions of cyclohexanone (see Fig. 1). The ring was considered by Mislow to possess a single rigid conformation as indicated by molecular models and supported by its proton nrnr spectra (2). We have used this ketone in a variety of studies involving the effects of stereochemistry on reactivity since it offered certain advantages over cyclohexanone as a model. One example involves a study of the hydrogen-deuterium exchange reaction for the axial and equatorial protons a to the carbonyl (3). In another, measurement of the variation in rates of addition of nucleophiles to 1 as a function of the stereochemistry of its a methyl and chloro derivatives provided evidence for the primary importance of torsional interactions as contributors to the antiperiplanar effect (4). The validity of the assumption of rigidity of 1, and thus its ability as a model to provide derivatives of known fixed stereochemistry, was examined for 3 and its methyl derivatives using X-ray crystallo...

Stereodependence of the Effect of α Substituents on the Face Reactivity of Conformationally Rigid Ketones toward Metal Hydrides. Is There an Antiperiplanar (Cieplak) Effect?

“…The greater rigidity of 1 and three of its α methyl derivatives was established on the basis of changes in energy with distortion from the most stable conformation as calculated by molecular mechanics using the MMX program, whose validity was confirmed by agreement between their calculated energy minima and those observed in the solid state …”

“…Previously we have described the preparation and conformational properties of the axial and equatorial methyl ( 2 , 3 ), chloro ( 4 , 5 ), and methoxy ( 6 , 7 ), derivatives of 1 in our study of substituent effects on the rates of their 18 O exchange . The two methylthio derivatives ( 8 , 9 ) were prepared from the reaction of the enolate of 1 with methyl methanethiolsulfonate.…”