N.M.R. Experiments on Acetals: XXI [1]. Barriers for Ring inversion in Some Geminal Substituted 1,3‐Dioxanes

Swaelens

Ketelaere

et al. 1971

Self Cite

Several 1,3-dioxanes, not possessing chair conformations (Chart I), are newly prepared. The PMR data are discussed in terms of possible conformations. Typically ZJ(OCH20) coupling constants are too positive in accordance with disturbed torsional relations between a(CL-H) bonds and adjacent p ( 0 ) lobes. A semiquantitative discussion, relating 2J with presumed torsion ring angles, is given. Thus values for 'J(OCH,O) in carbon disulfide are: -5 . 5 cps for a twist conformation, with (idealized) torsion angles of 30' and 60" (form (14)); -3.4 cps for a conformation between a chair form a half-chair form and sofa form (resp. (16) and (17)); and -4.7 cps for a form close to a classical boat (19) with torsion angles of 0' and 60'.(*) H-2e is easily recognized because this H-atom is involved in long-range coupling [6,7], causing a broadening or splitting of the lines.

NMR Experiments on Acetals. Part 28. 1,3‐Dioxanes with Non‐Chair Conformations and the ²J(Och₂O) Criterium

Swaelens

Ketelaere

et al. 1971

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Preparation of 4,6‐Methyl, Alkyl‐1,3‐Diselenanes; 4,6‐Methyl, Alkyl‐1,3‐Thiaselenanes and 4,6‐Methyl, Alkyl‐Oxaselenanes

Geens

1971

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A general method for the preparation of 4,6-disubstituted 1,3-diselenanes and corresponding thiaselenanes and oxaselenanes is reported starting from appropriate (secondary) /3-diols. The corresponding mono-or di-mesylates (alt. 8-dibromides) are treated with sodium selenocyanate and/or sodium sulfocyanate in HMPT below 80°C. The resulting alkyl B-(di)seleno(sulfo) cyanates are subsequently reduced with sodium in liquid ammonia, followed by formalation in strong acidic medium. The resulting different positional and geometric isomers could not be completely separated by GC. Small amounts of accompanying 1,2-diselenolanes may disturb class-separation, but they can be removed after LAH treatment of the reaction mixture.Heterocyclic ringsystems such as 1,3-dioxanes, 1,3-dithianes and 1,3-oxathianes have been recently the subject of conformational studies at these laboratories [I, 21. Also selenium containing heterocycles became interesting. The syntheses of 1,3-thiaselenanes and 1,3-oxaselenanes are not reported till now [cf. 41. The actual paper describes a general method for the preparation of such (substituted) systems. A. SYNTHESIS OF 1,3-DISELENANESA possible route, as followed from the preparation worked out at these laboratories for 1,3-dithianes [2], was first tried. This would consist in the displacement of suitable P-diol mesylates by sodium selenolacetate instead of thiolacetate [2]. Attempts to prepare selenolacetic acid by reaction between acetylchloride and hydrogen selenide (in pyridine) however failed, as reported also by Jensen [3]. I ,3-Diselenane itself was previously [4] obtained by reaction of 1,3-dibromopropane (I A; 'R=ZR=3R=4R=H) with sodium selenosulfate in aqueous ethanol, affording (1 F), which by treatment with hydrogen peroxide results in polytrimethylenediselenide. On reaction with potassium in liquid ammonia the resulting potassium salt of propane-l,3-diselenol (1 G) is further immediately converted by treatment with formaldehyde in 4 N HCI into 1,3-diselenane with an overall yield of 25 Yo.The scheme failed however in an attempt to prepare 5,5-diMe-1,3-diselenane ( 2 C ; 4R=6R=H4 ; 5R=5'R=Me), the appropriate dibromide (1 A; 1R=4R=H; 2R=3R=Me) being unattacked by sodium selenosulfate; as a result of the low nucleophilicity of the reagent in aqueous alcohol. Moreover the dibromide used is a neopentyl-typed substrate.

NMR experiments on acetals. Part 52. Chair chair inversion in some symmetrically substituted gem‐dimethoxy‐cyclohexanes

Beule

1974

Abstract.Barriers for ring inversion in 4,4-di0-, 4,4-di c.propy1-, and 4,4-pentamethylene-1,l-dimethoxycyclohexane have been evaluated using different methods (Table 1). The close parallelism of the observed effect on dGZ caused by gem. substitution in the present compounds as compared with correspondingly substituted lI3-dioxanes let to suppose that strain in rotations along C-C bonds neighbouring the substitution site may contribute to the observed trends.The barriers for topomerization (ring inversion) of three 4,4-disubstituted-1,l-dimethoxycyclohexanes were determined by NMR-technique. For this purpose the methoxysignals were studied at temperatures varying between +20and -lOO°C with a Varian HA 100 spectrometer.Samples consisted in a mixture of 500 p 1 carbon disulphide, 20 ~1 dichloromethane and 40 p l of the acetal. Temperatures were determined with a methanol sample as modified by Van Geet (1). The fast-exchange spectra show a sharp single methoxy-signal; below the coalescence temperature two equally intense signals are observed.Using the formulas of Alexander ( 2 ) (with J = 0 for the methoxysignals) a lineshape analysis was completed. This was done by iterative computer processes, comparing the calculated lineshape with experimental data such as the intensity-ratio Imax/Imin, the shift difference d v , the half bandwidth W i and finally the experimental lineshape given point by point (3). In our case the "point by point''-method failed for fast exchange signals, so that only the bandwidth method could be used for temperatures above the Tc-range. the "point by point''-method (3) account was taken for the fact that the shift difference dvOMe does not reach a constant value at low temperatures.when the effect of the exchange phenomena was outranged, a temperature dependency of Av still remained (11). Only in Indeed,-2 9 5 -