282 81 (1962) RECUEIL

I. Pentamethylene sulphide 4 12-13" 11. 1,2-Dithiane 5 30-31"

111. 1,3-Dithiane 54-55" IV. 2-Methyl-I ,3-dithiane 7

VI. 2-Phenyl-l,3-dithiane 7 74" V. 2,2-Dirnethyl- I ,3-dithiane

VII. 1 ,4-Dithiane 111" I (subl.)

547.842 INVESTIGATIONS INTO THE CONFORMATION OF DITHIANES

BY

H. T. KALFF AND E. HAVINGA Laboratory of Organic Chemistry, The University, Leiden

The present communication reports the results obtained from investi- gations on dithianes, an extension of the studies made in this laboratory on substituted cyclohexanes 1, 2 and dioxanes 3.

The compounds were synthesized by methods indicated in the literature (cf. Table I).

Table I Physical constants of the dithianes studied

I 140-141" 1.78

2.57 2.10

38-40"/1 2.04 88-89"/16 2.01

2.02 0.4

Compound Dipole moment (D) in benzene1 value lit.

1.71

0.2 10

For the investigations into the conformational characteristics of the dithianes the following methods were applied : 1. Dipole moments were calculated from measurements in dilute solutions

W. Kwestroo, F. A. Meyer and E. Havinga, Rec. trav. chim. 73, 717 (1954); W. Kwesfroo, Thesis Leiden, 1954. E. C. Wessels, Thesis Leiden, 1960. C. A. Alfona, C . Romers and E. Havinga, Tetrahedron Letters 10, 16 (1959). H. T. Clarke, J. Chem. SOC. 101, 1805 (1912). A. Schoberl and H. Grade, Ann. 614, 66 (1958). D. T. Gibson, J. Chem. SOC. 132, I, 12 (1930). W. Aufhenrieth and K. Wolff, Ber. 32, 1375 (1899).

* 0. Masson, J. Chern. SOC. 49, 233 (1886). C. W. N. Cumper and A . I. Vogel, ibid. 1959, 3521.

lo K. Calderbank and R. J . W. Le Fgvre, ibid. 1949, 199.

Investigations into the conformation of dithianes 81 (1962) RECUEII, 283

values lit. (in ethanol)

22613 I 290 l4 2.50 l4 248 lS 2.70 l5 250 l5 2.69 l5 251 l5 2.84 lS

238 254 l5 3.15 2.90 l5 260 265 2.80 2.60

226 l6 , 2.48 l R

1ma.x I l og&

I

in benzene, using the extrapolation formulae of Halverstadt and Kumler ll. The values obtained for pentamethylene sulphide and 1,Cdithiane are in good agreement with those reported in the literature. 2. Infrared absorption spectra were obtained of the compounds in the solid (KBr discs) or liquid state, using a Unicam SP 100 spectrophotometer. Some of these spectra have been recorded l2 *. 3. Ultraviolet absorption spectra (cf. Table 11) were taken of solutions in methanol and in cyclohexane with the aid of a Cary recording spectro- photometer, model 14 PM 50 *.

Table I1 Ultraviolet absorption characteristics of the dithianes

Compound

I I1

111 IV V

VI

VII

1max

- methanol 1 cyclohexane I

(230) 290

232 248 (232) 250

250 259 265

290 232 250

(231) 252 25 1

260 265

2.50 2.53 2.67 2.60 2.69

2.83 2.81 2.62

226 [ 226 1 2.51

As is well known, cyclohexanes occur preferably in the chair conforma- tion. For many other six membered saturated rings this conformation also seems to be the most stable one. 1,2-Dithiane 1’ occurs in the chair form and the same has been proved by X-ray analysis for 1,2-dithiane-3,6- dicarboxylic acid 18 and 1 ,Cdithiane 199 2 0 . The data obtained from electron ~ _ _ _ __

* The spectra will be reproduced in the forthcoming thesis of one of us (H.T.K.). l1 I . F. Halverstadt and W. D. Kumler, J. Am. Chem. SOC. 64, 2988 (1942). l2 a ) J. KIoubek and V. Ettel, Coll. Czech. Chern. Comm. 26, 517 (1961).

b) Amer. Petr. Inst. Research Program 44, Infrared Spectra. l3 E. A . Fehnef and M. Carmack, J. Am. Chem. SOC. 71, 84 (1949). l4 J. A . Barltrop, M. Calvin and P . M. Hayes, ibid. 76, 4348 (1954). lS E. E. Campaigne and G. F. Schaefer, B61. C61. Quim. Puerto Rico 9, 25 (1952);

Chern. Abstr. 46, 10884* (1952). l R H. Mohler and J. Sorge, Helv. Chim. Acta 23, 1200 (1940). l7 G. Claeson, G. M. Androes and M, Calvin, J. Am. Chem. SOC. 83, 4351 (1961);

A. Liittringhaus, S. Kabuss, W. Maier and H. Friebolin, 2. Naturforsch. 16b, 761 (1961). l8 0. Foss and L. Schotte, Acta Chern. Scand. 11, 1424 (1957). l9 R . E. Marsh, Acta Cryst. 8,91 (1955). 2o G. Y. Chao and J. D. McCullough, ibid. 13, 727 (1960).

284 H . T. Kalff and and co., Investigations into the conformation of dithianes

diffraction and Raman spectra also give support to the assumption of the occurrence of the chair conformation for 1,Cdithiane 21, 22.

The values found for the dipole moments of 1 ,Zdithiane and 1,4-dithiane (-2.6 D and 0.4 D) are in agreement with those calculated for the chair form (-2.7 D and -0 D, respectively). The assumption of the predomi- nance of the chair form in solution is the simpliest that fits in with the data; however, more complicated possibilities such as the occurrence of an equi- librium mixture of various conformations including a substantial percentage of the flexible form, may not be excluded.

To our knowledge, 1,3-dithiane has not yet been the subject of conforma- tional studies. For the sake of orientation a comparison may be made with 1,3-dioxane. The dipole moment of this compound has been determined recently by Arbousow 23 (1.91 D ) and by Walker and Davidson 24 (2.13 D). Arbousow calculates a value of 1.91 D for the chair and 2.34 D for the boat conformation, from which he concludes that 1,3-dioxane occurs in the chair conformation.

The values obtained for the dipole moments of 1,3-dithiane and its derivatives are all very close to each other (2.01 D - 2.10 D), which may be considered an indication that these compounds occur in the same - probably the chair - conformation.

The ultraviolet absorption spectra 13, l4 show that cyclic thio-compounds have an absorption maximum at longer wavelength than non-cyclic com- pounds. In a linear disulphide the absorption maximum is at 250 mp. The maximum of 1,2-dithiane is at 290 mp. In the case of 1,3-dithio-ethers the same regularity is observed. Linear compounds have their maxima at N 235 mp, whereas 1,3-dithiane and its derivatives exhibit a maximum at N 250 mp . It should be noted that 1,3-dithiane has a second small absorp- tion maximum at 232 mp, whereas 2-methyl-I ,3-dithiane shows an inflection and 2,2-dimethyl-1,3-dithiane does not exhibit any specific absorption at this wavelength.

Summarising, all available data on dithianes seem to be consistent with the preponderance in solution of the chair form, although here, as in many other cases, the possible occurrence of stretched flexible forms should not yet be excluded.

(Received October 28th. 1961).

21 0. Hassel and H. Viervoll, Acta Chem. Scand. 1, 149 (1947). 32 K. Hayasaki, J. Chem. SOC. Japan 81, 1645 (1960), English summary: A 112. 23 B. A. Arbousow, Bull. SOC. chim. France 1960, 1311. 24 R. Walker and D. W. Davidson, Can. J. Chem. 37, 492 (1959).