3,5-Dithiatricyclo[5.2.1.0^2,6]decan-4-one

Abstract

The X-ray structure of the title compound has been determined and the structure shows an exo-configured planar dithiolanone ring. This is in contrast to the few previous dithiolanones to be characterised crystallographically, which are all twisted.

1. Introduction

The title compound 1 was first mentioned in 1985 [1], when it was formed by mercuric acetate oxidation of the corresponding 1,3-dithiolane-2-thione 2 (Scheme 1). However, in that report it was treated as a synthetic intermediate and no analytical or spectroscopic data were given. A short time later, a second preparative method was described [2] involving reaction of the 1,2,3-trithiolane 3, formed from norbornene 4 and sulfur [3], with dichlorocarbene under phase-transfer conditions. In this report it was suggested that the trithiolane 3 reacts with dichlorocarbene to give the thione, 2, but the mechanism by which 2 was converted into 1 was unclear. Perhaps the simplest preparation of 1 involves direct treatment of norbornene 4 with diisopropyl xanthogen disulfide and the radical initiator azobis(isobuytronitrile) (AIBN), which affords 1 directly in 74% yield [4]. Although compound 1 has been characterised by melting point, ¹H and ¹³C-NMR and IR spectroscopy, and elemental analysis [2], its X-ray structure has not been investigated. We have obtained compound 1 repeatedly in low yield as a by-product arising from oxidative degradation of the adduct, 5, formed from norbornene 4 and Bu₃P × CS₂. The ylene 5 can be exploited synthetically in a Wittig reaction with aldehydes to give 2-alkylidene-1,3-dithiolanes [5,6,7] or, in the presence of extra CS₂, in a 1,3-dipolar cycloaddition process with dipolarophiles such as dimethyl acetylenedicarboxylate (DMAD) to give dihydrotetrathiafulvalenes [8]. Furthermore, it can also undergo hydrolysis to give the corresponding 1,3-dithiolane [9]. We describe here determination of the molecular and crystal structure of 1 by X-ray diffraction. The structure is compared with the few crystal structures previously reported for 1,3-dithiolan-2-ones.

2. Results

A sample of compound 1 suitable for X-ray diffraction was obtained from chromatographic purification of the product obtained by reaction of 5 with propargylaldehyde, HC≡C–CHO, and CS₂ (<5% yield). The resulting molecular structure is shown in Figure 1 and selected bond lengths and angles are given in

Table 1. Selected bond lengths and angles.

Bond	Length/Å	Angle	Value/°
C(1)–C(2)	1.570(8)	C(1)–C(2)–S(2)	113.2(4)
C(1)–S(1)	1.805(6)	C(2)–S(2)–C(8)	99.7(3)
S(1)–C(8)	1.753(6)	S(2)–C(8)–O(8)	122.5(5)
C(8)–O(8)	1.206(7)	O(8)–C(8)–S(1)	123.2 (5)
C(8)–S(2)	1.775(6)	S(2)–C(8)–S(1)	114.4(3)
S(2)–C(2)	1.798(6)	C(8)–S(1)–C(1)	100.4(3)
		S(1)–C(1)–C(2)	112.3(4)

. The structure features an exo-configured dithiolanone ring, which is essentially planar with the expected long C-S bond lengths and correspondingly smaller internal angles at sulfur. The crystal structure features a centrosymmetric arrangement of four molecules in the unit cell (Figure 2). Rather surprisingly, a search of the Cambridge Structural Database (CSD, March 2020 update) revealed only five previous structures containing the 1,3-dithiolan-2-one fragment with at least one sp³ carbon in the ring (Figure 3). These are the parent compound 6 [10], the polychlorinated compound 7 in which the dithiolanone ring is tetrasubstituted [11], the bicyclic diester 8 with a trans-disubstituted dithiolanone ring [12], and the Diels–Alder dimer of 4,5-bis(methylene)-1,3-dithiolan-2-one 9 and the derived cobalt complex 10 [13], both of which have the dithiolanone ring fully substituted but with one spiro sp³ centre and one sp² centre.

While comparison of the internal angles at sulfur and at C=O show all six compounds to have rather similar values, the key difference observed between the molecular structure of 1 and the previous structures 6–10 is the degree of planarity in the dithiolanone ring. This is readily quantified by examining the torsion angle S-C-C-S for the six compounds (

Table 2. Key angles in the structures of dithiolanones (°).

Compound	C–S–C Internal Angle	S–C(O)–S Internal Angle	S–C–C–S Torsion Angle	Reference
1	100.4(3), 99.7(3)	114.4(3)	1.9(5)	This work
6	96.61(5), 96.57(5)	113.41(6)	44.44(9)	[10]
7	95.7(2), 94.6(2)	114.8(2)	48.2(3)	[11]
8	94.7(2), 94.5(2)	113.6(2)	51.3(3)	[12]
9	99.67(8), 98.62(8)	114.1(1)	11.6(2)	[13]
10	97.9(2), 97.2(3)	113.7(3)	36.2(4)	[13]

). While the carbonyl C(2) carbon is completely planar in all cases, it is only in the case of 1 where the whole dithiolanone ring is fused cis to the rigid bicyclo[2.2.1] skeleton that all five ring atoms are essentially coplanar, corresponding to a S-C-C-S torsion angle near to zero. In all the other cases this torsion angle is significantly greater, corresponding to a twisting of the ring, and it appears that even in the absence of substituent effects in 6 this twisted conformation is preferred.

In summary, the X-ray crystal structure of the norbornane-fused 1,3-dithiolan-2-one 1 shows it to adopt an exo configuration with a planar heterocyclic ring, in contrast to the twisted shape adopted by the small number of other 1,3-dithiolan-2-ones for which crystal structures are known.

3. Experimental

3,5-Dithiatricyclo[5.2.1.0^2,6]decan-4-one (1): Crystal data for C₈H₁₀OS₂, M = 186.28 g mol⁻¹, colourless prism, crystal dimensions 0.25 × 0.12 × 0.05 mm, orthorhombic, space group Pna2₁, a = 13.134(6), b = 6.336(3), c = 9.999(4) Å, V = 832.1(6) ų, Z = 4, D_calc = 1.487 g cm⁻³, T = 93 K, R1 = 0.0430, Rw2 = 0.1051 for 1350 reflections with I > 2σ(I), and 101 variables, R_int 0.0530, Goodness of fit on F² 1.122. Data were collected using graphite monochromated Mo Kα radiation λ = 0.71075 Å and have been deposited at the Cambridge Crystallographic Data Centre as CCDC 1989785. The data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/getstructures. The structure was solved by direct methods and refined by full-matrix least-squares against F² (SHELXL Version 2018/3 [14]). Hydrogen atoms were assigned riding isotropic displacement parameters and constrained to idealised geometries. The data did not allow determination of the polarity of the axis.