Addition of phenylisocyanate to 3,5-diphenyl-1,2-dithiolium-4-olate gives a thioketone

Article abstract of DOI:10.1016/j.tetlet.2017.02.076

The Type B mesoionic heterocycle, 3,5-diphenyl-1,2-dithiolium-4-olate, reacts with phenylisocyanate producing a thioketone derivative rather than a pseudo-semi-conjugated heterocyclic mesomeric betaine. The structure of the thioketone product was confirmed by an X-ray crystallographic investigation.

Full text of DOI:10.1016/j.tetlet.2017.02.076

Accepted Manuscript
Addition of phenylisocyanate to 3,5-diphenyl-1,2-dithiolium-4-olate gives a
thioketone
Michael R. Probert, Christopher A. Ramsden, Stephen P. Stanforth
PII:
S0040-4039(17)30263-0
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.02.076
TETL 48687
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
6 February 2017
22 February 2017
24 February 2017
Please cite this article as: Probert, M.R., Ramsden, C.A., Stanforth, S.P., Addition of phenylisocyanate to 3,5-
diphenyl-1,2-dithiolium-4-olate gives a thioketone, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/
j.tetlet.2017.02.076
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Addition of phenylisocyanate to 3,5-diphenyl-1,2-dithiolium-4-olate gives a thioketone.
Michael R. Probert,a Christopher A. Ramsdenb and Stephen P. Stanforthc*
a School of Chemistry, Bedson Building, Newcastle University, Newcastle-upon-Tyne, NE1 7RU, UK
b
Lennard-Jones Laboratories, School of Physical and Geographical Sciences, Keele University,
Staffordshire, ST5 5BG, UK
c Department of Applied Sciences, University of Northumbria, Newcastle-upon-Tyne, NE1 8ST, UK
* steven.stanforth@northumbria.ac.uk.
Abstract: The Type B mesoionic heterocycle, 3,5-diphenyl-1,2-dithiolium-4-olate, reacts with
phenylisocyanate producing a thioketone derivative rather than a pseudo-semi-conjugated
heterocyclic mesomeric betaine. The structure of the thioketone product was confirmed by an X-ray
crystallographic investigation.
Keywords: mesomeric betaine, mesoionic, semi-conjugated
Only two examples (4, X = O, S, Fig. 1) of six-membered Class 5 heterocyclic mesomeric betaines
(HMBs)1,2 have been described in the literature. A claim,3 based on unpublished work by Ollis and
Morgan,4 suggested that the green crystalline product obtained from the five-membered Class 5
HMB 6 and phenylisocyanate is the six-membered pseudo-semi-conjugated HMB 4 (X = O). We have
reinvestigated this product and now report a revision of the published structure.
A classification of heterocyclic mesomeric betaines has recently been proposed from which five
discrete classes of HMB are identified.1,2 This classification system has enabled the similarities and
differences in the chemistry and properties of a diverse range of HMBs to be rationalised. Two
classes, semi-conjugated (Class 3) and pseudo-semi-conjugated (Class 5) HMBs, were not recognised
as discrete types of heterocycle in an earlier classification system.3 A feature of these heterocycles of
some interest is the observation that (i) they are associated with a higher degree of classical and
magnetic aromaticity but (ii) lower thermodynamic stability than their corresponding isomeric
heterocycles, including those possessing fully covalent rings.2,5
The formulae 1 and 3 (Fig. 1), in which the heteroatoms X and Y each contribute 2π-electrons to the
molecules’ π-electron system, are general examples of six-membered semi-conjugated HMBs
(structure 1) and pseudo-semi-conjugated HMBs (structure 3). To the best of our knowledge,
heterocycle 2 is the only recorded example of a semi-conjugated HMB of general structure 1; this
structure was confirmed by X-ray crystallography.6 As far as we are aware, only two examples
1

(heterocycles 4, X = O, S) of pseudo-semi-conjugated HMBs related to structure 3 have been
disclosed.4,7 In contrast to the 6-membered heterocycles 1 and 3, the 5-membered Type B mesoionic
heterocycles,8,9 as represented by the general formula 5, provide numerous examples of pseudo-
semi-conjugated HMBs. In view of the relative paucity of structural information relating to the 6-
membered heterocycles 1 and 3 and their potentially interesting properties, 2,5 we report in this
Letter a re-investigation into the reported synthesis of heterocycle 4 (X = O).
Figure 1. Semi-conjugated and pseudo-semi-conjugated HMBs.
The Type B mesoionic compound, 3,5-diphenyl-1,2-dithiolium-4-olate 6, was prepared by the
reaction of 1,1,3-tribromo-1,3-diphenylpropane-2-one with potassium ethylxanthate in DMF at
room temperature following a literature procedure.10 Compound 6 was heated with
phenylisocyanate in xylene at reflux, producing a green compound11 for which the structures 4 (X =
O)4 and 8 (X = O)12 had previously been assigned (Scheme 1). An X-ray crystallographic investigation
(Fig. 2) has showed the structure of the green compound to be the thioketone derivative 8 (X = O),13
confirming Rowson’s original structural assignment.12 The thioketone derivative 8 (X = O) adopts a
Z’=2 structure in the Pbca space group. The model shows a network of weak hydrogen bonds
pervading the system. The two independent molecules are related by non-crystallographic inversion
with the sense of rotation of the phenyl rings, to the central plane, in opposite orientations.
The spectroscopic properties are also consistent with the thioketone structure 8 (X = O).11 In
particular, the infrared spectrum shows absorptions consistent with the presence of both thione
(1359 cm-1) and keto (1780 cm-1) functional groups. Additionally, the green colour is consistent with
its formulation as a thioketone derivative, e.g. thiobenzophenone is blue.14 Similar green products
reported by Rowson using 4-methylphenyl- , 4-chlorophenyl- and 2-naphthylisocyanate undoubtedly
have the corresponding thioketone structures, correctly assigned by Rowson.12
The actual product 8 (X = O) is readily rationalised by the formation of adduct 7 (X = O), possibly in
two steps, followed by opening of the dithiolene ring and elimination of sulfur (Scheme 1).
Cycloadditions analogous to that proposed for the formation of product 8 (X = O) can be found in the
chemistry of dehydrodithizone 9 which gives analogous bicycloadducts with the electrophilic
dipolarophiles tetraphenylcyclopentadienone and dimethyl acetylenedicarboxylate, followed by
opening of the original ring.3,15
o
Rowson also obtained a crystalline product (dark green needles, mp 201-203 C (decomp.)) using
phenylisothiocyanate instead of phenylisocyanate.12 This product shows only the thione absorption
(1360 cm-1) with no evidence of a carbonyl group. This is almost certainly the thione 8 (X = S), as
originally formulated by Rowson,12 and not the HMB 4 (X = S), as later claimed.7
2

In conclusion, we have shown that the product obtained from the reaction of 1,2-dithiolium-4-olate
6 and phenylisocyanate is thioketone 8 (X = O), as proposed in the original unpublished study by
Rowson.12 The product formed using phenylisothiocyanate almost certainly has the analogous
thioketone structure 8 (X = S) proposed by Rowson.12 As far as we are aware, this means that no
examples of the Class 5 HMBs of general structure 3 or their aza analogues are known. There is no
reason why stable derivatives of this type should not be accessible and they present a challenging
gap in the heterocyclic literature.
Ph
N
C
X
_
_
X
X
S
O
O
Ph
N
O
N
xylene,
reflux
N
N
Ph
Ph
Ph
Ph
Ph
N
+
N
Ph
S
S
Ph
S
S
Ph
Ph
+
S
6
7
S
8
9
Scheme 1. A potential pathway for the reaction of compound 6 with phenylisocyanate and
phenylisothiocyanate.
Figure 2. One of the independent molecular units from the single crystal X-ray structure of
heterocycle 8 (X = O). Anisotropic displacement parameters are shown at the 50% probability level.
Acknowledgements
3

The authors thank Diamond Light Source for access to synchrotron facilities (beamline I19) and the
EPSRC UK National Mass Spectrometry Facility at Swansea University for high-resolution mass
spectra.
References and notes
1. Ramsden, C. A. Tetrahedron 2013, 69, 4146-4159.
2. Ramsden, C. A. Prog. Heterocycl. Chem. 2016, 28, 1-25.
3. Ollis, W. D.; Ramsden, C. A.; Stanforth, S. P. Tetrahedron 1985, 41, 2239-2329.
4. Morgan, D. O. PhD thesis, University of Sheffield, 1984.
5. Ramsden, C. A.; Oziminski, W. P. Tetrahedron 2014, 70, 7158-7165.
6. Neugebauer, F. A.; Fischer, H.; Krieger, C. Tetrahedron Lett. 1984, 25, 629-632.
7. See structures 386 and 387 in reference 3.
8. Ollis, W. D.; Ramsden, C. A. Adv. Heterocycl. Chem. 1976, 19, 1-122.
9. Newton, C. G.; Ramsden, C. A. Tetrahedron 1982, 38, 2965-3011.
10. Cannon, J. R.; Potts, K. T.; Raston, C. L.; Sierakowski, A. F.; White, A. H. Aust. J. Chem. 1978, 31,
297-311.
11. Xylene (10 mL) was added to heterocycle 6 (0.20 g, 0.70 mmol) giving a purple mixture.
Phenylisocyanate (0.20 mL, 1.85 mmol) was then added and the mixture heated at reflux (isomantle)
with gentle agitation for 10 minutes. During the heating period, the colour of the mixture gradually
changed from purple to green. The mixture was filtered whilst hot and green crystals of compound 8
o
(X = O) (0.14 g, 48%), mp 198-199 C, were produced almost immediately from the cooling filtrate.
1
Ref. 4 reports mp 199-202 oC and ref. 12 reports mp 199-200.5 oC. H-NMR (400 MHz, CDCl3) δ 7.44-
7.41 (2H, m, ArH), 7.36-7.30 (3H, m, ArH), 7.22-7.14 (4H, m, ArH), 7.05-6.99 (4H, m, ArH), 6.96-6.91
(2H, m, ArH); 13C-NMR (100 MHz, CDCl3) δ 212.9, 152.7, 145.2, 142.5, 133.9, 132.7, 131.1, 130.1,
129.9, 129.4, 129.0, 128.7, 128.4, 127.7, 127.2, 126.1; IR νmax (diamond anvil) 1780, 1556, 1486, 1390,
1359, 1230, 1132, 1056 cm-1; HRMS found 358.0901. C22H16NO2S [M+H]+ requires 358.0896; λmax
(MeOH) 390 nm (ɛ 9810). Copies of NMR spectra are given in the supplementary information.
12. Rowson, G. P. PhD thesis, University of Sheffield, 1976.
13. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre,
CCDC1523715. Copies of the data can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [Fax: (international) +44 1223 336033; e-mail:
deposit@ccdc.cam.ac.uk.
14. Abeles, R. H.; Hutton, R. F.; Westheimer, F. H. J. Am. Chem. Soc. 1957, 79, 712-716.
15. Boyd, G. V.; Norris, T.; Lindley, P. F.; Mahmoud, M. M. J. Chem. Soc. Perkin Trans 1 1977, 1612-
1615.
4

•
•
The reaction of phenylisocyanate with a mesoionic heterocycle produces a novel thioketone
The structures of products from similar reactions to that above have been either corrected
or confirmed
•
X-ray crystallography and spectral data confirm the thioketone structure
5