Anionic sigmatropic-electrocyclic-Chugaev cascades: accessing 12-aryl-5-(methylthiocarbonylthio)tetracenes and a related anthra[2,3-b]thiophene

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# Anionic sigmatropic-electrocyclic-Chugaev cascades: accessing 12-aryl-5-(methylthiocarbonylthio)tetracenes and a related anthra[2,3- b ]thiophene

Laurence Burroughs, John Ritchie, Mkhethwa Ngwenya, Dilfaraz Khan, William Lewis
and Simon Woodward *

Full Research Paper [Open Access](http://www.beilstein-journals.org/bjoc/about/openAccess.htm)

Address:

School of Chemistry, The University of Nottingham, University Park,

Nottingham NG7 2RD, United Kingdom

Email:
Simon Woodward * [- simon.woodward@nottingham.ac.uk](mailto:simon.woodward@nottingham.ac.uk)

- Corresponding author

Keywords:

allenes; anionic Chugaev rearrangement; anionic sigmatropic

rearrangement; tetracene properties; X-ray structures

_Beilstein J. Org. Chem._ 2015, _11,_ 273–279.

[doi:10.3762/bjoc.11.31](http://dx.doi.org/10.3762%2Fbjoc.11.31)

Received: 23 December 2014

Accepted: 30 January 2015

Published: 20 February 2015

Associate Editor: T. P. Yoon

License and terms: see end of document.

## Abstract

1,4-Diols resulting from the double addition of ArCCLi (Ar = Ph, substituted phenyl, 2-thienyl) to _ortho_ -C6H4(CHO)2 undergo
cascades to tetracenes on simple admixture of LiHDMS, CS2 and MeI. Acene formation proceeds by [3,3]-sigmatropic rearrangement of xanthate anions followed by 6π electrocyclisations. The reactions are terminated by E2 or anionic Chugaev-type elimina
tions. Structural packing motifs and electronic properties are reported for the tetracenes.

## Introduction

In recent years polyacenes, especially tetra- and pentacenes,

have been in the vanguard of new field effect and other organic

electronic based devices [1,2]. Although the simple parent

acenes have useful device characteristics in their own right, it is

often desirable to be able to tune this performance by use of

suitable substituted variants [3,4]. Unfortunately, attaining such

derivatives rapidly through simple chemistry is often problem
atic [5,6]. Cross-coupling approaches (formally an excellent ap
proach for acene library preparation) [7-13] are often hindered

by the insolubility, or poor availability, of the parent

haloacenes. Conversely, stepwise synthesis of a family of acene

derivatives from various acyclic precursors is normally very

step intensive. The prevalence of these issues in the synthesis of

substituted tetracenes caused Lin [14], building on the anthracy
cline natural product work of Saá [15], to introduce a 1,2-bis
allene cascade approach for rapid access to tetracene sulfoxides

in 2007 (Scheme 1).

In 2012 Liu used a Pd-based strategy to provide 12-substituted

5-aryltetracenes [after final trapping with RB(OH)2)] [16]. Both
of these reactions rely on the formation of bis-allenes 2, attained

by Lin through 2,3-sigmatropic rearrangement of 1a [14] or by
SN2’ carbonate displacement in 1b by Pd 0 (PPh3)2 in Liu’s case

[16]. To circumvent reversibility of these pericyclic annulation

273

_Beilstein J. Org. Chem._ 2015, _11,_ 273–279.

Scheme 1: Use of bis-allene intermediates 2 for rapid access to substituted tetracenes [14,16].

strategies Lin relied on PhSOH elimination while Liu relied on

ubiquitous palladium β-hydride steps leading to tetracenes 3 and

4 . We are interested in very efficient routes to tetracene deriva
tives containing one or more thiolate (SH) groups for the use in

highly electrically conducting organics. In this regard we were

attracted by a single result in the early literature [17] showing

that traces of allenes related to 2 (X = SCOSMe) were acces
sible via nominal [3,3]-sigmatropic rearrangements of

xanthates. As the thiocarbamate products derived from these are

predicted to be easily hydrolised to thiolates this potentially

offers a simple route to a protected SH analogue of 3 . Lin’s

chemistry [14] cannot be used as no simple method to modify

SOPh to SH is available. We proposed that use of starting ma
terial xanthate 1c should provide suitably protected 5-thio
tetracene derivatives directly (Scheme 2).

The required [3,3]-sigmatropic rearrangements and subsequent

6π elecrocyclisations of 1c have precise stereochemical require
ments (Scheme 2). Only the _meso_ diastereomer of 1c is

predicted by Woodward–Hoffman analyses [18] to deliver _anti_
6 that is required for facile E2 elimination leading to the desired

tetracene 7 under thermal conditions. However, the initially

required 1c are typically attained as ca. 1:1 _rac_ / _meso_ mixtures

and this might be expected to limit the potential yield of 7 to

only 50% under simple heating (in the absence of other factors).

Houk has demonstrated that both electronic donor or acceptor

and steric effects favour placing the larger/most electronically

biased substituent ‘outwards’ in disrotatory 6π processes [19].

This might also depopulate 5 limiting the final yield of 7 .

However, the following factors suggested to us the viability of

Scheme 2: (i) traces of allenes have been observed when pre
paring xanthates from propargylic alcohols [17]; (ii) the relative van der Waals volumes of SOPh, Ph, CS2 − and C(=S)SMe
(104.2, 76.9, 63.4, and 82.0 Å 3, respectively [20]) and related

electronic properties [σ(SOMe) +0.52, σ(Ph) +0.06, σ(SCOMe)

+0.39 [21]] and the work of Lin [14] suggest that significant

populations of intermediate 5, with ‘inward Ph’ should be

Scheme 2: Proposed access to aryl substituted 5-thiolatotetracene derivatives.

274

accessible; (iii) even if a _rac_ -diol is used in the cascade, the

possibility of aromatisation of 6 through Chugaev [22] _syn_

elimination. Finally the system of Scheme 2 provides a unique

opportunity to probe if these rearrangements do indeed proceed

from the neutral xanthates 1c or via the previously unprece
dented 1d - 2d - 6d anionic cascades.

## Results and Discussion

Investigation of the chemistry of Scheme 2 commenced with

the preparation of the required diols 8 through simple acetylide

addition to _o_ -phthalaldehyde (60–91% yield, see Supporting

Information File 1). All of the additions proceeded in high

yield, but under all conditions tried, no strong bias to either the

_rac_ or _meso_ diastereomer could be realised. The _meso_ enriched

diastereomer of 8a could be realised by treating _rac_ / _meso_

mixtures of bis-lithium alkoxide of 8a with freshly prepared an
hydrous NBu4F (2 equivalents) [23] (Scheme 3). Acid quench
of the resultant purple dianion leads to ca. 5:1 _meso_ : _rac_ 8a . We

assign this transformation to an equilibrium between dialkoxide

9 and the benzylic anion 10 . Intramolecular proton delivery via

cyclic transition state 11 is proposed to favour the _meso_

dialkoxide prior to protonic quench. Samples of _rac_ enriched 8a

were prepared from Sonogashira coupling of _anti_ enriched 8j .

The latter could be prepared directly from _o_ -bromobenzalde
hyde as shown (Scheme 3) with ca. 1:7 _syn_ : _anti_ enrichment by

recrystallisation from CHCl3. The enantiotopic ArCH signals of
_rac_ - 8a are split into separate signals upon treatment with

Eu(facam)3 confirming it to be the C2 chiral diastereomer while
no equivalent splitting in 1 H NMR samples of 8a prepared from

purple 11 (in line with it being the _meso_ diastereomer). These

assignments are in line with the finding of Saá [15]. The chem
ical shifts of the methine C _H_ OH proton in _rac_ - 8a (δH 6.20) and
_meso_ - 8a (δH 6.33) reflect an equivalent trend in diols 8b–f
where two distinct sets of equivalent signals are seen δH

Scheme 3: Equilibration to _meso_ species.

_Beilstein J. Org. Chem._ 2015, _11,_ 273–279.

6.14–6.20 and δH 6.23–6.35. On this basis we assign the higher
chemical shift signal to the _meso_ diastereomer.

Cascade optimisation (Table 1) was carried out using 8a in THF

unless otherwise stated. Typically diol 8a (ca. 1:1 _rac_ : _meso_ )
was deprotonated at an initial low temperature ( _T_ 1 ), then treated

sequentially with CS2 and MeI before finally being brought to a
second higher temperature ( _T_ 2 ) to facilitate aromatisation

leading to 7a (see Supporting Information File 1 for full optimi
sation details). Simply allowing −78 °C solutions of the

dialkoxide to warm slowly to ambient temperature in the pres
ence of excess CS2/MeI provided small amounts of tetracene 7a
(Table 1, run 1). Formation of the xanthate is favoured at

−30 °C but this is slowed at −50 °C (Table 1, runs 2 and 3). The
IR νmax [cm −1 ] ( _rac_ - 1c : 1035; _meso_ - 1c : 1036) of run 2 are
consistent with the isolation of xanthate [17].

Table 1: Optimisation of yield of tetracene 7a . a

Run Base _rac_ : _meso_ b _T_ 1

[°C]

_T_ 2

[°C]

[%] c

1 NaH 1:1.1 −78 22 23 9

2 NaH 1:1.1 −30 40 95
3 NaH 1:1.1 −50 40 45 30

4 NaH 1:1.1 0 60 30 43

5 LiHDMS 1:1.1 −50 40 39 38
6 LiHDMS 1:1.1 0 60 30 b 60

7 KHDMS 1:1.1 0 60 9 50
8 LiHDMS 1:4 0 60 5 b 38
9 LiHDMS 8:1 0 60 5 b 89

aUsing 8a (0.45 mmol) in THF (5.0 mL), with base (2.0 equiv), CS2
(3.0 equiv), MeI (8.0 equiv), see Supporting Information File 1 for
details. b Determined by NMR spectroscopy. c Isolated yields, except
where noted.

275

All attempts to convert the neutral xanthate 1c (either _rac_ or

_meso_ from Table 1, run 2) to tetracene 7a under thermal or

photochemical conditions failed. Either 1c was recovered, or it

slowly decomposed under forcing conditions (>200 °C; or

180–365 nm Hg lamp). Exceptionally, traces of 7a were

detected in reactions eletro-catalysed by Bauld’s catalyst (tris(4
bromophenyl)ammoniumyl hexachloroantimonate) [24] at

25 °C but these showed very poor chemoselectivity.

Conversely, rapid one-pot heating of a mixture of all the reac
tion components maximises the yield of 7a (Table 1, runs 3–6).

These results very strongly suggest unprecedented anionic

[3,3]-sigmatropic rearrangement starting from 1d ; another addi
tion to the body of evidence for the importance of charge upon

sigmatropic rearrangements [25,26]. In the subsequent cascade

the second 6π electrocyclisation appears rate limiting. The yield

of 7a in run 6 (60%) indicates conversion via _syn_ - 6d (unprece
dented anionic Chugaev elimination) is possible to some extent.

If only E2 termination of the cascade was possible (i.e., via

_anti_ - 6 ) a maximum yield of 52% 7a should be realised from the

1:1.1 _rac_ : _meso_ sample of 8a used. This idea is strongly

supported by runs 8 and 9 and the observation that replacing

MeI with other alkylating agents (EtBr, BnBr) resulted in only

traces of tetracenes. Of the bases screened (see Supporting

Information File 1), LiHDMS was superior, only its potassium

analogue gave comparable performance (Table 1, run 7).

Table 2: Preparation of derivatives. a

_Beilstein J. Org. Chem._ 2015, _11,_ 273–279.

In all reactions of Table 2 there is some unrecovered material.

One common byproduct is an intensely red compound detected

at high _R_ f (0.82, 4:1 pentane/CH2Cl2) in TLC analyses
conducted under argon. The very high air sensitivity of this

compound prevents its characterisation but it is tentatively

ascribed to a mixture ( 12, Scheme 4) of hydroquinone and its
monomethylether on the basis of partial 1 H NMR spectrum and

ESI mass spectra.

The poorly performing runs of Table 1 also show a broad
isolable red band ( _R_ f ca. 0.18, 4:1 pentane/CH2Cl2) whose 1 H
and 13 C NMR and MS data were intractable. For example, the
1H NMR spectrum shows only a broad envelope of signals at

δH 7.95–7.20; a number of very similar isomeric species seem
to be present. To cast light on these issues model alcohol 13

was treated with NaH/CS2/MeI at −78 °C and the mixture
allowed to come to ambient temperature. This resulted in the

smooth formation of allene 14 . In particular, the presence of
allenic and C=O signals in the 13 C NMR spectrum at 212.1 and

188.5 ppm and the absence of any alkyne C≡C resonances in

the region δC 80–90 are indicative of this transformation. All
attempts to isolate 14 resulted only in the rapid formation of red

oils whose mass spectra show molecular ions at ( 14 ) _n_ ( _n_ = 1–3).
Attempted direct crystallisation provided only trace amounts of

cyclobutene 15 which is otherwise unstable in solution (see

Compound Precursor diol ( 8 ) _rac_ : _meso_ ratio R 1 R 2 Yield ( 7 ) [%]

7a 1.6:1.0 H H 85

7b 1.9:1.0 OMe OMe 47
7c 4.8:1.0 CF3 H 56
7d 1.8:1.0 OMe H >99

7e 1.6:1.0 F H 29
7f 1.0:1.1 H CF3 44
7g 1.0:1.2 H OMe 38

7h 1.0:1.6 H _t-_ Bu 22

7i 1.0:1.0 - - 38
7j 1.0:2.0 b – – 50

aFrom diol precursor (0.45 mmol) in THF (5.0 mL), with LiHDMS (2.0 equiv), CS2 (3.0 equiv), MeI (8.0 equiv), isolated yields. bEquivalent _anti_ : _syn_
ratio for 8j .

276

Scheme 4: Competing reaction pathways.

Supporting Information File 1). Its extensive decomposition

prevented other characterisation. Based on this model system, it

is proposed that intermolecular [2 + 2] reactions of bis-allenes,

similar to results in other reported allenic rearrangements to

rubrene [27,28], 2c,d result in the formation of numereous

stereoisomeric oligomers resulting in the broad uninformative

NMR spectra in the cascade byproducts. The structure of 15 is

unremarkable except for the presence of a highly elongated

C–C bond (1.63 Å) brought on through the steric congestion of

the adjacent quaternary centers. A similar situation has been

reported [29].

The use of the optimal conditions provided a series of acene

derivatives (Table 2). All reactions resulted in chromatographi
cally stable red microcrystalline solids. As anionic Chugaev

elimination appeared the preferred aromatisation route from the

studies of Table 1 (compare runs 6, 8 and 9), preparations of

7a–c strongly benefit from higher _rac_ : _meso_ ratios that increase

the population of the equivalent _syn_ - 6 intermediates

(Scheme 2). Steric congestion in the anion Chugaev transition

Scheme 5: Stacking motifs in 7a, d, f  - h and j . Y = COSMe.

_Beilstein J. Org. Chem._ 2015, _11,_ 273–279.

state appears to favour this as all these compounds are isolated

in good to excellent yields. Conversely 7e–h are isolated in

lower yields due to a combination of higher _meso_ content in

8e–h (leading less efficient E2 elimination) and lower steric

promotion in the anion Chugaev elimination. Steric, rather than

electronic, factors seem to affect the reaction most as evidenced

by the quantitative yield of 7d compared to 7b (47%), 7c

(56%), 7f (44%) and 7g (38%). The decreasing yields suggest

that _meta_ substitution promotes the 6π cyclisation while _para_

electronic affects are minor and unhelpful according to the

observed trend. Increasing the reaction temperature, in attempts

to facilitate E2 elimination, was generally not useful as this led

only to increased amounts of inert xanthates through sulfur

alkylation. However, in the case of 7h this approach did allow

us to reach 50 ± 4% yields (range for 6 runs).

Compounds 7a, d, f–h and j were subjected to single-crystal

X-ray crystallography. This confirmed the molecular connec
tivity but more importantly allowed insight into their crystal

packing features (Scheme 5 and Supporting Information File 1)

277

across the family of structures. Pairs of 7a associate with slip
stack pairing (Cπ···Cπ 3.51–3.72 Å). Each of these ( 7a )2
‘dimers’ is linked to the next through π contacts to the xanthate

methyl (Cπ···MeS 3.38 Å). The ‘gaps’ in the columns are filled
by an additional motif (Cπ···Cπ 3.32–3.59 Å) almost perpendicular to the stacking. In 7d a lattice of ( 7d ) _n_ chains propagates
through C(11)π···MeS (3.39 Å) contacts. Adjacent chains
overlap to produce the partial brickwork stack motif showing

Cπ···Cπ 3.51–3.60 Å between the most electron rich and deficient aryl rings. Offset stacking ribbons are found in 7f (i.e.,

graphic ‘a’ is above ‘b’, etc.). The closest contacts are C···Cedge
at 3.82–3.96 Å and Cπ···F–CF2 3.2 Å. Perpendicular ribbons
propagate through the crystal linked by inter-digitated xanthates

or CF3 groups. Structure 7f is the only one of the di/trisubstituted family not to show local _C_ 2 symmetry in intermolecular
paring of the tetracenes. The structures of 7g, h (Scheme 5) are

closely related to those of 7d and 7a, respectively. Finally, the

least substituted tetracene 7j forms ribbons of herringbone

structures.

Estimates of the HOMO–LUMO data for 7 were taken from UV

and CV measurements (see Table 3 and Supporting Informa
tion File 1), as well as by DFT calculations. Tetracenes 7d and

7f show the widest range in HOMO–LUMO perturbation while

_E_ g opt. is ca. 0.4 eV lower that _E_ g calcd. across the series. We
could not attain the reduction potentials of 7 but from the onset

of oxidation data we could estimate the HOMO energy levels in

7 and these followed the same trend as _E_ HOMO calcd. Preliminary testing of vacuum deposited thin polycrystalline films

(ca. 800 nm) of 7a and 7j showed dielectric behaviour
(σ <10 −10 S cm −1 ) indicating that additional derivitisation and

radical cation salt formation is required for the attainment of

high electrical conductivity, as in the case of tetrathiotetracene

[30].

Table 3: Electro-optic properties of 7a–j .

_Beilstein J. Org. Chem._ 2015, _11,_ 273–279.

## Conclusion

Typical [3,3]-sigmatropic rearrangements of xanthates are

normally considered to proceed via neutral species (such as 1c ).

The tetracenes 7 herein are not formed this way, instead the evi
dence here strongly suggests that the required [3,3]-6π–6π elec
trocyclic cascades takes place via anionic xanthate species

before final capping with methyl iodide. Final aromatisation

through E2 or the anionic equivalent of the Chugaev reaction

are also both viable. As neutral Chugaev reactions normally

require very high temperatures this alternative approach is

attractive as only moderate temperatures are required

(60–80 °C). This procedure allows rapid access to mono sulfur
containing acenes, and is applicable to small scale library syn
thesis. Only low cost reagents are required and otherwise diffi
cult to synthesise hindered 1,3,4,12-tetrasubstituted species can

be made straightforwardly.

## Supporting Information

Supporting Information File 1

Experimental procedures, characterisation data, X-ray

structures, data for the DFT calculations, and NMR spectra.

[[http://www.beilstein-journals.org/bjoc/content/](http://www.beilstein-journals.org/bjoc/content/supplementary/1860-5397-11-31-S1.pdf)

[supplementary/1860-5397-11-31-S1.pdf]](http://www.beilstein-journals.org/bjoc/content/supplementary/1860-5397-11-31-S1.pdf)

## Acknowledgements

This project has received funding from the European Union’s

Seventh Programme for research, technological development

and demonstration under grant agreement No 308768. We are

grateful to Prof. Martins Rutkis (Institute of Solid State Physics,

University of Latvia) for the measurements on 7a / h . DK thanks

the Higher Education Council of Pakistan for a Fellowship. LB,

JR and SW acknowledge support by the EPSRC UK NSCCS

Compound E½ (ox.) [V] a HOMO/ LUMO calcd. [eV] b νmax (vis) [nm] c _E_ g opt. [eV] d _E_ g calcd. [eV] b

7a +0.52 −5.16/−2.51 287 2.27 2.65

7b +0.22 −5.21/−2.57 297 2.23 2.63

7c +1.07 −5.23/−2.93 297 2.18 2.60

7d +0.29 −4.89/−2.36 296 2.09 2.55

7e +0.65 −5.58/−2.96 286 2.27 2.62

7f +0.76 −5.61/−2.98 290 2.24 2.63

7g +0.43 −5.20/−2.69 290 2.28 2.51

7h +0.55 −5.33/−2.69 289 2.26 2.65

7i +0.60 −5.27/−2.32 283 2.49 2.95

7j +0.62 −5.10/−2.40 283 2.36 2.70

aBy cyclic voltammetry, referenced against Fc/Fc+. bDFT: Calculated with the B3LYP-6-31G(d,p) basis set using Gaussian 09 Rev.D.01. cIn CH2Cl2.
dDetermined from the onset (Tauc) of the lowest energy visible absorption band.

278

Computational Service. We thank Dr. Darren Walsh (Univer
sity of Nottingham) for the help with the electrochemical

studies. We thank European Thermodynamics for their involve
ment in the programme (JR) and use of the I19 Diamond

Facility [31] is acknowledged.

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