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

Article abstract of DOI:10.3762/bjoc.11.31

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 eliminations. Structural packing motifs and electronic properties are reported for the tetracenes.

Full text of DOI:10.3762/bjoc.11.31

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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
Address:
Beilstein J. Org. Chem. 2015, 11, 273–279.
School of Chemistry, The University of Nottingham, University Park,
Nottingham NG7 2RD, United Kingdom
doi:10.3762/bjoc.11.31
Received: 23 December 2014
Accepted: 30 January 2015
Published: 20 February 2015
Email:
Simon Woodward* - simon.woodward@nottingham.ac.uk
* Corresponding author
Associate Editor: T. P. Yoon
Keywords:
© 2015 Burroughs et al; licensee Beilstein-Institut.
License and terms: see end of document.
allenes; anionic Chugaev rearrangement; anionic sigmatropic
rearrangement; tetracene properties; X-ray structures
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 rearrange-
ment 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, step intensive. The prevalence of these issues in the synthesis of
have been in the vanguard of new field effect and other organic substituted tetracenes caused Lin [14], building on the anthracy-
electronic based devices [1,2]. Although the simple parent cline natural product work of Saá [15], to introduce a 1,2-bis-
acenes have useful device characteristics in their own right, it is allene cascade approach for rapid access to tetracene sulfoxides
often desirable to be able to tune this performance by use of in 2007 (Scheme 1).
suitable substituted variants [3,4]. Unfortunately, attaining such
derivatives rapidly through simple chemistry is often problem- In 2012 Liu used a Pd-based strategy to provide 12-substituted
atic [5,6]. Cross-coupling approaches (formally an excellent ap- 5-aryltetracenes [after final trapping with RB(OH)2)] [16]. Both
proach for acene library preparation) [7-13] are often hindered of these reactions rely on the formation of bis-allenes 2, attained
by the insolubility, or poor availability, of the parent by Lin through 2,3-sigmatropic rearrangement of 1a [14] or by
haloacenes. Conversely, stepwise synthesis of a family of acene SN2’ carbonate displacement in 1b by Pd0(PPh3)2 in Liu’s case
derivatives from various acyclic precursors is normally very [16]. To circumvent reversibility of these pericyclic annulation
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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 predicted by Woodward–Hoffman analyses [18] to deliver anti-
ubiquitous palladium β-hydride steps leading to tetracenes 3 and 6 that is required for facile E2 elimination leading to the desired
4. We are interested in very efficient routes to tetracene deriva- tetracene 7 under thermal conditions. However, the initially
tives containing one or more thiolate (SH) groups for the use in required 1c are typically attained as ca. 1:1 rac/meso mixtures
highly electrically conducting organics. In this regard we were and this might be expected to limit the potential yield of 7 to
attracted by a single result in the early literature [17] showing only 50% under simple heating (in the absence of other factors).
that traces of allenes related to 2 (X = SCOSMe) were acces-
sible via nominal [3,3]-sigmatropic rearrangements of Houk has demonstrated that both electronic donor or acceptor
xanthates. As the thiocarbamate products derived from these are and steric effects favour placing the larger/most electronically
predicted to be easily hydrolised to thiolates this potentially biased substituent ‘outwards’ in disrotatory 6π processes [19].
offers a simple route to a protected SH analogue of 3. Lin’s This might also depopulate 5 limiting the final yield of 7.
chemistry [14] cannot be used as no simple method to modify However, the following factors suggested to us the viability of
SOPh to SH is available. We proposed that use of starting ma- Scheme 2: (i) traces of allenes have been observed when pre-
terial xanthate 1c should provide suitably protected 5-thio- paring xanthates from propargylic alcohols [17]; (ii) the rela-
tetracene derivatives directly (Scheme 2).
tive van der Waals volumes of SOPh, Ph, CS2− and C(=S)SMe
(104.2, 76.9, 63.4, and 82.0 Å3, respectively [20]) and related
The required [3,3]-sigmatropic rearrangements and subsequent electronic properties [σ(SOMe) +0.52, σ(Ph) +0.06, σ(SCOMe)
6π elecrocyclisations of 1c have precise stereochemical require- +0.39 [21]] and the work of Lin [14] suggest that significant
ments (Scheme 2). Only the meso diastereomer of 1c is populations of intermediate 5, with ‘inward Ph’ should be
Scheme 2: Proposed access to aryl substituted 5-thiolatotetracene derivatives.
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Beilstein J. Org. Chem. 2015, 11, 273–279.
accessible; (iii) even if a rac-diol is used in the cascade, the 6.14–6.20 and δH 6.23–6.35. On this basis we assign the higher
possibility of aromatisation of 6 through Chugaev [22] syn chemical shift signal to the meso diastereomer.
elimination. Finally the system of Scheme 2 provides a unique
opportunity to probe if these rearrangements do indeed proceed Cascade optimisation (Table 1) was carried out using 8a in THF
from the neutral xanthates 1c or via the previously unprece- unless otherwise stated. Typically diol 8a (ca. 1:1 rac:meso)
dented 1d–2d–6d anionic cascades.
was deprotonated at an initial low temperature (T1), then treated
sequentially with CS2 and MeI before finally being brought to a
second higher temperature (T2) to facilitate aromatisation
Results and Discussion
Investigation of the chemistry of Scheme 2 commenced with leading to 7a (see Supporting Information File 1 for full optimi-
the preparation of the required diols 8 through simple acetylide sation details). Simply allowing −78 °C solutions of the
addition to o-phthalaldehyde (60–91% yield, see Supporting dialkoxide to warm slowly to ambient temperature in the pres-
Information File 1). All of the additions proceeded in high ence of excess CS2/MeI provided small amounts of tetracene 7a
yield, but under all conditions tried, no strong bias to either the (Table 1, run 1). Formation of the xanthate is favoured at
rac or meso diastereomer could be realised. The meso enriched −30 °C but this is slowed at −50 °C (Table 1, runs 2 and 3). The
diastereomer of 8a could be realised by treating rac/meso IR νmax [cm−1] (rac-1c: 1035; meso-1c: 1036) of run 2 are
mixtures of bis-lithium alkoxide of 8a with freshly prepared an- consistent with the isolation of xanthate [17].
hydrous NBu4F (2 equivalents) [23] (Scheme 3). Acid quench
of the resultant purple dianion leads to ca. 5:1 meso:rac 8a. We
Table 1: Optimisation of yield of tetracene 7a.a
assign this transformation to an equilibrium between dialkoxide
Run Base
rac:mesob T1
T2
1c
7a
[%]c
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 1H 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 CHOH 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
[°C] [°C] [%]c
1
2
3
4
5
6
7
8
9
NaH
NaH
NaH
NaH
1:1.1
1:1.1
1:1.1
1:1.1
−78 22
−30 40
−50 40
23
95
45
30
39
30b
9
9
–
30
43
38
60
50
38
89
0
60
LiHDMS 1:1.1
LiHDMS 1:1.1
KHDMS 1:1.1
LiHDMS 1:4
LiHDMS 8:1
−50 40
0
0
0
0
60
60
60
60
5b
5b
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. bDetermined by NMR spectroscopy. cIsolated yields, except
where noted.
Scheme 3: Equilibration to meso species.
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Beilstein J. Org. Chem. 2015, 11, 273–279.
All attempts to convert the neutral xanthate 1c (either rac or In all reactions of Table 2 there is some unrecovered material.
meso from Table 1, run 2) to tetracene 7a under thermal or One common byproduct is an intensely red compound detected
photochemical conditions failed. Either 1c was recovered, or it at high Rf (0.82, 4:1 pentane/CH2Cl2) in TLC analyses
slowly decomposed under forcing conditions (>200 °C; or conducted under argon. The very high air sensitivity of this
180–365 nm Hg lamp). Exceptionally, traces of 7a were compound prevents its characterisation but it is tentatively
detected in reactions eletro-catalysed by Bauld’s catalyst (tris(4- ascribed to a mixture (12, Scheme 4) of hydroquinone and its
bromophenyl)ammoniumyl hexachloroantimonate) [24] at monomethylether on the basis of partial 1H NMR spectrum and
25 °C but these showed very poor chemoselectivity. ESI mass spectra.
Conversely, rapid one-pot heating of a mixture of all the reac-
tion components maximises the yield of 7a (Table 1, runs 3–6). The poorly performing runs of Table 1 also show a broad
These results very strongly suggest unprecedented anionic isolable red band (Rf ca. 0.18, 4:1 pentane/CH2Cl2) whose 1H
[3,3]-sigmatropic rearrangement starting from 1d; another addi- and 13C NMR and MS data were intractable. For example, the
tion to the body of evidence for the importance of charge upon 1H NMR spectrum shows only a broad envelope of signals at
sigmatropic rearrangements [25,26]. In the subsequent cascade δH 7.95–7.20; a number of very similar isomeric species seem
the second 6π electrocyclisation appears rate limiting. The yield to be present. To cast light on these issues model alcohol 13
of 7a in run 6 (60%) indicates conversion via syn-6d (unprece- was treated with NaH/CS2/MeI at −78 °C and the mixture
dented anionic Chugaev elimination) is possible to some extent. allowed to come to ambient temperature. This resulted in the
If only E2 termination of the cascade was possible (i.e., via smooth formation of allene 14. In particular, the presence of
anti-6) a maximum yield of 52% 7a should be realised from the allenic and C=O signals in the 13C NMR spectrum at 212.1 and
1:1.1 rac:meso sample of 8a used. This idea is strongly 188.5 ppm and the absence of any alkyne C≡C resonances in
supported by runs 8 and 9 and the observation that replacing the region δC 80–90 are indicative of this transformation. All
MeI with other alkylating agents (EtBr, BnBr) resulted in only attempts to isolate 14 resulted only in the rapid formation of red
traces of tetracenes. Of the bases screened (see Supporting oils whose mass spectra show molecular ions at (14)n (n = 1–3).
Information File 1), LiHDMS was superior, only its potassium Attempted direct crystallisation provided only trace amounts of
analogue gave comparable performance (Table 1, run 7).
cyclobutene 15 which is otherwise unstable in solution (see
Table 2: Preparation of derivatives.a
Compound
Precursor diol (8) rac:meso ratio
R1
R2
Yield (7) [%]
7a
7b
7c
7d
7e
7f
1.6:1.0
1.9:1.0
4.8:1.0
1.8:1.0
1.6:1.0
1.0:1.1
1.0:1.2
1.0:1.6
1.0:1.0
1.0:2.0b
H
H
85
47
56
>99
29
44
38
22
38
50
OMe
CF3
OMe
F
OMe
H
H
H
H
CF3
OMe
t-Bu
–
7g
7h
7i
H
H
–
7j
–
–
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