Synthesis, crystal structure and reactivity of η2-thiophyne Ni complexes

Article abstract of DOI:10.1039/c8cc00705e

The first synthesis and crystal structures of five-membered ring aryne complexes, Ni(dcpe)(η²-thiophyne) and Ni(dcpe)(η²-benzo[b]thiophyne), are described. The aryne fragments retain considerable aromatic character and the two coordi

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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DOI: 10.1039/C8CC00705E
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Synthesis, crystal structure and reactivity of 2–thiophyne
Ni complexes
Misaki Terada,a Yuji Nishii,*b Masahiro Miura*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The first synthesis and crystal structures of five-membered ring
Although arynes are fragile species which cannot be isolated
aryne complexes, Ni(dcpe)(2-thiophyne) and Ni(dcpe)(2- in the free states, a number of aryne-metal complexes have
benzo[b]thiophyne), are described. The aryne fragments retain been synthesized and characterized, providing convincing
considerable aromatic character and the two coordinated carbon evidence for the triple bond character of the aryne
atoms exhibit distinct reactivity due to the asymmetric nature.
fragments.11,12 Among the reported synthetic protocols, an
intramolecular transmetallation strategy where ortho-
metalated boronic esters are readily converted into the
corresponding aryne complexes by a simple base treatment is
highly practical. The strategy was first developed by Bennett
and Wenger for the synthesis of benzyne complexes of nickel
and palladium (Scheme 1A).13 Recently, Hosoya and co-workers
adopted the method to achieve the preparation of various
benzyne complexes from ortho-borylated aryl triflates (Scheme
1B).14
Arynes are highly reactive, transient molecules that are widely
adopted as useful synthetic intermediates for the construction
of elaborate aromatic frameworks.1 Arynes are formally
generated by removing two adjacent substituents from the
parent aromatic molecules, and best described to have a
strained C–C triple bond in the ring systems.2 In sharp contrast
to the fact that various six-membered arynes are readily
accessible, it is obviously a challenging task to handle smaller
ring size arynes due to the severe ring strains.3 Historically, a
five-membered ring aryne, 2,3-didehydrobenzofuran,4 was first
postulated half a century in advance of the threshold of
benzyne chemistry5,6 to rationalize the cine-substitution with
ethoxide. However, its intermediacy has been questioned
because such cine-substitution reactions can proceed via non-
aryne mechanisms.7 Diels-Alder adducts and cycloadducts are
also known to be obtained without aryne intermediates, so that
isolation of these adducts has provided ambiguous evidences
for the actuality of five-membered arynes. Flash vacuum
thermolysis of the parent carboxylic anhydrides has been
emerged as the first experimental evidence for the generation
of five-membered arynes.8 Such harsh conditions are not
sufficient for practical use in organic synthesis, and little
achievements have been made in the chemistry of five-
membered arynes to date.9,10
Scheme
1 Synthesis of benzyne complexes via intramolecular
transmetallation.
We envisaged that five-membered arynes would be isolable
upon coordinating to an appropriate transition metal, and we
herein report the first isolation and crystal structure of five-
membered aryne complexes Ni(dcpe)(
Ni(dcpe)(
2-benzo[b]thiophyne).15,16 Preliminary study on the

2-thiophyne) and
a.Department of Applied Chemistry, Graduate School of Engineering, Osaka
University Suita, Osaka 565-0871 (Japan)

E-mail: miura@chem.eng.osaka-u.ac.jp
reactivity of the thiophyne fragments is also demonstrated.
At the outset, the synthesis of precursors for Ni-aryne
complexes was carried out (Scheme 2). A Ni(II) complex 2a was
synthesized by oxidative addition of the corresponding boronic
ester 1a onto Ni(cod)2 in the presence of a bidentate phosphine
b.Frontier Research Base for Global Young Researchers,
Graduate School of Engineering, Osaka University, Suita,
Osaka 565-0871 (Japan)
y_nishii@chem.eng.osaka-u.ac.jp
† Electronic Supplementary Information (ESI) available:
See DOI: 10.1039/x0xx00000x
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ligand 1,2-bis(dicyclohexylphosphino)ethane (dcpe).
benzo[b]thiophene analogue 2b was prepared from a boronic yields (Scheme 3), and the structures were confirmed by X-ray
ester 1b in the same procedure. The structures of 2a and 2b crystallography to be monomeric 
2-aryne complexes 3a and 3b
Journal Name
A
The generated complexes were successfully isolated in good
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DOI: 10.1039/C8CC00705E
were confirmed by X-ray diffraction measurements.
(Figure 2). In their 13C NMR spectra, the carbon atoms directly
bonded to nickel resonance around 150 ppm (149.6 and 156.6
ppm for 3a, 152.6 and 154.9 ppm for 3b) as double doublets
signals. This sufficiently evident that these complexes retain
their monomeric structure in the solution state as well.
Scheme 3 Preparation of complexes 3a and 3b
.
Scheme 2 Preparation of complexes 2a and 2b. Molecular structures
are drawn with 50% thermal ellipsoid. Hydrogen atoms and cyclohexyl
groups are omitted for clarity.
Other phosphine ligands such as PPh3, PCy3, and 1,2-
bis(diphenylphosphino)ethane (dppe) afforded complicated
mixtures and the isolation of desired complexes was
unsuccessful. We also examined the synthesis of analogous Pd
complexes from Pd2(dba)3 or CpPd(
complexation was observed.
-allyl); however, no clean
The precursors were then treated with KOtBu in THF-d8 and
the reactions were monitored by 31P NMR measurement. Two
doublet peaks characteristic of the complex 2a (62.1 and 68.5
ppm; Jpp = 25.9 Hz) were instantaneously replaced by a set of
doublet peaks (81.6 and 85.8 ppm; Jpp = 7.3 Hz), which could be
Fig. 2 Molecular structure of 3a (left) and 3b (right) with 50% thermal
ellipsoid. Hydrogen atoms are omitted for clarity and only major
orientation of the disordered groups is shown for 3a
.
The complex 3a decomposed with a half-life of approximately
60 hours in C6D6; however, fate of the thiophyne fragment was
unestablished. In sharp contrast, the benzo-fused analog 3b
remained unchanged at least for 4 days at ambient temperature.
Higher temperature was required to trigger the complete
decomposition (60 °C, 24 hours).17 This peculiar stability might
be rationalized by the presence of an extra benzene ring which
provides steric barrier and the extended-conjugation.
assigned to the corresponding
1, left). A similar result was obtained with the complex 2b

2-thiophyne Ni complex (Figure
;
immediate replacement of original peaks (58.7 and 61.9 ppm;
Jpp = 27.5 Hz) by doublet peaks (81.6 and 85.8 ppm; Jpp = 8.1 Hz)
was observed (Figure 1, right). These species were further
reacted with an excess amount of trifluoroacetic acid (TFA) to
give thiophene and benzo[b]thiophene, respectively, along with
Ni(dcpe)(OCOCF3)2, supporting the formation of aryne
complexes.17
Selected geometric parameters of the complexes 3a
, 3b,
and Ni(dcpe)(

2-benzyne)18 are listed in Table 1. Because 3b
has two independent molecules in the unit sell, the parameters
are inscribed separately for each structure (part 1 and 2). The
complexes
3 exhibit small dihedral angles between P1–Ni–P2
and C1–Ni–C2 planes, and sum of the bond angles around the
Ni atom is close to 360 degrees in their solid state. These
structural features indicate that both 3a and 3b have the
trigonal planer geometry where the midpoint of the
coordinated triple bond occupies one coordination site as
similar to the reported Ni-aryne complexes.13,14,18 The C1–C2
bond length of the complex 3a is considerably elongated from
the “free” 2,3-thiophyne and is rather close to the value of
thiophene C–C double bond (Figure 3). This is consistent with
Fig. 1 31P{1H} NMR spectra for the reaction of complex 2a (left) and 2b
(right) with KOtBu in THF-d8.
2 | J. Name., 2012, 00, 1-3
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the idea that the thiophyne fragment retains delocalized yield. In contrast, no clean transformation was observed when
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DOI: 10.1039/C8CC00705E
aromatic
nickelacyclopropene character as
contributor. A smiler trend was observed in the complex 3b
nature;
thus,
3a
possesses
thieno- complexes 3 were reacted with alkyl bromides or allyl halides.
a
major resonance
,
whose aryne fragment bears structural resemblance to
benzo[b]thiophene.
Table 1 Selected bond lengths [Å] and angles [deg.] of 3a, 3b, and
Ni(dcpe)(2-benzyne)
Ni(dcpe)
3a
3b (part 1)
3b (part 2)
(2-benzyne)
1.870(4)
1.868(4)
2.140(1)
2.152(1)
1.332(6)
91.8(1)
Ni–C1
1.851(8)
1.849(9)
2.137(2)
2.153(2)
1.335(10)
91.62(9)
42.3(3)
1.903(5)
1.843(5)
2.156(2)
2.131(1)
1.335(7)
91.41(5)
41.7(2)
110.8(4)
116.9(4)
9.08
1.860(4)
1.888(4)
2.142(1)
2.148(1)
1.329(6)
91.15(5)
41.5(2)
Ni–C2
Ni–P1
Scheme 4 Reaction of complexes 3a and 3b with MeI. Molecular
structures are drawn with 50% thermal ellipsoid. Hydrogen atoms and
cyclohexyl groups are omitted for clarity.
Ni–P2
C1–C2
P1-Ni-P2
C1-Ni-C2
S-C1-C2
When complex 3a was treated with an excess amount of I2,
both of the Ni–C bonds reacted to give 2,3-diiodothiophene
along with a small amount of 2,3,5-triiodothiophene (Scheme
5).
41.7(2)
110.3a
113.5(3)
114.2(4)
10.84
C1-C2-C3 118.0a
θb
4.07
c
359.93
358.97
360.02
360.0
a Average of two disordered groups without considering the errors.
b Dihedral angle between the P1-Ni-P2 and C1-Ni-C2 planes.
c The sum of the bond angles around the Ni atom.
Scheme 5 Reaction of complexes 3a with iodine.
Bennett reported that the aryne moiety of Ni(dcpe)(2
-
benzyne) can be replaced by diphenylacetylene to give the
corresponding alkyne-coordinated complex.18 We carried out
relevant experiments using complexes 3a and 3b; however,
such a ligand exchange was not observed even with the use of
an excess amount of the alkyne. Additionally, no significant
amount of Diels-Alder adducts was detected when the
complexes
3 were heated or treated with an oxidant such as
cerium ammonium nitrate or a ferrocenium salt in the presence
of cyclopentadiene or furan derivatives. These results indicate
the reluctance of
Ni center.
On the other hand, 3a smoothly reacted with dimethyl
acetylenedicarboxylate to give a benzo[b]thiophene derivative
Fig. 3 Comparison of selected bond lengths and angles: crystal structure
of 3a (left, angles are average of two disordered groups); optimized
structures of thiophene (middle) and 2,3-thiophyne (right) by DFT
calculation (B3LYP/6-31G(d)). Hydrogen atoms are omitted for clarity.

2-thiophyne fragment to dissociate from the
We further investigated the reactivity of the Ni-aryne
complexes. Methylation of the thiophyne fragment of 3a with
iodomethane resulted in the formation of a Ni(II) complex 4a
(Scheme 4). The methyl group was installed predominantly at
the C2 position (refer to Fig. 2 for the atom labeling), whereas
Ni-benzyne complexes were reported to react with
electrophiles non-selectively to give almost 1:1 mixture of
regioisomers.14 This is probably because the cleavage of stable
Ni–C1 (2-thienyl) bond was kinetically unfavorable. Reaction of
complex 3b with iodomethane was also examined, and the
corresponding C2-methylated complex 4b was afforded in 48%
5
which was formed probably through two-fold alkyne insertion
and subsequent reductive elimination.19 Interestingly, reaction
of 3b with bis(trimethylsilyl)acetylene resulted in the formation
of an alkyne-coordinated complex
the mechanistic detail remains unclear, X-ray diffraction
analysis unambiguously revealed the connectivity of the alkynyl
moiety at
6 as a single isomer. While
 position of the thiophene ring (Scheme 6).
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Journal Name
For a review, see: H. Heaney, Chem. Rev., 1962,V6ie2w, A8r1tic.le Online
5
6
(a) J. D. Roberts, H. E. Simmons Jr., L. DAO. IC: 1a0r.l1s0m39it/hC8aCnCd0C07.0W5E.
Vaughn, J. Am. Chem. Soc., 1953, 75, 3290; (b) G. Wittig and L.
Pohmer, Angew. Chem., 1955, 67, 348; (c) G. Wittig and L.
Pohmer, Chem. Ber., 1956, 89, 1334; (d) J. D. Roberts, D. A.
Semenow, H. E. Simmons Jr. and L. A. Carlsmith, J. Am. Chem.
Soc., 1956, 78, 601; (e) J. D. Roberts, C. W. Vaughan, L. A.
Carlsmith and D. A. Semenow, J. Am. Chem. Soc., 1956, 78
,
611; f) R. Huisgen and R. Knorr, Tetrahedron Lett., 1963, 1017.
For a review, see: M. G. Reinecke, Tetrahedron, 1982, 38, 427.
(a) M. G. Reinecke and J. G. Newsom, J. Am. Chem. Soc., 1976,
98, 3021; (b) M. G. Reinecke, J. G. Newsom and L.-J. Chen, J.
Am. Chem. Soc., 1981, 103, 2760.
A reaction using 2,3-thiophyne generated by lithiation of 2-
thienyl triflate was documented though its intermediacy was
not discussed; T. Truong, M. Mesgar, K. K. A. Le and O.
Daugulis, J. Am. Chem. Soc., 2014, 136, 8568.
7
8
9
Scheme 6 Reaction of complexes 3a and 3b with alkynes. Molecular
structures are drawn with 50% thermal ellipsoid. Hydrogen atoms and
cyclohexyl groups are omitted for clarity.
10 For related reports on 3,4-didehydrothiophene (cumulene),
see; (a) X.-S. Ye, W.-K. Li and H. N. C. Wong, J. Am. Chem. Soc.,
1996, 118, 2511; (b) X.-S. Ye and H. N. C. Wong, J. Org. Chem.,
1997, 62, 1940.
In summary, we achieved the synthesis of Ni 
2-thiophyne
11 For reviews of aryne-metal complexes, see: (a) S. L. Buchwald
and R. B. Nielsen, Chem. Rev., 1988, 88, 1047; (b) M. A.
Bennett and H. P Schwemlein, Angew. Chem., Int. Ed. Engl.,
1989, 28, 1296; (c) M. A. Bennett and E. Wenger, Chem. Ber.,
1997, 130, 1029; (d) A. M. Dyke, A. J. Hester and G. C. Lloyd-
Jones, Synthesis, 2006, 4093. (e) M. A. Bennett, Aust. J. Chem.,
2010, 63, 1066.
12 For selected examples of cycloalkyne-metal complexes, see:
(a) G. B. Robertson and P. O. Whimp, J. Organomet. Chem.,
1971, 32, C69; (b) M. A. Bennett, G. B. Robertson, P. O. Whimp
and T. Yoshida, J. Am. Chem. Soc., 1971, 93, 3797; (c) S. L.
Buchwald and R. B. Nielsen, Chem. Rev., 1988, 88, 1047; S. L.
Buchwald, R. T. Lum, R. A. Fisher and W. M. Davis, J. Am. Chem.
Soc., 1989, 111, 9113.
complexes as the first example of metal complexes bearing a
five-membered aryne. The structures were successfully
determined by single-crystal X-ray diffraction measurements 20
and their structural resemblance to the related benzyne-metal
complexes were disclosed. Two carbon atoms in the thiophyne
moiety exhibited distinct reactivity reflecting the asymmetric
nature of the aryne moiety. These results represent the
convincing evidence for the actuality of five-membered
heteroarynes and their potential utility in synthesizing complex
heteroaromatics.
This work was supported by a Grant-in-Aid for Scientific
Research from JSPS (Grant Number JP 17H06092, Specially
Promoted Research) to M. M. The authors appreciate Prof. Dr.
Kazushi Mashima and Dr. Haruki Nagae (Graduate School of
Engineering Science, Osaka University) for X-ray diffraction
measurements.
13 M. A. Bennett and E. Wenger, J. Am. Chem. Soc., 2002, 124
,
8348.
14 Y. Sumida, T. Sumida, D. Hashizume and T. Hosoya, Org. Lett.,
2016, 18, 5600.
15 A five-membered Cu-aryne complex was proposed as an
intermediate for the trimerization of N-methyl-2-iodoindole:
J. Bergman and N. Eklund Tetrahedron, 1980, 36, 1439.
16 For
recent
examples
of
3-

2-thiophyne
(2,3-
Conflicts of interest
The authors declare no competing financial interest.
dimetalathiophene) cluster complexes, see (a) M. I. Hossain,
M. D. H. Sikder, S. Ghosh, S. E. Kabir, G. Hogarth and L. Salassa,
Organometallics, 2012, 31, 2546; (b) M. K. Hossain, S.
Rajbangshi, A. Rahaman, M. A. H. Choedhury, T. A. Siddiquee,
S. Ghosh, M. G. Richmond, E. Nordlander, G. Hogarth and S. E.
Kabir, J. Organomet. Chem., 2014, 760, 231; (c) M. M. Uddin,
N. Begum, S. Ghosh, J. C. Sarker, D. A. Tocher, G. Hogarth, M.
G. Richmond, E. Nordlander and S. E. Kabir, J. Organomet.
Chem., 2016, 812, 197.
Notes and references
1
For selected reviews, see; (a) H. H. Wenk, M. Winkler and W.
Sander, Angew. Chem., Int. Ed., 2003, 42, 502; (b) H. Pellissier
and M. Santelli, Tetrahedron, 2003, 59, 701; (c) A. E. Goetz and
N. K. Garg, J. Org. Chem., 2014, 79, 846; (d) C. M. Gampe and
E. M. Carreira, Angew. Chem., Int. Ed., 2012, 51, 3766; (e) P.
M. Tadross and B. M. Stoltz, Chem. Rev., 2012, 112, 3550; (f)
S. Yoshida and T. Hosoya, Chem. Lett., 2015, 44, 1450; (g) F. I.
M. Idiris and C. R. Jones, Org. Biomol. Chem., 2017, 15, 9044.
The ortho-benzyne is better described as a strained alkyne
albeit the cumulene and diradical structures are considerable
resonance contributors; (a) J. W. Laing and R. S. Berry, J. Am.
Chem. Soc., 1976, 98, 660; (b) H. Jiao, P. von R. Schleyer, B. R.
Beno, K. N. Houk and R. Warmuth, Angew. Chem. Int. Ed. Engl.,
1997, 36, 2761; (c) P. G. Wenthold, R. R. Squires and W. C.
Lineberger, J. Am. Chem. Soc., 1998, 120, 5279.
17 For detail, see the Electronic Supplementary Information.
18 M. A. Bennett, T. W. Hambley, N. K. Roberts and G. B.
Robertson, Organometallics, 1985,
19 (a) M. A. Bennett and E. Wenger, Organometallics, 1996, 15
5536; (b) K. R. Deaton and M. S. Gin, Org. Lett., 2003, , 2477.
4, 1992.
,
5
20 The CCDC numbers for the complexes are 1523690 (2a),
1523691 (2b), 1529001 (3a), 1529002 (3d), 1529003 (4a),
2
1540560 (4b), and 1817952 (6). These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
3
4
A. E. Goetz, S. M. Bronner, J. D. Cisneros, J. M. Melamed, R. S.
Paton, K. N. Houk and N. K. Garg, Angew. Chem., Int. Ed., 2012,
51, 2758.
R. Stoermer and B. Kahiert, Ber., 1902, 35, 1633.
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