Metal-induced tautomerization of oxazole and thiazole molecules to heterocyclic carbenes

Article abstract of DOI:10.1039/b900955h

Oxazole and thiazole molecules N-coordinated to manganese(i) are transformed into their corresponding 2,3-dihydrooxazol-2-ylidene and 2,3-dihydrothiazol-2-ylidene carbene tautomers by acid-base reactions, and subsequently transmetalated to gold(i), via is

Full text of DOI:10.1039/b900955h

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Metal-induced tautomerization of oxazole and thiazole molecules
to heterocyclic carbenesw
´
Javier Ruiz* and Bernabe F. Perandones
Received (in Cambridge, UK) 16th January 2009, Accepted 27th February 2009
First published as an Advance Article on the web 23rd March 2009
DOI: 10.1039/b900955h
Oxazole and thiazole molecules N-coordinated to manganese(I)
are transformed into their corresponding 2,3-dihydrooxazol-2-
ylidene and 2,3-dihydrothiazol-2-ylidene carbene tautomers by
acid–base reactions, and subsequently transmetalated to gold(I),
via isolable heterometallic intermediates.
these complexes as carbene transfer agents are also shown
herein.
Azoles 1a,b easily coordinate to Mn(I) when reacting with the
precursor complex fac-[Mn(OClO3)(CO)3(bipy)] (3) to afford
the cationic complexes 4a,b (Scheme 2). As can be seen in the
X-ray diffraction study carried out on 4b (Fig. 1),z the azole
ligands in these complexes are N-coordinated. 4a,b are readily
deprotonated at the C2 carbon atom of the azole heterocycle
when treated with KtOBu to afford neutral derivatives 5a,b
containing oxazolyl and thiazolyl ligands. Subsequent protona-
tion at the nitrogen atom gives the carbene complexes 6a,b in
quantitative yields, completing the metal and acid–base induced
tautomerization of the original azole molecules 1a,b. It should
be mentioned that a low yield transformation of methylthiazole
and benzothiazole chromium complexes into N-alkyl thiazolin-
2-ylidene carbenes, involving deprotonation of the coordinated
azole by LiBu and subsequent alkylation, had been previously
described,11c,d although the experimental approach did not
appear to be selective.
Oxazole (1a) and thiazole (1b) are simple molecules of great
relevance in heterocyclic chemistry and important structural
elements in many natural products.1 Their corresponding
carbene tautomers, 2,3-dihydrooxazol-2-ylidene (2a) and
2,3-dihydrothiazol-2-ylidene (2b), derived from a 1,2-hydrogen
shift (Scheme 1) are not stable, and this has been rationalized
on the base of theoretical calculations,2 deuterium exchange
NMR measurements3 and argon matrix experiments.4 On the
other hand, whereas a large number of NHC complexes of
imidazol-2-ylidene type are known,5 many of them successfully
applied in homogeneous catalysis,6 relatively few examples of
coordination complexes containing carbene ligands of oxazol-
2-ylidene or thiazol-2-ylidene types have been described.
Methods for preparing oxazol-2-ylidene carbene complexes
are limited to intramolecular cyclization of functionalized
hydroxy isocyanides7 or reaction of epoxides with hydrogen
isocyanide complexes,8 the classical route involving in situ
deprotonation of oxazolium salts being very infrequently
encountered in the literature.9 A rare case of cyclization of
ketones with coordinated isocyanide has been described.10
Thiazol-2-ylidene carbene complexes are frequently prepared
by reaction of lithiated thiazoles with the appropriate metallic
fragment and subsequent alkylation or protonation of the
corresponding thiazolyl derivative.11 Methods implying
in situ deprotonation of thiazolium salts have also been used.12
Whereas complexes containing carbene 2b are known,11b to
the best of our knowledge, so far there has been no report of a
complex containing the oxazolin-2-ylidene carbene 2a itself.
We have recently reported that coordinated imidazoles can be
transformed into their corresponding NHC complexes by
means of acid–base treatments.13 We have now found that
this methodology can be applied to oxazole (1a) and thiazole
(1b) molecules coordinated to manganese(I), which can easily
be converted to oxazolin-2-ylidene (2a) and thiazolin-2-ylidene
(2b) ligands, respectively. Preliminary results on the use of
Transformation of compounds 4a,b into 6a,b according to
Scheme 2 was monitored by IR spectroscopy (see ESIw). This
showed a strong shift to low frequencies (about 30 cmꢀ1 on
average) in the nCO bands on passing from 4a,b to 5a,b, and a
new change to high frequencies when forming 6a,b after
protonation. The stronger donor character of the azolin-2-
ylidene ligands with respect to the azole tautomers is evidenced
by the lower values of the nCO bands of 6a,b with respect to
4a,b.
Amongst the spectroscopic data of compounds 6a,b (ESIw) it
is worth noting the low field singlet signal appearing in the
13C{1H} NMR at 216.8 (6a) and 225.5 ppm (6b) corresponding
to the carbene carbon atom. Fig. 2 shows the crystal structure
of the cationic complex 6b, allowing a comparison with that of
Departamento de Quı´mica Orga´nica e Inorga´nica, Facultad de
´
Quımica, Universidad de Oviedo, 33006 Oviedo, Spain.
E-mail: jruiz@uniovi.es; Fax: +34985103446; Tel: +34985102977
w Electronic supplementary information (ESI) available: Experimental
details and analytical and spectroscopic data. CCDC 716673 (4b),
716674 (6b), 716675 (7b) and 716676 (8b). For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b900955h
Scheme 1 Tautomerization of oxazole (1a) and thiazole (1b) mole-
cules to 2,3-dihydrooxazol-2-ylidene (2a) and 2,3-dihydrothiazol-2-
ylidene (2b) carbenes, respectively.
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Fig. 2 Molecular structure of the complex 6b, shown with 50%
thermal ellipsoids. Hydrogen atoms of the bipy ligand are omitted
for clarity. Selected interatomic distances (A) and angles (deg):
Mn1–C2
C4–C5
=
2.024(2), C2–N1
1.339(3), S3–C4
=
1.333(2), N1–C5
=
=
1.387(2),
1.720(2);
=
= 1.726(2), C2–S3
Mn1–C2–N1 = 128.7(1), Mn1–C2–S3 = 124.9(1), C2–N1–C5 = 118.3(2),
N1–C5–C4 = 111.8(2), S3–C4–C5 = 109.6(1), C2–S3–C4 = 93.9(9),
N1–C2–S3 = 106.4(1).
Scheme 2 Formation of azolin-2-ylidene carbene complexes of Mn(I)
6a,b and subsequent transmetalation process to afford Au(I) carbene
complexes 8a,b.
N-coordinated to manganese, leading uncoordinated oxygen
and sulfur heteroatoms. The preference of Au(I) metal ion,
which is softer than Mn(I), to bind carbene carbon atom
instead of nitrogen atom appears to be the driving force for
generating these heterometallic species, which can also be
considered as N-metalated azolin-2-ylidene carbene complexes
of gold(I). In the case of 7b an X-ray diffraction study has been
undertaken (Fig. 3), confirming the proposed structure for this
intermediate species. The Mn–N1 bond length (2.095(2) A) in
7b is slightly longer than that in the precursor thiazole complex
4b (2.074(4) A), suggesting some weakening of that bond in the
heterometallic intermediate, which anticipates the loss of the
manganese fragment in the hydrolysis of the complex. Thus,
the treatment of 7b with HClO4 in CH2Cl2 as solvent affords
the gold(I) carbene complex 8b (Scheme 2), together with the
starting manganese(I) complex fac-[Mn(OClO3)(CO)3(bipy)]
Fig. 1 Molecular structure of the complex 4b, shown with 30%
thermal ellipsoids. Hydrogen atoms of the bipy ligand are omitted
for clarity. Selected interatomic distances (A) and angles (deg):
Mn1–N1
S3–C4
=
2.074(4), N1–C2
=
1.321(6), C2–S3
=
=
1.691(5),
1.380(6);
=
1.705(5), C4–C5
= 1.361(7), C5–N1
Mn1–N1–C2 = 124.3(3), Mn1–N1–C5 = 124.9(3), N1–C2–S3 = 114.5(3),
C2–S3–C4 = 90.3(2), S3–C4–C5 = 110.2 (4), C4–C5–N1 = 114.3(4),
C5–N1–C2 = 110.7(4).
4b (Fig. 1). The Mn–C2 (2.024(2) A) bond length in 6b is
appreciably shorter than the Mn–N1 (2.074(4) A) distance in
4b, in consonance with the stronger donor character of the
carbene ligand already showed in the IR data of these com-
plexes (see above). The bond lengths and angles within the azole
heterocycle are similar in both complexes, though it should be
pointed out that C5–N1–C2 and S3–C2–N1 bond angles in the
thiazole cycle are smaller when this features N1- (4b) and
C2-coordination (6b), respectively.
Complexes 6a,b can be used as carbene transfer agents from
manganese to gold, in such a way that allows the isolation of
heterometallic intermediates. Thus, treatment of dichloro-
methane solutions of 6a,b with a stoichiometric amount of
[AuClPPh3] in the presence of KOH yields complexes 7a,b
after 1 h of stirring at room temperature (see ESI for full
experimental detailsw).
Fig. 3 Molecular structure of the complex 7b, shown with 50% thermal
ellipsoids. Hydrogen atoms of the bipy and PPh3 ligands are omitted for
clarity. Selected interatomic distances (A) and angles (deg): Mn1–N1 =
2.095(2), N1–C5 = 1.390(4), N1–C2 = 1.336(4), C2–S3 = 1.742(3),
S3–C4
= 1.702(3), C4–C5 = 1.348(4), C2–Au1 = 2.040(3),
Au1–P1 = 2.301(9); Mn1–N1–C2 = 129.5(2), Mn1–N1–C5 = 117.7(2),
C2–N1–C5 = 112.6(3), N1–C5–C4 = 115.6(3), N1–C2–S3 = 110.2(2),
C2–S3–C4 = 92.4(2), S3–C4–C5 = 109.2(2), N1–C2–Au1 = 132.9(2),
S3–C2–Au1 = 116.9(2), C2–Au1–P1 = 176.4(9).
Complexes 7a,b contain oxazolyl and thiazolyl bridging
ligands, respectively, which are C-coordinated to gold and
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heterocyclic carbene with respect to those in complex 7b.
In conclusion, we report herein an experimental approach
for the synthesis of oxazolin-2-ylidene and thiazolin-2-ylidene
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of the corresponding oxazole and thiazole heterocycles
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a-Granda and
Acknowledgement is made to the Spanish Ministerio de
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´
funding, Project CTQ2006-10035). B. F. P. thanks the
MICINN for an FPU grant (PGE and European
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Notes and references
z Crystal data for 4b: C17H13Cl3MnN3O7S, M = 564.66, monoclinic,
P21/n, a = 10.1874(5), b = 12.8131(5), c = 16.3439(7) A, b = 93.621(2)1,
V = 2129.15(16) A3, Z = 4, T = 100(2) K, 33 014 measured reflections,
3647 independent reflections (Rint = 0.0672), R1 = 0.0755, wR2 = 0.2284
(all data). 6b. C16H11F6MnN3O3PS,
a = 8.6199(3), b = 10.0390(3), c = 11.5074(3) A, a = 78.487(1),
b = 80.485(2), g = 84.597(2)1, V = 960.41(5) A3, Z = 2, T = 100(2) K,
15 455 measured reflections, 3416 independent reflections (Rint = 0.0275),
ꢀ
525.25, triclinic, P1,
M
=
R1 = 0.0338, wR2 = 0.0648 (all data). 7b C34H25AuF6MnN3O3P2S,
M
ꢀ
983.48, triclinic, P1,
=
a
=
11.0938(3),
b
=
11.4700(4),
c = 15.1096(5) A, a = 94.041(2), b = 101.191(2), g = 112.355(2)1,
V = 1722.13(10) A3, Z = 2, T = 100(2) K, 57 416 measured
reflections, 6738 independent reflections (Rint = 0.0305), R1 = 0.0398,
wR2 = 0.0519 (all data). 8b C21H18AuClNO4PS, M = 643.81,
monoclinic, P21/n, a = 9.7030(3), b = 16.6091(6), c = 13.9180(4) A,
b = 101.338(2)1, V = 2199.22(12) A3, Z = 4, T = 100(2) K, 57 620
measured reflections, 4051 independent reflections (Rint = 0.0314),
R1 = 0.0516, wR2 = 0.0658 (all data).
12 G. C. Vougioukalakis and R. H. Grubbs, J. Am. Chem. Soc., 2008,
130, 2234.
13 J. Ruiz and B. F. Perandones, J. Am. Chem. Soc., 2007, 129, 9298.
14 The oxazolin-2-ylidene carbene complex of gold(I) 8a is formed from
7a in a similar way, as detected by NMR spectroscopy, through in
this case purification of the compound was not feasible owing to the
appearance of variable amounts of secondary products.
ꢁc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 2741–2743 | 2743