Expanded-ring and backbone-functionalised N-heterocyclic carbenes

Article abstract of DOI:10.1002/ejic.200901182

An unsaturated seven-membered amidinium salt 7 decomposes in the presence of metal salts under basic conditions. However, 7 readily forms a Diels-Alder cycloadduct with CpH from which the Rh^I complexes may be prepared. Thus, structurally elabor

Full text of DOI:10.1002/ejic.200901182

SHORT COMMUNICATION
DOI: 10.1002/ejic.200901182
Expanded-Ring and Backbone-Functionalised N-Heterocyclic Carbenes
Manuel Iglesias,[a] Dirk J. Beetstra,[a] Kingsley J. Cavell,*[a] Athanasia Dervisi,*[a]
Ian A. Fallis,*[a] Benson Kariuki,[a] Ross W. Harrington,[b] William Clegg,[b]
Peter N. Horton,[c] Simon J. Coles,[c] and Michael B. Hursthouse[c]
Keywords: Carbene ligands / Cycloaddition / Rhodium / Ligand design / Isomers
An unsaturated seven-membered amidinium salt 7 decom- structurally elaborate, sterically crowded carbene ligand
poses in the presence of metal salts under basic conditions.
However, 7 readily forms a Diels–Alder cycloadduct with
CpH from which the RhI complexes may be prepared. Thus,
complexes bearing peripheral unsaturated functionality are
available in a short versatile synthesis.
via the chloride salts 2 and 3 (Scheme 1) by the reaction
of the parent amidines[4,5] 1 and (E)-1,4-dichlorobut-2-ene
under mild conditions (see Experimental Section). Reaction
temperatures below 60 °C resulted in essentially exclusive
formation of seven-membered dihydrodiazepinium deriva-
tives in moderate to good yields. Higher dilutions failed to
improve the yield of this reaction, whilst increased tempera-
tures (up to 100 °C in sealed tubes) resulted in 1:1 mixtures
of the desired seven-membered products 2 or 3, and the
vinyl-substituted five-membered products 4 or 5, presum-
ably via competing SN2 and SN2Ј mechanisms.
The unsaturated chloride salt 2 was structurally charac-
terised, and the structure of the cation is shown in Fig-
ure 1(a). The unsaturated C3–C4 bond brings about an ad-
ditional rigidity in the NHC ring, which results in a de-
crease of the Cring–N···N–Cring torsion angle and displaces
the alkene carbon atoms C3 and C4 away from the NC(H)-
N plane. The NC(H)N angle is similar to that of the “par-
ent” saturated seven-membered salt (127.4°). The olefinic
backbone of salts 2, 3, 6 and 7 offers potential for the fur-
ther manipulation of the carbene ligand framework. The
reactions reported here, which probe the reactivity of the
alkene backbone, represent just a sample of those under
study within our group. For example, electrophilic haloge-
nation of the alkene backbone of the amidinium salt 6 in
dichloromethane solution was achieved by the addition of
bromine at ambient temperature to afford the amidinium
dibromide 8. Attempts to dehydrohalogenate 8 by the elimi-
nation of 2 equiv. of HBr resulted only in decomposition.
This is not unexpected, as the fully unsaturated product of
this reaction would be expected to be an anti-Hückel 8-π
electron system. The structure of one of the isomers of the
racemate is depicted in Figure 1(b). The antiperiplanar dis-
position of the bromine atoms, with a Br–C–C–Br torsion
angle of 178.3°, forces 8 into a twisted-chair conformation,
with axial bromine atoms, resulting in a substantial increase
Introduction
N-Heterocyclic carbenes (NHCs) have established them-
selves as key ligands in organometallic chemistry and catal-
ysis.[1] To date most work has focused on unsaturated, for-
mally imidazolium-derived, five-membered-ring NHCs, but
more recently other ring sizes have been explored, and the
novel properties of these expanded-ring NHCs are currently
attracting much interest.[2,3] We have recently developed a
mild, efficient, functional-group-tolerant synthetic method-
ology, based on the method of Bertrand,[2d] which allows
the synthesis of a range of structurally elaborate carbene
precursors.[4] These studies highlighted large N–C–N angles
for these ligands, which has the effect of pushing the N–R
substituents towards the metal center, thus providing steric
protection not offered by smaller-ring NHCs.[3–5] This in-
crease in steric congestion appears to lead to enhanced per-
formance in selected NHC-complex-catalysed reactions.[6]
In this paper we examine methods of manipulating the li-
gand environment in NHC carbene precursors by electro-
philic or Diels–Alder additions to olefinic seven-membered
amidinium salts.
Results and Discussion
To examine the possibility of incorporating further func-
tionality into large-ring systems, we have prepared the un-
saturated seven-membered-ring NHC precursors 6 and 7
[a] School of Chemistry, Cardiff University, Main Building,
Park Place, Cardiff, CF10 3AT, UK
Fax: +44-29-20874030
E-mail: fallis@cardiff.ac.uk
[b] School of Chemistry, Newcastle University,
Newcastle upon Tyne, UK NE1 7RU
[c] University of Southampton,
Highfield, Southampton, SO17 1BJ, UK
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200901182.
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Expanded-Ring and Backbone-Functionalised NHCs
azo et al.[7b] has also indicated that a 1,2-elimination path-
way was possible in five-membered systems. Hence further
ligand modifications could only be achieved by manipula-
tions of the amidine salts themselves.
Figure 1. (a) Displacement ellipsoid (30%) plots of the amidinium
cation of the chloride salt 2; selected bond lengths [Å] and angles
[°]: N1–C1 1.318(3), C1–N2 1.319(3), N1–C2 1.482(3), N2–C5
1.485(3), C2–C3 1.486(4), C4–C5 1.486(4), C3–C4 1.328(4); N1–
C1–N2 127.4(2), C1–N2–C15 117.49(19), C1–N1–C6 117.41(19);
C2–N1···N2–C5 1.3. (b) Displacement ellipsoid (30%) plots of the
amidinium salt 8; selected bond lengths [Å] and angles [°]: N1–C1
1.317(5), C1–N2 1.304(5), N1–C2 1.475(5), N2–C5 1.457(5), C3–
Br1 1.964(6), C4–Br2 1.980(6); N1–C1–N2 133.7(4), C1–N2–C15
114.1(4), C1–N1–C6 113.0(3); C2–N1···N2–C5 8.7.
As a means of further functionalizing the system we rea-
soned that the salts 6 and 7, as electron-deficient (i.e. cat-
ionic) alkenes, should readily undergo Diels–Alder ad-
ditions with suitable dienes.
Thus, for example, reaction of methanol solutions of 6
Scheme 1. Reagents, conditions and isolated yields: (i) (E)-1,4-
dichlorobut-2-ene, MeCN, K2CO3, 60 °C, R = Mes (40%); R = or 7 with an excess of cyclopentadiene in a sealed tube at
Dipp (44%). (ii) NaBF4, 89–94%. (iii) Br2, CH2Cl2, 70%. (iv)
60 °C afforded the Diels–Alder cycloaddition products 9
DBU, MeCN. (v) Range of bases and metal cation sources. (vi)
and 10, respectively, as the endo products in high yields. 1H
Cyclopentadiene, MeOH, R = Mes (82%); R = Dipp (86%). (vii)
NMR spectroscopy confirmed the formation of only one
[Rh(acac)(CO)2], KN(SiMe3)2, Et2O, R = Mes (68%, sum of rota-
of the two possible isomers (endo or exo), and X-ray data
(Figure 2) confirmed the formation of the endo isomers for
both salts 9 and 10. The cis substitution on the backbone
mers); R = Dipp (66%). (viii) CDCl3, H2(g), 79%.
of the N–C(H)–N angle to 131.0(3)° over that of 6. Charac- in 9 and 10 does not significantly change the geometry of
teristically, no (carbene)metal complexes derived from 2, 3, the salt, which is in contrast to our previously reported
6, 7 and 8 could be prepared. Reaction of these amidinium seven-membered trans-substituted 5,6-dioxolane salt.[3] The
salts with a range of bases and metal precursors resulted N–C–N angles obtained for 9 (128.2°) and 10 (127.7°) are
only in decomposition of the amidinium salts. These obser- comparable to that of 2 (127.4°).
vations are in agreement with those reported by Nechaev et
The RhI complexes, 11 and 12, were prepared in good
al.,[7a] who suggested that the decomposition of similar yields by in situ reaction of the amidinium salts 9 and 10,
seven-membered materials occurred by a 1,4-Hoffman eli- respectively, with KN(SiMe3)2 in the presence of the metal
mination to form neutral acyclic amidines. In an earlier ac- precursor [Rh(acac)(CO)2]. Compound 11 was isolated as a
count of 1,3-bis(1-pyrolidino)imidazolin-2-ylidenes, Alcar- 50:50 mixture of rotamers (11a/11b) (Scheme 1), as defined
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K. J. Cavell, A. Dervisi, I. A. Fallis et al.
SHORT COMMUNICATION
of the isomers. In contrast, for complex 12 the increased
bulk of the Dipp groups results in the formation of only
one of the two possible isomers in solution, as evidenced
by a single resonance for the acac methine group at δ =
1
5.18 ppm in the H NMR spectrum, and two acac methyl
1
resonances in the H and 13C NMR spectra. In addition,
four signals are observed for the isopropyl methyl groups,
indicating a static structure at room temperature, with no
rotation about the Rh–NHC or N–Ar bonds observed. Fig-
ure 3(a) illustrates the structure of 11a, the isomer in which
the carbene double bond resides on the “acac side”; for 12
the sole “carbonyl side” isomer is shown [Figure 3(b)]. In
both cases the carbene ring adopts a conformation, which
Figure 2. Displacement ellipsoid plots (30%) of the amidinium
salts 9 (a) and 10 (b). Selected bond lengths [Å] and angles [°] for
9: C1–N1 1.287(15), C1–N2 1.358(14), C2–C3 1.489(14), C2–N1
1.519(11), C3–C4 1.582(12), C4–C5 1.485(15), C5–N2 1.519(11);
N1–C1–N2 128.2(5), C3–C2–N1 110.4(10), C2–C3–C4 115.4(11),
C6–C3–C4 102.6(8), C8–C4–C3 101.3(8); selected H atoms have
been omitted for clarity. Selected bond lengths [Å] and angles [°]
for 10: N29–C28 1.317(3), N15–C28 1.316(3), C43–C44 1.568(4),
C47–C48 1.322(5); N15–C28–N29 127.7(2), C28–N29–C30
116.8(2), C16–N15–C28 117.8(2), C42–C43–C49 112.9(3), C45–
C44–C46 112.7(3).
by the relative orientation of the carbene alkene; the 13C
NMR spectrum shows two carbene resonances at δ = 206.9
(1JRhC = 53.6 Hz) and 206.7 (1JRhC = 54.3 Hz) ppm, and
two carbonyl doublet resonances at δ = 190.5 (1JRhC
=
49.5 Hz) and 189.6 (1JRhC = 49.4 Hz) ppm. In the 1H NMR
spectrum two peaks of equal intensity are observed for the
methine group of the acac ligand. Assuming that the car-
bene ligand is perpendicular to the coordination plane, in
isomer 11a the alkene would be pointing towards the acac
ligand and in 11b towards the carbonyl ligand. On heating,
the 1H NMR spectrum no broadening of the acac CH
peaks of 11 was observed, suggesting no interconversion
between the two isomers at temperatures up to 353 K.
However, in a spin saturation transfer experiment, the irra-
diation of the CH proton of the acac ligand in one isomer
resulted in the expected disappearance of this peak and a
17% decrease in the intensity of the corresponding CH res-
Figure 3. Displacement ellipsoid plots (30%) of (a) 11a (one of two
independent but essentially identical molecules) and (b) 12. Se-
lected bond lengths [Å] and angles [°] for 11: Rh1–O2 2.076(3),
Rh1–O3 2.076(2), Rh1–C1 1.785(4), Rh1–C7 2.003(3); N1–C7–N2
118.8(3), O2–Rh1–O3 87.52(10). Selected bond lengths [Å] and
angles [°] for 12: Rh1–O2 2.067(2), Rh1–O3 2.064(2), Rh1–C1
2.000(3), Rh1–C35 1.798(3); N1–C1–N2 120.0(3), O2–Rh1–O3
onance in the other isomer, indicating slow interconversion 89.10(8).
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Expanded-Ring and Backbone-Functionalised NHCs
01, GR/R06458/01) and for funding the National Crystallography
Service and the Science and Technology Facilities Council (STFC)
for access to synchrotron facilities. We thank the Cardiff Institute of
Tissue Engineering and Repair (CITER) for MS instrumentation.
forces the ligand aryl rings to be closer together over one
face of the NCN core. Thus, in 12 the distance between the
iPr groups on the double bond side is shorter (3.8 Å) than
for the pair on the opposite side (5.9 Å), permitting the less
hindered face to tilt over the cis ligand. This allows weak
H-bonding between the methine iPr protons and the acac
oxygen donors and the iPr methyl protons on the other face
of the carbene with the oxygen atom of the carbonyl ligand.
The asymmetry of the aryl groups results from a large
ArCN···NCAr torsion angle of 14.5° in 12, due to pyrami-
dalisation of the nitrogen atoms on coordination to the rho-
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–
Cring plane in 12). Stirring of a chloroform solution of 11
at ambient temperature under hydrogen (2 bar) for 48 h
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olefin to afford 13 as a mixture of rotamers (salt 7, without
metal present, is of course inert under similar conditions).
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Conclusions
Unsaturated amidinium salts such as 6 and 7, although
incompatible with direct carbene formation or complex-
ation, do lend themselves to Diels–Alder adduct formation
and hence permit the formation of structurally elaborate,
sterically hindered ligands. These amidinium salts are in
turn suitable for complex formation which facilitates the in-
corporation of synthetically versatile functionality into the
ligand backbone. In essence an intermediate Diels–Alder
step has been used to “decouple” the ligand backbone alk-
ene reactivity from the NCN carbene core. This study also
highlights the potential value of large-ring NHCs as ligands
in catalytic hydrogenation. Their application in this area has
recently been reported.[10] Other catalytic applications, and
further ligand modifications will be reported shortly.
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Hursthouse, S. J. Coles, K. J. Cavell, A. Dervisi, I. A. Fallis,
Organometallics 2007, 26, 4800.
[4] M. Iglesias, D. J. Beetstra, J. C. Knight, L.-l. Ooi, A. Stasch,
S. J. Coles, L. Male, M. B. Hursthouse, K. J. Cavell, A. Dervisi,
I. A. Fallis, Organometallics 2008, 27, 3279.
[5] M. Iglesias, D. J. Beetstra, B. Kariuki, K. J. Cavell, A. Dervisi,
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[6] Manuel Iglesias, PhD Thesis (“Expanded ring carbenes – syn-
thesis coordination and catalysis”), Cardiff University, 2008.
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Experimental Section
Refer to the Supporting Information for all synthetic and charac-
terisation details. CCDC-723035 (2), -7233036 (8), -725799 (9),
-723033 (10), -725798 (11) and -723034 (12) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): Full experimental details for the synthesis and characterisation
of all materials.
[8] F. E. Hahn, C. Holtgrewe, T. Pape, M. Martin, E. Sola, L. A.
Oro, Organometallics 2005, 24, 2203.
[9] a) H. Kwart, K. King, Chem. Rev. 1968, 68, 415; b) G. G.
Smith, F. W. Kelly, Prog. Phys. Org. Chem. 1971, 8, 75.
[10] a) A. Binobaid, M. Iglesias, D. J. Beetstra, B. Kariuki, A.
Dervisi, I. A. Fallis, K. J. Cavell, Dalton Trans. 2009, 7099.
Received: December 7, 2009
Acknowledgments
We thank the Engineering and Physical Sciences Research Council
(EPSRC) for support (EP/C528638/1, EP/C53090X/1, GR/S86105/
Published Online: March 11, 2010
Eur. J. Inorg. Chem. 2010, 1604–1607
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