N-Heterocyclic Carbenes : From Laboratory Curiosities to Efficient Synthetic Tools

469 Pages·2010·26.5 MB·English

by  Silvia Díez-González

RSC Catalysis Series
Edited by Silvia Díez-González
N-Heterocyclic Carbenes
From Laboratory Curiosities to Efficient Synthetic Tools
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N-Heterocyclic Carbenes
From Laboratory Curiosities to Efficient Synthetic Tools
RSC Catalysis Series
Series Editor:
Professor James J Spivey, Louisiana State University, Baton Rouge, USA
Advisory Board:
Krijn P de Jong, University of Utrecht, The Netherlands, James A Dumesic,
University of Wisconsin-Madison, USA, Chris Hardacre, Queen’s University
Belfast, NorthernIreland,EnriqueIglesia, Universityof California atBerkeley,
USA, Zinfer Ismagilov, Boreskov Institute of Catalysis, Novosibirsk, Russia,
JohannesLercher,TUMu¨nchen,Germany,UmitOzkan,OhioStateUniversity,
USA, Chunshan Song, Penn State University, USA
Titles in the Series:
1: Carbons and Carbon Supported Catalysts in Hydroprocessing
2: Chiral Sulfur Ligands: Asymmetric Catalysis
3: Recent Developments in Asymmetric Organocatalysis
4: Catalysis in the Refining of Fischer-Tropsch Syncrude
5:OrganocatalyticEnantioselectiveConjugateAdditionReactions:APowerful
Tool for the Stereocontrolled Synthesis of Complex Molecules
6: N-Heterocyclic Carbenes: From Laboratory Curiosities to Efficient Syn-
thetic Tools
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N-Heterocyclic Carbenes
From Laboratory Curiosities to Efficient
Synthetic Tools
Edited by
Silvia Dı´ez-Gonza´ lez
Department of Chemistry, Imperial College London, London, UK
Institute of Chemical Research of Catalonia (ICIQ), Tarragona, Spain
RSCCatalysisSeriesNo.6
ISBN:978-1-84973-042-6
ISSN:1757-6725
AcataloguerecordforthisbookisavailablefromtheBritishLibrary
r RoyalSocietyofChemistry2011
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Foreword
A journalist recently asked me if I foresaw, when my group prepared the first
stable carbene, that this topic would become a field in its own right. My
answer was clear-cut: certainly not! Indeed, in 1988 and even for a few years
after, I believed that these species were too fragile to be really useful, and
that they would remain laboratory curiosities. I was not totally wrong as far
as our group’s first stable carbene is concerned, and it is obvious that Ardu-
engo’s carbenes, the famous NHCs, have been responsible for the fantastic
development of this field of chemistry.
AscanbeseeninthisBook,duringthelastfifteenyearsorso,followingthe
pioneering work by Herrmann et al., carbenes have mostly been used as
ancillary ligands for the preparation of transition metal-based catalysts.
Comparedtophosphorusligands,carbenestendtobindmorestronglytometal
centres,avoidingthenecessityfortheuseofexcessligandincatalyticreactions.
The corresponding complexes are often less sensitive to air and moisture, and
haveprovenremarkablyresistanttooxidation.Itisnoteworthythat,although
thefirstcarbene–transitionmetalcomplexeswerepreparedasearlyas1915by
Chugaev (Fischer and Maasbo¨l were the first to fully characterize a carbene–
metal species), the recent developments in their application in catalysis have
considerablybeenfacilitatedbytheavailabilityofcarbenesstableenoughtobe
bottled.Moreover,theexistenceofmetal-freecarbeneshasallowedtheiruseas
organocatalysts.
It is amazing to realize that the first method of taming a carbene was by
attaching it to a transition metal, whereas nowadays carbenes can be used to
stabilize transition metal centres that otherwise are not accessible. Similarly
striking are some recent developments, which show that carbenes, which were
consideredforyearsastheprototypesofreactiveintermediates,canbeusedfor
stabilizing highly reactive main group species.
Forthefuture,sincetherobustnessofcarbenecomplexesislargelyduetothe
presenceofstrongcarbon–metalbonds,othertypesofstablelow-valentcarbon
species are highly desirable, and I believe that a second generation of carbon-
based L ligands will soon appear.
v
vi Foreword
Lastly, I wish to say here that, for Bo Arduengo and myself, the enormous
amount of results summarized in this excellent Book is a wonderful gift.
Professor Guy Bertrand
Distinguished Professor of Chemistry
UCR-CNRS Joint Research Chemistry Laboratory,
Department of Chemistry, University of California, USA
Preface
N-Heterocyclic carbenes (NHCs) are certainly peculiar chemical entities.
Having been considered as transient species, at the most, for many years,
they are currently at the heart of numerous advances in different chemical
fields. One of the most conspicuous peculiarities of NHCs is the ongoing
debate about their Lewis representation, particularly when used as ligands in
metal complexes. A number of representations can be found indis-
tinguishably in the literature and, just as in fashion, the representation of
choice is normally dependent on the most popular version at the time, and/
or the research group implicated (Figure 1, five-membered ring diamino-
carbenes are represented as examples). Representation A was soon aban-
doned with early data showing the poor p back-donating ability of NHCs,
making NHC–M bond single in nature (at least with cyclic diamino-
carbenes).1 Although the current picture of NHCs is far more complex and it
is now known that p back-donation can represent over 30% of the bonding
(hardly negligible!),2 the analytical data stills point relentlessly towards
NHC–M single bonds. Similarly, the representation of unsaturated NHCs as
aromatic derivatives (B) has gradually decreased following reports on the
predominance of the carbenic form over ylidic resonance structures.3 Even if
subsequent studies pointed towards a cyclic electron stabilization, the result-
ing aromatic character of imidazol-2-ylidenes would be substantially smaller
than in benzene or imidazolium salts.4 Most probably, structure E might be
A B C D E
N N N N N N N N N N
R R R R R R R R R R
M M M M
M
Figure 1 Possiblerepresentationsof[(NHC)M]complexes.
RSCCatalysisSeriesNo.6
N-HeterocyclicCarbenes:FromLaboratoryCuriositiestoEfficientSyntheticTools
EditedbySilviaDı´ez-Gonza´lez
rRoyalSocietyofChemistry2011
PublishedbytheRoyalSocietyofChemistry,www.rsc.org
vii
viii Preface
+
N N N N N N
R R R R R R
H M
azolium salt free carbene NHC–metal complex
Figure 2 Chosenrepresentationsforthepresentbook.
N N N N N N N N
IPh ITol SITol IXy
N N N N N N N N
IPr SIPr IMes SIMes
Cl Cl
N N N N N N N
N N
ClIMes SNap TPT IBn
(Enders' carbene)
Me Me Me Me Me Me
Me N N Me Me N N Me Me N N Me N N N N N N
IMe SIMe MeIMe MeIEt IiPr MeIiPr
Me N N N N N N Me N N N N Me
IiPrMe ItBu SItBu ItBuMe IBuMe
O O
N N N N N N N N
n n
ICy SICy IAd IBiox5+n
(Arduengo's carbene)
Figure 3 StructuresandacronymsofNHCs.
the one closer to the real bonding situation in these species; however, it is
also the least straightforward to use. Actually, C and D are the most popular
versions nowadays, despite the fact of symbolizing a trivalent carbon bond
(D) in a confusing manner for any chemist unfamiliar with the field.
Preface ix
Inthepresentbook,versionDhasbeenconsistentlyused.Thisisunarguably
an arbitrary decision and does not pretend to make any statement in this
debate. For the reader’s convenience, the general representations used for
azoliumsalts, free NHCs and NHC/metalcomplexes aredepicted inFigure 2.
Also, the structures of all NHC acronyms used throughout the different
chapters can be found in Figure 3.
Regardless of their importance, NHCs are not the only peculiarity of this
book. When I was first contacted by the RSC about a book project, I was so
astonished I concluded it had to be an internet scam. How could I expect to
receivesuchaninvitationwithmyincipientcareer?Butonceitwasclearthatno
onewouldbeaskingformycreditcardinformation,Iwassettomakethebest
bookonNHCstodate . . . oratleasttotry!Myrelative‘juniority’(whichwill
be inexorably cured over time) makes me even prouder of having gathered
fourteen researchers I knew and respected to join me in this exciting project.
FromhereIwouldliketothankandcongratulatethemallfortheoutstanding
result of the joint efforts.
OnelastpeculiarityIwouldliketoshareisthewishIcherishforthisbook.In
spiteofbeingalonganddemandingjourney,Iactuallyhopethisbookwillbe
soon outdated! Despite their high popularity, the chemistry of NHCs is far
from mature and their reactivity, even beyond catalysis, will certainly provide
more mind-blowing advances. Personally, I am looking forward to knowing
what will happen next.
Dr Silvia Dı´ez-Gonza´lez
References
1. (a)K.O¨ feleandC.G.Kreiter,Chem.Ber.,1972,105,529–540; (b)K.O¨ fele
and M. Herberhold, Z. Naturforsch B., 1973, 28, 306–309.
2. X.Hu,Y.Tang,P.GantzelandK.Meyer,Organometallics,2003,22,612–
614.
3. For instance, see: (a) D. A. Dixon and A. J. Arduengo III, J. Phys. Chem.,
1991, 95, 4180–4182; (b) A. J. Arduengo III, H. Bock, H. Chen, M. Denk,
D.A.Dixon,J.C.Green,W.A.Herrmann,N.L.Jones,M.WagnerandR.
West, J. Am. Chem. Soc., 1994, 116, 6641–6649.
4. J.F.Lehmann,S.G.Urquhart,L.E.Ennios,A.P.Hitchcock,K.Hatano,
S. Gupta and M. K. Denk, Organometallics, 1999, 18, 1862–1872.