Synthesis of ureas from titanium imido complexes using CO2 as a C-1 reagent at ambient temperature and pressure

Article abstract of DOI:10.1039/c1ob06576a

The coordinatively unsaturated 12-electron complex dichloro t-butylimido bispyridine titanium(iv) (2a) has been shown to react with CO₂ to give N,N-bis-t-butyl urea. Two analogous sterically hindered coordinatively saturated 14-electron complexes dichloro t-butylimido trispyridine titanium(iv) (10a) and dichloro 2,6-(i-Pr)₂phenylimido trispyridine titanium(iv) (10b) also gave their corresponding symmetrical ureas upon treatment with CO₂. Further experiments support the intermediary of metallocycles formed from heterocumulene metathesis reactions. The unsymmetrical urea N-benzyl, N-t-butyl urea (11) was produced from treatment of 2,6-(i-Pr) ₂phenylimido trispyridine titanium(iv) (10b) with CO₂ and interception with BnNH₂. Equimolar quantities of N,N- bistrimethylsilybenzylamine or N,N-bistrimethylsilyphenethylamine were shown to promote the reaction between t-butylimido bispyridine titanium(iv) (2a) and CO₂ to give near quantitative yields of symmetrical urea. Other symmetrical ureas could be produced from TiCl₄, amine and CO ₂ in moderate to quantitative yields depending on the stoichiometry of amine present.

Full text of DOI:10.1039/c1ob06576a

Organic &
Biomolecular
Chemistry
www.rsc.org/obc
Volume 10 | Number 7 | 21 February 2012 | Pages 1313–1468
ISSN 1477-0520
PAPER
J. C. Anderson and R. B. Moreno
Synthesis of ureas from titanium imido complexes using CO2 as a C-1 reagent
at ambient temperature and pressure

Organic &
Biomolecular
Chemistry
Dynamic Article Links
Cite this: Org. Biomol. Chem., 2012, 10, 1334
www.rsc.org/obc
PAPER
Synthesis of ureas from titanium imido complexes using CO2 as a C-1 reagent
at ambient temperature and pressure†
James C. Anderson* and Rafael Bou Moreno
Received 14th September 2011, Accepted 25th October 2011
DOI: 10.1039/c1ob06576a
The coordinatively unsaturated 12-electron complex dichloro t-butylimido bispyridine titanium(IV) (2a)
has been shown to react with CO2 to give N,N-bis-t-butyl urea. Two analogous sterically hindered
coordinatively saturated 14-electron complexes dichloro t-butylimido trispyridine titanium(IV) (10a)
and dichloro 2,6-(i-Pr)2phenylimido trispyridine titanium(IV) (10b) also gave their corresponding
symmetrical ureas upon treatment with CO2. Further experiments support the intermediary of
metallocycles formed from heterocumulene metathesis reactions. The unsymmetrical urea N-benzyl,
N-t-butyl urea (11) was produced from treatment of 2,6-(i-Pr)2phenylimido trispyridine titanium(IV)
(10b) with CO2 and interception with BnNH2. Equimolar quantities of N,N-bistrimethylsilybenzyl-
amine or N,N-bistrimethylsilyphenethylamine were shown to promote the reaction between
t-butylimido bispyridine titanium(IV) (2a) and CO2 to give near quantitative yields of symmetrical urea.
Other symmetrical ureas could be produced from TiCl4, amine and CO2 in moderate to quantitative
yields depending on the stoichiometry of amine present.
Introduction
There is currently a search for alternatives to chemical feedstocks
for the fine chemicals industry that do not rely upon oil.
Carbon dioxide has emerged as a renewable candidate as it is
abundant, non-toxic, non-flammable and is the waste product of
many industrial processes. However, it is thermodynamically and
kinetically very stable and its use as a general C-1 reagent poses
significant problems.1 Carbon dioxide is currently used industrially
for the synthesis of urea, inorganic carbonates, methanol, salicylic
acid, cyclic carbonates and polycarbonates. For these processes
the CO2 is always pressurised or is in the solid state.1b,2 While
there have been considerable advances in the activation and use
of CO2 as a renewable C-1 source in the copolymerisation of
CO2 and epoxides,3 there is a need for the development of other
processes that can function at ambient temperature and pressures
of CO2. The synthesis of ureas from primary and secondary amines
with CO2 at atmospheric pressure and ambient temperature still
remains a challenge.4
We have been interested in heterocumulene synthesis through
the direct combination of CO2 as a C-1 reagent with metal
imido complexes to form isocyanates (Scheme 1). The converse
reaction, to generate a metal imido complex (and CO2) from the
combination of isocyanates and metal oxo complexes is a well
known method for the synthesis of metal imido complexes.5 The
reactions proceed by a [2 + 2] cycloaddition to generate a titanium-
Scheme 1 The formation of a carbamate metallocycle by [2 + 2] reactions.
carbamate metallocycle 1. The lability of these sorts of complexes
normally leads to the elimination of carbon dioxide. Examples of
the forwards reaction described in Scheme 1 are rare,6 but titanium
has shown some general reactivity in this reaction.7–9 Cycloaddi-
tion reactions of titanium imido complexes with CO2 have been
reported for a number of titanium complexes by Mountford7 and
others.8,9 The formation of the thermodynamically stable titanium
oxo complex is normally the driving force for the reaction and its
isolation the focus of most studies.6
Mindiola and co-workers reported the formation of an iso-
cyanate from the reaction between a latent low coordinate
titanium imido complex and CO2 (eqn (1)).8 Unlike other imido
systems, excess CO2 did not lead to six-membered ring aryl
imidodicarboxylates.7c Isocyanates are also known to react with ti-
tanium imido complexes to give carbodiimides.7g,10 It has also been
reported that certain titanium imido complexes react with carbodi-
imides to give symmetrical metallocyclo–guanidine complexes.11
In this paper we report our studies between Mountford-like
12- and 14-electron titanium imido complexes with CO2 to
give ureas as a preparative reaction. Further experiments are
presented that support a reaction mechanism involving the
in situ formation of metallocycle intermediates via heterocumulene
metathesis.
Department of Chemistry, University College London, 20 Gordon Street,
London, UK, WC1H 0AJ. E-mail: j.c.anderson@ucl.ac.uk
† Electronic supplementary information (ESI) available: Experimental and
full characterisation data. See DOI: 10.1039/c1ob06576a
1334 | Org. Biomol. Chem., 2012, 10, 1334–1338
This journal is
The Royal Society of Chemistry 2012
©

electron complexes 10 by transimination of t-butyl complex 10a
with anilines. Unfortunately, attempted transimination reactions
with alkyl amines did not go to completion and we were unable
to purify without degradation. Removal of the apical pyridine
ligand under high vacuum had been demonstrated by Mountford
for the conversion of certain sterically hindered titanium imido
complexes, such as 10b to 2b.12 From our work we found
that the less sterically hindered imido complexes prepared by
transimination did not undergo this reaction, only the sterically
hindered imido complex 10a lost a pyridine ligand under high
vacuum to give complex 2a (Scheme 3). Not surprisingly there
seems to be a steric effect in this ligand loss, which for our purposes
was preparatively restrictive.
(1)
Results and Discussion
Recently we reported the synthesis of carbodiimides by a
heterocumulene metathesis reaction between isocyanates and
Ti(NtBu)Cl2Py2 (2a),12 which led us to explore the possible
activation of CO2 by this complex.10b We envisaged that reaction
of 2a with CO2 could lead to an isocyanate like the Mindiola work
(eqn (1)).8 In the event, treatment of 2a with 1.5 atm of CO2 in
CH2Cl2 at rt for 5 h followed by work up with 2 M HCl gave
symmetrical urea 3a in 8% yield after aqueous hydrolysis (eqn
(2)). We inferred from this result that the central carbonyl carbon
was derived from CO2.
(2)
Scheme 3 The synthesis of 12-electron titanium imido complexes.
Treatment of coordinatively saturated 14 electron complexes 10
with CO2 mostly gave no reaction except for the sterically hindered
analogues 10a and 10b that gave symmetrical urea 3a and 3b in
14% and 98% unoptimised isolated yields, respectively (eqn (3)).
We speculate that the reaction proceeds by the 12 electron imido
complexes 2 and that loss of a pyridine ligand from 10a, b occurs
during the reaction as a consequence of steric compression from
the large t-Bu and 2,6-(i-Pr)2Ph groups on nitrogen. Loss of a
pyridine ligand would be necessary to allow coordination of CO2.
We presume the mechanism is similar to that formulated by
Mindiola8c and which we had used to explain the formation
of carbodiimides from isocyanates and Ti(NtBu)Cl2Py2 (2a).10b
Isocyanate 5 (and presumably 6), formed from the collapse of
metallo carbamate 4, could react with 2 to form either of the two
metallocycles 7 (from [2 + 2] cycloaddition across RN C of 5)
or 8 (from [2 + 2] cycloaddition across C O of 5). Potentially
complexes 7 and 8 could interconvert. Hydrolysis of 7 or 8
during work up would lead to symmetrical urea 3. Alternatively,
metallocycle 8 could break down to diimide 9 that could then be
hydrolysed during work up to 3 (Scheme 2).
(3)
In order to try and decipher whether urea 3 was produced
from hydrolysis of complexes 7 or 8, or formed by hydrolysis of
carbodimide 9 (Scheme 2) a range of nucleophiles were added to
the reaction (eqn (3)). Complex 10b was allowed to react with CO2
as before and then treated separately with nucleophiles EtOH,
H2O and BnNH2 in an attempt to intercept any intermediate
before hydrolysis (Scheme 4). If these nucleophiles were to react
with the imide type ligand of 7 or 8 in an acyl substitution type
mechanism we would expect to isolate a carbamate, an amine
Scheme 2 Possible mechanisms for the conversion of CO2 to ureas.
In order to investigate whether analogous imido complexes of
2 performed this transformation we required a flexible synthesis.
We found we could not purify other alkyl imido analogues of
complex 2a synthesised using the Mountford route to 2a.12 The
use of anilines also gave no products. Attempted transimination
of the 12-electron complex 2a with alkyl amines and anilines was
uniformly unsuccessful. We then investigated another method of
Mountford12 and prepared a range of coordinatively saturated 14
Scheme 4 Activation of CO2 by 10b.
This journal is
The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 1334–1338 | 1335
©

Table 1 Evolution of eqn (4) with different ratios of 10a and 10b
(via decarboxylation of carbamic acid) and an unsymmetrical
urea, respectively. Reaction with EtOH led to degradation, but
treatment with NaOEt led to symmetrical urea 3b in 68% yield.
Treatment with H2O led to no reaction. The fact that no urea
was observed by quenching with H2O or EtOH suggests no
carbodiimide 9 is present as these are favourable reactions.
When the reaction was quenched by addition of benzylamine,
unsymmetrical urea 11 was isolated in 18% yield.
Initial ratio
10a : 10b
16 h ratio
10a : 10b
40 h ratio
10a : 10b
20 : 80
28 : 72
52 : 48
48 : 52
60 : 40
77 : 23
4 : 96
0 : 100
0 : 100
26 : 74
29 : 71
43 : 57
64 : 36
12 : 88
37 : 63
40 : 60
50 : 50
70 : 30
Formation of the symmetrical urea 3b from the treatment of
10b with CO2 and H3O+ work up suggests either hydrolysis of
metallocycle 7 or 8 or hydrolysis of a symmetrical carbodiimide
9. Formation of the same urea from treatment with ethoxide
ion, however, suggests displacement of the imido ligand from
7 or 8 by the oxygen nucleophile (ethoxide ion). The fact that
no carbamate was generated from this latter experiment suggests
that any intermediate isocyanate 5 is consumed rapidly in the
reaction, presumably by reaction with another molecule of imido
complex 2. The isolation of the unsymmetrical urea 11 from
treatment with BnNH2 suggests either the interception of a
postulated metallocycle like 4, which based on the results for the
oxygen nucleophiles would seem unlikely, or more likely an acyl
substitution type reaction with the analogous complexes 7 or 8.
The fact the oxygen nucleophiles react in a different manner may
be a facet of titanium’s oxophilicity.
Experiments were conducted in C6D6.
followed by workup with 2 M HCl led only to the unsymmetrical
urea 12b.
(4)
Intermediate complexes 7 and 8 are identical to those postulated
in our previous report for the synthesis of ureas from the reaction
of complex 2a and isocyanates.10b Treatment of 2a with tert-
butylisocyanate in an NMR experiment showed only a constant
~1 : 1 mixture of an intermediate, believed to be 7 or 8 and
carbodiimide product over 16 h with no impurities [intermediate
(30%) : 2a (43%) : bis-N,N¢-tbutyl carbodiimide (23%) : 6 (3%)].7b
Quenching an identical preparative experiment led to the isolation
of symmetrical urea 3a in 62% yield (Scheme 5).13 When the
reaction was quenched with NaOEt, 3a was isolated in 82% yield,
but when quenched with BnNH2 unsymmetrical urea 12c was
isolated in 51% yield. No symmetrical urea 3a was detected in the
crude reaction mixture or isolated product of this latter reaction.
The formation of only 12b suggests that one of the complexes
reacts faster with CO2 than the other. The isocyanate formed
then reacts with the other complex to form a mixed urea after
hydrolysis. In order to assess this a series of NMR experiments
were conducted with varying ratios of complexes 10a and 10b
(Table 1).
In all cases the proportion of complex 10b increases with time.
These results suggest that the t-butyl compex 10a reacts quicker
than 10b with CO2, presumably to produce t-butylisocyanate.
Attempted isolation of the putative titanium oxo complex was
never achieved in this or any other experiments and could be due
to the propensity of Ti O bonds to oligomerise.6–9 Complex 10b
then reacts with the t-butylisocyanate, in preference to CO2, to
give complex 7 or 8 and upon hydrolysis 12b. From experiments
above (eqn (3) and Scheme 5) we also think the reaction between
10a and tBuNCO is slow (vide infra).13 In support of these
conclusions, treatment of complex 10b with CO2 in an NMR
tube (C6D6, rt, 16 h) led to the formation of only ~10% of 2,6-
di-isopropylphenylisocyanate, indicating this is a slow reaction.
Also treatment of 10b with t-butylisocyanate in CH2Cl2 for 48 h at
rt followed by quenching with dilute acid led to 12b in 29% yield.
Treatment of 10a with 2,6-di-isopropylphenylisocyanate produced
no urea after prolonged stirring. These control experiments were
consistent with the proposed order of events.
Scheme 5 The investigation of the intermediate.
The similarities in these two sets of experiments (Scheme 4
and 5) suggests a common intermediate, which we propose is
either complex 7 or 8. As supported by our previous work,10b
the treatment of either 2a or coordinatively saturated 14-electron
complexes 10a, b with CO2 gave symmetrical ureas with no
evidence for the formation of carbodiimide 9.
To investigate the reaction further we decided to investigate
the reaction of a mixture of complexes 10a and 10b with CO2.
Theoretically a mixture of 3 ureas 3a, 3b and 12b could be
expected (eqn (4)). In the event, treatment of 2 different mixtures
of complexes 10a and 10b with CO2 (1.5 atm) for 5 h at rt in CH2Cl2
The attempted synthesis of non aromatic 12- and 14-electron ti-
tanium imido complexes via transimination (Scheme 3) had proved
unsuccessful. The reactions most often did not go to completion
and we were unable to separate the new imido complexes without
degradation. Due to these difficulties in purification we submitted
some crude reaction mixtures to reaction with CO2 (eqn (5)).
1336 | Org. Biomol. Chem., 2012, 10, 1334–1338
This journal is
The Royal Society of Chemistry 2012
©

Using one equivalent of amine gave yields roughly one third
of that when using 3 equivalents. Excess amine (6 equivalents)
seemed to be deleterious to the reaction. Theoretically, excess
amine could intercept the isocyanate and effectively 1 equivalent of
TiCl4 would only be needed. We found that a large excess of amine
(300 equiv) and pyridine (230 equiv) gave near quantitative yields
of symmetrical urea 3 based upon 1 equivalent of TiCl4. Control
experiments where the reactions were repeated in the absence of
TiCl4 led to no reaction.
If the mechanism for the formation of the urea is as we
hypothesised (Scheme 3), then there exists an opportunity to try
and encourage turnover of the titanium oxide byproduct to react
with amine and form an imido complex and thus make the process
catalytic in Ti. Preliminary experiments to activate the presumed
Ti O bond by the addition of varying amounts of TMSCl
were unsuccessful, giving only stoichiometric quantitative yield
as observed above (eqn (7)). Other Lewis acids, such as Mg(OTf)2,
Bu3BOTf, Bu3SnCl and B(C6F5)3, were also tested under the same
reaction conditions, but they inhibited the reaction and no urea
or other identifiable organic adduct was formed.
(5)
The yields were low and gave mixtures of ureas due to
the mixtures of complexes present. Another known method of
performing the desired transimination to form new complexes is
to use bis-silylated amines.14 Treatment of 10a with either of two
bistrimethylsilylamines (1 equivalent) and addition of CO2 in situ
generated symmetrical urea 3a in almost quantitative yield (eqn
(6)). Clearly no transimination took place, but the silylated amines
are promoting the reaction in some way (compare with eqn (3)).
Their exact role in this reaction remains unknown. A tertiary
amine Et3N (1 equivalent) was also found to activate the reaction,
but less efficiently with a reduced yield of 37%.
(6)
Conclusions
Our initial observation that the 12-electron complex
Ti(NtBu)Cl2Py2 (2a) gave symmetrical urea 3a upon treatment
with CO2 (eqn (2)) led us to investigate this reaction further.
We found that only two sterically hindered 14-electron imido
complexes Ti(NtBu)Cl2Py3 (10a) and Ti(NPh(2,6-iPr)2)Cl2Py3
(10b) underwent the same reaction (Equation 3). As these
were also the only two complexes that we could form the
Another conceivable route to non-aromatic imido–titanium
complexes is their generation from TiCl4 and amines, similar to the
method used by Mountford12 and which we used to synthesize the
t-butyl imido complex 2a. This route gave us the desired complexes,
but the formation of byproducts of similar solubility made their
purification difficult. The crude complexes were thus treated with
CO2 in situ to give moderate yields of the symmetrical ureas
(eqn (7) and Table 2). Three equivalents of amine were initially
added as we believed at least two equivalents would be needed to
sequester HCl. We postulate that a 12- or 14-electron titanium
complex is formed followed by heterocumulene metathesis to
form isocyanate. Heterocumulene metathesis between the formed
isocyanate and another equivalent of complex would give a
metallocycle intermediate (7 or 8) that upon hydrolysis gives
symmetrical urea 3. This mechanism requires 2 equivalents of
TiCl4 to form one equivalent of urea.
analagous 12-electron complexes 2a,
b by removal of a
pyridine ligand under vacuum (Scheme 3), we propose that this
particular cumulene metathesis reaction with CO2 requires a
coordinatively unsaturated 12-electron complex. Further studies
with Ti(NPh(2,6-iPr)2)Cl2Py3 (10b) that tried to intercept reaction
intermediates after treatment with CO2 by the addition of H2O
or EtOH gave no identifiable products. Treatment instead with
NaOEt led to symmetrical urea and addition of BnNH2 led to the
unsymmetrical urea (Scheme 4). The formation of symmetrical
urea from treatment with CO2 and acid workup suggests
hydrolysis of the corresponding carbodiimide 9 or hydrolysis of
an intermediate complex 7 or 8, of which the existence of the
latter is supported by some NMR data from our previous work.7b
Formation of symmetrical urea from treatment with NaOEt
and unsymmetrical urea from treatment with BnNH2 are both
consistent with displacement of the urea like ligand from 7/8
by ethoxide ion and an acyl substitution type mechanism by
the amine at the central urea carbon of the bidentate ligand in
7/8. No products derived from direct reaction with isocyanate
(7)
a
Table 2 Synthesis of ureas with CO2
3
R
Yield urea% 3 equiv RNH2
Yield urea% 1 equiv RNH2
Yield urea% 6 equiv RNH2
Yield ureab% 300 equiv RNH2
a
c
d
e
t-Bu
7
48
56
36
2
16
17
14
26
34
42
19
99
99
99
99
Bn
Ph(CH2)2
Cy
a Isolated yields. b 230 equiv of pyridine used.
This journal is
The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 1334–1338 | 1337
©

5 were detected. We generated the same intermediate 7/8 from
our previous work by the treatment of Ti(NtBu)Cl2Py2 (2a)
with t-BuNCO and treated this mixture with aqueous acid,
ethoxide ion and BnNH2 to repeat the results above (Scheme
5). This is strong evidence for the pathway of this reaction
proceeding through intermediate 7 or 8 or some equilibrating
mixture of the two. NMR experiments showed the t-Bu imido
14-electron complexes 10a to be more reactive towards CO2 than
the analagous 2,6-(i-Pr)2Ph complex 10b (Table 1). The formation
of symmetrical urea from 2a upon treatment with CO2 could be
made nearly quantitative by the addition of bis-silylated amines
(eqn (6)). The exact role of the silylated amines is unknown at
present, but could be due to some Lewis acid effect by the silicon
groups and we are investigating this further. This could represent
a very efficient synthetic method for preparing ureas from an easy
to make complex and CO2. However, at present the generality of
this process was restricted by our inability to prepare pure alkyl
(aside from t-Bu) analogues of the complexes 2 and 10. Using an
in situ preparation from TiCl4 and amines we were able to isolate
symmetrical ureas of alkyl amines (eqn (7), Table 2). Interestingly
the yields were moderate with an optimised 3 equivalents, but
near quantitative with a vast excess of amine. This again is
a curious result we are investigating further. Initial attempts
to make the process catalytic in titanium by converting the
presumed titanium oxo complex back to an imido complex have
proven unsuccessful, but is the subject of our current and future
endeavours.
3 (a) M. R. Kember, A. Buchard and C. K. Williams, Chem. Commun.,
2011, 47, 141; (b) D. J. Darensbourg, Chem. Rev., 2007, 107, 2388;
(c) G. W. Coates and D. R. Moore, Angew. Chem., Int. Ed., 2004, 43,
6618; (d) H. Sugimoto and S. Inoue, J. Polym. Sci., Part A: Polym.
Chem., 2004, 42, 5561; (e) K. Nozaki, Pure Appl. Chem., 2004, 76, 541;
(f) C. Koning, J. Wildeson, R. Parton, B. Plum, P. Steemna and D. J.
Darensbourg, Polymer, 2001, 42, 3995.
4 (a) N. Yamazaki, F. Higashi and T. Iguchi, Tetrahedron Lett., 1974, 13,
1191; (b) A. Lorenzo, E. Aller and P. Molina, Tetrahedron, 2009, 65,
1397; (c) R. Nomura, M. Yamamoto and H. Matsuda, Ind. Eng. Chem.
Res., 1987, 26, 1056.
5 (a) D. C. Bradley, M. B. Hursthouse, K. M. A. Malik, A. J. Nielson and
R. L. Shirt, J. Chem. Soc., Dalton Trans., 1983, 2651; (b) L. K. Johnson,
S. C. Virgil and R. H. Grubbs, J. Am. Chem. Soc., 1990, 112, 5384; (c) A.
Antin˜olo, S. Garc´ı-Lledo´, J. M. Ilarduya and A. Otero, J. Organomet.
Chem., 1987, 335, 85; (d) W. A. Hermann, G. Welchselbaumer, R.
A. Paciello, R. A. Fischer, E. Herdtweck, J. Okuda and D. W. Marz,
Organometallics, 1990, 9, 489; (e) D. D. Devore, J. D. Lichtenhan, F.
Takusagawa and E. A. Maata, J. Am. Chem. Soc., 1987, 109, 7408;
(f) A. J. Nielson, M. W. Glenny, C. E. F. Rickard and J. M. Waters, J.
Chem. Soc., Dalton Trans., 2000, 4569; (g) W. B. Cross, J. C. Anderson,
C. Wilson and A. J. Blake, Inorg. Chem., 2006, 45, 4556.
6 (a) R. E. Blake Jr, D. M. Antonelli, L. Henling, W. P. Schaefer, K. I.
Hardcastle and J. E. Bercaw, Organometallics, 1998, 17, 718; (b) R.
Royo and J. Sa´nchez-Nieves, J. Organomet. Chem., 2000, 597, 61; (c) S.
C. Bart, C. Anthon, F. W. Heinmann, E. Bill, N. M. Edelstein and K.
Meyer, J. Am. Chem. Soc., 2008, 130, 12536.
7 (a) D. Swallow, J. M. McInnes and P. Mountford, J. Chem. Soc., Dalton
Trans., 1998, 2253; (b) A. J. Blake, J. M. McInnes, P. Mountford,
G. I. Nikonov, D. Swallow and D. J. Watkin, J. Chem. Soc., Dalton
Trans., 1999, 379; (c) A. E. Guiducci, A. R. Cowley, M. E. G. Skinner
and P. Mountford, J. Chem. Soc., Dalton Trans., 2001, 1392; (d) S.
R. Dubberley, A. Friedrich, D. A. Willman, P. Mountford and U.
Radius, Chem.–Eur. J., 2003, 9, 3634; (e) C. L. Boyd, T. Toupance, B.
R. Tyrrell, B. J. Ward, C. R. Wilson, A. R. Cowley and P. Mountford,
Organometallics, 2005, 24, 309; (f) C. L. Boyd, E. Clot, A. E. Guiducci
and P. Mountford, Organometallics, 2005, 24, 2347; (g) A. E. Guiducci,
C. L. Boyd and P. Mountford, Organometallics, 2006, 25, 1167; (h) S.
C. Dunn, N. Hazari, A. R. Cowley, J. C. Green and P. Mountford,
Organometallics, 2006, 25, 1755.
8 (a) F. Basuli, B. C. Bailey, J. Tomaszewski, J. C. Huffman and D.
J. Mindiola, J. Am. Chem. Soc., 2003, 125, 6052; (b) F. Basuli, B.
C. Bailey, L. A. Watson, J. Tomaszewski, J. C. Huffman and D. J.
Mindiola, Organometallics, 2005, 24, 1886; (c) U. J. Kilgore, F. Basuli,
J. C. Huffman and D. J. Mindiola, Inorg. Chem., 2006, 45, 487.
9 (a) D. S. Glueck, J. Wu, F. J. Hollander and R. G. Bergman, J. Am.
Chem. Soc., 1991, 113, 2041; (b) S.-H. Hsu, Jr.-C. Chang, C.-L. Lai,
C.-H. Hu, H. M. Lee, G.-H. Lee, S.-M. Peng and J.-H. Huang, Inorg.
Chem., 2004, 43, 6786.
Acknowledgements
We thank the University of Nottingham, University College
London and Flexible Foam Research Ltd for funding, Mr. T.
Hollingworth, Mr. D. Hooper, Mr J. Hill and Dr L. Harris for
providing mass spectra and Dr T. Liu and Ms G. Maxwell for
microanalytical data.
Notes and references
1 (a) H. Arakawa, M. Aresta, J. N. Armour, M. A. Barteau, E. J.
Beckman, A. T. Bell, J. E. Bercaw, C. Creutz, E. Dinjus, D. A. Dixon,
K. Domen, D. L. DuBois, C. A. Eckert, E. Fujita, D. H. Gibson, W.
A. Goddard, D. W. Goodman, J. Keller, G. J. Kubas, H. H. Kung, J. E.
Lyons, L. E. Manzer, T. J. Marks, K. Morokuma, K. M. Nicholas, R.
Periana, L. Que, J. Rostrup-Nielson, W. M. H. Sachtler, L. D. Schmidt,
A. Sen, G. A. Somorjai, P. C. Stair, B. R. Stults and W. Tumas, Chem.
Rev., 2001, 101, 953; (b) M. Aresta and A. Dibenedetto, Dalton Trans.,
2007, 2975; (c) T. Sakakura, J.-C. Choi and H. Yasuda, Chem. Rev.,
2007, 107, 2365; (d) Carbon Dioxide as Chemical Feedstock, ed. M.
Aresta, Wiley-VCH, Weinheim, 2010.
10 (a) P. J. Walsh, F. J. Hollander and R. G. Bergman, Organometallics,
1993, 12, 3705; (b) J. C. Anderson and R. B. Moreno, Tetrahedron,
2010, 66, 9182 and references therein.
11 Some examples are: (a) M. B. Dinger and W. Henderson, Chem.
Commun., 1996, 211; (b) P. Mountford, Chem. Commun., 1997, 2127.
12 A. J. Blake, P. E. Collier, S. C. Dunn, W.-S. Li, P. Mountford and O.
V. Shishkin, J. Chem. Soc., Dalton Trans., 1997, 1549 and references
therein.
13 Retrospectively we think that the increased yield of 3a in this experiment
compared to that in eqn (3) can be explained by the extended reaction
time of 16 h versus 5 h .
14 J. B. Alexander, R. R. Schrock, W. M. Davis, K. C. Hultzsch, A. H.
Hoveyda and J. H. Houser, Organometallics, 2000, 19, 3700.
2 N. MacDowell, N. Florin, A. Buchard, J. Hallett, A. Galindo, G.
Jackson, C. S. Adjiman, C. K. Williams, N. Shah and P. Fennell, Energy
Environ. Sci., 2010, 3, 1645.
1338 | Org. Biomol. Chem., 2012, 10, 1334–1338
This journal is
The Royal Society of Chemistry 2012
©