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Dalton Transactions
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COMMUNICATION
Dalton Transactions
With NHCtBu in THF we observe the direct formation of 3, hydrolysis of the diphosphene. In the latter case, Vwieew sAprtieclceuOlnalitnee
2
in almost 90% spectroscopic yield, along with 5% of 4NHCtBu
that the close proximity of NHC and diphosphene results in an
DOI: 10.1039/C9DT04690A
2
and 5% of 2NHCtBu
Catalytic use of NHCtBu also leads induced polarization of the P=P and hence the NHCs are able
10
2
.
2
quantitatively to the formation of 3. On the other hand with to catalyze the hydrolysis reaction. The enhancement of
NHCDip we observe the formation of 3 (20%) along with reactivity of the diphosphene is catalytic vis-à-vis the carbene.
2
Me
iPr
iPrMe
unreacted diphosphene, 1 (80%), even after two days. With Thus, NHCMe , NHC , NHC , NHC
are catalytic at low
are catalytic at rt.
4
2
2
NHCMes , 1 remains unchanged in reaction with one equivalent temperature while NHCiPr Me , NHC
tBu
2
2
2
2
of water even after 5 days.
Hydrolysis reactions of the native NHCs revealed that while the
The fact that NHCiPr Me does not catalyze the reaction at sterically unhindered carbenes hydrolyze instantly at rt, the
2
2
low temperatures while NHCMe does, raised the question if bulkier carbenes hydrolyze more slowly, providing
a
4
the reactivity of the NHCs with water itself might be straightforward explanation for the differing conditions
detrimental to the catalytic turnover. We therefore necessary for the catalytic turnover. In addition, the NHCs
investigated the hydrolysis reactions of all these NHCs in THF used in this study are those that are commonly used by a
(Scheme 5).10 To our surprise, we found that the hydrolysis of broad-range of chemists. The systematic comparative
Me
NHCiPr Me with respect to NHC
is very slow; even after one hydrolysis study of these NHCs will be important in their
2
2
4
month the reaction is not complete (Table 1). Therefore, choice as reagents/catalysts.
NHCiPr Me can catalyze the reaction at rt. In order to correlate
This project was funded by intramural funds at TIFR
2
2
the relative basicity of the NHCs with their propensity towards Hyderabad from the Department of Atomic Energy (DAE),
hydrolysis, we have estimated their proton affinities (PAs) in Government of India, India and Research Group Linkage
the gas-phase via the Gibbs free energies for the proton Programme (between TIFR Hyderabad, India and Saarland
detachment (PDFE) which supports our experimental University, Germany), AvH Foundation, Germany. S.K.P. and
findings.10
V.C. are thankful to the Department of Science and
Technology, New Delhi, India, for a National J. C. Bose
fellowship. We are thankful to Professor Dr. Biprajit Sarkar,
Freie Universität Berlin, Berlin Germany for the HRMS for
CHO
R'
N
R'
H
R
N
O
R
R
R
N
R'
H2O
+
+
H
N
R
R
N
NH
compound 4·NHCiPr Me . We are grateful to the reviewers for
2
2
R"
R"
R"
their critical insights to improve the quality of the manuscript.
A
B
Scheme 5. Hydrolysis reactions of NHCs.
Conflicts of interest
Table 1. Reaction of different N-heterocyclic carbenes (NHCs) with H2O.
There are no conflicts to declare.
NHCR'R''R
Conversion
Ratio of A and B
Reference
2
SI No.
of NHCR'R''R
2
(%) /
Reaction
Time at
RT10
Notes and references
1
(a) D. Bourissou, O. Guerret, F. P. Gabbaï, G. Bertrand, Chem.
Rev., 2000, 100, 39−91; (b) V. Nesterov, D. Reiter, P. Bag, P.
Frisch, R. Holzner, A. Porzelt, S. Inoue, Chem. Rev. 2018, 118,
9678−9842; (c) A. Doddi, M. Peters, M. Tamm, Chem. Rev.,
2019, 119, 6994−7112.
4
1
2
NHCMe
100 / 1 hr
60:40
46:54
In this study
In this study
NHCiPr
60 / 1
month
100 / 1 hr
Me
2
2
2
2
3
4
N-Heterocyclic Carbenes in Transition Metal Catalysis. Editor,
Glorius, F. Springer, 2007.
D. M. Flanigan, F. Romanov-Michailidis, N. A. White, T. Rovis,
Chem. Rev., 2015, 115, 9307−9387.
(a) J. A. M. Lummiss, C. S. Higman, D. L. Fyson, R. McDonald,
D. E. Fogg, Chem. Sci. 2015, 6, 6739−6746; (b) M. N.
Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature, 2014,
510, 485−496.
3
4
NHCMe
Not able to assigned
the 1H NMR
spectrum
Not able to assigned
the 1H NMR
spectrum
In this study
In this study
NHCiPrMe
100 / 1 hr
2
5
6
7
NHCiPr
100 / 1 hr
100:0
15a
15a
15b
NHCtBu
< 1 / 5 days
0:100
25:75
2
5
6
A. T. Biju, N. Kuhl, F. Glorius, Acc. Chem. Res., 2011, 44,
1182−1195.
NHCDip
57 / 10
days
95 / 5 days
2
(a) K. Leszczyńska, K. Abersfelder, A. Mix, B. Neumann, H.-G.
Stammler, M. J. Cowley, P. Jutzi, D. Scheschkewitz, Angew.
Chem. Int. Ed., 2012, 51, 6785−6788; (b) M. J. Cowley, V.
Huch, H. S. Rzepa, D. Scheschkewitz, Nat. Chem., 2013, 5,
876−879; (c) A. Jana, I. Omlor, V. Huch, H. S. Rzepa, D.
Scheschkewitz, Angew. Chem. Int. Ed., 2014, 53, 9953−9956;
(d) A. Maiti, D. Mandal, I. Omlor, D. Dhara, L. Klemmer, V.
Huch, M. Zimmer, D. Scheschkewitz, A. Jana, Inorg. Chem.,
2019, 58, 4071−4075.
2
8
NHCMes
60:40
15a
In conclusion, we report the influence of different NHCs on
the reaction of water to a diphosphene. The sterically less
encumbered and more basic NHCs bind to the diphosphene
and thus afford adducts, which are in equilibrium with the
diphosphene in solution. The other type of NHCs are sterically
encumbered, less basic and do not bind to the diphosphene.
However, in both cases the presence of NHCs induces
7
D. Dhara, P. Kalita, S. Mondal, R. S. Narayanan, K. R. Mote, V.
Huch, M. Zimmer, C. B. Yildiz, D. Scheschkewitz, V.
Chandrasekhar, A. Jana, Chem. Sci., 2018, 9, 4235−4243.
4 |Dalton Trans., 2016, 00, 1-4
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