Angewandte
Chemie
DOI: 10.1002/anie.201409969
Small-Molecule Trapping Hot Paper
Frustrated Lewis Pair Activation of an N-Sulfinylamine: A Source of
Sulfur Monoxide**
Lauren E. Longobardi, Vanessa Wolter, and Douglas W. Stephan*
Abstract: Inter- and intramolecular P/B frustrated Lewis pairs
are shown to react with an N-sulfinylamine to form PNSOB
linakages. These species can be regarded as phosphinimine–
borane-stabilized sulfur monoxide complexes, and indeed
these species act as sources of SO, effecting the oxidation of
PPh and delivering SO to [RhCl(PPh ) ] and an N-hetero-
3
3 3
cyclic carbene.
R
eactive intermediates are integral to a variety of commonly
used reactions, yet their fleeting existence is often difficult to
[1]
confirm. Sulfur monoxide has been detected in the atmos-
pheres of planets and comets in outer space but is an
unstable compound on earth. Early reports describe the
trapping and characterization of SO in an argon matrix but
it was only first observed only in the early 1930s. Since then,
chemists have exploited transition metal complexes to
stabilize this reactive diatomic molecule, although few
examples are known. The most common strategy for
delivering SO has been extrusion from episulfoxides or
trisulfide oxides, although most methods require elevated
temperatures or suffer from low yields of the SO-trapped
[
2]
[3]
Scheme 1. Reactions of FLPs with SO , PhNCO, and PhNSO.
2
[
4]
[5]
leaving the C=N double bond intact (Scheme 1). Based on
these findings, we were inspired to investigate the chemistry
of analogous systems with N-sulfinylamines (R-N=S=O),
[
6]
because they are isoelectronic with SO , easily prepared in
2
[15]
high yields from the corresponding amine and SOCl2, and
have been shown to be versatile building blocks for a variety
[7]
[16]
products.
More recently, the advent of frustrated Lewis pairs
of more complex products.
Very recently, Erker et al.
+
reported that their Zr /P FLP system reacts with N-sulfinyl-
amines, whereby the phosphine undergoes addition at N, and
the Zr is shown to coordinate both the S and O centers
[
8]
(
FLPs) has prompted the exploration of main group
+
compounds as reagents for activating small molecules.
Indeed, sterically demanding combinations of Lewis acids
[
17]
(Scheme 1).
[
9]
[10]
[11]
and bases have been shown to activate H , CO2, CO,
N O, and more recently, SO2.
2
Initially, a combination of P(tBu) and B(C F ) with one
equivalent of p-TolylÀN=S=O (1; p-Tolyl = 4-methylphenyl)
2
3
6 5 3
[
12]
[13,14]
The groups of Erker and
Stephan reported that the combination of P(tBu)3 and
B(C F ) under an atmosphere of SO add across one of the
6
5
3
2
S=O bonds to yield zwitterionic [(tBu) PÀ(S=O)ÀOÀ
3
B(C F ) ] (Scheme 1). In related work, Erker and co-workers
6
5 3
explored the reactivity of isocyanates with their linked FLP
system, in which P/B was found to add across the C=O bond,
[*] L. E. Longobardi, V. Wolter, Prof. Dr. D. W. Stephan
Department of Chemistry, University of Toronto
80 St. George St, Toronto, Ontario, M5S 3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
Prof. Dr. D. W. Stephan
Chemistry Department, Faculty of Science
King Abdulaziz University
Scheme 2. Synthesis of 2–4.
Jeddah 21589 (Saudi Arabia)
[
**] We gratefully acknowledge the support of NSERC of Canada. D.W.S
is grateful for the award of a Canada Research Chair. L.E.L. gratefully
acknowledges NSERC for a CGS-D scholarship, and the Walter C.
Sumner Foundation for a memorial fellowship. We thank Christo-
pher Caputo for insightful discussions and Michael Boone for X-Ray
crystallography support.
resulted in the formation of a new species 2 (Scheme 2). A
11
sharp singlet in the B NMR spectrum at d = À2.1 ppm and
3
1
1
a singlet at d = 83.8 ppm in the P{ H} NMR spectrum led us
to tentatively formulate product 2 as the FLP adduct,
however the nature of the binding could not be unambigu-
ously determined from spectroscopy. X-Ray diffraction
analysis revealed the structural formulation of 2 to be the
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
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