Inorganic Chemistry
Communication
decomposition of terminal cyclopropene products during the
concentration and workup of the reaction mixture rather than
with the cyclopropenation step. In fact, a reaction of
weak and strong CO···Cu bonds involving the ester moiety,
in the two systems. The CO stretch of [(CF ) Tp]Cu(Cyp-
3
2
−
1
2) in IR displays a 76 cm reduction from that of the free
Cyp-2 due to this latter O···Cu interaction, while the
[
(CF ) Bp]Cu(NCMe) with independently prepared Cyp-7
3 2
and Cyp-8 in CH Cl indicated the decomposition of these
corresponding lowering in [(CF
3
)
2
Bp]Cu(Cyp-2) is only 34
Bp]Cu(Cyp-3) and [(CF Bp]-
Cu(Cyp-4) also show a reduction in C=O stretch by 23 and 28
2
2
−
1
cyclopropenes to yet unidentified products, even above −50
C. Copper-mediated cyclopropene ring-opening as well as
carbometalation chemistry is well-known, and the products
cm (see ESI). The [(CF
)
3
2
)
3 2
°
−
1
cm , respectively, relative to the corresponding value in their
free cyclopropenes.
30,32−36
depend on the nature of the copper source.
Thus, on
the basis of these observations, we improved the procedure by
using a slightly larger ratio of alkyne:EDA followed by
[
(CF ) Bp]Cu(3‐hexyne) + Cyp‐2
3 2
treatment with H S to deactivate the catalyst before the
workup of the reaction mixture, which led to Cyp-7 and Cyp-8
in 56% and 37% isolated yields, respectively.
2
F [(CF ) Bp]Cu(Cyp‐2) + 3‐hexyne
Kc = 0.12
Kc = 9.1
3
2
(
(
1)
2)
Challenges with the cyclopropenation process of terminal
[
(CF ) Bp]Cu(Cyp‐2) + 3‐hexyne
3
2
alkynes using [(CF ) Bp]Cu(NCMe) prompted us to test the
3
2
more sterically demanding tris(pyrazolyl)borate catalyst
F [(CF ) Bp]Cu(3‐hexyne) + Cyp‐2
3
2
58
[
(CF ) Tp]Cu(NCMe). Gratifyingly, it produced Cyp-8 in
3 2
excellent, 93% yield. It could be that the resulting Cyp-8 is less
prone to decomposition by [(CF ) Tp]Cu(NCMe) due to the
steric crowding at the copper. Interestingly, the cyclo-
3
2
[
(CF ) Bp]Cu(NCMe) + Cyp‐2
3
2
propenation of the internal alkyne, 3-hexyne by [(CF ) Tp]-
F [(CF ) Bp]Cu(Cyp‐2) + MeCN
Kc = 11.0
3
2
3 2
Cu(NCMe) was somewhat less effective relative [(CF ) Bp]-
3
2
(3)
Cu(NCMe) under the same conditions (NMR yields of Cyp-
: 61% and 71% respectively for the two catalysts), perhaps
because of the steric effects. Nevertheless, the differential
reactivity is a useful observation and provides opportunities for
further catalyst optimizations.
Considering that copper-catalyzed cyclopropenation of
alkynes producing cyclopropenes likely involve several ligand
exchanges, we also assessed the relative binding affinities of
MeCN, Cyp-2 and 3-hexyne with [(CF ) Bp]Cu using isolable
trations were measured using NMR spectroscopy at various
2
3
2
3
2
Next, we investigated the coordination chemistry of
[
(CF ) Tp]Cu with cyclopropenes. The in situ generated
3 2
[
(CF ) Tp]Cu with Cyp-2 afforded a 1:1 cyclopropene
3
2
temperatures. The equilibrium constants K for eq 1 and the
c
complex of copper(I), [(CF ) Tp]Cu(Cyp-2) in 59% yield
(
3
2
1
3
1
control experiment noted in eq 2 (to ensure equilibrium was
achieved under reverse conditions) are 0.12 and 9.1,
respectively, at 243 K, which are consistent for a system in
equilibrium for the forward and reverse directions. The data
indicate that the alkyne preferentially binds to copper(I) over
the cyclopropene, despite the predisposition of strained
alkenes for metal ion coordination. Both the precursor alkyne
and product cyclopropene also have a greater affinity to
Scheme 1). The C{ H} NMR data of [(CF ) Tp]Cu(Cyp-
3 2
2
(
1.6 ppm, Table S1), in contrast to [(CF ) Bp]Cu(Cyp-2),
3 2
indicating less involvement of the olefinic moiety in the adduct
formation. Indeed, the X-ray crystal structure of [(CF ) Tp]-
3
2
Cu(Cyp-2) revealed it to be a solely O-bonded Cyp-2 via the
2
ester group rather than an η -cyclopropene complex (Figure
2
). A comparison of metrical parameters of [(CF ) Tp]Cu-
3 2
[
(CF ) Bp]Cu fragment than acetonitrile. For example, K for
3
2
c
(
Cyp-2) to [(CF ) Bp]Cu(Cyp-2) summarized in Table 1
3 2
eq 3 is 11.0 at 243 K.
nicely illustrates the effect of Cu(I) on the cyclopropene CC
In summary, we have isolated for the first time, a group of
copper η -cyclopropene complexes using a highly fluorinated,
2
bis(pyrazolyl)borate auxiliary ligand, and a Cu−O bonded
linkage isomer using a bulkier tris(pyrazolyl)borate ligand
support, and characterized them using multiple methods
including X-ray crystallography. The cyclopropenes used in
this work as well as several others were also obtained in
reasonable to excellent yields by the copper catalyzed
cyclopropenation process involving the same ligand supports.
We are currently probing these interesting complexes more
deeply and the chemistry of cyclopropenes with other
important metal ions.
ASSOCIATED CONTENT
sı Supporting Information
■
*
Materials, methods, catalysis, spectroscopic and struc-
Figure 2. Molecular structure of tris(pyrazolyl)borate ligand
supported [(CF ) Tp]Cu(Cyp-2).
3
2
D
Inorg. Chem. XXXX, XXX, XXX−XXX
Noonikara-Poyil, Anurag
