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part (entries 12–13, Table 1). The classic fashion of forming copper
hydride using tBuOK as catalytic additive was compared with
LiNPtBu3 (1b) as co-catalyst. 83% NMR yield of desired product
could be obtained, however, the hydrogenation product was also
observed with 8% in the crude reaction mixture (entry 14, Table 1).
The hydrogenation product can also be obtained when SIMes and
tBu3P were used as ligand, with 15% and 10% NMR yield respec-
tively (entry 2 and entry 11). Therefore, the precursor SIPrCuCl
(c9) was chosen as suitable partner for phosphoranimide (1b) with
89% of isolated yield for the next study of substrate scope (entry 9,
Table 1).
metric reactions of [NHC]CuCl ([NHC] = IPr or SIPr) with LiNPtBu3
(1b) proceeded in dry THF instead of benzene due to the poor sol-
ubility of lithium salt (1b). Fortunately, the NHC coordinated cop-
per(I) phosphoranimide complexes were obtained as colourless
solids (3, 4) with good isolated yields (Fig. 1). The solubility of
these copper(I) complexes is extremely outstanding in hydrocar-
bon solvent such as pentane and hexane at room temperature.
Complexes 3 and 4 were characterized by NMR, elemental analysis
and X-ray crystallography. The latter technique revealed the
monometallic, two-coordinate nature of 3 and 4 (structures shown
in Fig. 1 with selected bond lengths and angles).
The phosphoranimide ligand in complex 3 and 4 approached
closer (Cu–N 1.78 Å) to the metal centre than in LiNPtBu3 (1b)13
(Li–N = 2.03 Å). The coordination geometry at Cu is close to linear:
the donor atoms of the NHC and phosphoranimide bind the Cu cen-
tre with a C(1)–Cu(1)–N(3) angle of 177–179°, which is similar to
IPrCuOtBu reported by the group of Prof. Sadighi.14 The cone angle
is a little bit smaller than the corresponding titanium complex,15
which could be rationalized by the steric repulsion between the
Substrates scope
Several terminal alkynes were tested with the co-catalyst of
LiNPtBu3 (1b) and SIPrCuCl (c9) (Table 2). A variety of terminal aryl
alkynes can undergo b-selective hydroboration to form the desired
vinylboronates, showing good tolerance for substituents on the
phenyl ring with yields of 74–99% (2b–2g, 2k). The alkyl-substi-
tuted vinylboronates can also be obtained under these reaction
conditions (2h–2i, 2l). An intramolecular olefin moiety remains
intact under this reaction conditions (2j, Table 2); ester and toslate
protected amide substrates can also undergo hydroboration to
obtain the corresponding products (2m and 2o13, Table 2).
t
NHC moiety and Bu group on phosphorus atom.
In order to examine the catalytic activity of the isolated copper
complexes, the hydroboration reaction of diphenylacetylene as
bulky substrate was also tried using complex 4 as catalyst to afford
the cis-vinylborate 2q with 78% isolated yield (Eq. (1)).16
Double hydroboration can be achieved as well by using diyne as
starting material to afford diboronate compound 2p with 81% iso-
lated yield at slightly higher temperature. The trans geometry of
C@C was confirmed by X-ray crystallography (see SI for full detail).
(1)
Preparation of copper(I) complex
With the optimized catalyst combination in hand, we
attempted to isolate the catalytically active species. The stoichio-
Preliminary mechanistic study was undertaken through a deu-
terium labeling experiment and NMR study of a stoichiometric
reaction. The terminal proton of the alkyne was kept intact during
the hydroboration process, which was verified by the deuterated
experiment using complex 4 as catalyst to afford the 82% isolated
yield of deuterated product 2r with 99% of deuterium retention
(Scheme 3). Stoichiometric reaction of complex SIPrCuNPtBu (4)
with 1 eq. of HB(pin) was studied by NMR (see SI for full detail).
SIPrCuH (5) could be observed from the 1H NMR spectrum with
the chemical shift of the copper hydride d = 1.93 ppm.5g This trans-
formation was not clean after 10 h and the hydride peak will disap-
pear as time extended. The by-product 6 can be observed as well
by 31P NMR analysis with chemical shift d = 39.5 ppm and its iden-
tity was confirmed by adding another 1 eq. of compound 6 synthe-
sized according to literature3 into the reaction mixture (Scheme 3).
Based on the experiments above, we proposed the catalytic
cycle for the reaction of alkyne hydroboration as below: the copper
hydride intermediate 5 can be obtained by the reaction of catalyst
4 with coordinated HB(pin), then the cis-insertion will occur to
give the vinylcopper species 10 followed by the quenching step of
reacting with HB(pin) to give the final b-selective vinylboronates
2 and regenerate SIPrCuH 5 (Scheme 4).7a,13
Table 2
Substrates scope of hydroboration reaction for terminal alkyne.a
In summary, we designed and synthesized the highly hydrocar-
bon soluble phosphoranimide [NHC]copper(I) complexes as the
effective catalyst for the alkyne hydroboration reaction. The stoi-
chiometric studies indicate that the copper hydride species could
be formed when met with organoborane compound. This demon-
strates a new way of forming the important metal hydride species.
The specification of the phosphoranimide ligand could be utilized
as a starting point into other base metals in periodic table to
develop a new generation of catalysts for organic transformations.
Some of the projects are being conducted in our laboratory.
a1 (0.20 mmol), HBpin (0.22 mmol, 1.1 eq.), SIPrCuCl (0.010 mmol, 5.0 mol%), 1b
(0.010 mmol, 5.0 mol%), benzene (2 mL, 0.1 M) at rt, for 13 h.
bThe reaction was conducted at 35 °C; Ts = 4-methyl-benzenesulfonyl.