Organometallics
Article
Varying the concentration of the reaction mixture revealed that
more dilute conditions gave a higher yield in the same reaction
time (Table 1, entries 6 and 7). This observation suggests a
possible bimolecular catalyst deactivation process. The
reaction proceeded to identical conversion in the presence of
one drop of Hg, providing one piece of evidence that catalysis
is likely homogeneous and not attributed to metallic cobalt
nanoparticles (Table 1, entry 4). No hydroboration activity
was observed when either the cobalt precatalyst 1 or the
the yields obtained for substrates with aryl chloride (2e) and
aryl bromide (2f) functionalities are diminished and decrease
in the order F > Cl > Br. The hydroboration of aliphatic
terminal alkenes, such as cyclohexylethylene, 1-hexene, and
allylbenzene, afforded the hydroboration products 2g, 2h, and
2i, respectively, in near-quantitative yield with no evidence for
branched products. Hydroboration also proceeded with the
more sterically hindered aliphatic substrate tert-butylethene,
albeit with a slightly lower yield of product 2j (83%). Chain-
walking was not observed in any of the aforementioned cases.
KBEt H activator was omitted from the reaction mixture
3
(
Table 1, entries 8 and 9). Finally, in a control reaction
The functional group tolerance of the 1/KBEt H catalytic
3
49
system was, however, found to have limitations (Figure 2). The
hydroboration of allyl phenyl ether resulted in only an 8% yield
of 2k under the optimized reaction conditions, and conversion
did not improve with longer reaction times, likely due to
deprotection of the allyl ether under the reaction conditions.
Catalytic hydroboration of 5-hexen-2-one with 1 equiv of
HBPin did not result in hydroboration of the alkene, but
suggested in a recent paper by Thomas and co-workers,
catalysis proceeded in a 96% yield in the presence of TMEDA
Table 1, entry 10) with no evidence for formation of the
(
11
3
S34), ruling out decomposition of HBPin to catalytically active
boron species. Ultimately, the optimized conditions chosen for
the remainder of our studies were 1 mol % 1, 2.1 mol %
KBEt H, in 3 mL of THF (4−6 mM catalyst concentration)
3
for 30 min (Table 1, entry 3).
2
:1 HBPin to ketone ratio, hydroboration of both the ketone
Encouraged by these initial results using styrene, we sought
to extend the substrate scope and investigate the limitations of
this cobalt-catalyzed reaction by using complex 1 as a
precatalyst for the hydroboration of a variety of alkene
substrates (Figure 2). The hydroboration of styrene derivatives
with both electron-withdrawing and electron-donating sub-
stituents in the para position proceeded in >90% yields,
converting to exclusively the anti-Markovnikov product in all
and alkene functional groups occurs, generating a 7:1 mixture
(
not proceed to form any detectable 2m, likely owing to the
ability of the pyridine moiety to coordinate to cobalt and
deactivate catalysis. Amine and ester functional groups were
also shown to interfere with catalysis, as catalytic hydro-
boration of 4-vinylaniline and 4-acetoxystyrene afforded no
conversion to 2o or 2p, respectively. Finally, internal alkenes
such as 2-octene were not amenable to the hydroboration
protocol and hydroboration products such as 2n or 2n′ were
not formed even with extended reaction times.
In comparing the catalytic activity of the 1/KBEt H system
3
to known cobalt hydroboration catalysts, we find that both the
activity and selectivity of the present system are excellent, but
the system is limited compared to other reported catalysts in
the context of functional group tolerance. Comparing the
system with Fout’s (CCC)CoN catalyst, it becomes clear that
2
1
/KBEt H is more active toward sterically hindered substrates,
3
dipp
but ( CCC)CoN is superior in functional group tolerance
2
30
as it tolerates ketone and allyl ether functionalities. Although
operates under similarly mild conditions and with similar
1
reaction times to (PNN)CoCl , the latter catalyst is functional
2
at a much lower catalyst loading (0.05 mol %) and is more
29
functional group tolerant. The activity of precatalyst 1 is
comparable to Chirik’s Co(I) alkyl catalysts, but a notable
difference is the inactivity of 1/KBEt H toward internal
3
16,17
olefins.
To assess whether the catalytic hydroboration activity was
specific to mononuclear (PPP)Co complexes, several addi-
tional complexes were screened as precatalysts for the
I
hydroboration of styrene (Table 2). The symmetric Co
dimer (PPP) Co was found to be inactive as a catalyst/
2
2
precatalyst for alkene hydroboration (Table 2, entries 1 and
43
II
2
). The Co hydride dimer [(PPP)CoH] was found to be
2
inactive as a catalyst for the hydroboration of styrene even
when KBEt H was added (Table 2, entries 3 and 4). On the
3
II
other hand, the chloride-bound Co dimer [(PPP)CoCl]2
does display a moderate catalytic activity when KBEt H is
3
added and produces the linear hydroboration product in a 61%
yield (Table 2, entry 6), implying that some quantity of
catalytically active species can be generated from this complex
Figure 2. Substrate scope for the catalytic hydroboration of alkenes
using 1 as a precatalyst.
1
027
Organometallics 2021, 40, 1025−1031