Journal of the American Chemical Society
Communication
Figure 4. aReaction conditions: alkyl iodides (0.6 mmol), B2cat2 (4.0 equiv), nBu4NBF4 (0.3 mmol), DMF (6.0 mL), Mg plate anode and carbon
cloth cathode, undivided cell, constant current I = 200 mA, 2.5 F/mol, 12 min, room temperature; pinacol (8.0 equiv), Et3N (1.5 mL), 1 h. Isolated
yields are reported. bReaction conditions: alkyl chloride (0.6 mmol), B2cat2 (4.0 equiv), LiClO4 (0.3 mmol), DMAc:TPPA (3:1, 6.0 mL), Mg plate
anode and carbon cloth cathode, undivided cell, constant current I = 60 mA, 4.0 F/mol, 64 min, room temperature; pinacol (8.0 equiv), Et3N (1.5
c
d
mL), 1 h. Isolated yields are reported. 30 mA, 128 min. 20 mA, 193 min.
preferentially, which could mediate electron transfer between
the cathode and alkyl halides.
chloride was obtained (32). Additionally, Polyboron com-
pounds have emerged as versatile reagents, but straightforward
approaches to their synthesis are limited.9 We describe herein
the preparation of various diborylated and triborylated
compounds by employing dibromides or alkene-bearing
monobromides (33−39), respectively.
Subsequently, after extensive evaluation of key parameters
including solvent, electrode, and electrolyte, we discovered that
the desired alkylboron product 1 can be obtained in 83% yield
using magnesium as the anode and carbon cloth as the cathode
(Table 1, entry 1). The cell voltage in this condition is only
around 1.1 V, which implies that a higher current can be
applied for this electroreductive process. We found that the
current can increase even further, from 10 mA to 200 mA, with
only a slight reduction in the product yield (entries 3 and 4),
highlighting the efficient electron transfer between the cathode
and substrate. Even so, cathodic electron transfer is still the
rate-determining step under these conditions. This reaction
could also work well when open to the air (cap removed, entry
5); however, water has a detrimental impact on the reaction
efficiency (entry 6). Finally, electricity was proved to be
essential for this transformation, as no product was detected in
its absence (entry 7). As high current densities are particularly
important for chemical throughput, therefore, this method
provides a fast and practical approach for the synthesis of
alkylborons.
With the optimized reaction conditions in hand, the scope of
alkyl bromides was investigated. As shown in Figure 3, a wide
range of primary, secondary, and tertiary alkyl bromides were
viable in this reaction, furnishing the desired alkyl boronic
esters (1−31) in moderate to excellent yields. Of note, a broad
range of functional groups, including acetal (5), boronate ester
(6), halides (7, 15, 16), trifluoromethyl group (8), esters (9,
10), cyanide (11), arenes and heterocycles (11−23), ethers
(17−20), and carbamate (30), were all compatible with this
transformation, which demonstrates the robustness of this
electrochemical protocol. Both monocyclic and polycyclic
bromides were also transformed efficiently into alkyl boronic
esters in good yields (26−31). Notably, the highly chemo-
selective borylation of an alkyl bromide in the presence of a
We next extended this borylation reaction to alkyl iodides.
This electroreductive borylation occurs with a high preference
for the C−I bond in the presence of various reducible
functional groups, including chloro- (42), alkynyl- (43),
ketone- (44), ester- (45), and nitrile- (46) (Figure 4); the
corresponding products were isolated in moderate to excellent
yields (41%−89% yields). The representative heterocycles
thiophene and carbazole, which usually are sensitive to
oxidative conditions, remained intact after reaction, affording
the desired products 50 and 51 in 76% and 90% yield,
respectively. Cyclic and acyclic secondary iodides were
borylated in 45%−78% yields (1, 26, 52−54). Tertiary alkyl
iodides were also amenable to this protocol, as exemplified by
1-iodoadamantane, which gave product 31 in 87% yield. As an
example of the application of the protocol to diiodides, 1,3-
diiodopropane delivered the diborylated product 6 in 40%
yield.
Encouraged by these results, we further attempted to apply
our developed approach to unactivated alkyl chlorides. More
inert and challenging unactivated alkyl chlorides could also be
boronated in acceptable yields (55−60). Silyl groups (56), aryl
fluoride (58), aryl ether (59), and carbazole (60) examples
were well tolerated. Compared with bromides and iodides,
organochlorides offer the following advantages: (1) abundant
and diverse structures in both commercial and natural sources;
(2) reduced toxicity compared with most available electro-
philes; (3) low sourcing and production costs on a large scale;
and (4) reasonable chemical stability in multistep sequences.10
The synthetic application of our developed method was
further demonstrated by the late-stage borylation of a series of
12987
J. Am. Chem. Soc. 2021, 143, 12985−12991