Organic Letters
Letter
a
a
Table 1. Effects of Different Reaction Conditions
Table 2. Scope of Different Alkenes (1)
a
1a (0.2 mmol), 2a (0.4 mmol), [Ni] (0.01 mmol), ligand (0.012
mmol), base (0.4 mmol), silane (0.4 mmol), solvent (1.5 mL), at 25
b
°C for 24 h. Yields determined by GC using n-dodecane as the
c
internal standard. Isolated yield.
suggested that other kinds of bases could not afford better
results (Entries 3 and 4, Table 1). Notably, this reaction was
sensitive to the solvent (Entries 5−7, Table 1 and Supporting
Information). After the screening, the mixed solution of
dimethylacetamide (DMA) and CH3CN was selected as the
best choice, which could improve the yield of a desired product
remarkably (Entry 7, Table 1). Then, the effect of different
nickel catalysts was studied. Compared with NiBr2·diglyme,
other kinds of Ni salts induced lower yields (Entries 8−10,
Table 1). Further studies indicated different kinds of ligands
could affect the results of this reaction obviously (Entries 11
and 12, Table 1). Among them, L1 was the best one. Finally,
the condition of entry 7 was chosen as the optimal condition.
With the optimal reaction condition in hand, the substrate
scope of different alkenes was investigated (Table 2). In
general, various alkenyl boronic esters and unactivated boron-
containing alkenes resulted in a diversity of allylboron esters
with modest to good yields and high regioselectivity. As shown
in Table 2, alkenyl boronic ester substrates, which were
installed with chloro, ether, ester, nitrile, and amide groups,
worked very well (4c to 4k, Table 2). The corresponding
products of these starting materials could be used for further
transformations. Besides 4,4,5,5-tetramethyl-1,3,2-dioxaboro-
nate-type alkenyl boronic esters, further studies indicated that
4,4,6,6-tetramethyl-1,3,2-dioxaboronate and 2,3-dihydro-1H-
naphtho[1,8-de][1,3,2]diazaboronate version substrates still
worked well (4l and 4m, Table 2). Importantly, unactivated
terminal and internal boron-containing could go through the
chain-walking-type reaction to afford the desired allylboron
esters (4o to 4r, Table 2).14
a
Standard conditions: 1 (0.2 mmol), 2a (0.4 mmol), Ni Br2·diglyme
(0.01 mmol), L1 (0.012 mmol), KF (0.4 mmol), (EtO)3SiH (0.4
b
mmol), DMA/CH3CN (30:1) (1.5 mL), at 25 °C for 24 h. Isolated
yield. The detailed oxidation process is shown in the Supporting
c
d
both alkenyl triflates and alkenyl bromide substrates proceeded
well with excellent regioselectivity (Table 3). Cyclic alkenyl
triflates with -Me, -F, -Ph, or -tBu could be tolerated with this
catalyzed reaction condition (5b to 5e, Table 3). Notably,
alkenyl bromides could be compatible with the reaction, and
starting materials with ortho-, meta-, or para-substituents at the
arene group could be converted into the desired product
smoothly (5f to 5i, Table 3). Further studies suggested the
benzene ring with an electron-donating group or an electron-
withdrawing group did not affect the efficiency of this
transformation (5j to 5m, Table 3). Importantly, the pyridine,
furan, and naphthyl motifs could survive (5n to 5p, Table 3).
The potentiality for a pharmaceutical molecule modification of
this reaction was well-demonstrated by these results.
The substrate scope of the nickel-catalyzed highly selective
hydroalkenylation transformation was further expanded to a
variety of alkenyl electrophiles 2. These results indicated that
B
Org. Lett. XXXX, XXX, XXX−XXX