With the optimal reaction conditions, we sought to investigate the substrate scope of this cobalt-catalyzed reductive alkynylation reaction. As shown in
Scheme 1, a broad scope of alkyl, alkenyl and aryl electrophiles was examined, providing moderate-to-good cross-coupling yields. Concerning alkyl electrophiles, both primary (
3, 7~
26) and cyclic or acyclic secondary (
27~
31) alkyl iodides were suitable substrates. 1-Iodoadamantane (
32), as an example of a tertiary alkyl halide with certain particularities, was also well suited. However, 2-iodo-2-methylpropane (
33) could not be converted to the desired product because of
β-H elimination side reactions. A variety of functional groups was accommodated under mild reductive cross-coupling reaction conditions, such as ether (
3, 7), ester (
8~
10), aryl fluoride (
11, 12), aryl chloride (
13, 14), trifluoromethyl (
15), cyano (
16), phosphonate (
17), sulfone (
18) and carbamate (
27, 28) groups. This reaction exhibited good compatibility with many synthetically valuable heterocyclic compounds. For example, phthalimide (
20), difluorobenzodioxole (
21), indole (
22) and thiophene (
23) were compatible during the coupling process. Concerning C(sp
2) electrophiles, alkenyl bromide (
41, 48), aryl iodides (
44~
47, 49), aryl bromides (
44, 46), and aryl triflate (
50) coupling partners all gave rise to the target products; the coupling efficiency of aryl bromides was a slightly lower than that of the corresponding aryl iodides. We also examined acyclic alkenyl bromides. A mixture of (
E)-(2-bromovinyl)benzene and (
Z)-(2-bromovinyl)benzene was selected as a representative example. This mixture was successfully converted to the corresponding (
E)-enyne product
43, with the recovery of (
Z)-alkenyl bromide. Alkynyl bromide containing other substituent groups rather than the silyl group could also be used, comprising aryl (
34) and alkyl (
35) substituted alkynyl bromides. The reductive C-C cross-coupling process between two electrophiles showed good chemoselectivity to many reactive sites. For instance, the alkenyl group (
19) posed no problem without any hydrogenation or hydroalkylation byproducts observed. Without strong bases, the amide group (
46) possessing a N-H bond was retained without significant
N-alkylation side reactions. In the example of 1-bromo-4-iodobenzene (
45), a phenylacetylene derivative carrying an aryl bromide group was obtained [
68]. Due to the exemplary functional group tolerance, this reaction has the applicability to introduce alkyne fragments into complex molecules, such as multiply-functionalized drug molecules or natural products. Moderate-to-good yields were attainable in the reactions with derivatives of camphanic acid (
36), isoxepac (
37), acefylline (
38), indomethacin (
39), tadalafil (
40) and probenecid (
49), underscoring the practicability of this method in the modification of biologically active molecules [
69,
70].