Technical Library

The nickel-catalyzed thioetherification of alkylsilicates wherein the initially generated alkyl radical converts a thiol to the thiyl radical, which then couples with a (het)aryl bromide was reported by the Molander group. Primary, secondary as well as tertiary thiols were all found to react well. This highly useful reaction was serendipitously discovered when the sp3-sp2 cross-coupling of 3-mercaptopropylsilicate with 4-phenyl-1-bromobenzene was attempted and 4-phenyl-1-propylthiobenzene was isolated rather than the expected 4-phenylphenylpropylthiol. Although this proved to be a useful entry into various propylthioaromatics and propylthioheteroaromatics (19 examples: 0 – 97%), it proved to be more general to employ the isobutylsilicate 9 in conjunction with different thiols to thioalkylate aryl and heteroaryl bromides (Scheme 15).14

Scheme 15

Triethylammonium bis(catecholato)iodomethylsilicate, preparable on multigram scale, reacted under photoredox conditions with olefins to provide the corresponding cyclopropanes in high yields. The reaction conditions avoid many of the undesirable aspects of other cyclopropanation protocols. The method is safe, scalable, and lacks the need for strong bases and low temperatures, and has excellent functional group tolerance.15 An investigation of the role of the halogen in three halomethylsilicates revealed the advantages of the iodomethylsilicate. An initial emphasis was place on the preparation of trifluoromethyl-substituted cyclopropanes due to the interest in trifluoromethyl pharmacons such as the calcium channel blocker ACT-709478 10.16 Under similar conditions the chloromethylsilicate reacted with various olefins to give the cyclopropane in yields ranging from 0 to 92% compared with the iodomethylsilicate with yields of 44 to 98% (Scheme 16).

Scheme 16