Advancement in Cascade [1,n]-Hydrogen Transfer/Cyclization: A Method for Direct Functionalization of Inactive C(sp3)-H Bonds

The cascade [1,n]-hydrogen transfer/cyclization, recognized one century ago, has received considerable interest in recent decades and great achievements have been made. This cascade process can functionalize C(sp(3))-H bonds directly into C-C, C-N, C-O bonds under the catalysis by Lewis acids, BrOnsted acids or organocatalysts, and even under thermal conditions. This methodology has shown preeminent power to construct 5- or 6-membered heterocyclic as well as all-carbon rings. In this review, various hydrogen donors and hydrogen acceptors are categorized and discussed.