Palladium-catalyzed benzylic C(sp3)–H arylation of o-alkylbenzaldehydes

ortho-C(sp3)-H arylation of aromatic aldehydes using 2-amino-N-methyl-acetamide as a L,L-type transient directing group

Pd-catalyzed ortho-C(sp³)-H arylation of aromatic aldehydes using 2-amino-N-methyl-acetamide as a simple, efficient and commercially available L,L-type transient directing group (TDG) is reported. The reaction exhibited excellent substrate compatibility and generated the desired products in moderate-to-high yields up to 78%. Further acid-catalyzed cyclization and dehydrative aromatization were also tested, and furnished some polycyclic aromatic hydrocarbons with excellent yields up to 96%. The X-ray crystal structure of a 2-methylbenzaldehyde ortho-C(sp³)-H palladation intermediate was obtained. Then, a plausible reaction mechanism involving the formation of a [5,6]-fused palladacycle was proposed. This approach offers valuable insights for exploiting novel L,L-type TDGs.

Transient Directing Groups in Metal-Organic Cooperative Catalysis

The direct functionalization of C-H bonds is among the most fundamental chemical transformations in organic synthesis. However, when the innate reactivity of the substrate cannot be utilized for the functionalization of a given single C-H bond, this selective C-H bond functionalization mostly relies on the use of directing groups that allow bringing the catalyst in close proximity to the C-H bond to be activated and these directing groups need to be installed before and cleaved after the transformation, which involves two additional undesired synthetic operations. These additional steps dramatically reduce the overall impact and the attractiveness of C-H bond functionalization techniques since classical approaches based on substrate pre-functionalization are sometimes still more straightforward and appealing. During the past decade, a different approach involving both the in situ installation and removal of the directing group, which can then often be used in a catalytic manner, has emerged: the transient directing group strategy. In addition to its innovative character, this strategy has brought C-H bond functionalization to an unprecedented level of usefulness and has enabled the development of remarkably efficient processes for the direct and selective introduction of functional groups onto both aromatic and aliphatic substrates. The processes unlocked by the development of these transient directing groups will be comprehensively overviewed in this review article.