The first visible light-induced Pd-catalyzed Heck reaction of α-heteroatom substituted alkyl iodides and -bromides with vinyl arenes/heteroarenes has been developed. This transformation efficiently proceeds at room temperature and enables synthesis of valuable functionalized allylic systems, such as allylic silanes, boronates, germanes, stannanes, pivalates, phosphonates, phthalimides, and tosylates from the corresponding α-substituted methyl iodides. Notably, synthesis of the latter substrates failed under existing thermally induced Pd-catalyzed conditions, which highlights the importance of visible light for this transformation.
Palladium catalysis induced by visible light is an emerging field of catalysis. In contrast to classical reactions catalyzed by Pd complexes in the ground state, which mostly proceed through two‐electron redox processes, the mechanisms of these new methods based on photoexcited Pd complexes usually operate through transfer of a single electron. Such processes lead to putative hybrid Pd/radical species, which exhibit both radical and classical Pd‐type reactivity. This Minireview highlights the recent progress in this rapidly growing area.
A photoinduced palladium-catalyzed 1,2-carbofunctionalization of
conjugated dienes has been developed. This mild modular approach,
which does not require employment of exogeneous photosensitizers and
external oxidants, allows for efficient and highly regio- and stereoselective
synthesis of a broad range of allylic amines from readily available
1,3-dienes, alkyl iodides, and amines. Employment of O- and C-nucleophiles
toward oxyalkylation and dialkylation products was also demonstrated.
A putative π-allyl palladium radical-polar crossover path is
proposed as a key event in this three-component coupling process.
The utility of this protocol is highlighted by its application for
derivatization of several amine-containing drugs.
The efficient Pd-catalyzed Heck reaction of diverse tertiary alkyl halides with alkenes has been developed. Unactivated tertiary alkyl halides efficiently react at room temperature under visible light irradiation with no exogenous photosensitizers required. For activated tertiary alkyl halides, the same catalytic system works well without light. These methods offer a general access to electronically diverse alkenes possessing quaternary and functionalized tertiary allylic carbon centers. The substituents at these centers include alkyl-, carbalkoxy-, tosyl-, phosphonyl-, and boronate groups. It was also shown that the end-game mechanism of this transformation may vary depending on the type of the substrates used.
The rhodium-catalyzed transannulation reaction between 1,2,3-thiadiazoles and alkynes, proceeding via intermediacy of the previously unknown Rh thiavinyl carbene, toward a highly efficient and regioselective synthesis of up to fully substituted thiophenes is described.
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