Triphenylphosphine-Catalyzed Alkylative Iododecarboxylation with Lithium Iodide under Visible Light

Abstract

Photoactivation of an electron donor-acceptor encounter complex in an organic solvent cage, a phenomenon that has been described in Mulliken theory, has been known for decades, but it has not been employed as a photoactivation step in the design of photocatalysis for organic synthesis until recent years. We report herein an iododecarboxylation reaction that applies this concept for photoactivation by using a catalyst to facilitate electron transfer and to suppress back electron transfer in the photoexcited state. Under irradiation of 456 nm blue light-emitting diodes, PPh3 catalyzes the iododecarboxylation of aliphatic carboxylic acid-derived N-(acyloxy)phthalimide with lithium iodide as iodine source. The reaction delivers primary, secondary, and bridgehead tertiary alkyl iodides in acetone solvent, and the alkyl iodide products were easily used to generate C-N, CO , C-F, and C-S bonds to allow various decarboxylative transformations without using transition-metal or organic dye-based photocatalysts. This protocol is applicable to redox-active esters derived from various natural products and pharmaceuticals.

Decarboxylation‐Initiated Intermolecular Carbon‐Heteroatom Bond Formation

“…The Barton‐type approach thus allows photochemical decarboxylations under mild conditions. The photochemical decarboxylative iodination of NHPI esters was independently disclosed by the groups of Chen [193] and Shang [194] (Scheme 17). Chen's procedure is enabled by N ‐heterocyclic carbene (NHC) catalysis with NaI as the iodine donator whereas Shang's protocol involves the use of PPh 3 in the presence of LiI [193] .…”

“…Notably, Chen's method works well for the synthesis of secondary alkyl iodides whereas the use of primary NHPI esters leads to competing formation of C( sp 3 )−(N)phth bonds by follow‐up reactions of the intermediately formed iodoalkanes. In this regard, the PPh 3 /LiI system seems advantageous since it gives access to aliphatic iodides even starting from complex molecules [194] . Both strategies are proposed to proceed via photoactivation of a tricomponent electron donor‐acceptor (EDA) complex leading to rapid decarboxylation and recombination of the alkyl radical with NHC/PPh 3 ‐stabilized iodine radicals to produce alkyl iodides.…”

Synthesis of Cyclic Anhydrides via Ligand‐Enabled C–H Carbonylation of Simple Aliphatic Acids

“…β‐Alkynyl ester 4 d was directly accessible in a synthetically useful yield (39 %) through a mild decarboxylative alkynylation with more general and diverse alkynyl Grignard reagents [16e] . For C−Y (Y=heteroatom) bond‐forming reactions, boryl (B(pin), 4 e ), [16f] silyl ( 4 f ), [16g] and iodo ( 4 g ) [16h] groups could be efficiently installed at the β position of the parent acid in modest yields (39–43 %), producing versatile linchpins for further elaboration. The CH 2 F fragment, a highly sought‐after bioisostere in medicinal chemistry, could be successfully introduced by decarboxylative fluorination, albeit in low yield ( 4 h , 11 %) [16i] .…”

Phosphorus Compounds as Precursors and Catalysts for Radical C−C Bond‐Forming Reactions

“…Very recently, Shang and co‐workers have demonstrated a decarboxylative iodination of phthalimide esters ( 137 ) using PPh 3 and LiI (Scheme 30). [59] This reaction shows a broad substrate scope, and can be used to generate primary, secondary and tertiary alkyl iodides. Attempts to synthesis the analogous alkyl chlorides and bromides from LiCl/Br were unsuccessful.…”