Apart from benzylic and propargylic fluorides, acyl fluorides that display a good balance of highly electrophilic reactivity and stability are also popular substrates for atom-economical C-F insertion reactions as the corresponding fluoroaroylation products are useful for drug discovery [
35,
36]. In 2020, Tobisu's group reported an elegant phosphine-catalyzed intermolecular acylfluorination of alkyonates
28, which provided rapid and straightforward access to a variety of highly functionalized monofluoroalkene derivatives
30 (
Scheme 7) [
37]. This transition metal free protocol operates at room temperature, thus permitting a good functional group tolerance. For example, a broad range of acyl fluorides
29 bearing electron-neutral, electron-deficient, or electron-rich groups readily participated in this reaction with (hetero)aryl-substituted alkynoates
28 to produce the corresponding monofluoroalkenes
30 in moderate to excellent yields (30%-94%). However, the alkyl-substituted alkynoates
28 were not compatible, which constituted the limitation of this method. It should be noted that the stereoselectivity of the reaction is substrate dependent. While most products were obtained as an inseparable
E and
Z isomers (1:1-1:1.6), the use of a 2-pyridyl alkynoate led to excellent
Z:
E selectivity (96:4 ~ > 99:1). This is possibly due to the stabilizing n-
π* interaction between the nitrogen lone pair of pyridyl ring and C=O
π* orbital. Based on the control experimental results and density functional theory (DFT) calculations, the authors proposed that the reaction might occur
via the addition of tertiary phosphines to alkyonates followed by the nucleophilic substitution with an acyl fluoride to generate the key P(V) intermediate
Int-Ⅱ, which was then underwent an unprecedented C-F bond-forming ligand coupling to afford the products
30 and release the catalyst.