Cu(I)-Catalyzed 6-endo-dig Cyclization of Terminal Alkynes, 2-Bromoaryl Ketones, and Amides toward 1-Naphthylamines: Applications and Photophysical Properties

Functional group substituted 1-naphthylamines, especially N-methylated ones, play important roles in numerous chemical and biological processes. However, these compounds' general and step economic syntheses are highly limited, which seriously restricts efforts to improve the properties and develop new functions for this kind of compound. In this report, we describe the development of an efficient, convenient, and general method for the synthesis of valuable functionalized 1-naphthylamines directly from readily available terminal alkynes, 2-bromoaryl ketones, and amides via Cu(I)-catalyzed benzannulation in a green solvent (i.e., water) under Pd- and ligand-free conditions. A total of 82 functionalized 1-naphthylamines, especially synthetically and biologically useful N-methylated compounds, are synthesized in isolated yields up to 95%. Some unique features of the reaction are as follows: (1) exclusive 6-endo-dig selectivity, (2) ready incorporation of a broad range of functional groups directly from easily available substrates, and (3) amides that can be used as aminating agents and that are excellent alternatives to toxic and/or odorous amines. Due to facile tuning of functional groups for the reaction, the products possess good electronic donor-acceptor structures and exhibit intriguing photophysical properties, such as tunable and polarity-sensitive fluorescence emission and large Stokes shifts (up to 258 nm). Utilizing the products' unique polarity sensitive fluorescence response, we successfully applied the 1-naphthylamine derivatives, such as compound 91, to image lipid droplets (LDs) and monitor cellular LDs growth. The previously mentioned advantages of this methodology, along with the mild conditions, simple operation, and scalable synthesis, may allow this novel reaction to be extended to varied applications in chemistry, biology, and materials science.