Letter Cite This: Org. Lett. 2018, 20, 1261−1264
pubs.acs.org/OrgLett
PONy Dyes: Direct Addition of P(III) Nucleophiles to Organic Fluorophores Alexey N. Butkevich,*,† Maksim V. Sednev,‡,† Heydar Shojaei, Vladimir N. Belov,* and Stefan W. Hell Department of NanoBiophotonics, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, Germany S Supporting Information *
ABSTRACT: Nucleophilic addition of phosphinic acid, phosphites, sodium dialkyl phosphites, phosphoramidites, phosphinites, and phosphonites to highly polarized or cationic fluorophores, followed by oxidation, results in new “PONy” dyes with auxochromic phosphinate, phosphonate, or phosphonamidate groups. The reaction was applied to a wide variety of coumarins, (thio)pyronins, and Nalkylacridinium and 5,6-dihydrobenzo[c]xanthen-12-ium salts as well as a meso-chlorinated BODIPY to provide compact dyes with redshifted absorption and emission bands and Stokes shifts up to 8200 cm−1. hile searching for new fluorophores suitable for nanoscale imaging of intracellular targets,1 we noticed that various organic dyes with electrophilic (cationic or highly polarized neutral) conjugated systems reacted with nucleophilic phosphorus(III) reagents such as phosphinic acid (H3PO2), phosphinites [ROPR′2], phosphonites [(RO)2PR′], phosphites [(RO)3P], or phosphoramidites [(RO)2PNR′2] (Scheme 1) to form phosphonylated leuco bases.2 Upon oxidation, these intermediates provided new fluorophores with red-shifted absorption and emission maxima and increased Stokes shifts as compared to the precursor dyes. The phosphonylated fluorophores were deemed particularly attractive due to their low molecular mass (typically MW < 500) and orange to near-
W
infrared emission. Due to their compact structures and the presence of phosphorus (P), oxygen (O), and nitrogen (N) atoms in close proximity, we nicknamed these compounds “PONy dyes”. These dyes can be prepared in cationic or zwitterionic forms, making them promising candidates for future development of cell-permeant fluorescent markers for living cells. In this paper, we highlight the diversity and easy synthetic accessibility of PONy dyes. In terms of the Pearson acid−base theory, a qualitative interpretation of the orbital overlap concept, nucleophilic P(III) reagents, such as trimethyl phosphite,3 are typical soft bases. On the other hand, the pyronin-type cationic dyes (pyronins, thio-, carbo-, and Si-pyronins) as well as acridinium salts (Figure 1b− e) and also electron-poor coumarins and dipyrromethenes (Figure 1, a,f) behave as soft acids due to high polarization and extended delocalization of the positive charge. In our case, the direct addition of P(III) nucleophiles (see Figure S1) to a variety of cationic or highly polarized fluorophores (Figure S2) was followed by a Michaelis−Arbuzov rearrangement4 and produced leuco-forms of phosphonylated dyes (Scheme 1). The latter could be isolated and used in further transformations or oxidized to the products demonstrating red-shifted absorption and emission bands and increased Stokes shifts (dye 2, Scheme 1). In the presence of a non-nucleophilic counterion, such as Cl− or ClO4−, the formation of phosphonylated leuco dyes was slow, resulting in low yields (
