Synthesis and Reactivity of 4,4-Dialkoxy-BODIPYs: An Experimental and Computational Study

A series
of boron-disubstituted O-BODIPYs were
synthesized, and their structures and spectroscopic properties were
investigated using both computational and experimental methods. Three
methods were investigated for the preparation of 4,4-dimethoxy-BODIPYs
bearing electron-donating or electron-withdrawing 8-aryl groups: method
A employs refluxing in the presence of NaOMe/MeOH, method B uses AlCl3 in refluxing dichloromethane followed by addition of methanol
as nucleophile, and method C involves activation of the BODIPYs using
TMSOTf in refluxing toluene followed by addition of methanol. The
yields obtained depend on the method used and the structure of the
starting BODIPYs; for example, 1a and 3a were most efficiently prepared using method C (98 and 70%, respectively),
while 2a was best prepared by method A (50%). Methods
B and C were employed for the synthesis of seven new 4,4-dialkoxy-BODIPYs.
4,4-Dipropargyloxy-BODIPY 1e reacted under Cu­(I)-catalyzed
alkyne–azide Huisgen cycloaddition conditions to produce 4,4-bis­(1,2,3-triazole)-BODIPY 4 in 78% yield. The substitution of the fluorides for alkoxy
groups on the BODIPYs had no significant effect on the absorption
and emission wavelengths but altered their fluorescence quantum yields.
Among this series of dialkoxy-BODIPYs, the 4,4-dipropargyloxy 1e and its corresponding bis­(1,2,3-triazole) 4 show the largest quantum yields in toluene and THF, respectively.