Solvent-Selective C-H Fluorination and Alkylative Dimerization of Pyrazolo[1,5-a]pyrimidines by Utilizing the Dual Role of Selectfluor as an Electrophilic Fluorinating Reagent and Oxidant

Selectfluor is an important reagent playing either the role of an oxidant or an electrophilic fluorine source for various organic transformations. Herein, we report a solvent-selective, C-H fluorination of pyrazolo[1,5-a]pyrimidines in MeCN using selectfluor (1 equiv) as the electrophilic fluorine source at room temperature for the synthesis of a wide variety of 3-fluoropyrazolo[1,5-a]pyrimidines in good yield. Interestingly, when the solvent was changed from MeCN to DMSO, pyrazolo[1,5-a]pyrimidines did not undergo C-H fluorination at room temperature; however, at high temperature, they underwent oxidative alkylative dimerization reaction, where selectfluor played the role of an oxidant and DMSO as the C-1 synthon to access bis(pyrazolo[1,5a]pyrimidin-3-yl)methanes in good yield. Thus, by choosing an appropriate solvent, two different classes of valuable products could be synthesised from pyrazolo[1,5-a]pyrimidines by utilising the different characteristics of selectfluor.