Trialkylphosphonium oxoborates as C(sp(3))–H oxyanion holes and their application in catalytic chemoselective acetalization

The use of trialkylphosphonium oxoborates (TOB) as catalysts is reported. The site-isolated borate counter anion in a TOB catalyst increases the availability of C(sp(3))–H to interact with electron donor substrates. The catalytic protocol is applicable to a wide range of substrates in the acetalization reaction and provides excellent chemoselectivity in the acetalization over thioacetalization in the presence of alcohols and thiols, which is otherwise hard to achieve using typical acid catalysts. Experimental and computational studies revealed that the TOB catalysts have multiple preorganized C(sp(3))–Hs that serve as a mimic of oxyanion holes, which can stabilize the oxyanion intermediates via multiple C(sp(3))–H non-classical hydrogen bond interactions.