Experimental and computational study of BF3-catalyzed transformations of ortho-(pivaloylaminomethyl)benzaldehydes: an unexpected difference from TFA catalysis

Abstract

Previously, we have studied the trifluoroacetic acid (TFA)-catalyzed rearrangements of unsubstituted and alkoxy-substituted ortho-(pivaloylaminomethyl)benzaldehydes and revealed the formation of rearranged, regioisomeric aldehydes along with dimer-like products (“TFA dimers”). In the present study, related reactions of ortho-(pivaloylaminomethyl)benzaldehydes are described with the difference that boron trifluoride diethyl etherate (BF₃·OEt₂) is used as the catalyst. Although in these reactions the formation of the same “TFA dimers” can be observed after a couple of hours reaction time, during further stirring these are transformed into a new dimer-like keto compound (“BF₃ dimer”) that gradually becomes the main product. Apart from this, an oxoindene-type by-product is also formed. The new products are characterized by detailed NMR studies and two of them also by single-crystal X-ray diffraction. DFT calculations support the mechanism proposed for the transformations and explain the differences observed in the product distribution.