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

Previously, we have studied the trifluoroacetic acid (TFA)-catalyzed rearrangements of unsubstituted and alkoxy-substituted -(pivaloylaminomethyl)benzaldehydes and revealed the formation of rearranged, regioisomeric aldehydes along with dimer-like products ("TFA dimers"). In the present study, related reactions of -(pivaloylaminomethyl)benzaldehydes are described with the difference that boron trifluoride diethyl etherate (BF·OEt) 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 ("BF 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.