Search

Expansion of the scope of the 1,3-diaza-Claisen rearrangement beyond bridged-bicyclic tertiary allylic amines has been investigated through a tethering strategy. Isothioureas tethered to tertiary allylic amines are converted to carbodiimides through a reaction with AgOTf/Et3N. Intramolecular cyclization of the tertiary allylic amine to the carbodiimide equilibrates with a zwitterionic intermediate. Heating the carbodiimide/zwitterion affords a rearrangement product. Heating carbodiimide/zwitterion with a deuterated allyl group results in the scrambling of the deuterium label, which is consistent with an ionic mechanism involving heterolytic cleavage of the allylic C-N bond, followed by trapping of the allyl cation at either terminal carbon. The ionic mechanism is attributed to silver salt contamination since pushing deuterium-labeled carbodiimide/zwitterion through silica gel prior to heating results in clean deuterium transposition consistent with a sigmatropic mechanism, and adding back silver salts results in deuterium scrambling. Overall, the tethering strategy broadens the scope of the rearrangement to simpler allylic substrates. Density functional theory (DFT) calculations of the sigmatropic rearrangement are in agreement with reactivity trends observed with reactions run under silver-free conditions.

Isothioureas tethered to bridged-bicyclic tertiary allylic amines can be converted to carbodiimides through reaction with Hg(II) salts. Intramolecular cyclization of the tethered tertiary allylic amines to the carbodiimides afford zwitterionic intermediates that undergo 1,3-diaza-Claisen rearrangements, affording highly substituted tricyclic guanidines.

Strained, tertiary, allylic, amine 2-benzyl-2-azabicyclo[2.2.1]hept-5-ene reacts with in situ generated carbodiimides in a 1,3-diaza-Claisen rearrangement to afford structurally interesting bicyclic guanidines. Use of more electron deficient carbodiimides makes these rearrangements more facile; however, there are not sufficient methods for the synthesis of highly electron deficient carbodiimides. Herein reported is the exploration of the synthesis of such carbodiimides from parent ureas and isothioureas and their use in 1,3-diaza-Claisen rearrangements.