Amino Substituted Bisketenes: Generation, Structure, and Reactivity

Hitherto unknown diamino-substituted bisketenes with both free (14) and tethered (16) amino substituents
have been generated by using laser flash photolysis for ring opening of the corresponding cyclobutenediones. The time-resolved kinetics of ring closure of the amino bisketenes back to the cyclobutenediones
were measured by IR or UV spectroscopy, and give first-order rate constants which vary by a factor of
7.5 × 104, and the bis(Me2N) bisketene 14 is the most reactive in ring closure that has been reported.
Rate constants for ring closure of these and previously observed bisketenes vary by a factor of 1013. The
dialkylamino bisketenes 16 (R = Me, n-Bu) with tethered substituents and restricted geometries are less
reactive than the bis(Me2N) bisketene 14 by factors of 1700 and 540, respectively. Computational results
obtained with DFT methods suggest angle strain in the tethered cyclobutenediones 15 inhibits facile
cyclization of bisketenes 16.