Borovika A, Tang P.-I, Klapman S, Nagorny P * University of Michigan, Ann Arbor,
USA
Thiophosphoramide-Based Cooperative Catalysts for Brønsted Acid Promoted Ionic Diels–Alder
Reactions.
Angew. Chem. Int. Ed. 2013;
52: 13424-13428
Key words
ionic [2+4] cycloaddition - cooperative catalysis - Diels–Alder reaction - thiophosphoramides
Significance
The Nagorny group reports an ionic [4+2] cycloaddition between α,β-unsaturated acetal
dienophiles 1 and dienes 2 to afford Diels–Alder adducts 3 in moderate to excellent yields. The reaction is promoted by a cooperative catalytic
system involving a strong Brønsted acid [PTSA (p-toluenesulfonic acid)] and a triple hydrogen bond donor thiophosphoramide (A). NMR and computational studies suggest that the key feature of the catalytic system
is the strong interaction between A and the sulfonate anion.
Comment
Ionic [4+2] cycloadditions (Gassman’s cycloadditions) have proven to be efficient
complements to traditional Diels–Alder reactions when challenging unactivated substrates
are involved. The authors report a variety of these reactions, which interestingly
do neither require a Lewis acid nor a highly ionic medium for the generation of the
reactive separated ion pair. The same objective is achieved by a cooperative catalytic
system in which the sulfonic acid generates the oxocarbenium species and in which
the thiophosphoramide co-catalyst ensures the formation of separated, highly reactive
counterions via three hydrogen bonds to the sulfonate anion.
