Organocatalytic Asymmetric Claisen Rearrangement

Key words

Claisen rearrangement - chiral guanidinium salts - H-bonding catalysts

Significance

Catalysis of the Claisen rearrangement by simple guanidinium salts such as N,N′-diphenylguanidinium salt 1 is reported. A broad variety of substituted allyl vinyl ethers (see selected examples) were suitable substrates for this interesting transformation. Moreover, highly enan­tioselective Claisen rearrangements of ester-substituted allyl vinyl ethers 2 have been achieved using C ₂-symmetric guanidinium salt 3 as the catalyst. Other types of substrates failed to give satisfying results in the asymmetric variant. The weakly coordinating counterion BArF^- was found to be crucial for catalytic activity of the respective guanidinium salt.

Comment

(Thio)ureas constitute a privileged class of H-bonding catalysts (see Review below). However, in catalytic Claisen rearrangements only modest rate accelerations have been realized by means of this catalyst class (see, for example: M. Hiersemann, T. Strassner and co-workers J. Org. Chem. 2007, 72, 4001). Although guanidinium BArF species exhibit similar pK _a values as N,N′-­diarylthioureas, they possess superior catalytic activity in this transformation. This work complements the metal-catalyzed asymmetric Claisen rearrangement developed by the Hiersemann group (Angew. Chem. Int. Ed. 2001, 40, 4700).

Review

M. S. Taylor, E. N. Jacobsen Angew. Chem. Int. Ed. 2006, 45, 1520-1543.