Related Organocatalysts

Superbases for Organic Synthesis Guanidines, Amidines, Phosphazenes and Related Organocatalysts Edited by Tsutomu Ishikawa 2009 John Wiley Sons, Ltd. ISBN 978-0-470-51800-7 

In 1985, Schwesinger [10] introduced phosphazenes (triaminoiminophosphorane skeletons), which contain a phosphorus atom [P(V)] bonded to four nitrogen functions of three amine, and one imine substituents, as organobases containing a phosphorus atom. They are classified as P bases, based on the number (n) of phosphorus atoms in the molecule [11]. 

Typical structure of Verkade s base and its basicity due to trans-annular P-N 

Computational calculation of their proton affinities indicates that these new generations are, as expected, stronger than the original ones [12,18], 

Haflinger, G. and Kuske, RK.H. (1991) The Chemistry of Amidines andimidates,yol.2(eds S. Patai and Z. Rapport), John Wiley Sons, Chichester, pp. 1-100. 

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Superbases for Organic Synthesis Guanidines, Amidines, Phosphazenes and Related Organocatalysts... 

Superbases for organic synthesis guanidines, amidines and phosphazenes and related organocatalysts / Tsutomu Ishikawa. p. cm. 

C. Laurence and J.-F. Gal, Lewis Basicity and Affinity Scales Data and Measurement, John WUey Sons, New York, 2010, is an outstanding reference with respect to both tabulated data and discussion of methodology. G. GUh and P. GUh, The Nature of the Hydrogen Bond, Oxford University Press, New York, 2009, provides an in-depth discussion of this topic. For an excellent review of superbases Superbases for Organic Synthesis Guanidines, Amidines, and Phosphazenes and Related Organocatalysts, Ishikawa, T., eds., Wdey, New York, 2009. 

Moreover, the same authors employed a closely related organocatalyst and the corresponding urea derivative to promote the enantioselective dynamic kinetic resolution of azalactones with allylic alcohol. In this case of substrates, the urea derivatives proved to be superior to their thiourea analogues and, most usefully, these catalysts were insensitive to the steric bulk of the amino acid residue, allowing alanine-, methionine- and phenylalanine-derived azalactones to undergo dynamic kinetic resolution with unprecedented levels of enantioselectivity, as shown in Scheme 9.4. Furthermore, the compatibility of these catalysts with thiol nucleophiles was exploited in the first enantioselective catalytic dynamic kinetic resolution of azalactones by thiolysis to furnish enantioenriched amino acid thioesters of potential use with moderate enantioselectivities (<64% ee). 

Figure 1.9 Various types of proline-related organocatalysts.

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