The tetracoordinate tetrahedral carbon has magnificently served biological systems for millions of years and our imaginations for but a century.1 We here open the problem of stabilizing tetracoordinate planar carbon. It would seem too much to hope for a simple carbon compound to prefer a planar to a tetrahedral structure.(1) J. H. van't Hoff, "Voorstel tot uitbreíding der tegenwoordig in de scheikunde gebruikte strukuurformules in de ruimte," Utrecht, 1874;
The stable carbene 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene has a pKa of 24 in (CD3)$30 and gives an elimination : substitution ratio with 2-bromopropane comparable to that of 1,5-diazabicyclo[4.3.0lnon-5-ene (DBN).Since their first isolation in 1991,l stable imidazol-2-ylidene carbenes? have attracted considerable a t t e n t i ~n . ~. ~We report preliminary results which show that one of these species, 1,3diisopropyl-4,5-dimethylimidazol-2-ylidene 1, is a stronger base than DBN 2, DBU 3 and the strongest proton sponge 4, (conjugate acid pK, 16.1 in 35% aqueous Me$30),4 although it does not rival some of the phosphazene bases developed by Scheme 1
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