Shorter and Easier Syntheses of Di-tert-butylketene and Related gem-Di-tert-butyl Compounds

The ketene tBu(2)C=C=O is prepared from tBu(2)C=O in three steps (performable as a two-stage operation) through elimination of HCl from the intermediate product tBu(2)CCl-CH=O. The acid tBu(2)CH-CO2H, obtainable in two, three, or four preparative stages from tBu(2)C=O, adds slowly to the ketene to produce the anhydride (tBu(2)CH-CO)(2)O. Elemental lithium together with ClSiMe3 converts tBu(2)CCl-CH=O into tBu(2)C=CH-OSiMe3, which is a durable precursor of tBu(2)CH-CH=O, making this aldehyde easily and cheaply available from tBu(2)C=O. By exclusion of alternative mechanistic possibilities, the reduction of tBu(2)CCl-CH=O by tBuMgCl is shown to involve at least one single-electron transfer, leading to the enolate tBu(2)C=CH-OMgCl, which can be converted into tBu(2)CH-CH=O (three steps from tBu(2)C=O) or into tBu(2)C=CH-OSiMe3. Hydride transfer from NaBH4 to tBu(2)CCl-CH=O affords tBu(2)CCl-CH2OH, the transformations of which provide an entertaining set of S(N)1-type reactions. Several other examples of carbenium-type behavior are encountered in this gem-tBu(2) system; they are attributed to steric congestion, which also impedes bond rotations in the anhydride and in two esters. A convenient route to tBu(2)CH-C=N (five steps from tBu(2)C=O) uses the conversion of tBu(2)C=CH-OSiMe3 into tBu(2)CH-CH=NOH. The slow thermal (Z)/(E) equilibration of tBu(2)CH-NH-CH=O reveals the ranking of ecliptic repulsions as H3C < tBu < tBu(2)CH.