Procedure. Solutions of ethyl diazoacetate in either neat 2-methyl-3-buten-2-ol (1), or 10% 1 in dry pentane, were irradiated (Hanovia 450 W Hg lamp, Pyrex) until the ethyl diazoacetate had completely reacted as indicated by the disappearance of the yellow color. Photolysis of ethyl diazoacetate produces carboethoxycarbene predominantly in the singlet state. Ethoxyketene is also produced through Wolff rearrangement. Products of addition of the alcohol substrate to ethoxyketene, insertion of the carbene into C-H and O-H bonds of substrate and solvent, addition of the carbene to the substrate double bond, and possibly carbene attack at the oxygen atom of the substrate hydroxy group to form an ylide were anticipated.
Products were identified through comparison with authentic samples or through isolation and spectroscopic analysis.
For additional description of products and the analysis of the reaction mixtures, please see:
Product arising from an ylide intermediate. The products and their distribution are unremarkable, except for the presence of product 5. Formation of 5 must involve a rearrangement, most reasonably rearrangement of an oxonium ylide intermediate. Our results thus confirm the formation of oxonium ylides in reactions of carboethoxycarbene with alcohols. They do not, however, require that the ylide be an intermediate in the process leading to O-H insertion.
As mentioned above, the rearrangement might occur as either a concerted 2,3-sigmatropic shift or a Stevens rearrangement of the ylide:
Stevens rearrangement might also lead to product 8. We have not yet identified this product in the reaction mixture, but we are currently preparing an authentic sample for purposes of comparison.
Products consistent with ylide rearrangement via the radical pair pathway were, in fact, observed in the reaction of :CH2 with substrate 1 (ref 3). The relative yields of O-H insertion, ylide rearrangement and double bond addition products are comparable in the reactions involving carboethoxycarbene vs :CH2.