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During the presentation of reaction mechanisms in an advanced organic chemistry course, the involvement of reactive intermediates is often discussed without offering physical proof of their existence. Laboratory courses provide a venue for the investigation of such species. For example, the Wittig synthesis (eq 1) of 1,1diphenyl-1-propene (3) from ethyltriphenylphosphonium iodide (1) and benzophenone requires the generation of phosphorus ylide intermediate 2 via deprotonation of 1. The existence of the ylide may be inferred by the formation of a deeply colored solution upon deprotonation. However, by running the reaction in an NMR tube, as described below, the ylide may be directly observed by ^sup 1^H NMR spectroscopy. Furthermore, since the ylide is generated in DMSO-d^sub 6^, a deuterium exchange between the solvent and the ylide at the C-1 position results, yielding 2-D from initially formed 2-H (eq 1), so that the final product, 3, is also deuterium-labeled. This reaction sequence, therefore, illustrates a convenient method for the synthesis of deuterated alkenes.
Experimental Procedure
The experiment took place during two 3-h classes. In the first session, the ylide intermediate was generated, observed by ^sup 1^H NMR spectroscopy, and quenched by the addition of benzophenone. The reaction mixture remained in the NMR tube until the following week's lab session, at which time the alkene was isolated, purified, and characterized as described in the procedure below. ^sup 1^H NMR spectra were obtained at 60 MHz using a Varian EM-360 NMR spectrometer. Samples were run in DMSO-d^sub 6^.
Materials (approximate cost per student)
10 mg of freshly washed NaH ($0.05)
0.5 mL of DMSO-d^sub 6^ ($1.07)
0.157 g (0.9 eq) ethyltriphenylphosphonium iodide1 ($0.10)
76 mg benzophenone ($0.05)
NMR tube, sand...