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product if much toluene remains in the sample or if the sample is applied to the column in a more polar solvent.
11. The spectral properties for 1-phenoxy-1-phenylethene are as follows: IR (film) cm-1: 1600, 1495, 1290, 1230; *H NMR (250 MHz, CDCI3) S: 4.45 (d, 1 H,J = 2.3), 5.05 (d, 1 H, J = 2.3), 7.06-7.11 (m, 3 H), 7.29-7.38 (m, 5 H), 7.66-7.70 (m, 2 H).
12. While the submitters observed no complications, the checkers were unable, even after many trials, to obtain the dihydrocoumarin adduct completely free of what appears to be the product of double bond migration to give the endocyclic enol ether. Initial results were quite erratic. However, use of the glassware treatment described in Note 1, suggested by Professor Pine, has consistently provided the desired compound contaminated with only a few percent of the unwanted isomer. Spectral properties for the 3,4-dihydro-2-methylene-2H-1-benzopyran are as follows: IR (film) cm-1: 1665, 1595, 1500, 1470, 1250, 990, 770; 1H NMR (250 MHz, CDCI3) S: 2.57 (t, 2 H, J = 6.5), 2.80 (t, 2 H, J = 6.5), 4.14 (s, 1 H), 4.55 (s, 1 H), 6.85-6.92 (m, 2 H), 7.03-7.07 (m, 1 H), 7.11-7.18 (m, 1 H); impurity (partial) 8: 1.88 (bs, 3 H), 3.39 (bs, 2 H), 4.70 (bs, 1 H).
The formation of carbon-carbon bonds through the condensation of carbonyl compounds with phosphoranes (the Wittig reaction) is a very useful method in organic synthesis.3 Allowing the convergence of a wide variety of substrates enables this reaction to provide considerable flexibility in product design. Yet, with limited exceptions,4 this process has not been effective for the transfer of methylene or alkylidenes to the carbonyl group of esters or other carboxylic acid derivatives. However, reaction of the titanium-aluminum complex (the Tebbe reagent)2 described here does transfer a methylene to the carbonyl group of esters, effecting the conversion of an ester to an enol ether.5
The Tebbe reagent functions as a nucleophilic carbenoid in its reactions with carbonyl groups. The carbenoid is activated in the presence of a Lewis base which presumably complexes with the aluminum atom. Tetrahydrofuran is the Lewis base in the reactions described above. If the reaction is performed in the absence of added tetrahydrofuran, the carbonyl oxygen atom can function as a weak Lewis base, although the methylenation process is considerably slower.
Vinyl ethers have also been prepared by addition of alkoxides to acetylene,6’7’8 elimination from halo ethers and related precursors,6'8 and vinyl exchange reactions.8 Reaction of an electrophilic tungsten carbenoid with methylene phosphorane or diazomethane also produces vinyl ethers.9 Enol ethers have resulted from the reaction of some tantalum and niobium carbenoids with esters,10 and the reaction of phosphoranes with electrophilic esters.4
Methylenation using the titanium-aluminum complex converts a variety of esters to enol ethers in good yields.5 Lactones are converted to synthetically useful exomethylene enol ethers. Carbon-carbon double bonds do not interfere with the methylenation reaction, although functional groups containing acidic hydrogen atoms do consume the reagent and should be protected. The carbonyl group of aldehydes and ketones reacts in preference to the ester carbonyl group in the methylenation process,5b but those groups can also be selectively protected.
Because of the expense of obtaining the Tebbe reagent in its pure form,11 an in situ method for its preparation and use was developed.12 The presence of the excess Lewis acidic by-product, dimethylaluminum chloride, in the in situ preparation may cause reactivity at other sites in the substrate or lower yields of desired products (e.g., 70% vs. 94% for Preparation A, and 65% vs. 85% for Preparation B).5b However the overall simplicity of the method can be advantageous with readily obtained substrates.
1. (a) Department of Chemistry, California State University, Los Angeles, CA 90032; (b) Participant in the NIH-MBRS program.
2. Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. J. Am. Chem. Soc. 1978, 100, 3611.
3. (a) Cadogan, J. I. G.; Ed. "Organophosphorus Reagents in Organic Synthesis", Academic Press: New York, 1979; (b) Maercker, A. Org. Reactions 1965, 14, 270.
4. (a) Uijttewaal, A. P.; Jonkers, F. L.; van der Gen, A. J. Org. Chem. 1979, 44, 3157; (b) LeCorre, M. Bull. Soc. Chim. Fr. 1974, 2005; (c) Bestmann, H. J.; Dornauer, H; Rostock, K. Chem. Ber. 1970, 103, 2011.
5. (a) Pine, S. H.; Zahler, R.; Evans, D. A.; Grubbs, R. H. J. Am. Chem. Soc. 1980, 102, 3270; (b) Pine, S. H.; Pettit, R. J.; Geib, G. D.; Cruz, S. G.; Gallego, C. H.; Tijerina, T.; Pine, R. D. J. Org. Chem. 1985, 50,1212.
6. Shostakovskii, M. F.; Trofimov, B. A.; Atavin, A. S.; Lavrov, V. I. Russ. Chem. Rei (Engl.) 1968, 37, 907; Chem. Abstr. 1968, 70, 28091z.
7. Rutledge, T. F. In "Acetylenes and Aliénés, Addition, Cyclization, and Polymerization Reactions", Reinhold: New York, 1969, p. 283.
8. Buehler, C. A.; Pearson, D. E. In "Survey of Organic Syntheses," Wiley: New York, 1977; Vol. 2, p. 335.