A Synthesis of 1,1-Diarylallenes by Lead Tetraacetate Decarboxylation

Abstract

Lead tetraacetate oxidation of vie dicarboxylic acids is a mild, versatile method for the formation of olefinic functions.² We now report that this reaction can be used for the synthesis of certain allenes by application to an appropriate dicarboxylic acid. Thus, treatment of 3-carboxy-4,4-diphenyl-3-butenoic acid (Ia) with 1.1 equiv of lead tetraacetate in 10:1 benzene-pyridine under a nitrogen atmosphere for 2 hr at 50° produced 1,1-diphenylallene³ (IIa) [ir 5.19 and 11.7μ; nmr δ 7.2 (s, 2) and 5.12 (m, 10)] in modest (ca. 20%) yield. The allene was easily separated from more polar by-products by chromatography on silica gel. The course of the reaction could be followed by CO₂ evolution, which was, however, less than theoretical. Similar results were obtained in pyridine as solvent and the inclusion of catalytic amounts of anhy cupric acetate did not materially affect the reaction. In a similar manner the p-methyl derivative Ib was transformed into allene IIb [17% yield; mp 80.5–82°; Anal. C, 92.6 H, 7.2; ir 5.2μ; nmr δ 2.3 (s, 2), 5.18 (s, 2) and 7.16 (AA'BB', m, 8); uv (hexane) δ_max 266 nm (σ 1.1 × 10⁵)] and the p-methoxy compound Ic into allene IIc [15% yield; mp 100–102°; Anal. C, 81.1; H, 66; ir 5.2m; nmr β 3.76 (s, 6), 5.13 (s, 2) and 6.94 (AA'BB' m, 8)]. A significant amount of