[6142-73-0] · C4H6 · Methylenecyclopropane · (MW 54.10)
(four-carbon building block useful for thermal, photochemical, and transition metal-mediated cycloadditions;1 epoxidation and rearrangement yields cyclobutanone;2 can be metalated3 or transformed to 2,4-dilithio-1-butene4)
Physical Data: bp 11 °C; d 0.85 g cm-3.
Solubility: sol most organic solvents.
Form Supplied in: colorless liquid.
Preparative Method: readily prepared by treatment of 3-chloro-2-methyl-1-propene with sodium t-butoxide (eq 1).2
Methylenecyclopropane (1) reacts as a dienophile in the Diels-Alder reaction (eq 2)5 and undergoes photochemical [2 + 2] cycloadditions with both electron-rich and electron-poor alkenes (eq 3).6 Neat thermolysis of (1) yields a mixture of [2 + 2] dimers (eq 4).7
Dipolar cycloadditions proceed smoothly and regioselectively between (1) and Diazomethane,8 Phenyl Azide,8 and nitrones (eq 5).9 The major nitrone cycloadducts such as (2) thermally rearrange to give 4-oxopiperidines (eq 6).9
Transition metal-mediated chemistry of (1) can involve several intermediate forms (5)-(7) (eq 7). Reactions involving Mo, Fe, Co, Rh, Ni, Pd, and Pt have been reported.1
Palladium-catalyzed dimerization of methylenecyclopropane gives a high yield of (8) (eq 8)11a (compare with eq 4).
Cycloadditions with 1-trimethylsilyl-1-alkynes10 and with a,b-unsaturated aldehydes, ketones, esters, nitriles, and sulfones have been extensively studied11 and representative results are shown in eq 9. Metal-catalyzed [2 + 2] reactions of (1) have also been reported.12
Under carefully optimized conditions, (1) can be condensed with Carbon Dioxide to give butenolide (12) in good yield (eq 10).13
Epoxidation of (1) yields the isolable oxaspiropentane, which exothermically rearranges to cyclobutanone (13) with catalytic Lithium Iodide (64% overall) (eq 11).2
Deprotonation of (1) occurs at C-2, yielding an allylic anion.3 This ambident nucleophile reacts exclusively at the ring position to yield 2-substituted methylenecyclopropanes (eq 12).
Treatment of (1) with Lithium in ether yields a stable solution of dilithiobutene (14) (70-75%) (eq 13).4
Scott McN. Sieburth & Gary Hiel
State University of New York at Stony Brook, NY, USA