Tropone Is a Mere Ketone for Cycloadditions to Ketenes

Abstract

Dedicated to Professor Rolf Huisgen on the occasion of his 85th birthday Tropone (1) reacts with ketenes 2 to yield [8 2] cycloadducts, the g-lactones 3. The concerted [8 2] cycloaddition path is formally symmetry-allowed, but we established that it is unfavorable. Careful lowtemperature NMR ( 1 H, 13 C, and 19 F) spectroscopies of the reaction of diphenyl ketene (2b) or bis(trifluoromethyl) ketene (2c) with tropone (1) allowed the direct detection of a b-lactone intermediates 5b,c and novel norcaradiene species 6b,c in head-to-head configurations. The [2 2] cycloadducts 5b,c equilibrated with the norcaradienes 6b,c. The b-lactones 5b and 5c were converted to the g-lactones 3b and 3c, respectively, in quantitative yields. The DFT calculations showed that the concerted [8 2] cycloaddition is unfavorable. The first step of the calculated reaction 1 2c is a cycloaddition which leads to a dioxetane intermediate. This initial [2 2] cycloadduct is isomerized to the b-lactone 5c via the first zwitterionic intermediate. The b-lactone 5c is further isomerized to the product g-lactone 3c via the second zwitterion intermediate. Thus, 3c is not formed via the well-established two-step mechanism including zwitterionic intermediates but via a five-step mechanism composed of a [2 2] cycloaddition and subsequent isomerization (Scheme 12).

[8+2] Cycloaddition Reactions in Organic Synthesis

“…Machiguchi and co-workers have recently shown that the reaction of ketenes with tropone does not proceed via a concerted pathway. 26 The cycloaddition reactions of tropone with phosphaalkynes were utilized for the synthesis of cage compounds 69 by Regitz and co-workers. 27 The reaction proceeds through a series of [4+2], [2+2+2], and [8+2] addition reactions followed by rearrangements as depicted in Scheme 15.…”

Copper-Catalyzed [4+2] Cycloaddition of 9H-Cyclohepta[b]pyridine-9-one and Electron-Rich Alkenes

“…Cycloadditions involving tropone ( 4 ) have been studied extensively over the past six decades. − Recently, Li and Yamamoto reported Lewis acid-mediated, inverse-electron demand Diels–Alder (IEDDA) reactions of tropone and its derivatives with vinyl ethers or ketene acetals to form [3.2.2] bicycles (Figure B) . The major cycloadduct ( 6 ) obtained bears the electron-donating substituent γ to the carbonyl group, whereas the minor product ( 7 ) bears the substituent β to the carbonyl.…”

Ni(NHC)]-Catalyzed Cycloaddition of Diynes and Tropone: Apparent Enone Cycloaddition Involving an 8π Insertion

“…This resonance structure explains the dipolar nature of tropone 3a and the large LUMO coefficients at the alpha-positions (Figure 1 c). 5 Therefore, tropone cycloaddition reactions usually lead to bond formations at the α-positions as in (6 + 2), 6 (6 + 3), 7 and (6 + 4) 8 cycloaddition reactions or in one α-position and the oxygen atom as in (8 + 2) 9 and (8 + 3) 10 cycloaddition reactions (Figure 1 a). 11 …”