The reaction of di(n-propyl)cyclopropenone ( 7), di(n-butyl)cyclopropenone ( 16), and bicyclo[12.1.0]pentadeca-1( 14)-en-7-yn-15-one (21) with CpCo(CO) 2 and CpCo(cod) yielded CpCo-complexed benzoquinone and cyclopentadienone derivatives. When 16 was reacted with CpCo(CO) 2 and a 10-fold surplus of 4-octyne, a mixture of CpCo-capped benzoquinone 25 and cyclopentadienone 26 was isolated. The reaction of cyclopropenone 16 containing a 13 C-labeled CO group with CpCo(CO) 2 yielded a CpCo-capped tetrakis(n-butyl)-p-benzoquinone with one 13 C nucleus per molecule as the main product. The mechanism of the formation of p-benzoquinone is discussed on the basis of the results of trapping and labeling experiments.

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Heating of dialkyl-or diarylcyclopropenones at 75 °C in the presence of copper(I) bromide (5 mol%) affords the corresponding 4-oxaspiro[2.4]hepta-1,6-dien-5-ones in good yields.The first synthesis of a cyclopropenone derivative, diphenylcyclopropenone, was reported simultaneously by Breslow et al. 1 and Vol'pin et al. 2 50 years ago. Since that time several reviews have appeared, which collected the chemistry and properties of cyclopropenones and the related cyclopropenylium derivatives. [3][4][5] Our interest in these, the smallest of aromatic ring systems, arose when we used a cyclopropenone 6 and the cyclopropenylium ring 7 as building units for cyclophanes and when we treated cyclopropenone derivatives with dicarbonyl(cyclopentadienyl)cobalt(I) CpCo(CO) 2 . 8 During these studies we noticed that cyclopropenone and its alkyl-and aryl-substituted derivatives dimerize upon heating (100 °C) to a spirolactone (Scheme 1). 9 The yields for the spirolactones vary between 40% and 50%. When diphenylcyclopropenone (1a) was treated with catalytic amounts of triethyl phosphite, 10 the corresponding spirolactone was obtained at room temperature in 30 days in 60% yield. Spirolactones are of considerable interest for their application in medicine. 11 In this short paper we report our experiments with various cyclopropenones and transitionmetal compounds.Our model system was dibutylcyclopropenone (1c) which was prepared from dec-5-yne and dichlorocarbene. In a first attempt we heated catalytic amounts of Pd(II), Pt(0) or Ru(0) with dibutylcyclopropenone (1c). In most cases the yield of the resulting spirolactone was rather low (Table 1). During these studies we noticed high yields when we heated (75 °C) a solution of 1c in 1,2-dichloroethane with catalytic amounts of copper(I) bromide (Table 1, entry 7). Applying these conditions to the different cyclopropenones 1a-d the yields varied between 97% (2a), 74% (2b), 84% (2c), and 58% (2d).To rationalize the reaction mechanism we assume the reduction of cyclopropenone 1 to its radical anion 3 as the first step (Scheme 2). The radical anion of 3a has been observed by ESR spectroscopy. 12 These measurements suggest that the unpaired electron is mostly located at the two carbon atoms of the double bond of the three-membered ring.In a second step the nucleophilic carbonylate adds to a second molecule of 1 to yield 4, which is oxidized by Cu(II) to 5. The latter species undergoes ring opening to 2. Similar additions to the carbonyl group of cyclopropenones and ring enlargements have been observed on various occasions. 3,5,7,9,13 All spirolactones were characterized by their analytical properties. In the cases of 2a and 2d we were able to grow Scheme 1 Preparation of spirolactones 2a-d from 1a-d O R R R R a R = Ph b R = n-Pr c R = n-Bu O O R R O O O 2 d 1 d a R = Ph b R = n-Pr c R = n-Bu 1 2 Δ Δ 2 2 Scheme 2 Possible mechanism for the generation of 2 from two equivalents of 1 in the presence of copper(I) O R R R R O O R R 1 2 + Cu I -Cu II O R R 3 O R R 1 R R 4 O R R + Cu II -Cu I O R ...

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