Retrodienic reactions XVII - a general route to allenes gem-diactivated by electroattracting groups
✍ Scribed by Abdelhak Hakiki; Zoubida Jabry; Jean-Louis Ripoll; André Thuillier
- Book ID
- 104242239
- Publisher
- Elsevier Science
- Year
- 1984
- Tongue
- French
- Weight
- 117 KB
- Volume
- 25
- Category
- Article
- ISSN
- 0040-4039
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✦ Synopsis
Ketone2 undergoes Knoevenagel condensation with five malonic derivatives_7, yielding, ufter flash-thermolysis, the gem-&activated alLenes l-5 in 20-50 % yields. _-The synthetic utility and ketene equivalence of allenes gem-disubstituted by electroattracting groups, in nucleophilic additions and (2 n t 2 n) or dipolar cycloadditions, has been demonstrated ' ; on the other hand, very few reports are dealing with the synthesis of these compounds : 1,1-bis(ethoxycarbony1) and (methoxycarbonyl)-3,3_diphenylpropadienes are stable and prepared by Wittig reaction ' , l,l-dicyanopropadiene_1 has been identified as its anthracene adduct in the thermolysis of dicyanomethylenecyclooctatriene 3 and l-cyano-l-ethoxy- carbonylpropadiene 2 characterized by low temperature nmr in the photolysis of a diazocompound '. A more general method, involving dehydrohalogenation of a-chloroethylidenemalonates or cyanacetates with triethylamine, conducts to <, 1-cyano-1-methoxycarbonylpropadiene,? and l,l-bis (ethoxycarbonyllpropadiene4, however, these compounds cannot be isolated and are directly reacted with nucleophiles, imines or diazoalkanes I. Flash thermolysis of compounds ,8 leads to allenes ,1.,_3 and_5. obtained practically pure in 50 % yield ; partial loss of ethylene and decarboxylation, unavoidable in the case of allenes containing ethoxycarbonyl groups, give impure products and lower yield (20 Xl for_2 and $. Relative stabilities of allenesL -5 follow the order-1 (polymerizing rapidly at O°C) < ,2-.2 '4 _. -.,5. (stable several days at room temp.1 ; ir and nmr spectra of allenesl -,2. are reported in the following table :
Table. 'H nmr and ir spectra of allenes 1 -5. -._ ._ allene 1 -5 ._ -'H nmr (6 ppm, CDCls) ir (solid film, -196°C) at -50°C (1. -2) or+25"C (4, 5) CgN c=c=c c=o <_ H ,';;i=*=( CDzEt 2 CN 6,02 1s) 2240 1965 5,95 (s, 2H) -4,41 (q, 2H1 -1,38