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Generation and Reactions of Lithiated tert-Butyl and 2,6-Di(tert-butyl)-4-methylphenyl Cyclopropanecarboxylates

✍ Scribed by Robert Häner; Thomas Maetzke; Dieter Seebach

Publisher
John Wiley and Sons
Year
1986
Tongue
German
Weight
709 KB
Volume
69
Category
Article
ISSN
0018-019X

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✦ Synopsis

trrf-Butyl and 2,6-di(tert-butyl)-4-methylphenyl (RHT) cyclopropanecarboxylates (4, 6, 24, 25) are lithiated with LiN(i-Pr), and t -BuLi, respectively. Reactions with alkyl halides, aldehydes, acyl chlorides, and heteroelectrophiles give a-substituted BHT esters which can be cleaved (t-BuOK/H,O/THF) to the corresponding carboxylic acids or reduced (LiAIH,/THF) to the cyclopropdnemethanols.

Though cyclopropane (la) itself is more acidic than the less strained cycloalkanes and open-chain alkanes (cf. lb) [l] [2], the relative acidity of the corresponding derivatives with conjugatively electron-withdrawing groups (EWG) may be reversed (2a less acidic than 2b), as shown by many mechanistic investigations with various EWG's (COR

). This effect is normally ascribed to the increase in strain upon introduction of a trigonal center into a three-membered ring [12] (compare 3a with 3b). Pyramidalized carbanion centers or C-atom rather than heteroatom-metalated structures (compare 3a with 3c) are, therefore, discussed for such species3). l b 2 a 2 b 3 a 3 b 3 c 3 d EWG = electron-withdrawing group

Alternatively, the carbanionoid species can be stabilized by decoupling to a diradical, see 3d [17] [18]. Hitherto, it was not possible to isolate suitable single crystals for X-ray structure analysis to obtain insight about the bonding in such species.

Besides these physical organic aspects, synthetic applications of EWG-substituted cyclopropyl nucleophiles have been studied extensively. A list of leading references to papers in which the successful generation and reactions with electrophiles of metalated ') , ) 3, Part of the projected Ph. D. thesis of R . H . , ETH Zurich.

Part of the Master's thesis of Th.M.. ETH Zurich, 1985. For the question as to whether a pyramidal ketonic carbanion exists or not, see [13-161.

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