Cyclopropylketenes: preparation and nucleophilic additions

Abstract

LEYI GONG, and THOMAS T. TIDWELL. Can. J. Chem. 69, 138 (1991). Phenylcyclopropylketene (4), tert-butylcyclopropylketene (5), and dicyclopropylketene (6) were formed by dehydrochlorination of the corresponding acyl chlorides by Et3N in THF, and are the first cyclopropylketenes to be isolated and purified. Reaction of 4 with n-BuLi and capture of the intermediate enolates with Me3SiC1 gave the stereoisomeric silyl en01 ethers c-PrCPh=C(OSiMe3)-n-Bu with a 79:21 preference for formation of the Z isomer resulting from nucleophilic attack syn to cyclopropyl, whereas the corresponding reaction of t-BuLi gave a 9:91 preference for attack anti to cyclopropyl. Some isopropyl-, cyclopentyl-, and cyclohexylketenes gave comparable results. Analyses of the relative sizes of the ketene substituents in the ground state by steric parameters, and of the product stabilities by molecular mechanics, both fail to predict the observed similarities in the results with different secondary alkyl groups. The hydration reactivities of 4 and 6 show that, in neutral H20/CH3CN, c-PrCPh=C=O is more reactive than i-PrCPh=C=O, a result ascribed as mainly due to the smaller size of cyclopropyl. c-Pr2C=C=0 has the same reactivity in neutral water as Et2C=C=0, but is 22 times less reactive with acid, a result attributed to the inability of the P-cyclopropyl groups to directly stabilize the cationic transition state for protonation.Key words: cyclopropylketenes, ketenes, nucleophilic addition, hydration kinetics. Chem. 69, 138 (1991). On a obtenu les phCnylcyclopropy1-(4), tert-butylcyclopropyl- (5) IntroductionThe conjugative interaction of the cyclopropyl group with adjacent r-systems has long been a source of fascination to chemists ( l a ) . Thus both vinylcyclopropane ( 1 ) ( l b , c ) and cyclopropanecarboxaldehyde ( 2 ) ( 1 d ) have favored conformations in which the cyclopropane ring is bisected by the plane of the adjacent sp2 carbon, so that r-conjugation between the cyclopropane ring and the side chain is possible. In 1 the s-trans conformation shown is favored with a 2.9 kcal/mol bamer ( 1 b ) to rotation to a non-bisected pauche conformation 1.2 kcal/mol ( 1 b ) higher in energy.'The ;-trans conformation is favored by r-conjugation whereas the gauche conformation is favored by reduced non-bonding interactions involving H2, H3, and H~, and by entropy ( l c ) . The r-stabilization is estimated as 1.4 kcal/mol ( l c ) . For 2 there is a slight preference for the syn conformer shown in the gas phase with a 4.3 kcal/mol barrier for conversion to the anti conformer ( 1 d ) .

Photochemical Reactions of 2,4-Diethyl-2,4-diphenyl-1,3-cyclobutanedione in Benzene and in Isopropyl Alcohol1

“…cis- and trans- 2,4-Dicyclopropyl-2,4-diphenyl-1,3-cyclobutanedione (11). The precursor, cyclopropyl phenyl ketene, was synthesized according to a five-step synthetic strategy described by Tidwell . After vacuum distillation, the ketene was heated to 120 °C for 3 days.…”

Inductive and Resonance Effects of Substituents on π-Face Selection

“…The prototype of such a center is C(1) of phenylcyclopentylketene or fluoroallene. , To explain the stereochemistry of alkyllithium additions to phenylcyclopentylketene, Scheme , one has to assume that it is controlled by at least two and possibly three effects. One is the steric strain.…”

“…Nonetheless, since the electron-donor capability of the exocyclic C−C bonds increases with the ring size (section 2.3.2), the increase in σ-hyperconjugative assistance to the syn approach on going from phenylcyclopropylketene to phenylcyclohexylketene could precipitate the reversal of π-face selectivity in additions of n -butyllithium. Indeed, ketene reactivity in acid-catalyzed hydration, which depends on the ability of the substituents to directly stabilize the cationic transition state for protonation, increases in the order c -Pr 2 CCO < Et 2 CCO . The alternative explanation might invoke the change in reactivity of ketenes caused by the inductive effect of the cycloalkyl groups (e.g., c -PrCPhCO is more reactive than i -PrCPhCO in neutral hydration in H 2 O/CH 3 CN 186 ).…”

Addition of heteroatom nucleophiles to ketene dimers

“…Methylphenylketene, ethylphenylketene, dimethylketene and TMS-ketene were prepared according to literature procedures. [31][32][33][34][35] TMS-quinine, Mequinidine and Me-quinine were synthesized according to literature procedure. 7,36 NMR spectra were recorded on a Bruker DPX Avance 200 spectrometer (200 MHz for 1 H and 50 MHz for 13 C) and on a Bruker Biospin AG 400 spectrometer (400 MHz for 1 H and 100 MHz for 13 C).…”