systems where IO2 may be involved including erythropoietic protoporphyrias2 and hematoporphyrin p h~t o c h e m o t h e r a p y .~~-~~ The relatively precise distance and angle information for 4-7 provided by N M R studies coupled with the triplet energy transfer rate constants determined by flash photolysis provide correlations among structural, dynamical, and photophysical paratneters which will be useful in developing or testing quantitative theories of energy transfer between chromophores within a single molecule.For example, it appears that a quantum mechanical calculation of orbital overlap would be necessary in order to correlate the above-mentioned electron transfer in polystyrene with structure. These molecular systems are also potentially useful for studying intramolecular electron transfer reactions; in fact, a carotenoporphyrin linked to a quinone has recently been found to form a long-lived charge-separated state upon excitation with visible light.56 (52) Mathews-Roth, M. M. -(4-hydroxyphenyl)-lO,l5,20-tris(4methylphenyl)porphyrin, 57412-08-5; 7'-apo-7'-(4-(iodomethyl)phenyl)-P-carotene, 95998-89-3; 5-(3-hydroxyphenyl)-lO,l5,20-tris(4methylphenyl)porphyrin, 57412-06-3; 5-(2-hydroxyphenyl)-lO,l5,20tris(4-methylphenyl)porphyrin, 57412-07-4; 5-(4-(3-hydroxypropoxylphenyl)-10,15,20-tris(4-methylphenyl)porphyrin, 95998-90-6; abromo-p-toluoyl chloride, 52780-16-2; 2,3-dimethoxybenzaldehyde, 86-51-1; p-tolualdehyde, 104-87-0; pyrrole, 109-97-7; propionic acid, 79-09-4; 2,6-dimethoxybenzaldehyde, 3392-97-0; ethylene oxide, 75-21-8; methyl a-bromo-p-toluate, 2417-72-3; P-apo-S'-carotenal, 1 107-26-2; oxygen, 7782-44-7. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ (56) Moore, T. A.; Gust, D.; Mathis, P.; Mialocq, J. C.; Chachaty, C.; Bensasson, R. V.; Land, E. J.; Doizi, D.; Liddell, P. A.; Lehman, W. R.; Nemeth, G. A.; Moore, A. L. Nature (London) 1984, 307, 630-632.Abstract: The condensation of acetaldehyde (1) to an equilibrium mixture of aldol ( 2 ) and crotonaldehyde (3) is second order in 1. An excess acidity analysis reveals that a water molecule is also involved in the rate-limiting step; the reaction is actually the base-assisted addition of vinyl alcohol to protonated 1, even in concentrated H2S04. A previous report of a kinetically first-order conversion of 1 to 3 is shown to be due to the presence of a fast-reacting oligomer of 1. The reaction of 1 in D2S04 leads to partially deuterated 3, a result ascribed to partial conversion of vinyl alcohol to deuterated 1. Hydrogen isotope exchange of 3 was also observed, but at a slower rate. The rates of enolization of 1 were studied by iodination and are consistent with previous results and the proposed mechanism. The interconversion of 2 and 3 is shown to proceed via the enol of 2; in this case the rate-limiting step is water attack on/water loss from protonated 3/2, not proton transfer at carbon.The acid-catalyzed condensation of acetaldehyde (1) to aldol

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Ester Aldehyde 1. The same procedure was used as was used to prepare (±)-l.From the diester 40 with [a]20D -22.4°: (-)-l; [a]20D -27.0°( c, 0.022, in CHClg).

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 ) .

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