Détermination cinétique de la nature du conformère réactif dans l'hydrolyse des orthoesters

Lamaty, G.; Moreau, Claude; Mouloungui, Zéphirin

doi:10.1139/v83-454

Cited by 3 publications

(4 citation statements)

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“…1,l-Diethoxy-3,4-dihydro-1 H-Zbenzopyran (9) and 1, l-diethoxy-3,4-dihydro-6-methoxy-1H-2-benzopyran (10) were from a previous study (15). Spiro ortho esters were prepared from these by ortho ester interchange.…”

Section: Methodsmentioning

confidence: 99%

“…We feel that the small amount of ester also comes from this cation. Under similar conditions the ortho ester 10 gives 85% lactone (15). This ortho ester undergoes only cleavage of the exocyclic ethoxy group (15) to produce a cation analogous to 15.…”

Section: Product Analysismentioning

confidence: 99%

“…For all four ortho esters a predominance (85-95%) of lactone plus diol is observed. The ester 19 can cyclize to the lactone but this process is slow compared to the ortho ester hydrolysis (14,15). The nmr spectra were recorded sufficiently quickly to provide the products of kinetic control.…”

Section: Product Analysismentioning

confidence: 99%

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Reversible ring opening in the hydrolysis of spiro ortho esters

McClelland¹,

Moreau²

1985

Can. J. Chem.

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. Hydrolysis kinetics are reported for four spiro ortho esters: 3,4-dihydro-6-methoxy-I H-2-benzopyran-1-spiro-2'-11,3'-dioxolane (13), its 11,3'-dioxane analog (14), and the 6-unsubstituted versions of each (11 and 12). For comparison, also included are the diethoxy analogs: 1 ,l-diethoxy-3,4-dihydro-6-methoxy-1H-2-benzopyran (10) and the 6-unsubstituted compound (9). Product analysis implicates an initial opening of the dioxolane or dioxane ring in the spiro ortho esters, as expected on the basis of stereoelectronic considerations. The intermediate dialkoxycarbocations can be observed in HCl solutions. A detailed analysis has been carried out for the 6-methoxy systems to provide the rate constants k, , the second-order rate constant for H+-catalyzed formation of the cation from the ortho ester, k2, the first-order rate constant for water addition to the cation, and k-, , the first-order rate constant for ring closing of the cation to reform the ortho ester. The two spiro ortho esters are shown in this analysis to undergo reversible ring opening in their hydrolysis, in that values of k-, are greater than k2. The differences, however, are not large, k-,/k2 being 1.2 (dioxolane, 13) and 3.8 (dioxane, 14). Comparison with the diethoxy ortho ester also reveals that the ring opening process (k, rate constants) is inherently more difficult with the dioxolane, although not with the dioxane. An argument involving lone pair orientation is advanced to explain this.ROBERT A. MCCLELLAND et CLAUDE MOREAU. Can. J. Chem. 63, 2673Chem. 63, (1985.On rapporte les donnks cinktiques relatives l'hydrolyse que quatre spiro ortho esters: le dihydro-3,4 mkthoxy-6 1 H-benzo-2 pyrannespiro-1 dioxolane-2'-1 ',3' (13), son analogue dioxanne-1 ' ,3' (14) ainsi que les homologues non substituts en position 6 (11 et 12). Pour fins de comparaisons, on a kgalement inclus les analogues dikthoxylks: le dikthoxy-1,1 dihydro-3,4 mkthoxy-6 1 H-benzo-2 pyranne (10) et le compost qui n'est pas substituk en position 6 (9). L'analyse des produits implique que l'ouverture initiale se fait au niveau des cycles dioxolane ou dioxanne des ortho esters spiro et ce conformCment A ce que laissent prtvoir les facteurs stkn5oklectroniques. On peut observer les dialkoxycarbocations intermtdiaires dans des solutions de HCl. On a effectuC une analyse dktaillke des systkmes mkthoxy-6 dans le but d'obtenir: (a) les constantes de vitesse k,, (b) la constante de vitesse d'ordre deux, k2, relative a la formation, catalyske par H+, du cation a partir de I'ortho ester, (c) la constante de vitesse d'ordre un relative a l'addition d'eau sur le cation et (d) k-I , la constante de vitesse d'ordre un pour la cyclisation du cation qui ramkne a I'ortho ester. Grlce A cette analyse on dtmontre que, au cours de leur hydrolyse, les deux spiro ortho esters subissent une ouverture reversible de cycle qui se traduit par le fait que la valeur de k-I est su@rieure ii k2. Cependant, la diffkrence n'est pas trbs grande; k-,/k2 est Cgal A 1,2 pour le dioxolane 13 et 3,8 pour le dioxanne ...

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Section: Methodsmentioning

confidence: 99%

Section: Product Analysismentioning

confidence: 99%

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Reversible ring opening in the hydrolysis of spiro ortho esters

McClelland¹,

Moreau²

1985

Can. J. Chem.

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“…La specificit6 observCe ne peut Etre expliqute qu'en postulant une difference entre les energies libres d'activation pour les reactions de coupure des diffirentes liaisons C-OR; c'est ainsi que dans un precedent travail (3), nous avons montrt a partir de rksultats cinetiques que l'hydrolyse des dikthoxy-2,2 tetrahydropyrannes s'effectuait par l'intermediaire du conformkreA et non par l'intermediaire du conformere F (Schema l), hypothkse d6ja retenue a partir de l'analyse des produits de reactions (4).…”

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Effet de groupe partant dans l'hydrolyse d'orthoesters cycliques: changement de conformère réactif dans l'hydrolyse des diéthoxy-2,2 tétrahydropyrannes

Lamaty¹,

Lorente²,

Moreau³

1983

Can. J. Chem.

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Requ le 17 mars 1983 GERARD LAMATY, PHILIPPE LORENTE et CLAUDE MOREAU. Can. J. Chem. 61, 2651 (1983). Les vitesses d'hydrolyse des orthoesters diithoxy-2,2 dihydro-3,4 benzopyrannes-1 substituts en position 6 (X = OMe, Me, H et C1) ont Ctt ditermintes a deux temperatures dans le solvant eauldioxanne 2: 1 en volumes. Le changernent de signe observe dans les constantes p de Hammett (pH,,+ = -0,8; p, , = +0,7) est caracttristique du depart d'un groupe phtnolate, c'est-a-dire de la rupture de la liaison C-0 endocyclique dans les systemes Ctudits ici; ces rtsultats indiquent un changement du conformere rtactif dans l'hydrolyse des diethoxy-2,2 tttrahydropyrannes. GERARD LAMATY, PHILIPPE LORENTE, and CLAUDE MOREAU. Can. J. Chem. 61, 2651 (1983). The rates of hydrolysis of 6-substituted 2,2-diethoxy 3,4-dihydro benzo-I-pyrans (X = MeO, Me, H and C1) were determined at two temperatures in waterldioxane 2: 1 by volume. The observed change of sign of the Hammett p constants (pH,,+ = -0.8; PAH = +0.7) is characteristic of a phenolate leaving group, i.e. of the rate-determining cleavage of the endocyclic C-0 bond in the systems here studied; these results clearly indicate a change in the nature of the reactive conformer in hydrolysis of 2,2-diethoxy tetrahydropyrans.\E t ,-.

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