The reactivity of oxocarbonium ions. 1. Detection as transient intermediates in the hydrolysis of ketals and ortho esters

. 57,1531 (1979). Kinetic investigations of the hydrolysis of the 2-phenyl-4,4,5,5-tetramethyl-l,3-dioxolenium ion and 2-phenyl-2-methoxy-4,4,5,5-tetramethyl-l,3-dioxolane furnish rate constants for all three reaction stages of the ortho ester hydrolysis: ( I ) generation of the dioxolenium ion, (2) hydration of this ion to form hydrogen ortho ester, and (3) breakdown of this species t o pinacol monobenzoate. The equilibrium constant for stage (2) can also be obtained. This study complements a previous investigation of 2-phenyl-2-alkoxy-l,3-dioxolanes where similar information was obtained.The rate constants for carbonyl oxygen exchange of the ester products of these reactions, pinacol monobenzoate and ethylene glycol monobenzoate, have been measured. This reaction is shown to proceed by a different mechanism to that normally associated with exchange of carboxylic acid derivatives: cyclization of the glycol monoester to form hydrogen ortho ester, followed by loss of the labelled exocyclic OH group to give 1,3-dioxolenium ion. Reversal of these steps, initiated by an unlabelled water molecule, results in exchange. The relationship of this mechanism with that of the ortho ester hydrolysis is obvious; it is shown that the exchange provides rate constants for the reverse of stage (3). This means that both the forward and reverse rates of this process have been obtained, and this provides the equilibrium constant.

Section: Ortho Ester Hydrolysissupporting

confidence: 93%

Carbonyl oxygen exchange of glycol monoesters. Rate and equilibrium constants for the formation of a tetrahedral intermediate

McClelland¹,

Ahmad²,

BOHONEK³

et al. 1979

“…With the acetal/ortho ester The picture with the phosphorane must, however, be different. The first noteworthy feature is the kH value for H+-catalysis, which is several orders of magnitude larger than any that have been observed with acetals and ortho esters, even with derivatives such as tropone diethyl ketal where very stable and observable (19) oxocarbocation intermediates are formed. With the acetals and ortho esters there is a general trend that the larger the kH value, the smaller the Bronsted a.…”

Section: General Acid Catalysismentioning

confidence: 80%

Hydrolysis of a monocyclic oxyphosphorane containing a five-membered ring

McGall¹,

McClelland²

1991

Self Cite

Can. J. Chem. 69,2075(1991. A kinetic study is reported for the hydrolysis of 2,2-diphenyl-2-rnethoxy-1,3,2-dioxaphosphoane 1. This phosphorane exists in aqueous solution in a pseudo acid-base equilibrium with an observable phosphonium ion, the rin opened (2'-hydroxyethoxy)diphenylmethoxyphosphonium ion 5. The equilibrium constant Ka ([l][Hi]/ [5]) is 9 X lo-' : values determined by kinetic and spectroscopic methods being in good agreement. This phosphonium ion is, however, not involved in the overall hydrolysis reaction, which proceeds via the thermodynamically less stable cyclic five-membered phosphonium ion derived by loss of the exocyclic methoxy group from the phosphorane, the 2,2-diphenyl-l,3,2-dioxaphospholan-2-ylium ion 6. This route for the overall hydrolysis is established by analysis of the products, and by the observation that the rate constant for the disappearance of 5 in acid solutions is 40 000 times greater than that for an analog that differs only in not being able to cyclize, the (2'-rnethoxyethoxy)diphenylmethoxyphosphonium ion 7. At all pH, the phosphorane 1 and the ring-opened phosphonium ion 5 exist in equilibrium, and the rate-limiting step in the overall hydrolysis is the cleavage of the exocyclic methoxy group to give the cyclic phosphonium ion 6, which is rapidly converted to products by reaction with water. The actual equilibration reaction involving 1 and 5 cannot be observed at any pH, even with stopped-flow spectroscopy. The non-catalyzed ring closure of the phosphonium ion 5 reforming the phosphorane 1 occurs with a rate constant of 200-500 s-', corresponding to an effective rnolarity of (2-5) X lo7 M for the intramolecular hydroxy group in this reaction. The rate-limiting exocyclic cleavage is assisted by H+ , with a very large rate constant 2 X 10' M-' s-I . Catalysis by general acids is also observed. The Bronsted plot has a slope a of 1.0 for the weaker acids, with a break for acids with pKa < 3. This "Eigen"-type behavior is proposed to arise from a transition state with little phosphonium ion character, in which the proton is almost completely transferred for the weaker acids.Key words: phosphorane, phosphate, phosphonium, hydrolysis.GLENN H. MCGALL and ROBERT A. MCCLELLAND. Can. J. Chem. 69,2075Chem. 69, (1991. On a effectuC une Ctude cinCtique de l'hydrolyse du 2,2-diphknyl-2-mCthoxy-1,3,2-dioxaphospholane, 1. En solution aqueuse, ce phosphorane existe sous forme d'kquilibre pseudo acidc-base avec un ion phosphonium qui peut Ctre observk, l'ion a cycle ouvert (2'-hydroxyCthyl)diphCnylmCthoxyphosphonium, 5. Les valeurs de la constante d'equilibre K, = [I:J[H+:l/[5] = 9 X lo-' qui ont Ct C dCterminCes par des mCthodes cinktiques et spectroscopiques sont en bon accord. Toutefois, cet ion phosphoniurn n'est pas impliquk dans la rCaction globale d'hydrolyse qui se produit par le biais de l'ion 2,2-diphCnyl-1,3,2-dioxaphospholan-2-ylium, 6, comportant un cycle a cinq chainons, qui est thermodynamiquement moins stable et qui est form6 par la perte d'un groupe mCthoxy exocycliqu...

“…For detailed analysis we have concentrated on the 6-methoxy compounds 10,13, and 14, where the cations are longer lived. Tht: intermediates with these systems have uv absorbance with A; , at 300 nm typical of p-methoxyphenyl dialkoxycarbocations (2,21,22). As shown in Fig.…”

Section: Product Analysismentioning

confidence: 90%

Reversible ring opening in the hydrolysis of spiro ortho esters

McClelland¹,

Moreau²

1985

Self Cite

. 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 ...