A methoxide-bridged dinuclear Zn(II) complex of 1,3-[N,N'-bis(1,5,9-triazacyclododecane)]propane (1-Zn(II)2:(-OCH3)) was prepared, and its catalysis of the cyclization of a series of 2-hydroxypropyl aryl phosphates (4a-g) was investigated in methanol at pH 9.8, T = 25degreesC by stopped-flow spectrophotometry. An X-ray diffraction structure of the hydroxide analogue of 1-Zn(II)2:(-OCH3), namely 1-Zn(II)2:(-OH), reveals that each of the Zn(II) ions is coordinated by the three N's of the triazacyclododecane units and a bridging hydroxide. The cyclizations of substrates 4a-g reveal a progressive change in the observed kinetics from Michaelis-Menten saturation kinetics for the poorer substrates (4-OCH3 (4g); 4-H (4f); 3-OCH3 (4e); 4-Cl (4d); 3-NO2, (4c)) to second-order kinetics (linear in 1-Zn(II)2:(-OCH3)) for the better substrates (4-NO2,3-CH3 (4b); 4-NO2, (4a)). The data are analyzed in terms of a multistep process whereby a first formed complex rearranges to a reactive complex with a doubly activated phosphate coordinated to both metal ions. The kinetic behavior of the series is analyzed in terms of change in rate-limiting step for the catalyzed reaction whereby the rate-limiting step for the poorer substrates (4g-c) is the chemical step of cyclization of the substrate, while for the better substrates (4b,a) the rate-limiting step is binding. The catalysis of the cyclization of these substrates is extremely efficient. The kcat/KM values for the catalyzed reactions range from 2.75 x 10(5) to 2.3 x 10(4) M-1 s-1, providing an acceleration of 1 x 10(8) to 4 x 10(9) relative to the methoxide reaction (k2OCH3, which ranges from 2.6 x 10(-3) to 5.9 x 10(-6) M-1 s-1 for 4a-g). At a pH of 9.8 where the catalyst is maximally active, the acceleration for the substrates ranges from (1 - 4) x 10(12) relative to the background reaction at the same pH. Detailed energetics calculations show that the transition state for the catalyzed reaction comprising 1-Zn(II)2, methoxide, and 4 is stabilized by about -21 to -23 kcal/mol relative to the transition state for the methoxide reaction. The pronounced catalytic activity is attributed to a synergism between a positively charged catalyst that has high affinity for the substrate and for the transition state for cyclization, and a medium effect involving a reduced polarity/dielectric constant that complements a reaction where an oppositely charged reactant and catalyst experience charge dispersal in the transition state.

A dinuclear Cu(II) complex of 1,3-bis-N(1)-(1,5,9-triazacyclododecyl)propane with an associated methoxide (2-Cu(II)(2):(-OCH(3))) was prepared, and its kinetics of reaction with an RNA model (2-hydroxypropyl-p-nitrophenyl phosphate (1, HPNPP)) and two DNA models (methyl p-nitrophenyl phosphate (3) and iso-butyl p-chlorophenyl phosphate (4)) were studied in methanol solution at (s)(s)pH 7.2 +/- 0.2. X-ray diffraction structures of 2-Cu(II)(2):(-OH)(H(2)O)(CF(3)SO(3)-)(3):0.5CH(3)CH(2)OCH(2)CH(3) and 2-Cu(II)(2):(-OH)((C(6)H(5)CH(2)O)(2)PO(2)-)(CF(3)SO(3)-)2 show the mode of coordination of the bridging -OH and H(2)O between the two Cu(II) ions in the first complex and bridging -OH and phosphate groups in the second. The kinetic studies with 1 and 3 reveal some common preliminary steps prior to the chemical one of the catalyzed formation of p-nitrophenol. With 3, and also with the far less reactive substrate (4), two relatively fast events are cleanly observed via stopped-flow kinetics. The first of these is interpreted as a binding step which is linearly dependent on [catalyst] while the second is a unimolecular step independent of [catalyst] proposed to be a rearrangement that forms a doubly Cu(II)-coordinated phosphate. The catalysis of the cleavage of 1 and 3 is very strong, the first-order rate constants for formation of p-nitrophenol from the complex being approximately 0.7 s(-1) and 2.4 x 10(-3) s(-1), respectively. With substrate 3, 2-Cu(II)(2):(-OCH(3)) exhibits Michaelis-Mentin kinetics with a k(cat)/K(M) value of 30 M(-1) s(-1) which is 3.8 x 10(7)-fold greater than the methoxide promoted reaction of 3 (7.9 x 10(-7) M(-1) s(-1)). A free energy calculation indicates that the binding of 2-Cu(II)(2):(-OCH(3)) to the transition states for 1 and 3 cleavage stabilizes them by -21 and -24 kcal/mol, respectively, relative to that of the methoxide promoted reactions. The results are compared with a literature example where the cleavage of 1 in water is promoted by a dinuclear Zn(II) catalyst, and the energetic origins of the exalted catalysis of the 2-Cu(II)(2) and 2-Zn(II)(2) methanol systems are discussed.

The methanolytic cleavage of a series of O,O-dimethyl O-aryl phosphorothioates (1a−g) catalyzed by a C,N-palladacycle, (2-[N,N-dimethylamino(methyl)phenyl]-C1,N)(pyridine) palladium(II) triflate (3), at 25 °C and sspH 11.7 in methanol is reported, along with data for the methanolytic cleavage of 1a−g. The methoxide reaction gives a linear log k2−OMe vs sspKa (phenol leaving group) Brønsted plot having a gradient of βlg = −0.47 ± 0.03, suggesting about 34% cleavage of the P−OAr bond in the transition state. On the other hand, the 3-catalyzed cleavage of 1 gives a Brønsted plot with a downward break at sspKa (phenol) 13, signifying a change in the rate-limiting step in the catalyzed reaction, with the two wings having βlg values of 0.0 ± 0.03 and −1.93 ± 0.06. The rate-limiting step for good substrates with low leaving group sspKa values is proposed to be substrate/pyridine exchange on the palladacycle, while for substrates with poor leaving groups, the rate-limiting step is a chemical one with extensive cleavage of the P−OAr bond. DFT calculations support this process and also identify two intermediates, namely, one where substrate/pyridine interchange has occurred to give the palladacycle coordinated to substrate through the S═P linkage and to methoxide (6) and another where intramolecular methoxide attack has occurred on the P═S unit to give a five-coordinate phosphorane (7) doubly coordinated to Pd via the S− and through a bridging methoxide linked to P and Pd. Attempts to identify the existence of the phosphorane by 31P NMR in a d4-methanol solution containing 10 mM each of 3, trimethyl phosphorothioate (a very slow cleaving substrate), and methoxide proved unsuccessful, instead showing that the phosphorothioate was slowly converted to trimethyl phosphate, with the palladacycle decomposing to Pd0 and free pyridine. These results provide the first reported example where a palladacycle-promoted solvolysis reaction exhibits a break in the Brønsted plot signifying at least one intermediate, while the DFT calculations provide further insight into a more complex mechanism involving two intermediates.

The rates and products of cleavage of methyl (2-chloro-4-nitrophenyl) phosphate (2) promoted by a dinuclear Zn(II) complex (3) of 1,3-bis-N,N'(1,5,9-triazacyclododecyl)propane along with 1 equiv of ethoxide were investigated in ethanol solution containing small amounts of water (8 mM or=1.6 x 10(17) times relative to the background hydroxide reaction, suggesting that complex 3 promotes the hydrolysis at least 1000 times more effectively than ethanolysis.

The di-Zn(II) complex of 1,3-bis[ N1, N1'-(1,5,9-triazacyclododecyl)]propane with an associated methoxide ( 3:Zn(II) 2: (-)OCH 3) was prepared and its catalysis of the methanolysis of a series of fourteen methyl aryl phosphate diesters ( 6) was studied at s (s)pH 9.8 in methanol at 25.0 +/- 0.1 degrees C. Plots of k obs vs [ 3:Zn(II) 2: (-)OCH 3] free for all members of 6 show saturation behavior from which K(M) and kcat (max) were determined. The second order rate constants for the catalyzed reactions (kcat (max)/K(M)) for each substrate are larger than the corresponding methoxide catalyzed reaction (k 2 (-OMe)) by 1.4 x 10(8) to 3 x 10 (9)-fold. The values of k cat (max) for all members of 6 are between 4 x 10(11) and 3 x 10(13) times larger than the solution reaction at s (s)pH 9.8, with the largest accelerations being given for substrates where the departing aryloxy unit contains ortho-NO 2 or C(O)OCH 3 groups. Based on the linear Brønsted plots of k cat (max) vs s (s)pKa of the phenol, beta lg values of -0.57 and -0.34 are determined respectively for the catalyzed methanolysis of "regular" substrates that do not contain the ortho-NO 2 or C(O)OCH 3 groups, and those substrates that do. The data are consistent with a two step mechanism for the catalyzed reaction with rate limiting formation of a catalyst-coordinated phosphorane intermediate, followed by fast loss of the aryloxy leaving group. A detailed energetics calculation indicates that the catalyst binds the transition state comprising [CH 3O (-): 6], giving a hypothetical [ 3:Zn(II) 2:CH 3O (-): 6] complex, by -21.4 to -24.5 kcal/mol, with the strongest binding being for those substrates having the ortho-NO 2 or C(O)OCH 3 groups.