La3+-Catalyzed Methanolysis of N-Aryl--lactams and Nitrocefin
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- 作者：Pedro J. Montoya-Pelaez ; Graham T. T. Gibson ; Alexei A. Neverov ; and R. S. Brown
- 刊名：Inorganic Chemistry
- 出版年：2003
- 出版时间：December 29, 2003
- 年：2003
- 卷：42
- 期：26
- 页码：8624 - 8632
- 全文大小：155K
- 年卷期：v.42,no.26(December 29, 2003)
- ISSN：1520-510X
文摘
The kinetics of the La3+-catalyzed methanolysis of N-phenyl-G SRC="/images/gifchars/beta2.gif" BORDER=0 ALIGN="middle">-lactam (2) and N-p-nitrophenyl-
ges/gifchars/beta2.gif" BORDER=0 ALIGN="middle">-lactam (3) as wellas that of nitrocefin (1) were studied at 25 ges/entities/deg.gif">C under buffered conditions. In the case of 2 and 3, the observedsecond-order rate constants (k2obs) for catalysis plateau at 
gif">pH 7.5-7.8, reaching values of 1 × 10-2 and 35 ×10-2 M-1 s-1 respectively. Potentiometric titrations of solutions of 2 × 10-3 M La(OTf)3 were analyzed in terms ofa dimer model (La3+2(-OCH3)n), where the number of methoxides varies from 1 to 5. The species responsible forcatalysis in the 
gif">pH range investigated contain 1-3 methoxides, the one having the highest catalytic activity beingLa3+2(-OCH3)2, which comprises 80% of the total La3+ forms present at its 
gif">pH maximum of 8.9. The catalysisafforded by the La3+ dimers at a neutral 
gif">pH is impressive relative to the methoxide reactions: at 
gif">pH 8.4 a 1 mMsolution of catalyst (generated from 2 mM La(OTf)3) accelerated the methanolysis of 2 by ~2 × 107-fold and 3 by~5 × 105-fold. As a function of metal ion concentration, the La3+-catalyzed methanolysis of 1 proceeds by pathwaysinvolving first one bound metal ion and then a second La3+ leading to a plateau in the kobs vs [La3+]total plots at all
gif">pH values. The kmaxobs pseudo-first-order rate constants at the plateaus, representing the spontaneousmethanolysis of La3+2(1-) forms, has a linear dependence on [-OCH3] (slope = 0.84 ± 0.05 if all 
gif">pH values areused and 1.02 ± 0.03 if all but the two highest 
gif">pH values are used). The speciation of bound 1 at a La3+concentrations corresponding to that of the onset of the kinetic plateau region was approximated through potentiometrictitration of the nonreactive 3,5-dinitrobenzoic acid in the presence of 2 equiv of La(OTf)3. A total speciation diagramfor all bound forms of La3+2(1-)(-OCH3)n, where n = 0-5, was constructed and used to determine their kineticcontributions to the overall 
gif">pH vs kmaxobs plot under kinetic conditions. Two kinetically equivalent mechanisms wereanalyzed: methoxide attack on La3+2(1-)(-OCH3)n, n = 0-2; unimolecular decomposition of the formsLa3+2(1-)(-OCH3)n, n = 1-3.