Ring-closure rate constant

Rate constants for reaction of cis-[Pt(NH3)2(H20)Cl]+ with phosphate and with S - and 5/ -nucleotide bases are 4.6xl0-3, 0.48, and 0.16 M-1s-1, respectively, with ring closure rate constants of 0.17 x 10 5 and 2.55x10-5s-1 for subsequent reaction in the latter two cases 220). Kinetic aspects of interactions between DNA and platinum(II) complexes such as [Pt(NH3)3(H20)]2+, ds-[Pt(NH3)2(H20)2]2+, and cis-[Pt(NH3)2(H20)Cl]+, of loss of chloride from Pt-DNA-Cl adducts, and of chelate ring formation of cis-[Pt(NH3)2(H20)(oligonucleotide)]"+ intermediates implicate cis-[Pt(NH3)2(H20)2]2+ rather than cis-[Pt(NH3)2 (H20)C1]+, as usually proposed, as the most important Pt-binder 222). The role of aquation in the overall scheme of platinum(II)/DNA interactions has been reviewed 223), and platinum(II)-nucleotide-DNA interactions have been the subject of molecular modeling investigations 178). 

A series of studies were also undertaken for substituted phenanthroline complexes of the type Mo(CO)5N—N. The ring-closure rate constant k2 in general decreases with increasing steric hindrance on the ligand, and the... 

Table 2 The experimental values of absorption, M(III) SP formation rate constant (kj), thermal ring closure rate constant (k p, equilibrium (K ) constant of the reversible reaction, relaxation time and colorability in...

There are also examples of electrocyclic processes involving anionic species. Since the pentadienyl anion is a six-7c-electron system, thermal cyclization to a cyclopentenyl anion should be disrotatory. Examples of this electrocyclic reaction are rare. NMR studies of pentadienyl anions indicate that they are stable and do not tend to cyclize. Cyclooctadienyllithium provides an example where cyclization of a pentadienyl anion fragment does occur, with the first-order rate constant being 8.7 x 10 min . The stereochemistry of the ring closure is consistent with the expected disrotatory nature of the reaction. 

Radical cyclization. Ring closure of hept-6-en-1 -y 1 radical yields two products, methyl-cyclohexyl radical (85 percent) and cycloheptyl radical (15 percent).39 The overall rate constant is 3.5 X 104 s l. What are the rate constants for each pathway ... 

Rate constants at 65°C for ring closure of co-alkenyl radicals in the exo-mode (71) ... 

In addition to nucleophilic reactions, Baldwin s rules also apply to homo-lytic and cationic processes. Table 21 lists rate constants for ring closure of lower -alkenyl radicals (71), in which intramolecular addition to the double bond occurs in the exo-mode (Beckwith, 1981). It is unfortunate that EM-... 

The reactions of [Ru(edta)(H20)] with adenine, and with adenosine and its phosphate derivatives amp, adp, and atp, involve ring closure in a reversibly-formed intermediate containing a unidentate incoming ligand. Both formation (cf. above) and aquation of the intermediates are, on the evidence of the AS values for the amp, adp and atp systems, associative. Rate constants for ring closure are between 0.6 and 4.4 s-1 (165). 

In Case 1 both the ring closure steps (with rate constants ks and kR) are faster than dissociation of the 7r-complexes to reform the alkenol and N-X+ species. Here the [S]/R] product ratio is determined only by the difference in the activation energy leading to the 7is- and rcR-complexes because immediately after these are formed the cyclization occurs. Thus, the chirality is set by the approach of the alkene to the halonium ion. 

The main tables are designated by a letter and a number (a full list is given in the Contents at the beginning of this chapter). Tables in the text, which mostly contain effective molarities taken from the main tables, are designated by a number only. Every individual reaction is identified by a reference letter and two numbers. Thus B.6.3 refers to the base-catalysed cyclization of cyclohexyl 2-hydroxyethyl phosphate which is the third entry in Table B.6. The EM for this reaction is given in Table 10 in the discussion of the effects of alkyl substitution on ring-closure, and the reference number quoted there. Full details of the calculation of the EM, the rate constant on which it is based, the conditions used and the authors concerned can then be found by consulting Table B.6. 

The second-order rate constants for thiocyanate anation vs pH are shown in Fig. 1.13. The full line represents (1.216) with the values shown in scheme (1.217). This profile had been earlier recognized in the ring closure of the three analogous pH-related forms of Co(III)-edta to give Co(edta) in which the edta is completely coordinated.In the Co(lll) case the reactivities of the three forms are much closer. A plot of A [H+] -1-[H+] is a quadratic curve from which / ah2> ah be obtained. 

Compare the ring closure shown in (4.111) with the corresponding process involving only a unidentate ligand in (4.112), rate constants at 25 °C. 

One obvious area in which anchimeric effects might materialize is in chelation reactions this is the underlying reason that chelation is dominated by the first step (Sec. 4.4.1). We have seen it operate in the formation of Pt(II) chelates (Sec. 4.7.6). The accompanying values of AS are less negative than is usual for the associative substitution in Pt(II), consistent with the intramolecular mechanism. The rate constant for the ring-closure reaction (6.1)... 

The authors attribute the B2 and B states to the isomers TC and TT and support this claim with the statement that the thermal ring-closure reaction for this compound that lasts for tens of seconds shows two components. This fact alone means that their assignment is unlikely because any species equilibrating in 21 psec would not show two decay components over tens of seconds. Also, the isomerization between TT and TC forms of typical napthopyrans has been shown to result in a spectral shift [76,77], which is not observed in this case. Further, for the same and similar compounds, the two mero-forms have clearly resolved noncoalesced H-NMR peaks [78,79] and the rate constant for... 

The sole adduct formed from thermolysis of 163e was A-butoxy-A-(hex-5-en-l-yl)benzamide (168e) rather than the methylcyclopentyl isomer. The rate constant for trapping must at least exceed that for ring closure of hexenyl at these temperatures (ca 5 X 10 8 ) ° . 

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