Intramolecular conversions

Scheme 2 Mechanism for the binding of ethidium cation (G) to two binding sites of DNA (A and B). For the intramolecular conversions, kAB and kBA are bimolecular rate constants where (G-DNA)a or (G-DNA)b react with a second DNA molecule leading to transfer of the guest.

One further example of selection rules for reactions is provided by the intramolecular conversion of an open-chain, conjugated polyene to a cyclic olefin with one less pair of n electrons. The simplest example is the butadiene-cyclobutene interconversion ... 

The radical cation decayed in the time scale of hours the reaction was attributed to an intramolecular conversion of the radical cation into a benzyl type radical, which subsequently decayed to produce stable recombination products. 

New syntheses of pyrido[2,3-f>]thieno[3,2-rf]-pyrimidine and -1,2,3-triazine have been achieved <02SC3493>. Intramolecular conversion of l-aryl-3,3-dialkyltriazenes into... 

Kinetics of a quite different type are observed for the reactions of Bromophenol Blue with aromatic amines. Aromatic amines are such weak bases that only the first acidic function of Bromophenol Blue is involved, and the product of the reaction is of type IV. The overall rate of formation of the ion-pair from the acid and the base is found to be many orders of magnitude less than the diffusion-controlled rate, and, for several amines, has a negative enthalpy of activation [97], These data, listed in Table 21, can be interpreted in terms of the rate-limiting step being the intramolecular conversion of the hydrogen-bonded species ROH—B to the ion-pair RO —HB+. Although the reaction consists of the... 

The most feasible Routes I-III for the synthesis of triribbed-functionalized a-dioximate clathrochelates (Scheme 12) were proposed in Ref. 65. The halogen-carbon bonds are reasonably active in nucleophilic substitution reactions, and the dihalogenoxime complexes are relatively stable (unlike dihalogenoximes, these complexes are available and undergo no intramolecular conversions... 

In the process of heating, polyorganosiloxane rubbers undergo a series of chemical transformations where, along with destruction and depolymerization reactions, intramolecular conversions, polymerization-type, exchange, and crosslinking reactions proceed. 

The major advances in this field have been brought about by the authors group at Osaka University. As a strategy for attaining highly selective reactions via acyllithium, we employed intramolecular conversion. A number of tactics as shown in Scheme 5-2 and following sections were devised. It should be mentioned that these reactions are no longer acylation via acyllithium but, rather, various type of enolate reactions derived from them. 

Scheme 5-2. Tactics for intramolecular conversion of acyllithium. 1) CO / EtjO, -78 °C MejSiO pSiMej...

Intramolecular Conversion of Acyllithium 139 Table 5-1. Preparation of acylsilanes and their enol sily ethers. 

Values of kp/gj changed only slightly In a solvent of high or low polarity. Is Is probably because of the very high electrophilic reactivity of which has a benzyl Iodide type structure. These observations lead to a conclusion that the big difference In kp gp values Is mainly due to the value of Xg In Equation 2. For example, Xg Is approximately 0.001 In CH3CN and 0.015 In PhCN. It Is Important to note that Xg Is governed by the rate of the Intramolecular conversion (k ) of a phosphonlum species to a covalent species 32. The mechanism of this type is an entirely new one which has not freen known In the Ionic polymerization. 

This mechanism involves formation of binuclear complexes (u-oxo complexes) which undergo intramolecular conversion to the peroxo dimer and then hydrolysis of the... 

Fig. (7). Intramolecular conversion of ptaquiloside to pterosin B, as proposed Alonso-Amelot [14].

See, for example, D. L. Bunker, /. Chem. Phys., 40,1946 (1963). Monte Carlo Calculations. IV. Further Studies of Unimolecular Dissociation. D. L. Bunker and M. Pattengill,/. Chem. Phys., 48, 772 (1968). Monte Carlo Calculations. VI. A Re-evaluation erf Ae RRKM Theory of Unimolecular Reaction Rates. W. J. Hase and R. J. Wolf, /. Chem. Phys., 75,3809 (1981). Trajectory Studies of Model HCCH H -P HCC Dissociation. 11. Angular Momenta and Energy Partitioning and the Relation to Non-RRKM Dynamics. D. W. Chandler, W. E. Farneth, and R. N. Zare, J. Chem. Phys., 77, 4447 (1982). A Search for Mode-Selective Chemistry The Unimolecular Dissociation of t-Butyl Hydroperoxide Induced by Vibrational Overtone Excitation. J. A. Syage, P. M. Felker, and A. H. Zewail, /. Chem. Phys., 81, 2233 (1984). Picosecond Dynamics and Photoisomerization of Stilbene in Supersonic Beams. II. Reaction Rates and Potential Energy Surface. D. B. Borchardt and S. H. Bauer, /. Chem. Phys., 85, 4980 (1986). Intramolecular Conversions Over Low Barriers. VII. The Aziridine Inversion—Intrinsically Non-RRKM. A. H. Zewail and R. B. Bernstein,... 

Upon addition of TMSCl, the yield dramatically increased from 21 % to 91 % for the palladium-catalyzed intramolecular conversion of methyl 3-oxo-6-heptenonate to 2-carbomethoxycyclo-hexanone (eq 96). In this particular case, however, hydrogen chloride, generated from hydrolysis of TMSCl with adventitious moisture, was identified as the active promoter. Thus, in a similar cyclization reaction, addition of hydrogen chloride, instead of TMSCl, provided the desired product with comparable yield (eq 97). 

The catalytic system proved not only applicable to alkyl hahdes, but also allowed for the intramolecular conversion of aryl halides. Interestingly, the corresponding Mizoroki-Heck-type cyclization products were formed selectively, without traces of reduced side-products (Scheme 10.27) [55]. Therefore, a radical reaction via a single electron-transfer process was generally disregarded for cobalt-catalysed Mizoroki-Heck-type reactions of aromatic hahdes. Instead, a mechanism based on oxidative addition to yield an aryl-cobalt complex was suggested [51]. 

A soln. of startg. sulfone and allyltrimethylsilane in dichloromethane treated at —78° with 1 M EtAlCl2 in hexane, after stirring at — 78° for 1 h the mixture warmed to 0°, and poured into 2 M HCl when reaction complete by t.l.c. (1-5 h) product. Y 60%. F.e., also 3-ethylenethioethers from 1,1-sulfonylthioethers and intramolecular conversions s. N.S. Simpkins, Tetrahedron Letters 29, 6787-90 (1988). 

The ability of aconitase to form both isocitrate and c/.y-aconitate is not well understood. It should be pointed out that, here again, the enzyme is capable of two different reactions an intramolecular conversion in the process of transforming citrate to isocitrate and a dehydration in the reaction transforming citrate into cw-aconitate. All attempts to separate these two catalytic properties have failed except in Aspergillus niger, in which two separate aconitases have been found. 

Since the configuration of the native protein can be stabilized by side-chain hydrogen bonds, we should expect these hydrogen bonds to influence the rate and equilibrium in the intramolecular conversion of native protein to denatured protein. As in previous chapters, it is not implied that side-chain hydrogen bonds are the only features of the tertiary structure which influence the stability of the native protein. Certainly, hydrophobic bonds also play an important role. However, as in the consideration of ionization, binding, and limited proteolysis, the purpose of the treatment here is to assess the effect of side-chain hydrogen bonds on the stability of the native protein. No doubt similar treatments will soon be developed for the influence of hydrophobic bonds on the stability of protein molecules. 

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