Reversible bond formation

Primary and secondary amines are susceptible to oxidation and replacement reactions involving the N—H bonds. Within the development of peptide synthesis numerous protective groups for N—H bonds have been found (M, Bodanszky, 1976 L.A. Carpino, 1973), and we shall discuss five of the more general methods used involving the reversible formation of... 

The effect of a substituent may be substantially modified by fast, concurrent, reversible addition of the nucleophile to an electrophilic center in the substituent. Ortho- and para-CS.0 and pam-CN groups have been found by Miller and co-workers to have a much reduced activating effect on the displacement of halogen in 2-nitrohaloben-zenes with methoxide ion [reversible formation of hemiacetal (143) and imido ester anions (144)] than with azide ion (less interaction) or thiocyanate (little, if any, interaction). Formation of 0-acyl derivatives of 0x0 derivatives or of A-oxides, hydrogen bonding to these moieties, and ionization of substituents are other examples of reversible and often relatively complete modifications under reaction conditions. If the interaction is irreversible, such as hydrolysis of a... 

The rapid and reversible formation of complexes between some metal ions and organic compounds that can function as electron donors can be used to adjust retention and selectivity in gas and liquid chromatography. Such coordinative interactions are very sensitive to subtle differences in the composition or stereochemistry of the donor ligand, owing to the sensitivity of the chemical bond towards electronic, steric and strain effects. A number of difficult to separate mixtures of stereoisomers and isotopomers have been separated by complexation chromatography. 

The head-to-tail-coupling reactions described above are potentially useful in the design of dynamic combinatorial libraries. Features of these reactions include the rapid and reversible formation of carbon-carbon bonds, multifunctional ene-imine building blocks, and formation of stereo centers upon ene-imine linkage. Support for template-directed synthesis utilizing ene-imine building blocks is the formation of a poly ene-imine species that could recognize 3 -GGA-5 sequences of DNA.48 It is noteworthy that some polyene-imines are helical and could form a triple helix with DNA. 

E.O. Fischer s discovery of (CO)sW[C(Ph)(OMe)D in 1964 marks the beginning of the development of the chemistry of metal-carbon double bonds (1). At about this same time the olefin metathesis reaction was discovered (2), but It was not until about five years later that Chauvln proposed (3) that the catalyst contained an alkylidene ligand and that the mechanism consisted of the random reversible formation of all possible metallacyclobutane rings. Yet low oxidation state Fischer-type carbene complexes were found not to be catalysts for the metathesis of simple olefins. It is now... 

A few other interesting and potentially important consequences of the reversible formation of five-membered zirconacycles include stereo- and regioselective skeletal rearrangement, as exemplified by Scheme 1.57 [197], and 1,3-C=C bond and Zr migration (Scheme 1.58) [191,192], supporting the associative mechanism for alkene displacement (Generalization 22 ). 

The second model involves the reversible formation of a complex between the iodine and the monomer, and in this context it is unimportant whether the iodine forms an n-complex, e.g., with the O-atom of a vinyl ether, or a 7t-complex with a double-bond or an aromatic ring of the monomer (styrene, N-vinylcarbazole) the only important point is that the complexed monomer is very much more reactive with the propagating ester group than the uncomplexed monomer, and that its formation is an equilibrium reaction. [Very detailed studies of the reactions of iodine with styrene [38] and with butyl vinyl ether (nBVE) [39, 40] have been reported in which the formation of various complexes is discussed.]... 

Olefin metathesis proceeds via reversible formation of metallacyclobutanes by [2 + 2] cycloaddition (Figure 1.7). The precise pathway for such a cycloaddition has been calculated for molybdenum complexes such as 1 (Figure 1.6) [9]. These calculations suggest that although Mo-C and C-C bond formation is concerted the Mo-C bond is formed more quickly than the C-C bond. It was also found, beautifully consistent with experimental results, that the activation barrier for [2 + 2] cycloaddition is lowered by increasingly electron-withdrawing alkoxy ligands. 

When secondary Grignard reagents are used, the coupling product sometimes is derived from the corresponding primary alkyl group.169 This transformation can occur by reversible formation of a nickel-alkene complex from the cr-bonded alkyl group. Reformation of the cr-bonded structure will be preferred at the less hindered primary position. 

Konarev DV, Khasanov SS, Otsuka A, Saito G (2002) The reversible formation of a single-bonded (C6o 2 dimer in ionic charge transfer complex Cp lr-C6o(C6H4Cl2)2- The molecular structure of (Cgo )2- J Am Chem Soc 124 8520-8521... 

Titanium-catalyzed cyclization/hydrosilylation of 6-hepten-2-one was proposed to occur via / -migratory insertion of the G=G bond into the titanium-carbon bond of the 77 -ketone olefin complex c/iatr-lj to form titanacycle cis-ll] (Scheme 16). cr-Bond metathesis of the Ti-O bond of cis- iij with the Si-H bond of the silane followed by G-H reductive elimination would release the silylated cyclopentanol and regenerate the Ti(0) catalyst. Under stoichiometric conditions, each of the steps that converts the enone to the titanacycle is reversible, leading to selective formation of the more stable m-fused metallacycle." For this reason, the diastereoselective cyclization of 6-hepten-2-one under catalytic conditions was proposed to occur via non-selective, reversible formation of 77 -ketotitanium olefin complexes chair-1) and boat-1), followed by preferential cyclization of chair-1) to form cis-11) (Scheme 16). 

FIGURE 3-7 Reversible formation of a disulfide bond by the oxidation of two molecules of cysteine. Disulfide bonds between Cys residues stabilize the structures of many proteins. 

When polyuridylic acid is photolyzed with radiation in the 250 p region, two types of photochemical reactions take place. One is the reversible formation of dimers between adjacent uracils and the other is a photohydration of the 5-6 double bond of the uracil. The position of the photochemical equilibrium of the first reaction is known to depend upon the wavelength of the radiation used and that of the latter is determined by... 

The reversal of the well-known transformation of sugars into pyrans has been detailed as a method for assembling simple monosaccharides from simple furans (71T1973). A compound of the 2-furylcarbinol type was converted by the Br2/MeOH procedure into a mixture of the cis and trans isomers of the corresponding 2,5-dimethoxy-2,5-dihydrofuran derivative (129). Mild acid hydrolysis of (129) resulted in cleavage of the acetal bonds with formation of the dicarbonyl compound (130) which underwent immediate cyclization to 2,3-dideoxy-DL-alk-2-enopyranos-4-ulose (131 Scheme 29). 

The reversible formation of a low-spin [Co (III) (NHS) n02 ]2+ complex within a Co (II) Y zeolite has been demonstrated by EPR spectroscopy. In this complex n is probably equal to five. A maximum of one cobalt complex per large cavity was farmed. The cobalt hyperfine structure shows that the unpaired electron is only 8% on the metal ion. Experiments utilizing 170 indicate that 02 enters the coordination sphere of the Co2+ ions and that the unpaired electron is largely associated with the oxygen molecule. The oxygen-17 hyperfine structure reveals that the two oxygen atoms are not equivalent hence, it is concluded that the oxygen is bonded as a peroxy-type superoxide ion. 

The reversible formation of a C-H bond by protonation of [HFe4C(CO)l2] to HFe4(CH)(CO)l2 (75, 7 7) is another instructive example of reactivity at the carbon atom. The exposed carbon atom in the anion proves to be a less basic site than is the methanol molecule, since the methylidyne cluster spontaneously deprotonates in methanol solution [Eq. (24)]. 

Kinetic evidence implicates a pre-association mechanism for catalysis that supports decarboxylation involving reversible formation of a complex of C02 and the carbanionic product.50 The catalyst is able to accelerate the reaction by competing for the carbanion. Such a situation would routinely be available in an enzyme active site.37 The complex cannot be observed spectroscopically because of its short lifetime and low concentration. However, catalysis after C C bond-breaking should alter the observed 12C/13C kinetic isotope effects (CKIE). 

One of the main principles for chiral separation used in modern capillary GC is the bonding of the optically active compounds via hydrogen bridges to a stationary-phase material. Feibush and Gil-Av [8] suggested a rapid and reversible formation of association complexes between carbonyl and amide functions of selector and selectand. The formation of diastereomeric associates yields complexes of different stability, depending on the relative configuration. The introduction of dipeptide and diamide phases leads... 

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