Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Bond lability

The classical view was that a-bonds between transition metals and carbon were generally weak, and that this weakness was connected with the possibility of electronic rearrangement during a bond mpture process Most recent authors have preferred the view that this is not so, but that such bonds are frequently labile for mechanistic reasons Some recent attempts have been made to relate bond lability to electronic configuration at the metal, but these are open to criticism, and it is time to attempt a more coherent treatment. [Pg.165]

Ultraviolet light photolysis of CpC (16c) leads to the formation of mono- and dihydrates and dimers of the cytosine residue. Since saturation of the 5,6-double bond labilizes the 4-amino group, deamination also takes place readily with the formation of hydrates and dimers also of UpC, CpU, and UpU. A chart of those products which have been separated and identified is shown in Figure 21.74 The starting material, CpC, was labeled with 32P. The products were separated by various... [Pg.237]

Some striking demonstrations of metal-metal bond lability are provided by cluster rearrangements due to protonation. This is the case for some anionic osmium clusters (cf. Section VI). It involves ligand activation for some tetrairon clusters (51-53). Thus, the clusters 9 and 11 open up upon protonation, and compensation for the lost iron - iron bonds in the products 10 and 12 comes from the bonding between one iron atom and a carbonyl oxygen. The relation of these unusual nucleophile-electrophile interactions to cluster-induced CO transformations is obvious. [Pg.175]

Baldwin and Christie (18) have proposed that the origin of this difference almost certainly derives from a stereoelectronic factor. In 64 and 65, the Cg - S bond is more nearly orthogonal to the B-lactam amide plane than the C — S bons is, with respect to the thiazolidine amide plane (cf. 66). The Cg - S bond is therefore weaker than the CA - S bond and is preferentially cleaved. With tricyclic a-lactam 61, due to the five-membered ring, the C - S bond becomes more nearly orthogonal to the thiazolidine amide plane (cf. 67). As a consequence, the ordering of bond lability is reversed and the C — S bond is cleaved more readily. The stereoelectronically controlled step 61 63 has led to a stereospecific synthesis of a penicillin derivative from a peptide precursor. [Pg.164]

The experiments with the varioug —M-Re(CO)3L species establishes that population of the a level is not required to achieve sufficient M-M bond lability to yield homolytic cleavage within the lifetime of the excited state. However, when the sigma bond order is reduced from one to approximately one-half by the depopulation of the level, the rate constant for M-M bond cleavage appears to be only 105 s-1. By way of contrast, M-M bond cleavage seems to occur with a rate of >1010 s 1 for species... [Pg.101]

The safety catch principle for the solid-phase preparation of C-ternninal modified peptides requires the bond between the handle and the first residue to be stable to the normal conditions of SPPS. However, at the end of the synthesis, a chennical transformation of the linker substituent makes the key bond labile to nucleophiles. The 4-sulfanylphenol 44b ti and the sulfonamide 45 (Kenner) resins are two examples of safety catch resins. Thus, in the first case, which is only compatible with the Boc/Bzl strategy, once the peptide chain is elongated, treatment of the peptide-resin with hydrogen peroxide converts the sulfide into the corresponding sulfone, which makes the bond labile to nucleophiles. In the second case, N-methylation with diazomethane leads to an N-methylated peptidyl-sulfonannide-resin, from which peptides can also be cleaved by nucleophiles. [Pg.706]

The degradation can be followed under mild conditions in solution starting from related cationic derivatives [20, 21]. Model compounds of bond-labilized... [Pg.816]

Even cis-trans and mer-fac isomers are usually well-separated in the spectrum 10 ppm for the isomers of PtClaBra " 208 ppm for Pt(NH3)2(Ha0)a, and 310 ppm for Pt(N0a) Cla " (12). Substitut-ion shifts are smallest for substitutions trans to ligands with high trans influences. Thus, in Pt(IV) nitrite complexes, a Br" for Cl substitution is ca. 50 ppm larger when trans to Cl compared to trans to NOa (12). The term trans influence is used to denote the dependence of NMR (ground-state) parameters on the nature of the trans ligand. This is distinct from the trans effect" which is related to bond labilization as measured by ligand substitution rates. [Pg.174]

All known reactions of PLP-containing enzymes can be described mechanistically in the same way-formation of a planar Schiff base or aldimine intermediate, followed by formation of a resonance-stabilized carbanion with a quinoid structure, as shown in Figure 20,15. Depending on the bond labilized, formation of the aldimine can lead to a transamination (as shown in Figure 20.15), to decarboxylation, to racemization, or to numerous side chain modifications. [Pg.1513]

That such bonding is operative in many carbon-metal bonds is indicated by the superior stability of phenyl and acetylenic derivatives of the transition metals (Section II. E) and the reduced reactivity of vinylboranes (92, 98). Trivinylborane, for example, has a reduced acceptor tendency towards ammonia and does not react with oxygen. Investigations have aimed at looking for similar effects with vinylstannanes (112) and vinylgermanes (108). Yet in alkynylgermanes and alkynylstannanes it appears that the adjacent triple bond labilizes the carbon-metal bond. Both types undergo... [Pg.87]

Azide and thiocyanate react with Res CI9 in a similar fashion to that described for cyanide (Section 43.5.3.1) to give salts of the [RCsC Xj] ", [ResClaX ] ", [RejCljXgf and [RcsClsXgf anions (X = N3 or NCS). - There is also evidence that [Re3(NCS),2] can be formed from RC3X, (X = Q or Br) under forcing reaction conditions, further reflecting the fact that the order of Re—Cl bond lability is Clt Cli,. ... [Pg.161]

Linkage specificity. In this case, the enzyme makes a specific type of bond labile. [Pg.206]


See other pages where Bond lability is mentioned: [Pg.246]    [Pg.380]    [Pg.394]    [Pg.285]    [Pg.250]    [Pg.52]    [Pg.275]    [Pg.170]    [Pg.398]    [Pg.4]    [Pg.43]    [Pg.360]    [Pg.30]    [Pg.97]    [Pg.110]    [Pg.231]    [Pg.161]    [Pg.364]    [Pg.382]    [Pg.3754]    [Pg.80]    [Pg.283]    [Pg.1659]    [Pg.371]    [Pg.14]    [Pg.1967]    [Pg.51]    [Pg.51]    [Pg.52]    [Pg.52]    [Pg.3753]    [Pg.7]    [Pg.921]    [Pg.3615]   


SEARCH



Exchange of labile bonded hydrogen

Halogens, labile bonded

Labile

Lability

© 2024 chempedia.info