Labile reactions

The tosyl protecting group of 4a is removed with sodium naphthalide at -60 °C, and an intramolecular nucleophilic attack of the nitrogen lone pair to the epoxide takes place spontaneously to afford the pyrrolidine hydroxy ester as a labile reaction product, which is immediately protected as the corresponding silyl ether 3a in 68%... 

In addition to the type of electron transfer reaction, shown in Equations 6.142-6.145, there are examples where pure MLCT excited states induce ligand substitutions by associative or dissociative mechanisms. A well-established example of a MLCT excited state-mediated ligand labilization reaction is shown in Equation 6.149.136... 

Recent developments in chemical kinetics have been in the study of rapid reactions. Chemical events with half-lives as short as 10-7 seconds have been studied with reasonable accuracy. It is obvious that special methods must be used to study such labile reactions. 

The chemical history of the a-peroxylactones is almost as long. Derivatives (3a, b) were postulated as labile reaction intermediates in the autoxidation of ketenes (Eq. 6) to explain the formation of ketones and carbon dioxide. Indeed, very recent work confirmed these claims, except that singlet oxygenation of ketenes is preparatively more effective. However, the first stable a-peroxylactone that was isolated and characterized was the tert-butyl derivative (4), obtained from the corresponding a-hydroperoxy acid via dicyclohexylcarbodiimide (DCC) dehydration (Eq. 7). In the meantime, a number of stable a-peroxylactones have been reported (Table 2). 

Pulse radiolysis is the radiation chemical analogue of flash photolysis. It is a fast-kinetics technique that enables transitory processes, initiated by the absorption of ionizing radiation, to be observed in time frames as short as the submicrosecond region. It permits the detection and characterization of short-lived intermediates, the determination of the kinetics of their decay, and a probing of reaction mechanisms. The technique finds use in the study of radiation effects on materials, and as a tool for the examination of mechanistic details. For inorganic systems, pulse radiolysis is used to characterize metal complexes in unusual oxidation states, to examine the kinetics and rates of ligand-labilization reactions and to elucidate the mechanism of electron transfer. 

Protonation at the P-carbon to yield a protonated inline that is subsequently hydrolyzed to pyruvate and ammonia provides a reasonable mechanism for the final steps in the p-elimination reaction catalyzed by tryptophanase. BZ may bind preferentially to this gem-diamine species thereby stabilizing a labile reaction intermediate (structure 5B). 

Were the ethylenediamine labilization random rather than rules-based, Reactions 2, 3, 4, 5, and 6 (Figure 7) would occur with bout equal probability, and one would predict 2/3 cis- relative to trans-[Co(en)2(H20)Cl] which agrees very poorly with observation. Furthermore, in order to obtain the observed value of 0.6 for < >nh3/< ci by random nitrogen labilization, Reaction 1 would have to have a probability of about 0.6 relative to that for nitrogen labilization, or a probability of about 3 relative to Reactions 2, 3, 4, 5, or 6. One then expects about 4/5 m-relative to frans-[Co(en)2(NH3) (H20)] which is about that observed no distinction between the random and the labilization rules-based mechanism is thus possible in this case. 

Carbenes are very reactive species. Controlling the reactivity-selectivity pattern of such labile (reaction) intermediates is a challenge of utmost importance if carbenes are to be used in organic synthesis. 

Scheme 6.6 Labilization reactions of yttrium amido-NHC compound 57.

Many inorganic and organic compounds possessing labile hydrogen atoms add acrylonitrile readily with the formation of compounds containing a cyancethyl grouping (—CH j. CH2. CN). This reaction is usually known as cyanoelhylation ... 

It has been tentatively suggested that one mechanism underlies the Willgerodt reaction and the Kindler modification of it. A labile intermediate is first formed which has a carbon—carbon bond in the side chain. The scheme is indicated below it postulates a series of steps involving the addition of ammonia or amine (R = H or alkyl), elimination of water, re addition and eUmination of ammonia or amine until the unsaturation appears at the end of the chain then an irreversible oxidation between sulphur and the nitrogen compound may occur to produce a thioamide. 

In the total synthesis of the naturally occurring big molecule of palytoxin, which has numerous labile functional groups, this coupling is the most useful for the creation of E, Z-conjugated diene part 653. In this case, thallium hydroxide as a base accelerates the reaction 1000 times more than KOH[523]. Even TECOj can be used instead of a strong base in other cases[524]. 

Since allylation with allylic carbonates proceeds under mild neutral conditions, neutral allylation has a wide application to alkylation of labile compounds which are sensitive to acids or bases. As a typical example, successful C-allylation of the rather sensitive molecule of ascorbic acid (225) to give 226 is possible only with allyl carbonate[l 37]. Similarly, Meldrum s acid is allylated smoothly[138]. Pd-catalyzed reaction of carbon nucleophiles with isopropyl 2-methylene-3,5-dioxahexylcarbomite (227)[I39] followed by hydrolysis is a good method for acetonylation of carbon nucleophiles. 

In thiazole series the application of Tchemiac s method (S). which consists of cyclizing the intermediary iminoketothioether, obtained by reaction of a thiocyanoketone upon a labile hydrogen compound, is suitable and occurs with an average yield of 50%. In selenazole series this cydization does not happen from selenocyanoketones and by using different acidic media (9). 

By reaction of a corresponding quaternary salt in the presence of a condensation agent such as amyl nitrite or with a heterocycioammonium salt possessing a labile group, such as Cl, I. or SR (6. 11) in the 2-position, cyanine dyes are obtained (Scheme 52). 

Active Raney nickel induces desulfurization of many sulfur-containing heterocycles thiazoles are fairly labile toward this ring cleavage agent. The reaction occurs apparently by two competing mechanisms (481) in the first, favored by alkaline conditions, ring fission occurs before desul-, furization, whereas in the second, favored by the use of neutral catalyst, the initial desulfurization is followed by fission of a C-N bond and formation of carbonyl derivatives by hydrolysis (Scheme 95). 

The conversion of 4,5-dicyanothiazoles to diketones has been attempted (91). A difference in reactivity between the two cyano groups has been observed the least labile is the group in the 5-position. These Grignard reactions are limited and lead to 4-acetyl-5-cyanothiazole (Scheme 25). 

Pos twe-Tone Photoresists. The ester, carbonate, and ketal acidolysis reactions which form the basis of most positive tone CA resists are thought to proceed under specific acid catalysis (62). In this mechanism, illustrated in Figure 22 for the hydrolysis of tert-huty acetate (type A l) (63), the first step involves a rapid equihbrium where the proton is transferred between the photogenerated acid and the acid-labile protecting group ... 

The rate of the reaction in such case is R = k [PH ], where P is the reactant (ie, a repeat unit bearing the acid-labile protecting group). 

Replacement of Labile Chlorines. When PVC is manufactured, competing reactions to the normal head-to-tail free-radical polymerization can sometimes take place. These side reactions are few ia number yet their presence ia the finished resin can be devastating. These abnormal stmctures have weakened carbon—chlorine bonds and are more susceptible to certain displacement reactions than are the normal PVC carbon—chlorine bonds. Carboxylate and mercaptide salts of certain metals, particularly organotin, zinc, cadmium, and antimony, attack these labile chlorine sites and replace them with a more thermally stable C—O or C—S bound ligand. These electrophilic metal centers can readily coordinate with the electronegative polarized chlorine atoms found at sites similar to stmctures (3—6). 

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