Reactive intermediates long-lived

The highly fractionated nature of the and Th series nuclides is illustrated by the measured activities in some representative waters in Figure 1. The highest activities are typically observed for Rn, reflecting the lack of reactivity of this noble gas. Groundwater Rn activities are controlled only by rapid in situ decay (Table 1) and supply from host rocks, without the complications of removal by adsorption or precipitation. The actinide U, which is soluble in oxidizing waters, is present in intermediate activities that are moderated by removal onto aquifer rocks. The long-lived... 

Cationic complexes are key intermediates in a great variety of organic transformations such as isomerizations, rearrangements, addition reactions, aromatic substitutions, polymerization and others. Long-lived cationic complexes are important structural models for these intermediates. Studies of such complexes by modem physical methods provide valuable insight regarding their structure and reactivity. 

Little has investigated monoactivated and doubly activated alkenes tethered to butenolide with respect to electroreductive cyclization [202]. The geminally activated systems 227 undergo cyclization to diastereomeric products 228 and 229 in an 1 1 mixture, whereas both the a,j8-unsaturated monoester and a,/ -unsaturated mononitrile fail to cyclize. Only saturation of the C-C double bond of butenolide is observed. The author explains these results by distinct reactivity and lifetime of the intermediate radical anions. The radical anions derived from the monoactivated olefins are less delocalized than those of 227 and therefore should be shorter lived and more reactive. In this case preferential saturation occurs. The radical anions derived from the doubly activated alkene 227 are comparatively long-lived and less basic and thus capable of attacking the C-C double bond of the butenolide moiety. A decrease in saturation, accompanied by a marked increase in cyclization, is observed. 

The postulation of trimethylene and tetramethylene diradicals as reactive intermediates involved in many thermal isomerization and fragmentation reactions has a long history,but not until 1994 had they ever been detected in real time. The validity of the diradical hypothesis was tested through femtosecond studies, and the tests provided dramatic evidence confirming that these short-lives species are indeed real, directly experimentally accessible chemical entities. 

The overall TOF of this reaction was unexpectedly low and may be due to deactivation by long lived intermediates blocking reactive sites on the bimetallic particles. We are currently attempting to determine the cause of this loss of activity. 

Photodecomposition of a 4-nitrophenyl moiety is a multiphoton process absorption of one photon leads to the singlet, which relaxes immediately into a long-lived triplet state. Capturing another photon transforms the singlet through a dissociative decay process into reactive intermediates, probably radicals, which react with nucleophilic sites. In the absence of a reactive species, these reactive intermediates will relax into the initial ground state 23 ... 

Under these conditions, silver-assisted electrocyclic ring opening provided the haloallyl cation, which was subsequently trapped by isocyanate anion. Interception of the cationic species with isocyanate was successful since bromide was removed from the reaction mixture as a precipitate (AgBr). Finally, treatment of intermediate 9 with methanol furnished the desired carbamate in 96% yield. This example demonstrates the usefulness of the silver(I)-mediated process. Removal of free halide from the reaction mixture affords a long-lived cationic species that can be captured by a different nucleophile, such as solvent, the silver(I) counteranion, or an intramolecular nucleophile. This reactivity has been exploited in many different ways throughout the years and is examined in greater detail later in this chapter. 

The reaction of CH2 with C02 was first postulated by Kistiakowsky and Sauer (13) as taking place via an a-lactone intermediate. The occurrence of this reaction was subsequently demonstrated by Milligan and Jacox (14) in low temperature matrices. These low temperature matrix isolation experiments, however, could not determine definitely the structure of the CH2 C02 intermediate. The result of our laser absorption experiment shows that the CO is vibrationally excited up to v — 4 with a distribution close to the one predicted by a statistical model assuming the existence of a long-lived CH2 C02 complex. This calculation, however, is insensitive to the structure of the complex assumed. Since the ground state triplet CH2 is known to be less reactive and kinetically behaves like CHj (15,16), which does not react readily with C02, the singlet A CH2 is assumed to be involved in the reaction. 

A clear evidence for a long-lived intermediate came from investigations of the stereoselectivity of the Paterno-Buchi reaction with cis and trans 2-butene as substrates, when acetone [52] or benzaldehyde [53] was used as carbonyl addends, complete stereo-randomization was observed. Acetelde-hyde and 2-naphthaldehyde showed stereoselective addition reactions which accounts for the singlet mi as the reactive state [43]. Fleming and Gao... 

For a long time, carbenes, neutral carbon species with a divalent carbon atom bearing six valence electrons, were considered to be too reactive to be isolated [1]. As a consequence, many chemists hesitated to make use of these compounds, especially as spectator ligands for transition metal chemistry. However, whereas the majority of carbenes are short-lived reactive intermediates, this picture does not hold for N-heterocyclic carbenes [2],... 

Living polymerization—A polymerization in which the reactive intermediates have very long lifetimes. An example is an anionic polymerization with no termination steps. 

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