Reactive intermediates carbenes, features

There are several unique features about PAC. First, PAC and the related methods are the only experimental techniques currently available, which can measure the heats of reaction of carbenes on the microsecond and faster time scale. This usually allows for an accurate determination of the heats of formation of these reactive intermediates. Second, PAC can monitor the reactions of transients which are optically transparent, i.e. do not have an UV-VIS optical absorbance. Hence, in addition to thermodynamics, PAC can also provide important kinetic information about these invisible species. 

Carbenes play important roles both as reactive intermediates and also as ligands consequently, considerable effort has been devoted to understanding their molecular and electronic structures. Special interest is associated with carbenes that feature the attachment of donor groups to the carbenic carbon since they behave as nucleophiles and, in some instances, can be isolated. Pioneering work on nucleo-... 

Both Figures 5.2 and 5.3 provide a starting point for the consideration of reactive intermediates and fiieir roles in organic reactions. In the current discussion, we want to focus on the structural features that affect the stability of reactive intermediates and then relate this understanding to the chemical course and kinetics of a reaction. In most cases these reactive intermediates will be ions in which a carbon atom formally bears the positive or negative charge or radicals and carbenes in which the nonbonded electrons are formally localized on a carbon atom. Certainly there are other carbon-centered reactive intermediates (such as carbynes and atomic carbon ), and... 

The reactivity of carbenes/carbenoids towards C=C bonds continues to attract much attention from theoretical and synthetic chemists alike. Calculations using different levels of theory have thus been conducted to better understand the reactivity of cyclo-propenylidene towards C=C bonds. This DFT study has suggested that the reaction involves two pathways from a common intermediate to give products featuring three-and four-membered rings, respectively. Computational methods applied for the first time to intra- and inter-molecular cyclopropanations involving oxiranyllithiums have provided mechanistic rationale for such carbenoid reactions. " While the intramolecular cyclopropanation of oxiranyllithiums equipped with an olefinic moiety (i.e., 1,2-epoxyhexene used as model substrate) proved to follow either a two-step carbolithiation pathway or a concerted methylene transfer, the latter route predominates for intermolecular cyclopropanation. 

S+3C] Heterocyclisations have been successfully effected starting from 4-amino-l-azadiene derivatives. The cycloaddition of reactive 4-amino-1-aza-1,3-butadienes towards alkenylcarbene complexes goes to completion in THF at a temperature as low as -40 °C to produce substituted 4,5-dihydro-3H-azepines in 52-91% yield [115] (Scheme 66). Monitoring the reaction by NMR allowed various intermediates to be determined and the reaction course outlined in Scheme 66 to be established. This mechanism features the following points in the chemistry of Fischer carbene complexes (i) the reaction is initiated at -78 °C by nucleophilic 1,2-addition and (ii) the key step cyclisation is triggered by a [l,2]-W(CO)5 shift. 

This article is an attempt at evaluating new important features of tin(II) chemistry the central point is the interrelationship between molecular structure and reactivity of molecular tin(II) compounds. To define these compounds more closely, only those are discussed which are stable, monomeric in solvents and which may be classified as carbene analogs21. Thus, not a complete survey of tin(II) chemistry is given but stress is laid on the structures and reactions of selected compounds. A general introduction to the subject precedes the main chapters. For comparison, also solid-state tin(II) chemistry is included to demonstrate the great resemblance with molecular tin(II) chemistry. Tin(II) compounds, which are either generated as intermediates or only under definite conditions such as temperature or pressure, are not described in detail. 

The elucidation of the mechanism for olefin metathesis reactions has provided one of the most challenging problems in organometallic chemistry. In Volume 1 Rooney and Stewart concluded that the carbene chain mechanism is now generally accepted for olefin metathesis reactions, but much remains to be learned about the formation and reactivity of metal-carbene intermediates, metallocycles, and especially the mechanistic aspects of chain initiations. Since that report, systems have been designed that begin to reveal the important mechanistic features of olefin metathesis. 

Davies has provided a detailed analysis of how the arrangement of four proli-nate ligands around a dirhodium core can lead to a metal carbene intermediate that reacts further with such high stereocontrol [12]. The two most striking features of the vinyl diazoester cyclopropanations are the excellent diastereoselec-tivity and the total lack of reactivity of frans-disubstituted alkenes. The former is accounted for by the structure of the vinyl diazoester which suggests that high... 

The effect of multiplicity of carbenes on their reactivity is most vividly marked in the following features rationalized by Skell et al. from experimental data [37-39]. First, the reaction of carbenes occurs in the singlet electron state at a much faster rate than in the triplet, with the absolute rates of typical reactions of addition to multiple bonds and of insertion into the C—H bonds exceeding, under normal conditions, the rate of intercombination conversion. Secondly, the singlet carbenes are characterized by one-step stereospecific addition to double bonds, as, for instance, in the cyclopropanation reaction, while the triplet carbenes react in a nonstereospecific way to form first an intermediate biradical through addition to one of the atoms of the double bond. The formation of a trimethylene radical, in the course of reaction of triplet methylene ( B ) with ethylene, has been confirmed by semiempirical [40, 41] and ab initio [42, 43] quantum chemical calculations. 

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