Triplet carbenes reactivity differences

Most of the reactions of triplet carbenes discussed in this chapter will deal with reactions in solution, but some reactions in the gas phase will also be included. Triplet carbenes may be expected to show a radical-like behaviour, since their reactions usually involve only one of their two electrons. In this, triplet carbenes differ from singlet carbenes, which resemble both carbenium ions (electron sextet) and carbanions (free electron pair). Radical like behaviour may, also be expected in the first excited singlet state Sr e.g. the state in CH2) since here, too, two unpaired electrons are present in the reactive intermediate. These Sj-carbenes are magnetically inert, i.e., should not show ESR activity. Since in a number of studies ESR spectra could be taken of the triplet carbene, the reactions most probably involved the Ti-carbene state. However, this question should be studied in more detail. 

The radical versus electrophilic character of triplet and singlet carbenes also shows up in relative reactivity patterns shown in Table 10.1. The relative reactivity of singlet dibromocarbene toward alkenes is more similar to that of electrophiles (bromination, epoxidation) than to that of radicals ( CC13). Carbene reactivity is strongly affected by substituents.61 Various singlet carbenes have been characterized as nucleophilic, ambi-philic, and electrophilic as shown in Table 10.2. This classification is based on relative reactivity toward a series of different alkenes containing both nucleophilic alkenes, such as tetramethylethylene, and electrophilic ones, such as acrylonitrile. The principal structural feature that determines the reactivity of the carbene is the ability of the substituents to act as electron donors. For example, dimethoxycarbene is devoid of electrophilicity toward... 

Reactivity Differences between Triplet and Excited Triplet Carbenes. 438... 

Simple MO pictures also explain the differences in the bond dissociation energies (BDE) of methane and ammonia, and explain different triplet carbene and nitrene reactivities toward hydrogen atom donors. 

A more widely held view, argued persuasively by Gaspar and Hammond (1964), is that carbenes of different multiplicity react in ways which are quantitatively, and in some cases qualitatively, different, and that the spin state of the carbene must be taken into account in any discussion of reactivity. Since, for many carbenes, spectroscopic evidence (Section IIB) indicates that the ground state is a triplet, low reactivity and high selectivity is often regarded as being associated with this spin state. Further, the triplet state is associated with non-stereospecific addition to olefins, whereas stereospecific addition is usually taken to indicate reaction by way of the singlet state. 

On a simple view of the reactivity of carbenes, and accepting the argument that singlet and triplet states show different chemical... 

Carbenes are highly reactive and undergo insertion into a-bonds, cycloaddition reactions, dimerization, complex formation and intramolecular reactions. The singlet carbene, which often acts as an electrophile, gives different products than the triplet carbene, which behaves as a radical. Despite their very different nature, they manage to produce the same product in some reactions. 

Carbenes will add to alkenes to form cyclopropanes. The mechanism of these insertion reactions depends on whether a singlet carbene (this contains a pair of reactive electrons in the same orbital) or a triplet carbene (this contains two unpaired and reactive electrons in different orbitals) adds to the double bond. 

The relationship between the reactivities of singlet and triplet carbenes in some of these processes has, however, attracted the interest of many investigators. Chemical evidence from such studies led to the conclusion that the singlet state can be intercepted due to its higher energy and that it displays a different chemistry from that of the triplet, since Skell and Woodworth developed their rule in the late 1950 s and indicated that it may also be applied to carbene reactions other than cyclopropanation. 

The low-temperature irradiation of the series of quinoline diazomethanes 543 has shown that these are good precursors of triplet carbenes. These carbenes have been shown to be present as two rotamers. The cyclopentadienylcarbene derivative 544 and fluorenylidene have also been shown to be triplets. It is clear that structural factors play an important role in determining the reactivity of carbenes. Typical of this is the behaviour of the dianthrylcarbene 545, which reacts differently from other diaryl carbenes. ... 

The rate constant of the triplet carbene with a typical quencher such as O2 and 1,4-cyclohexadiene can be employed as a more quantitative measure of the reactivity. However, neither ko nor chd appreciably correlated with the D value (Table 2). Presumably, simple linear correlations with spin delocalization factors may not be expected for the reaction of triplet carbenes (diradicals) to form the corresponding monoradicals since the extents of the delocalization of unpaired electrons should be different between the two states. An additional... 

The kinetic data obtained using typical triplet quenchers should be more reliable to estimate the reactivity difference between the two carbenes especially in this case in which the two carbenes decay in different pathways. The data, especially that the rate constant for H abstraction from CHD observed with the 2-isomer are two orders of magnitude greater than that of the 1 -isomer, clearly indicate that the 1-isomer is considerably less reactive than the 2-isomer (Table 7). 

The reactivity of a carbene is strongly influenced by its multiplicity—that is, whether it is a singlet or triplet. As noted above, some carbenes are produced as singlets, while others are formed as triplets or may convert to triplets before reaction. Singlet and triplet carbenes exhibit similar reaction types, but there are some important differences between them. Because it has both an empty p orbital (like a carbocation) and a nonbonded pair of electrons (like a carbanion), the singlet carbene exhibits both carbocation and carbanion character. However, the triplet carbene behaves more as a diradical. These characteristics influence the types and stereochemistries of carbene reactions. 

Carbenes are prominent in synthetic and polymer chemistry as well as in interstellar and atmospheric chemistry [2-5]. The great interest stems largely from the chemical differences between singlet and triplet carbenes, particularly in their reactivity and rich spectroscopy and photochemistry. The latter were recentiy discussed and summarized in a review by Kable et al. [6]. Laser-induced fluorescence (LIE) studies in particular have yielded a wealth of structural and spectroscopic data as well as useful information on the nature of the potential energy surface of the excited A state, such as barriers to dissociation and... 

The reactivity and lifetime of triplet PN and triplet PC differed remarkably in the inner phase of 6. Whereas PC reacts with 6 already at 15K and persists probably only a few minutes at this temperature, the lifetime of incarcerated PN is 13.6min at -3°C. Both PC and PN preferentially insert into inward-pointing acetal C-H bonds of 6 to produce 115 and 134, respectively (Figures 41 and 43). The difference in reactivity toward C-H insertion between triplet carbenes and an isoelectronic triplet nitrenes is well known and is believed to proceed through a nitrogen rehybridization in the rate-limiting H-abstraction step of the nitrene. Rehybridization is not needed in the carbene reaction. 

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