Temperature dependence triplet carbenes

This key paper was followed by a flurry of activity in this area, spanning several years." " "" A variety of workers reported attempts to deconvolute the temperature dependence of carbene singlet/triplet equilibria and relative reactivities from the influence of solid matrices. Invariably, in low-temperature solids, H-abstraction reactions were found to predominate over other processes. Somewhat similar results were obtained in studies of the temperature and phase dependency of the selectivity of C-H insertion reactions in alkanes. While, for example, primary versus tertiary C-H abstraction became increasingly selective as the temperature was lowered in solution, the reactions became dramatically less selective in the solid phase as temperatures were lowered further. Similar work of Tomioka and co-workers explored variations of OH (singlet reaction) versus C-H (triplet reaction) carbene insertions with alcohols as a function of temperature and medium. Numerous attempts were made in these reports to explain the results based on increases in triplet carbene population... 

This law means that the intensity becomes smaller as the temperature is raised. Deviations from linearity could indicate a temperature-dependent equilibrium between a triplet and a singlet. Conversely, a linear Curie plot is taken as evidence for a triplet ground state far below the singlet, but such observations require that the carbene be stable to a certain extent at elevated temperature. 

An advantage of Sander s method is that EA and IP can be calculated for any carbene, and do not depend (as does w xy) 011 specific set of alkenes or carbenes. Thus, Sander is able to examine species he refers to as super-electrophilic carbenes. [82] These include difluorovinylidene, 29 [84] and 4-oxo-2,3,5,6-tetrafluorocyclohexa-2,5-dienylidene, 30, a highly electrophilic triplet carbene. [85] With EA= 2.26 (observed) or 2.56 eV (computed), [84] carbene 29 is electrophilic enough to insert into the C-H bond of methane, and even the H-H bond of hydrogen at temperatures as low as 40 K. This type of reactivity is also observed [85] with 30 (computed EA= 3.61 eV). [82]... 

Alkynes are formed as by-products but do not come from decomoosition of the cyclobutadienes. The ratio of products formed depends very much on the solvent and temperature and it appears that the cyclobutadienes arise from singlet carbenes and the alkynes from triplet carbenes [54]. 

In order to determine whether QMT may contribute to the overall reaction of diarylcarbenes with hydrogen atom donors in solution at ambient temperature, kinetic isotope effects for the benzylic hydrogen atom abstractions of the triplet states of several diarylcarbenes with toluene-toluene-i g in fluid solution were determined over the temperature ranges of —75 to 135 °C. The results are very much dependent on the structure of the carbene (Table 9.11)." The differential... 

Irradiation of the homochiral pyrazoline (51) leads to the elimination of nitrogen and the stereospecific formation of the cyclopropane (52) (Jimenez et al). The reaction proceeds by way of triplet diradical intermediates but evidently, closure to give the cyclopropane ring is faster than C-C bond rotation. The multiplicity of the diradical from the low-temperature photolysis of dihydropyr-rolo[3,4-d(]pyridazines such as (53) is reported to be wavelength dependent and even at 77K, the singlet and triplet states do not equilibrate to a measurable extent (Bush et ai). The results reported by Motschiedler et al. from the triplet sensitised irradiation of 1,2-diaryldiazoethanes in methanol solution go some way to establishing that 1,2-H and 1,2-C shifts in a spin-equilibrated carbene compete... 

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