Free triplet carbenes

This is not surprising, since triplet carbenes are free radicals. But singlet carbenes can also give this reaction, though in this case only halogen atoms are abstracted, not hydrogen. 

The term methylene is exclusively reserved for CH2. Spectroscopic investigations demonstrated that photolysis of diazo compounds in fact produces free carbenes. A flash photolysis of diazomethane gave methylene whose spectrum could be recorded 3i>. ESR-spectra were taken of a series of triplet carbenes which had been obtained by direct irradiation of diazo compounds in various matrices at low temperatures (see p. 97). 

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. 

Triplet carbenes have a singly occupied p orbital, as is the case for radicals, and hence react like those radicals. Hydrogen atom transfer reactions are fundamental reaction pathways of triplet carbenes. The reaction of a triplet carbene with a hydrocarbon is quite analogous to the free radical hydrogen atom transfer process (Scheme 9.6). 

The reaction of triplet carbenes with a persistent nitroxide such as 2,2,6,6-tetra-methylpiperidine A -oxide (TEMPO, 84) to form benzophenone would be spin allowed and >100-kcal/mol exothermic (Scheme 9.26). The reaction has a few parallels in free radical chemistry, such as the reaction of tert-butoxyl with carbon monoxide (to yield CO2) or with phosphorus (111) substrates to yield P(V) products. " ... 

Triplet carbenes can abstract hydrogen or other atoms to give free radicals, e.g, CH2 + CHjCHj--------------------------------> CH,. +. CH2CH3... 

The photolysis of diazoalkanes both in the gas phase and in solution is a carbenoid reaction. Moreover, the results of EPR-spectroscopic investigations (Section IIB) demonstrate that triplet carbenes can be generated by irradiation of diazoalkanes. That the reactive intermediates in carbenoid reactions are free carbenes is usually taken as self-evident. While such an assumption is probably wholly justified in most cases, it is worth remembering that both in the gas phase and in solvents such as n-hexane, the electronic absorption spectra of simple diazoalkanes show definite fine structure (Bradley etal., 1964a). This implies that the photo-excited state is bonding (Hoffmann, 1966) and consequently may have a life-time long enough to enable it to react directly with another molecule... 

Triplet carbenes can abstract hydrogen or other atoms to give free radicals, for example. 

We can think of the singlet state of a free carbene, in terms of localized hybrid orbitals, as a bent molecule (6) harboring three sp2 orbitals (one of which is doubly occupied and non-bonding) and an empty, perpendicularly-situated 2p orbital. A triplet carbene (really a diradical) would be a less bent species (7) with two singly-occupied, orthogonal 2p orbitals and two sp orbitals that are involved in bonding to substituents. 

Whereas free singlet carbenes are rather unselective with respect to formation of cyclopropane 22 or ylide 23 and the cyclopropane is favored under conditions that populate the triplet state of a carbene (see Section I.2.I.2.4.2.6.2.), the metal carbenes generated with copper or rhodium catalysts display a selectivity for functional groups which are more nucleophilic than a double bond. Thus, no cyclopropanes are obtained from dialkylallylamines allyl sulfides -allyl dithioacetals , and allyl selenides under carbenoid conditions (copper or rhodium catalysts). 

Diphenylcarbene (diphenylmethylene) can be generated from diphenyldiazomethane (373) by direct irradiation or by triplet sensitization.1156 The intermediate multiplicity then controls the subsequent reactions the singlet carbene inserts into the O—H bond of methanol, whereas the triplet carbene adds to an a I Irene (Scheme 6.173). It has been found that singlet and triplet diphenylcarbenes are in rapid equilibrium relative to the rates of reactions.1157,1158 Competitive quenching experiments (to obtain k1 and kTS) and laser flash spectroscopy (Section 3.7 to obtain k2 and kST) allowed the determination of the free energy difference between the singlet and triplet states of carbene ( 20kJ mol J). 

The stereochemistry of these cycloadditions is so specific that it may be used as a diagnostic test for the identification of singlet and triplet carbenes. If the reaction is conducted in the presence of triplet quenchers, substances such as butadiene, which selectively remove any triplet carbenes, the addition is again stereospecific. Reactions involving free carbenes are very exothermic because two new a bonds... 

Sol 3. (d) Diazo compounds liberate the free carbenes under photochemical conditions that are in singlet states. This process can be directed toward the formation of a triplet carbene using a triplet sensitizer like benzophenone. For triplet carbenes, the aUcene insertion reaction is nonstereospecific. Therefore, the reaction of Z-but-2-ene with triplet carbene gives a mixture of cisHrans-1,2-dimethylcyclopropane. 

A further method available for the study of triplet carbenes exclusively is ESR spectroscopy. CIDNP experiments are used as well to investigate the spin state of carbenes in solution. Generation of a carbene from 3H-diazirine or diazo compounds by thermolysis or photolysis with evolution of nitrogen yields a free carbene (Schemes 1 and 2). 

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