Free singlet carbenes

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). 

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. 

Closs and Trifunac, 1970 Baldwin and Andrist, 1971 Lepley and Closs, 1972 Bethell and McDonald, 1977). The formation of free radicals from aromatic carbenes is often easily detected by the fast laser spectroscopic techniques discussed earlier. The radicals generally have characteristic absorption spectra and reactivity patterns that make their identification certain. The direct insertion reaction of singlet carbenes is not expected to generate free radicals. 

Alternatively, if some of the reaction occurs by the hydrogen abstraction-recombination route proceeding through free radicals, then some mono-deuteriated (crossed) products will be formed (14). The former result is taken to be characteristic of the reaction of a singlet carbene, the latter that of a triplet. 

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. 

A second characteristic reaction of carbenes is addition to olefins to yield cyclopropanes. Singlet carbenes might react as either nucleophiles or electrophiles triplets may be expected to behave like free radicals- The data in Table 5.9, showing the increase in rate of addition on substitution of electron-donating... 

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 philicity of singlet carbenes is an important concept to classify carbenes that was systematically studied by Moss. [9-11] The relative reactivity (selectivity) of a series of singlet carbenes in cyclopropanation reactions with electron rich and electron poor carbenes was used to quantify the carbene philicity. An empirical carbene philicity scale with a parameter niQ- (where X and Y are the substituents at the carbene center) was defined (Figure 1). Electrophilic carbenes show Wqxy values below 1, nucleophilic carbenes above 2, and ambiphiles are between. [10] Ambiphilic carbenes act as an electrophile towards electron-rich alkenes and as a nucleophile towards electron-poor alkenes. The niQ- Y values obey an empirical linear free energy relationship with the Taft substituent parameters and Oj. This allows to estimate the niQ- Y values of unknown carbenes. 

Gloss and Moss have suggested the term carhenoid to describe intermediates that exhibit reactions quahtatively similar to those of carbenes without necessarily being free divalent carbon species. When carbenes are coordinated with metals they are known as carbenoids that have modified reactivities. lodomethylzinc iodide is often referred to as a carhenoid, because it resembles a carbene in its chemical reactions (Simmons-Smith reaction) (Scheme 5.14). Rhodium and copper carbenoids are unstable whereas some transition metals such as tungsten and chromium form stable and isolable carbenoids called metallocarhenes or Fischer carbenes. Carbenoids are structurally related to singlet carbenes and possess similar reactivity. 

In addition to various FLPs, other metal-free catalysts for H2 activation found recently include singlet carbenes [31-37], digermyne [38-46], and distannyle compounds [47]. Theoretical calculations [35] revealed that a singlet carbene has a vacant p orbital and a lone pair in a nonbonding orbital, similar to a transition... 

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