Carbenes, generation singlet

Reactions involving free carbenes are very exothermic since two new theoretical treatment of the addition of singlet methylene to ethylene suggests that there is no activation barrier.168 The addition of carbenes to alkenes is an important method for synthesis of many types of cyclopropanes and several of the methods for carbene generation listed in Scheme 10.8 have been adapted for use in synthesis. Scheme 10.9, at the end of this section, gives a number of specific examples. 

An interesting variation appears when furan reacts with the allenic carbene generated by the action of potassium ter-butoxide upon l-bromo-3,3-dimethylallene. Though the yield is only 9%, one product is reasonably assigned a structure (Scheme 52) that could hardly be approached by way of a cyclopropane intermediate. The authors comment that in an allenic carbene in the singlet state two electrons will be accommodated in the sp... 

Take the reaction of carbenes generated by photolysis of diazo compounds (Scheme 9.1), for example. Direct irradiation of a diazo compound (12) is believed to generate the carbene initially in singlet state ( 3) via the singlet excited state of the precursor. Triplet sensization, on the other hand, is presumed to give the triplet... 

Singlet carbenes generated in an 02-doped inert gas matrix also react with O2 to give the corresponding oxides (80b, Scheme 9.22). For example, chlorophenyl-carbene (67) reacts with O2 to give the oxides. However, the reaction is much... 

The comparison of thiophene with thioethers on the one hand and with enol thioethers on the other, in regard to its behaviour towards conventional electrophiles, has been made in Section 3.02.2.3. Attack on carbon is the predominant mode of reaction (Section 3.14.2.4) reaction at sulfur is relatively rare (Section 3.14.2.5). Carbenes are known to act as electrophiles attack at both carbon and sulfur of thiophene has been reported. The carbene generated from diazomalonic ester by rhodium(II) catalysis attacks the sulfur atom of thiophene, resulting in an ylide. It has also been shown that the carbenoid species derived by thermolysis of such an ylide functions as an electrophile, attacking the a-carbon of a second molecule of thiophene (Section 3.14.2.9). Singlet nitrene is electrophilic. However, in contrast to carbenes, it invariably attacks only the carbon atom (Section 3.14.2.9). 

The two unshared electrons in singlet carbenes have opposite spins. Carbenes generated by a-elimination are singlets because the two unshared electrons are derived from the same MO. Triplet carbenes, in which the two unshared electrons have parallel spins, are usually generated by photolysis of diazo compounds. Triplet carbenes are discussed in Chapter 5. 

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). 

Carbenes are readily obtained by the photodecomposition of diazo-compounds. The reactions of carbenes generated in this way in rigid matrices at low temperatures have been reviewed. Addition of singlet methylene to acetonitrile affords the nitrile ylide... 

These results are quite distinct from the chemistry of phenyl carbene (PC) [100]. The chemistry of PC in hydrocarbons is independent of initial singlet or triplet carbene generation. Equilibration of singlet and triplet PC is faster than their intermolecular reactions. 

The effect of substituents on the temperature dependence of a-carbonyl-carbene reactivity has been examined using carbenes generated by low-temperature photolysis of methyl diazophenylacetate. A correction to the literature on the photoreaction of isopropylidene diazomalonate (98) with 1,3,3-trimethyl-cyclohexane (99) has been reported. The photoproduct, originally thought to be a cyclopropane derivative, has now been shown to be the cyclobutanone (100), the formation of which presumably involves a photo-Wolff rearrangement as illustrated in Scheme 11. Substituent effects observed in the product distribution of diazo-amide photochemistry have been ascribed to conformational factors the jS-lactam, oxindole, and Wolff rearrangement products appear to arise directly from the excited singlet state of the sym-Z form of the diazo-amide itself. 

In some instances when the carbene centre is adjacent to a ring, opening or fragmentation takes place. This is exemplified in one of the attempts to synthesize tetrahedrane from the carbene produced on irradiation of 484. Here, cyclization did not occur and instead the formation of two alkynes 485 and 486 results . Such ring opening appears to be the result of the generation of a triplet carbene. The singlet path is followed by 487 and the... 

Until the last decade, product studies formed the main evidence for carbene formation singlet carbenes formed cyclopropanes from alkenes stereospecifically, while triplet carbenes formed cyclopropanes non-stereospecifically. Formation of a cyclopropane (though not by addition to an alkene) via a carbocation route was demonstrated and, more recently, it has been shown that p values for insertion-addition selectivity and for cyclopropanation stereoselectivity vary as to photochemical or thermal generation of the carbene. The authors of this latter study suggest that a ground state diazo compound could be masquerading as a carbene in its thermal reaction with olefins, possibly by electrocyclic... 

In the case of the photochemical reaction, irradiation of diazomethane in the presence of cw-2-butene provides cw-l,2-di-methylcyclopropane with no detectable amount of the tram isomer (eq 45). This reaction is thought to proceed via a singlet carbene. However, if the same reaction is carried out via a triplet carbene, generated via triplet sensitization, then a 1.3 1 mixture of tram to cis dimethylcyclopropane is observed (eq 46). The yields in the photochemical reaction are typically lower than the metal-mediated processes, and are usually accoii5)anied by more side products. 

The stereospecificity of carbene additions has been widely used in experiments designed to determine the multiplicity of a carbene generated under given conditions. Stereospecific addition is taken as evidence for the involvement of the singlet species. The alternate result, formation of a mixture of the possible isomers, requires additional information for complete interpretation. Lack of stereospecificity implies some triplet carbene, but permits simultaneous involvement of the singlet species. 

Simmons-Smith) Various Methods Can Be Used to Generate Singlet Carbenes... 

The generation of carbenes by reaction of metastable singlet carbon atoms with aldehydes and ketones has been described in detail the distribution of intramolecular rearrangement products is identical with that observed from toluene-p-sulphonylhydrazone-derived carbenes. Suitably substituted carbenes generated in this manner, e.g. that prepared from (4), can be trapped by olefins, when the cyclopropane (5) produced retains the original olefin geometry, indicative of a singlet species. 

The nitrogen analogs of carbenes are called nitrenes. As with the carbenes, both singlet and triplet electronic states are possible. The triplet state is usually the ground state, but either species can be involved in reactions. By far the most commonly applied means of generating a nitrene is photolysis or thermolysis of an azide. This method is clearly analogous to formation of a carbene from a diazo compound. The types of azides in which the decomposition has been studied... 

As to other carbenes, some react as triplets, some as singlets, and others as singlets or triplets, depending on how they are generated. There are, however, molecules that generate persistent triplet carbenes. 

There is a limitation to the use of stereospecificity of addition as a diagnostic test for singlet or triplet carbenes.When carbenes are generated by photolytic methods, they are often in a highly excited singlet state. When they add to the double bond, the addition is stereospecific but the cyclopropane formed carries excess... 

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