Alkenes triplet carbenes

Addition reactions with alkenes to form cyclopropanes are the most studied reactions of carbenes, both from the point of view of understanding mechanisms and for synthetic applications. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. With triplet carbenes, an intermediate 1,3-diradical is involved. Closure to cyclopropane requires spin inversion. The rate of spin inversion is slow relative to rotation about single bonds, so mixtures of the two possible stereoisomers are obtained from either alkene stereoisomer. 

Fig. 10.1. Mechanisms for addition of singlet and triplet carbenes to alkenes.

Singlet and triplet carbenes exhibit different properties and, to great extent, show markedly different chemistry . For example, a singlet carbene will add to a c/ -disubstituted alkene to produce only cis-disubstituted cyclopropane products (and to a rara-disubstituted alkene to produce only /rara-disubstituted cyclopropane products), while a triplet carbene will add non-stereospecifically to produce a mixture of cis and trans products. 

Surprisingly, the critical experiment has been done infrequently over the last one-half of a century The requirements for an experiment that truly speaks to the issue at hand are that one be able to see the results of addition of both spin states of a single carbene, and these requirements rarely have been met. For example, the direct irradiation of methyl diazomalonate leads to the stereospecific addition expected of a singlet carbene, whereas the photosensitized decomposition of the diazo compound leads to formation of the triplet carbene and loss of the stereochemical relationship originally present in the reacting alkene. Rotational equilibration in the intermediate seems to be complete, as it makes no significant difference whether cis or trans alkene is used as starting material (Scheme 7.9). ... 

Product Studios. Alkenes are known as diagnostic reagents for spin state of reacting carbenes. Thus, the reaction of a singlet carbene with an olefin usually results in the formation of a cyclopropane through stereospecific addition to the C—C double bond, while a triplet carbene gives rise to a nonstereospecific addition product (see Section 7). 

Triplet carbenes show enhanced reactivity toward alkenes that can stabilize the intermediate radical center. For example, the reactivity of 1,3-butadiene toward DPC is shown to be some a 100 times larger than that of 1-hexene. ... 

Problem 9.29 (a) Use the following observations to discuss stereospecificity of carbene addition, (b) Suggest mechanisms for addition of (i) singlet and (ii) triplet carbenes to alkenes. 

In the presence of alkenes, photolysis of alkyl (silyl)diazoacetates leads mainly to the formation of cyclopropanes as diastereomeric mixtures4,111,112. With (Z)- and ( )-but-2-ene, the cyclopropanation is not completely stereospecific with respect to the double bond configuration, but gives a small amount of the wrong isomer these results point to the participation of a triplet carbene in the cyclopropanation reaction. Allylic C,H insertion products are also formed their yield increases in the series 1,1-, 1,2-, tri- and tetrasubstituted C=C bond. With 2,3-dimethyl-but-2-ene, the allylic C,H insertion product is formed at the complete expense of the cyclopropane. 

Triplet diphenylcarbene has a Tl/2 of 2 ps in the presence of 1.0 M isoprene (Eisenthal et al., 1980). The lifetime of singlet phenylchlorocar-bene ranges from 5 to 500 ns in the presence of 1.0 M alkenes with various substituents (Turroetal., 1980). In 1.0M methanol singlet fluorenylidene and singlet diphenylcarbene have half-lives of 0.77 ns and 0.02 ns respectively (Zupancic and Schuster, 1980 Eisenthal et al., 1980). Triplet carbenes react very rapidly with 02. 

Carbenes, as synthetic species, are fascinating chemicals, and the synthesis of cyclopropanes by a [2 +1] cycloaddition of alkenes with carbenes represents an extremely fruitful approach. Recently, highly effective intramolecular [2 + 1] cycloadditions, novel triplet sensitizers, metal-catalyzed cyclopropanations, and novel precursors of carbenes have been developed. 

In the photochemistry of a-diazo carbonyl compounds, singlet carbenes undergo a Wolff rearrangement while triplet carbenes react with alkenes to afford cydopropanes. 

The alkene insertion reaction is stereospecific only for singlet carbenes. For triplet carbenes, the reaction is nonstereospecific. Though carbenes formed thermally from diazoalkenes must initially be singlets, photochemistry is one way to provide the energy needed for their transformation to the more stable triplet. 

The mechanism of this nonspecific reaction must be different. In fact, a concerted reaction is impossible for triplet carbenes because of the spins of the electrons involved. After the carbene adds to the alkene in a radical reaction, the diradical (triplet) intermediate must wait until one of the spins inverts so that the second C-C bond can be formed with paired electrons. This intermediate also lives long enough for C-C bond rotation and loss of stereochemistry. 

In principle, triplet carbene insertions should follow a two-step radical pathway analogous to their insertion into alkenes. However, very few triplet carbene insertions into C-H bonds have been observed, and the stereochemical consequence of the two-step mechanism (which should result in mixtures of stereoisomers on insertion into a C-H bond at a stereogenic centre) has never been verified. 

Moreno and coworkers published a study on the triplet carbene-ethene addition reaction. This process should involve two steps, namely the formation of a triplet trimethylene 1,3-diradical as an intermediate followed by intersystem crossing and formation of 1. The intermediate may live long enough to permit rotation at the CC bonds. In this way, the stereochemistry of the alkene will be lost and a non-stereospecific addition takes place. Moreno and coworkers calculated for the first step of the reaction C 2C i) H2C = CH2 a barrier of 11 kcalmol" and a reaction energy of -26 kcalmoF at the MP2/3-21G level. (It has to be mentioned in this connection that the 3-21G basis is far too small to lead to reliable energies in correlation calculations and therefore results are just of a qualitative nature ) Activation energies varied from 5 to 17 kcalmoF if CH2 was replaced by the triplet state of CH(CN), CH(BeH) and CHLi ... 

The stereochemistry of reactions between carbenes and alkenes is determined by the states of the carbenes (when generated), whereby singlet carbenes react in a stereospecific one-step concerted process whilst triplet carbenes lead to a mixture of products via a diradical intermediate (Figure 6.58). Consequently, since fluorocarbenes are singlets in the ground state (Table 6.2), cyclopropanation of alkenes is often stereospecific [91] (Figure 6.59) (for more examples, see Sections A and B). 

The monosubstituted (trifluoromethyl)carbene has only been generated by irradiation of 2,2,2-trifluorodiazocthane, which is prepared by nitrosation of 2,2,2-trifluoroethylamine.1,3 175 The course of reactions with alkenes is dependent on concentration and pressure conditions. Thus, insertion products and pyrazolines may be obtained in competition with [2+1] cycloaddition giving the cyclopropane system. Yields of the latter are often moderate and, depending on singlet or triplet carbene formation, the reaction is not always stereoselective (Table 14). 

In contrast with fluorenylidene, singlet-triplet equilibrium in the carbene derived by photoelimination of nitrogen from the diazodihydroboraanthracene (57) is slow. Direct irradiation affords singlet carbene which reacts with propan-2-ol to give the ether (58), whereas 2-acetonaphthone-sensitized irradiation yields triplet carbene which undergoes non-stereospecific addition to alkenes. Products obtained by irradiation of 3-diazo-2,5-diphenylpyrrole can arise via the intermediacy of singlet and triplet carbenes. Decomposition of the pyrrole (59) in the presence of benzene, for example, yields the 2H-cycloocta[c]pyiTole (60) as the major product. 

Stereospeoifioity (or lack of it) in the addition of a carbene to an alkene can be a good test of whether the carbene reacts as a singlet or triplet lack of stereospeoifioity in a carbene addition almost certainly indicates that a triplet carbene is involved, but the fact that an addition is stereospeoifio doesn t mean that the carbene mustbe a... 

The addition of a triplet carbene to an alkene can be considered to be rather like a radical addition to a double bond. The concerted addition of a singlet carbene, on the other hand, is a pericyclic reaction, and from Chapter 3 5 you should be able to classify it as a [1 -h 2 ] cycloaddition, addition to alkenes of triplet carbenes is a radical reaction addition of singlet carbenes is a [1-h2] cycloaddition... 

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