Cyclopropane electronic states

Cyclopropane (D3 , symmetry) has a degenerate pair of in-plane e orbitals (S, A). Vertical ionization leads to a doubly degenerate E state, and JT distortion results in two nondegenerate electronic states, A and 82 (Cav symmetry), corresponding to two different molecular structures. The Ai state (orbital S singly occupied) corresponds to a structure with one lengthened C—C bond it is lowest in energy for many cyclopropane radical cations (Fig. 6.10). 

The MOs and electronic states of carbene have been discussed in Chapter 7. The orbital and state correlation diagrams for addition of CH2 to ethylene is shown in Figure 14.9. The Walsh bonding picture for the MOs of cyclopropane requires that the and a MOs of the ethylene also be included in the diagram. The a2 and least-motion pathway preserves a vertical plane of symmetry (as well as the other elements of the C2v point group), and the... 

Cyclopropane bond angle, 16 bonding, 84-85 correlation diagrams, 207 electronic states, 207 hybridization, 16 point group of, 5 structure of, 16, 84 Walsh orbitals, 85 Cyclopropanes... 

The addition of dihalocarbenes to double bonds occurs stereospecifically to give cyclopropane derivatives [68J. The reaction appears to have electrophilic character even though the electronic state of the carbene is uncertain (cf. p. 341). Again the direction of attack on the double bond is subject to steric control. The diene (2) for example, reacts with di-bromocarbene to give first the 2ct,3[Pg.294]

In this reaction, triplet methylene inserts in the ethene molecule to produce an energized cyclopropane molecule, which may then either isomerize to propene or may be collisionally deactivated to give cyclopropane. Molecules may also be activated photochemically. Although most photochemical processes involve more than one electronic state of the molecule, it is possible in some cases to produce molecules in their electronic ground states with high vibrational excitation, and these may subsequently isomerize or dissociate. A typical example is the photoexcitation of cycloheptatriene followed by its isomerization to toluene. 

The electronic state responsible for reactions II and III is not so well known and it is almost certain that there are essential differences in this respect between the different cyclic ketones. On the basis of the different pressure dependence of propene and cyclopropane formation in the direct photolysis of cyclobutanone, McGee concluded that cyclopropane is formed from the excited singlet state, while propene is considered to be a triplet product. 

Transient carbenes display a rich and diverse chemistry as stoichiometric reagents, for example, in reactions such as olefin cyclopropanation, C-H insertion, dimerization, 1,2-migration, and so on. Carbenes are important in several synthetic methods and are growing in importance, especially the intramolecular versions. Carbenes are electron deficient, and unless strong resonance interaction is possible the reactions will be electrophilic. The chemical behavior of a carbene depends to some extent on its method of preparation, electronic state, and also on the presence or absence of certain metals or metallic salts. The state in which the carbene is produced depends on the method of generation, that is, singlets... 

The addition reaction with alkenes is the best-studied reaction of carbene intermediates, both from the point of view of understanding carbene mechanisms and for synthetic applications. The usual course of reaction of a carbene with an alkene results in the formation of a cyclopropane, an observation that is true for both the singlet and the triplet state of most carbenes. The alternative electronic states... 

The effect of multiplicity of carbenes on their reactivity is most vividly marked in the following features rationalized by Skell et al. from experimental data [37-39]. First, the reaction of carbenes occurs in the singlet electron state at a much faster rate than in the triplet, with the absolute rates of typical reactions of addition to multiple bonds and of insertion into the C—H bonds exceeding, under normal conditions, the rate of intercombination conversion. Secondly, the singlet carbenes are characterized by one-step stereospecific addition to double bonds, as, for instance, in the cyclopropanation reaction, while the triplet carbenes react in a nonstereospecific way to form first an intermediate biradical through addition to one of the atoms of the double bond. The formation of a trimethylene radical, in the course of reaction of triplet methylene ( B ) with ethylene, has been confirmed by semiempirical [40, 41] and ab initio [42, 43] quantum chemical calculations. 

The electronic state of the carbene -whether it is singlet or triplet - may have important consequences for how it reacts. For example, a singlet carbene adding to a double bond to produce a cyclopropane ring does so in one stage, so any stereochemistry of the double bond is preserved in the product. If the carbene has a triplet state it acts as a diradical, and the reaction proceeds in two stages. Any stereochemistry about the double bond will not be preserved in the final product... 

From the point of view of both synthetic and mechanistic interest, much attention has been focused on the addition reaction between carbenes and alkenes to give cyclopropanes. Characterization of the reactivity of substituted carbenes in addition reactions has emphasized stereochemistry and selectivity. The reactivities of singlet and triplet states are expected to be different. The triplet state is a diradical, and would be expected to exhibit a selectivity similar to free radicals and other species with unpaired electrons. The singlet state, with its unfilled p orbital, should be electrophilic and exhibit reactivity patterns similar to other electrophiles. Moreover, a triplet addition... 

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... 

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