Carbenes triplet methylene

Phenylcarbene (la). Just as in triplet methylene (CH2), in triplet phenylcarbene (3A"-la) one electron occupies the p-jr atomic orbital on the carbene carbon and one electron occupies the in-plane a hybrid orbital. However, in the lowest singlet state of CH2 and of phenylcarbene ( A -la), both electrons occupy the hybrid a orbital, because this orbital is substantially lower in energy than the p-jt AO. 

The mechanism proposed for carbene-abstraction and carbene-insertion reactions is based on the calculations of Dewar (MINDO/2) and Hoffmann (extended Hiickel) Hoffmann dealt only with the concerted reactions of singlet carbenes, whereas Dewar discussed both singlet and triplet carbene reactions. The calculations of Dewar s ) for the reaction of triplet methylene with methane gave the following results ... 

Hoffmann was the first to apply the concept of orbital symmetry to the cycloaddition of carbenes to olefins. This concept, which is based on EH-calculations was demonstrated for the [H-2]-cyclo-addition of triplet methylene to ethylene. 

It was demonstrated in the previous section that calculated IR spectra are able to reproduce the experimental spectra of highly reactive molecules to the point that they are extremely useful to the experimentalist in confirming the synthesis of such species. As will be seen in this section, the same holds true for reactive intermediates. We undertook the calculation of IR spectra of both singlet and triplet methylene in part to test the reliability of computed ab initio spectra of reactive intermediates such as carbenes. Among the known carbene intermediates, methylene is perhaps the most studied. It represents the simplest carbene, consisting... 

The origin of the difference lies in the fact that triplet carbenes are biradicals (or diradicals) and exhibit chemistry similar to that exhibited by radicals, while singlet carbenes incorporate both nucleophilic and electrophilic sites, e.g., for singlet and triplet methylene. 

Although singlet and triplet imidogen have similar absorption spectra, singlet and triplet methylene do not. In fact, most carbenes have rather poor chromo-phores for UV-vis detection by LEP and must be visualized by trapping with pyridine to form ylides. The exceptions are arylcarbenes, which have n n transitions localized on the aromatic n system. " ... 

It is of interest to compare the properties of substituted carbenes with those of methylene itself. Theoretical calculations predict that as a linear carbene triplet begins to bend, the orbitals containing the unpaired electrons must rehybridize and gain some 5 character. The... 

Energy-resolved rate constant measurements near the threshold for triplet methylene formation from ketene have been used to provide confirmation of the fundamental hypothesis of statistical transition state theory (that rates are controlled by the number of energetically accessible vibrational states at the transition state).6 The electronic structure and aromaticity of planar singlet n2-carbenes has been studied by 7t-electron coupling perturbation theory.7 The heats of formation of three ground-state triplet carbenes have been determined by collision-induced dissociation threshold analysis.8 The heats of formation of methylene, vinylcarbene (H2C=CHCH), and phenylcarbene were found to be 92.2 3.7, 93.3 3.4, and 102.8 33.5 kcal mol-1, respectively. 

The simplest carbene CH2 ( methylene, Figure 3.14, center) is a bent molecule with an H,C,H bond angle of 135° and has a triplet ground state. The singlet CH2 is less stable by 8 kcal/mol. Its free electron pair occupies the sp AO (because in this orbital it is nearer to the nucleus and therefore more stabilized than in the 2pz AO), and the H,C,H bond angle amounts to 105° (two electrons in the sp AO as compared to one electron in the sp AO of triplet CH2 cf. the discussion of VSEPR theory in Section 1.1.1). 

Carbenes are highly reactive electron-dehcient species generally having short lifetimes (less than Is).Tliey have been isolated only in matrices at very low temperatures (77 K or less). The two nonbonded electrons of a carbene may be either paired or unpaired. The species is termed a singlet if they are paired and a triplet if they are unpaired. The parent of carbenes is methylene ( CH2). 

It is informative to compare methylene and nitrene in terms of simple molecular orbital theory and then to extend this comparison to their aromatic derivatives phenyl carbene and phenyl nitrene. Hoffman et al. [28] have described the molecular orbitals of CH2 in the following way. Methylene has two nonbonding molecular orbitals, one is an in-plane, a type, hybrid orbital, the other is an out-of-plane pure p type n orbital. Singlet methylene has a bond angle of 105° and the in-plane, sigma, orbital is doubly occupied. Triplet methylene has both a singly occupied n and a orbital and a bond angle of 135°. 

Photolysis of diazo compounds (305) generates nitrogen and the carbene 289. Photolysis of diazomethane (311, R = H), for example, generates methylene (H2C ),242 Benzophenone is often added as a sensitizer for the photolysis of diazomethane. Under these conditions, triplet methylene is formed via intersystem crossing (5i Ti)243 (sec. II.IO.B). Energy transfer from triplet benzophenone to triplet diazomethane followed, and triplet diazomethane decomposed to triplet methylene.244 a similar thermal reaction gives the carbene and the usual carbene reactions.245... 

Study of diaryl and arylcarbenes. [7] Wasserman and co-workers [8] used EPR speetroseopy to demonstrate that triplet methylene has a bond angle of 135 as first predieted by Foster and Boys. [9] EPR spectroscopy is now a standard tool for the study of triplet carbenes. 

Table 7.3 gives experimental values of the fine-structure constants D and E for some aromatic molecules in their excited triplet states Ti and for the carbene CH2 (methylene) in its electronic ground state, which is a triplet state. In molecules or in molecular crystals which have a three- or more-fold axis of symmetry that is chosen to be the the z axis, it is = (712)- this cylindrical symmetry, it follows from Eq. (7.5b) that E = 0. Eor benzene molecules, however, E O. This shows that benzene in its triplet state Ti no longer has a 6-fold axis of symmetry. The molecule is so distorted owing to its Jahn-TeUer instability in the Ti state that it has - like the higher aromatics - at most a twofold axis of symmetry. E is thus a measure of the deviation from cylinderical symmetry. 

Fig. 24. Geometry of approach of triplet methylene to ethylene. The dots indicate successive positions of the inethylenic carbon atom. The carbene is lying in a plane perpendicular to the paper. Lines a and h indicate the initial and final position of the hydrogen atoms at the central carbon atom of the resulting CH2CH2CH2 diradical. The dotted lines indicate the reaction path for rearrangement of the diradical through the 7t complex C 239)...

The simplest carbene is methylene, CH2, which has an sp hybrid orbital and a p orbital in addition to the two C—H bonds. As a 6e species, CH2 has 4e in the two C—H bonds and therefore two electrons remain to be placed in the sp and p orbitals on carbon. We will consider that both electrons are placed in the lower-lying sp orbital to give a singlet carbene, leaving the p orbital empty (see Fig. 11.1a). In the free carbene, the triplet state, where the two unpaired electrons have parallel spins, is also important, however. 

The ring forming reactions described in this section, with the exception of the reaction of triplet methylene (carbene, CH2), the second example in Table 6.5, follow the symmetry rules outlined in Chapter 4. Thus, the simplest example of an allowed (2 -H 2) addition is shown the first item in Table 6.5. 

The triplet carbene, with its two nonbonding electrons of the same spin, must produce an intermediate that cannot close to a cyclopropane until a spin is somehow changed. Such spin flips are not impossible, but they are usually slow on the time scale of molecular processes. In particular, they are slow compared with the very rapid rotations about carbon-carbon single bonds. Therefore the stereochemistry of the original alkene need not be maintained in the eventual product of the reaction, the cyclopropane (Fig. 10.48). If rotation about the carbon-carbon bond is faster than the rate at which the spin quantum number of one electron changes (spin flip), it will not matter whether cis or trans alkene is used in the reaction with triplet methylene. The product will be a mixture of cis and trans 1,2-dialkylcyclopropanes in either case. 

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. 

Among radicals of the second type, carbenes (methylenes) are well studied. The simplest carbene is methylene CH2, which exists in two forms singlet and triplet. In singlet CH2 the HCH angle is equal to 103°, rc n = 0.112 nm, = 393 kJ/mol. In triplet CH2 all three atoms lie on one line, rc—h = 0.103 nm. 

Divalent carbon species called carbenes are capable of fleeting existence. For example, methylene, CH2, is the simplest carbene. The two unshared electrons in methylene can be either spin-paired in a single orbital or unpaired in different orbitals. Predict the type of hybridization you expect carbon to adopt in singlet (spin-paired) methylene and triplet (spin-unpaired) methylene. Draw a picture of each, and identify the valence orbitals on carbon. 

The comparatively small size of the simplest carbene (methylene) ensures that it has a definite mobility in frozen inert matrices, which leads to the formation of dimerization products under these conditions. It became possible only in 1981 to detect in the spectra of the diazomethane photolysis products bands at 1115 cm (Ar matrix) and 1109 cm (Xe matrix) which were attributed to the deformation vibration of methylene in its ground triplet state (Lee and Pimentel, 1981). 

---