Methylene, carbene singlet

In contrast to methylene, carbenes which contain an oxygen atom bonded to the carbenic center are known to exist as singlet ground states. The rational for this is that the lone pair electrons on the adjacent oxygen atom have a resonance stabilizing effect on the electron deficient carbenic center ... 

Chapter 7. On the other hand, methylene (rCH ) and alkyl substituted methylenes (carbenes, CHR CR2 R H) can be readily prepared either by the thermal (or photochemical) decomposition of the corresponding diazo compounds (Equation 6.51) themselves prepared from an amine (Chapter 10) or ketone (or aldehyde) (Chapter 9) precursor. Alternatively, for a species that behaves as if it were singlet methylene (rCH ), the treatment of methylene iodide (diiodomethane, I2CH2) with copper (Cu)-activated zinc (Zn) metal (Equation 6.52) can be used (the Simmons-Smith reaction ). 

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. 

Given the zwitterionic natnre of single carbenes, the possibility exists for coordinating solvents such as ethers or aromatic compounds to associate weakly with the empty p-orbital of the carbene. Several experimental stndies have revealed dramatic effects of dioxane or aromatic solvents on prodnct distribntions of carbene reactions. Computational evidence has also been reported for carbene-benzene complexes. Indeed, picosecond optical grating calorimetry stndies have indicated that singlet methylene and benzene form a weak complex with a dissociation energy of 8.7kcal/mol. ... 

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. 

Enthalpies of formation for the singlet and triplet states of methylene were obtained from the photodissociation of ketene.131 The data for CH2 (3Bi) were recently confirmed by methods which do not rely on ketene.132,133 In a widely applicable procedure, threshold collision energies for the loss of halide ion from RR C-X- were combined with gas phase acidities of RR CH-Cl to give AHf (RR C ) (Eq. 11).134 Similarly, gas phase acidities of the radicals RR CH were combined with ionization energies of the radical anions RR C -, or electron affinities of the carbenes RR C (Eq. 12).135136... 

The third member, trimethylenemethane (3), had some relevance to our studies on carbenes, since besides methylene and its simply substituted derivatives trimethylenemethane 3 is one of the few molecules having a triplet ground state.22 Also the experience with 3 could be of help in order to deal with the singlet/triplet differentiation in matrix-isolated carbenes. We learned that, if the calculated IR spectra of the singlet and triplet molecule are sufficiently different, it might be possible to determine the multiplicity of the matrix-isolated species by comparison with the experimental IR spectrum. In this context it is also worth mentioning that we were able to measure the matrix IR spectrum of 3, but a special technique (irradiation in halogen-doped xenon matrices) had to be developed in order to achieve a concentration of 3 sufficient for its IR detection.23... 

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. 

Carbenes with two orbitals, occupied by one electron each, have a total spin number of s = 1 the multiplicity is therefore 2s -f-1 =3, i.e., a triplet state. This is the usual ground state of a carbene. If however, the two orbitals are not degenerate, the multiplicity can become 1, i.e., a singlet state is possible. Methylene — and other carbenes — can, in principle, be linear or bent. The binding is achieved by using the 15 hydrogen Is and the 2s and 2p carbon orbitals. 

Photolysis of diazomethane in carbon-tetrachloride in the presence of benzophenone yields 1,1,1,2-tetrachloroethane showing an enhanced absorption due to the triplet carbene. The direct photolysis of diazomethane proceeds via singlet methylene CIDNP-studies of the photolysis of methyl-diazoacetate, for which a radical pair mechanism was suggested, were recently challenged 2). 

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 ... 

A highly reactive compound containing a neutral divalent carbon with two nonbondmg electrons (ie.,. CR2 or a substitution derivative). The nonbonding electrons can have parallel spins (triplet state) or antiparallel spins (singlet state). The parent species, iCRz, is also known as methylene. A number of carbene derivatives have been used as photoaffinity labels of proteins. Irradiation of 3 -0-(4-benzoyl)benzoyl-ATP will cause 70% inactivation of mitochondrial Fi-ATPase. ... 

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... 

Carbenes and related compounds are among those reactive intermediates for which gas-phase experimental data exist. Some of those are compared to calculated geometries in Table 5-17, drawn from a larger collection provided in Appendix A5 (Tables A5-42 to A5-49). Except for methylene (CH2), where both singlet and triplet states have been considered, only singlet-state molecules have been examined. The usual theoretical models have been assessed. Mean absolute errors in bond lengths and angles based on the full data set have also been provided. 

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. 

Benzene derivatives such as m-methylanisole (40) can be converted to distonic carbene ions. Reaction of 40 with O occurs with loss of H2, generating the conventional carbene anion 41 this anion reacts with molecular fluorine by dissociative ET, followed by nucleophilic attack of F on the methyl group, forming 42. In contrast to phenyhnethylene, 42 has a singlet ground state however, upon protonation it gives rise to the triplet state of m-hydroxyphenyl-methylene. This interesting reaction can be viewed as a spin-forbidden proton-transfer reaction. 

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