A CftEH Feature
Parti
Carbene chemistry Carbenes are Important in the synthesis of cyclopropanes and tar more highly strained small ring compounds and, in fact, there's hardly a substrate, from steroids to elemental nitrogen, that hasn't been "hit" with a carbene or. Robert A. MOSS, Rutgers the State university, New Brunswick, N.J.
D
uring the recent interest in the chemistry of divalent carbon species, or carbenes, it's often been difficult to recall that we are experiencing a renaissance and not a birth. Thus, the concept of a divalent carbon intermediate is of similar vintage as those of the trivalent carbon intermediates—carbonium ions, carbon radicals, and carbanions. As early as 1835, J. B. Dumas suggested routes to methylene, CH 2 . In 1862, A. Geuther postulated the intermediacy of dichlorocarbene, CC12, in the basic hydrolysis of chloroform, a concept that was practically expressed in the Reimer-Tiemann synthesis of phenolic aldehydes in 1876, 74 years before Jack Hine's pioneering modern studies in 1950 and later of haloform hydrolysis which put the idea into respectable use. By the turn of the century, J. U. Nef had proposed a general methylene theory. H. Staudinger's studies of diazo compounds in 1910-16 were an important contribution. In 1942, H. Meerwein showed that the photochemical decomposition of diazomethane in diethyl ether led to the attack of a CH 2 fragment on the ether's C—H bonds, with the formation of ethylpropyl and ethylisopropyl ethers. This discovery led to Doering's study about 20 years ago of the reaction in benzene solution (products: toluene and cycloheptatriene) and, in an important generalization, to his study of CH 2 attack on saturated hydrocarbons. Of this Doering said: "Methylene must be classed as the most indiscriminate reagent known in or-
The modern use of the term carbene was collaboratively conceived by W. von E. Doering, S. Winstein, and R. B. Woodward "in a nocturnal Chicago taxi and later delivered diurnally in Boston"—at an ACS meeting, by Doering and Knox in April 1951. Although divalent carbon compounds are also called methylenes and -ylidenes, the carbene nomenclature is the most popular. This is the first of this two-part Feature; part two will follow week-after-next; however, it will be reprinted as a single article. 60 C&EN JUNE 16, 1969
ganic chemistry." Though the accuracy of his statement depends partly on the definitions of "reagent" and "organic chemistry", there's no gainsaying the stimulation to research provided by these remarkable studies. With credit to the brilliant work of Hine, a watershed was reached in 1954 when Doering and A. K. Hoffmann showed that CC12 or CBr2 could be intercepted by olefins with the formation of cyclopropanes.
This reaction had been implicit in the earlier study of benzene and CH 2 , and in unpublished work (in which CH 2 had been added to cyclohexene, yielding norcarane), but it was the Doering-Hoffmann paper, particularly, which was followed by a burst of activity, including P. S. Skell's stereochemical and kinetic studies of CBr2 additions to olefins in the mid-1950 , s. Similar studies of CC12 and CH 2 shortly came from Doering's laboratory. Simultaneously and suddenly, new paths opened in both synthetic and theoretical organic chemistry. These paths are now highways. Carbenes are important in the synthesis of cyclopropanes and far more highly strained small ring compounds. Their use in this area has let us see just how far our ideas about carbon hybridization can be pressed. There is hardly a substrate, from steroids to elemental nitrogen, that hasn't been "hit" with a carbene. In the theoretical area, carbenes are of continuing interest to spectroscopists and ldneticists, as well as to those specialists whose experiments involve only a computer and a telephone hot-line. Largely through the efforts of Professors P. P. Gaspar and G. S. Hammond, it's now known that in 1944, in what must surely have been highly classified wartime research, the redoubtable chemist, D. Duck, suggested: "If I mix CH 2 with NH 4 and boil the atoms in osmotic fog, I should get speckled nitrogen." Structural
theory
According to theory, there ought to be two chemically accessible kinds of methylene, singlet and triplet. As con-
structed with carbon 2s and 2p and hydrogen l.s orbitals, triplet methylene is a linear or near-linear species. The carbon atom is ideally pictured as sp hybridized; two of its four valence electrons are disposed in the sp hybrid or bitals, where they are involved in bonding to the hydrogen atoms. The other two valence electrons are distributed, one each, in the equivalent, mutually perpendicular ρ orbitals. These electrons have parallel spins. The mole cule is thus a diradical, or of triplet multiplicity. Singlet methylene is a bent species, in which the carbon atom is presumed to approximate sp2 hybridization. Two of the three sp2 hybrid orbitals, each containing an electron, are used in forming the C—H bonds, whereas the third hybrid orbital contains two paired electrons. The final carbon orbital remains ρ in character, and unpopulated. Not only are the nonbonding orbitals not equivalent in singlet methylene, but because one is doubly occupied while the other is vacant a superimposed carbonium ion-carbanion picture emerges that is richly suggestive to organic chem ists. These are simplified, post-hoc descriptions. Far more sophisticated pictures are drawn by the theoreticians. One of the difficult a priori questions is: Which state, singlet or triplet, is the ground state of methylene? In a recent and relatively simple approach, extended Hiickel theory was used to consider the change in energy that would accompany the bending of a linear methylene, with one electron in each nonbonding orbital. The resulting graph of energy vs. H—C—H angle was compared with a similarly derived graph for the bending of a linear methyl ene in which both electrons had been paired in one of the nonbonding orbitals. The graph for the electron-paired methylene has an energy minimum at an H—C—H angle of about 115°, whereas the angle corresponding to the energy minimum of the triplet methylene is about 155°. More detailed recent calculations suggest angles of 108° for singlet and 138° for triplet methylenes. The energy mini mum of the paired (singlet) methylene is about 13.8 kcal/mol lower than that of the triplet methylene. How ever, the calculation doesn't consider the electron repulsion energy that must be overcome to pair two electrons in a single orbital. This energy is sufficiently greater than
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Triplet methylene has an H—C—H angle close to 180° and two unpaired electrons in mutually perpendicular ρ orbitals. Singlet methylene has an H—C—H angle nearer 120° (actually 103°). This species has no unpaired electrons and, in fact can be regarded as a superimposition of a carbonium ion and a carbanion. The ρ orbital is perpendicular to the plane con taining the three atoms and the electron pair
13.8 kcal/mol, so that the triplet methylene is actually ex pected to be the ground state by about 9 to 10 kcal/mol. The actual energy gap between CH2 isn't known with certainty, but estimates put it between 1.5 and 48.5 kcal/mol. Applying this computational treatment to simple alkyl and arylcarbenes leads to analogous predictions of triplet ground states. All the triplets are predicted to be bent, with bond angles between 140 and 160°. Although bend ing a linear, sp hybridized carbene causes progressive en ergy splitting between the then no longer equivalent nonbonding orbitals, the splitting, at bond angles close to 180°, can easily be less than the electron repulsion energy that must be overcome to pair the two carbene electrons in one orbital. Thus, the bent carbene can be a triplet. Extreme bending ( < 120° ) should make the energy gap between the nonbonding orbitals larger than the electron repulsion energy, in which case the electrons should pair JUNE 16, 1969 C&EN
61
in the lower energy orbital, and the carbene's ground state should be a singlet. Calculations definitely suggest bent, singlet ground states for difhiorocarbene, CF 2 . The uv and ir spectra of ground state CF 2 show it to be highly bent and, with little doubt, of singlet multiplicity. The most important physical data on CH 2 structure were obtained by G. Herzberg, who studied the electronic spectra of the methylene species produced during the flash photolyses of CH 2 N 2 , CHDN 2 , and CD 2 N 2 . Analysis of the results demonstrated the production of two kinds of CH2—a linear or near-linear species with a C—H bond length of 1.03 À, which absorbed uv radiation at about 1414 A, and a bent species, bond angle 102 to 103°, C - H bond length 1.12 A, which absorbed radiation in the 5500to 9500-A region. Moreover, in the presence of a large excess of nitrogen, the absorption of the short-wavelength, linear CH 2 increased at the expense of that of the longwavelength bent, CH 2 . This experiment pointed to triplet CH 2 - ( : l ) CH 2 -as the ground state. The short-lived bent or singlet CH2—(1)CH2—was being degraded through collisions with the moderator, nitrogen, and dropping to the more stable, linear, or triplet state. A great deal of experimental work in carbene spectroscopy has been reported during the past few years. Uv, ir, and esr spectroscopy have been used. Uv studies of CF 2 , CFCl, CC1 2 , CHF, and CHCl established bent, singlet ground states for these carbenes. Ir studies at very low temperatures, of CC1 2 , CBr 2 , and CHCl, isolated in inert matrix supports, also indicated singlet ground states. Considerable uv spectroscopic effort is presently going into the study of arylcarbenes, which have have been found to possess triplet ground states. In the case of diphenylcarbene, fluorescence, fluorescence excitation, and absorption spectroscopy have been brought to bear on the structural problem. The techniques of absorption spectroscopy will undoubtedly have as great an impact on the study of carbene reactivity as they have already had on the study of carbene structure. The use of esr spectroscopy is necessarily restricted to triplet carbenes. Since triplet ground states appear to be general for aryl·- and diaryIcarbenes, however, a large area of investigation is being actively exploited. Esr signals from such carbenes are observed after photolyses of suitable precursors in single-crystal or glass hosts at a low temperature—77° K. These carbenes are probably generated as singlets but, trapped in a solid host at a very low temperature, they decay to their triplet ground state more rapidly than they react with their surroundings. The triplets are extremely long-lived under the experimental conditions—the triplet epr signals persist for hours as long as the temperature is kept at 77 °K. In many cases, it is possible to generate the same triplet from different precursors, so as to leave little doubt that it is really the spectrum of the carbene that is being observed. That the esr spectra of the triplets are those of ground state species follows from the extreme longevity of the signals at 77 °K. Very detailed structural information can be derived by analysis of the epr signals. Two of the most interesting and general discoveries in this area are that triplet aryl- and diarylcarbenes are appreciably bent, and that one of the two carbene electrons is delocalized over the aromatic π system, whereas the other is essentially localized on the carbenic center. Thus the ττ system of an arylcarbene triplet should resemble that of the corre sponding free radical. Uv studies of triplet diphenyl62 C&EN JUNE 16, 1969
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"PCH2 (photolysis of CH 2 N 2 in the liquid phase) to ds-butene and frans-butene yield only cis-1,2dimethylcyclopropane and Jrans-l,2-dimethylcyclopropane, respectively. These addition reactions are stereospecific; the geometrical relationship of the olefinic substituents is preserved in the product cyclopropanes: W +("CHZ
The position of insertion is shown, and the numbers in dicate the yields, adjusted for the relative number of protons of each bond type. Only slight preferences for the 3° (1.51) and 2° (1.22) C - H bonds appear. One may ask whether this nearly random (1>CH2 insertion is "direct," or whether some "intermediate," such as a radical pair, is involved. Photolysis of CH 2 N 2 in liquid, 14 C-labeled isobutene led to 2-methylbutene-l in which the label had scrambled to the extent of only 1%.
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