Reactive Intermediate Chemistry

486 ATOMIC CARBON

presumably arises from ring opening of an intermediate oxacyclohexatriene formed by DBA of C to 87 (Eq. 47).

3.6.5. C H Insertion versus Double-Bond Addition in the Reaction of Carbon with Aromatics. The factors that determine if C atoms react with an aromatic by C H insertion or DBA are not yet understood. Benzene and substituted benzenes are postulated to react by C H insertion although detailed labeling studies have only been carried out in benzene, toluene, and tert-butylbenzene. No C H insertion is observed in the reaction of carbon with 71, while C H insertion is a minor pathway when carbon reacts with 76 and 87.

3.7. Deoxygenation by Atomic Carbon

3.7.1. Reaction of Carbon Atoms with Alcohols and Ethers. The electrophilicity of atomic carbon and the exothermicity of carbon monoxide formation in its reactions facilitates attack on, and removal of oxygen by C atoms. Deoxygenation of ethers, alcohols, and carbonyl compounds has been reported. This process is generally a highly exothermic reaction, which is likely to generate products with excess energy.

The reaction of nucleogenic and arc generated C atoms with alcohols gives products of deoxygenation, O H, and C H insertion. For example, cocondensation of arc generated C with methanol gives the products in Eq. 48.91 Experiments with specifically deuterated methanols confirm that the acetaldehyde is a product of a C H insertion while the dimethoxymethane results from two consecutive O H insertions. Deoxygenation produces CO and an unidentified fragment (presumably methyl radicals).

H

ð48Þ

The deoxygenation of ethers by C atoms was first studied by McKay and Wolfgang92 who reported a high yield of 11CO in the reaction of 11C with oxirane. Deoxygenation of oxiranes was subsequently reported for both arc and chemically

Figure 10.2. Carbenes that have been produced by the deoxygenation of carbonyl compounds. Numbers below the structures refer to the literature references in the text.

While this can lead to interesting new carbene reactions, it may also complicate comparisons with the same carbene generated by conventional methods.

A dramatic example of the production of energetic carbenes by C atom deoxygenations is provided by the deoxygenation of the tolualdehydes 96a–c to the corresponding tolylcarbenes 97a–c at 77 K (Eq. 55).104 When the m- and p- tolylcarbenes 97b and 97c are generated from the diazo compounds and pyrolysed above 650 C, they rearrange to the o-tolylcarbene (97a), which is trapped as benzocyclobutene or rearranges further to 1-phenylethylidene, and thence to styrene.105

490 ATOMIC CARBON

The fact that this same rearrangement occurs in 97a–c produced by deoxygenation at 77 K is strong evidence for energetic carbenes in this system.

Deoxygenation of formaldehyde,106 methylformate,107 and phosgene107 leads to methylene, methoxycarbene, and dichlorocarbene respectively, which can be trapped (Eq. 56). The methylene produced from reaction of chemically generated C atoms with formaldehyde adds nonstereospecifically to the 2-butenes in the gas phase (Eq. 57).106 Since yields are not effected by added O2 and the reaction becomes stereospecific upon addition of 200 Torr N2, triplet methylene is not involved. The fact that increasing the pressure of the 2-butene does not alter the stereochemistry leads to the conclusion that nonstereospecificity does not result from isomerization of energetic dimethylcyclopropane products. Instead, the reaction is postulated to produce CH2 in its excited (1B1) singlet state, a species in which nonstereospecific addition to alkenes has been predicted.108 The addition of the inert N2 degrades the excited singlet to its ground (1A1) state, which then adds stereospecifically. Since increasing the pressure of the 2-butene to 200 Torr only increases reactant concentration, nonstereospecific addition prevails. Since deoxygenation of formaldehyde to CH2 (1A1) is exothermic by 98 kcal/mol and CH2 (1B1) lies 21.1 kcal above CH2 (1A1), production of excited singlet CH2 is energetically feasible in this system.

X1 = X2 = H;

X1 = X2 = Cl;

X1 = H, X2 = OCH3;

ð57Þ

N2

CH2(1A1)

When cyclopentanone (98a) is deoxygenated to cyclopentanylidene (99a) with C atoms produced both in an arc and by tetrazole decomposition, an unusual carbene cleavage to ethylene and allene occurs along with rearrangement to cyclopentene (Eq. 58).109 Since this cleavage is not observed when 99 is generated by tosylhydrazone decomposition, it has been attributed to energetic 99. Calculations (B3LYP/6-311þG(d)þZPC) indicate that the cleavage proceeds via the open shell singlet 99, which cleaves to biradical 100. The lack of concert in the cleavage reaction is demonstrated by the fact that cis-3,4-dideuterocyclopentane (98b) is deoxygenated to cis-3,4-dideuterocyclopentylidene (99b), which cleaves nonstereospecifically to cisand trans-1,2-dideuteroethylene.109

XX

Carbene generation by C atom deoxygenation has been useful in answering questions concerning the intermediacy of free carbenes in certain systems. For

example, tert-butylcarbene (101) from several precursors gives 1,1-dimethylcyclo- propane (102) by C H insertion, and 2-methyl-2-butene by C C insertion.100a,110

However, calculations ([QCISD(T)/6-31þG(2d,p)]// MP2/6-31G(d)) indicate that C C insertion should not be competitive with C H insertion in this carbene.111 In agreement with this prediction, deoxygenation of 2,2-dimethylpropanal (103) by C atoms, at 77, 158, and 195 K, yields only the C H insertion product (Eq. 59).111 A similar result is observed when free carbene 101 is generated by cheleotropic extrusion from a tricyclopropane.103

Other free carbenes that have been generated in this way and their reactions compared with those from other precursors include bicyclo[2.2.2]octylidene,112 cyclopropylmethylcarbene,113 and 1-phenylpropylidine (104).114 In the case of 104, the same ratio of (E) to (Z)-1-phenylpropene was produced when the carbene was generated by deoxygenation of the corresponding ketone and by the phenylcarbene rearrangement (Eq. 60).114 Thus, the method represents a viable alternative to

492 ATOMIC CARBON

more conventional routes to carbenes with the caveat that the high energies involved in these deoxygenations may bring about unexpected reactions.

C

ð60Þ

104

+

3.7.3. Phenylnitrene by the Deoxygenation of Nitrosobenzene. The cocondensation of carbon with nitrosobenzene (105) yields aniline (106), azobenzene (107), azoxybenzene (108), and phenylisocyanate (109) (Eq. 61).115 In analogy with carbonyl deoxygenations to carbenes, this reaction is postulated to give singlet phenylnitrene 1110, which decays to 3110. Hydrogen abstraction by 3110 gives 106, dimerization yields 107, and reaction of 105 with 3110 produces 108. It seems likely that 109 arises from addition to the N O double bond in a reaction competitive with deoxygenation. Attempts to trap azacycloheptatetraene (111), the product of ring expansion of 1110 are successful when the C þ 105 reaction is carried out at 195 K but not at 77 K (Eq. 61).

This temperature effect reflects the known fact that ring expansion of 1110 to 111 is only competitive with isc to 3110 at higher temperatures.116 The reaction of C atoms

with pyridine is also a route to singlet and triplet 110 presumably via the pyryidylcarbenes (112) (Eq. 62).115

N

3.8. Other Reactions of Carbon Atoms with Lone Pairs

Since carbon monosulfide is a higher energy species than carbon monoxide, desulfurizations by atomic carbon are expected to be less energetic. This prediction has been born out in a series of experiments by Skell et al.96 For example, desulfurization of tetrahydrothiophene leads to ethylene and cyclobutane in a 10:1 ratio. The fact that cyclobutane is generated in the desulfurization of tetrahydrothiophene but not in the deoxygenation of THF, has been attributed to the lower exothermicity of the desulfurizations. The generation of carbenes, presumably with less energy than those from deoxygenation, by desulfurization of thiocarbonyl compounds has not yet been attempted.

Reaction of arc generated carbon with aziridines generates alkenes in a nonstereospecific manner along with concurrent production of HCN.117 The reaction of arc generated carbon with trimethylamine appears to proceed by attack on the nitrogen lone pair to give ylid 113, which has limited stability at 77 K.118 Addition of oxygen to a 77 K matrix of carbon þ trimethylamine results in the formation of carbon monoxide, indicating that 113 functions as a triplet C atom donor (Eq. 63).

4. MOLECULAR BEAM STUDIES OF CARBON ATOM REACTIONS

There have been several recent reports of the interaction of C atoms with substrates in molecular beams.27 In these experiments, a beam of C atoms is generated by laser evaporation of graphite, the energy of the beam is selected, and the beam is crossed with a substrate beam of known energy. Products are identified mass spectrometrically and their energies measured. Under the single collision conditions of these investigations, initial products cannot dissipate their energy and a C H bond cleavage invariably occurs (Eq. 64). The observed energy release in the reaction is then correlated with high quality calculations to deduce the structure of the product.

Substrates that have been studied using these beam techniques include acetyl-

ene,119 ethylene,120 propene,121 propyne,122 allene,123 propargyl radicals,124 2-butyne,125 1,2-butadiene,126 H2S,127 and benzene.27,128 The reaction of C(3P)

with benzene is of interest as initial reactivity is different from that observed in other carbon þ benzene reactions discussed earlier. In this case, initial addition gives complex 114 between triplet carbon and benzene. Ring opening of 114 to

494 ATOMIC CARBON

triplet 53 is followed by loss of H to give the 1,2-dehydrocycloheptatrienyl radical 115 (Eq. 65).

C

In molecular beam studies of C atom reactions, it is also possible to select higher energy portions of the beam that contain C(1D). This selection has been accom-

plished in a study of the reaction of singlet carbon with acetylene that proceeds with loss of hydrogen to give 116 (Eq. 66).119a In contrast, C(3P) reacts with acetylene by two channels producing both 116 and 117 (Eq. 67).119b

ð67Þ

Since molecular beam investigations of C atom reactivity differ from other C atom studies with regard to dissipation of reaction exothermicity, it is difficult to compare them with other methods. A hallmark of carbon molecular beam studies is the fact that addition of C is followed by loss of H. This reaction path is unimportant in other reactions of C atoms.

5. CONCLUSION AND OUTLOOK

It is clear that atomic carbon exhibits a rich and exotic chemistry. Indeed, as one of the most energetic species in organic chemistry, such behavior is anticipated. Only a small portion of this chemistry has been explored, leaving a vast area of C atom chemistry awaiting further research. It is probable that the ideal photochemical precursor to C atoms has not yet been developed. Such a molecule would produce C upon gas-phase photolysis at convenient wavelengths.

The continuing interest in carbene chemistry and the fact that C atoms serve as convenient carbene precursors makes this research area attractive. Although carbonyl compounds are conveniently deoxygenated to carbenes, the analogous deoxygenation of acid chlorides and amides to generate chloroand aminocarbenes has not been explored. The fact that many carbenes produced by this method will have excess energy and may be in electronically excited states should provide the basis for many exciting experiments. The chemistry of energetic nitrenes from

REFERENCES 495

carbon þ nitroso compounds should be investigated. The deoxygenation of ethers should lead to many radical pairs that can be compared with the same radical pair generated in thermolysis reactions. Deoxygenation of metal carbonyls seems a viable route to metal carbide complexes. The chemistry of carbenes produced by C atom desulfurizations should be investigated. These are just a few of the many possible future directions in C atom chemistry. Future researchers should be encouraged by the fact that is difficult to imagine a molecule that will not react with atomic carbon.

SUGGESTED READING

For a discussion of the formation of biomolecules in carbon atom reactions see: P. B. Shevlin, D. W. McPherson, and P. Melius, ‘‘The Reaction of Atomic Carbon with Ammonia. The Mechanism of Formation of Amino Acid Precursors,’’ J. Am. Chem. Soc. 1983, 105, 488 and G. Flanagan, S. N. Ahmed and P. B. Shevlin, ‘‘The Formation of Carbohydrates in the Reaction of Atomic Carbon with Water,’’ J. Am. Chem. Soc. 1992, 114, 3892.

For a discussion of the possible intermediacy of electronically excited carbenes in carbon atom reactions see: G. Xu, T.-M. Chang, J. Zhou, M. L. McKee, and P. B. Shevlin, ‘‘An Unusual Cleavage of an Energetic Carbene,’’ J. Am. Chem. Soc. 1999, 121, 7150.

The reaction of carbon atoms with aromatic rings is discussed by B. M. Armstrong, F. Zheng, and P. B. Shevlin, ‘‘The Mode of Attack of Atomic Carbon on Benzene Rings,’’ J. Am. Chem. Soc., 1998, 120, 6007. This reactivity may be contrasted with that observed when a beam of C(3P) reacts with benzene as described by I. Hahndorf, Y. T. Lee, R. I. Kaiser, L. Vereecken, J. Peeters, H. F. Bettinger, P. R. Schreiner, P. von R. Schleyer, W. D. Allen, and H. F. Schaefer, III. ‘‘A combined crossed-beam, ab initio, and Rice–Ramsperger–Kassel– Marcus investigation of the reaction of carbon atoms C(3Pj) with benzene, C6H6(X 1A1g) and d6-benzene, C6D6(X 1A1g),’’ J. Chem. Phys. 2002, 116, 3248.

For a combined experimental and computational approach to carbon atom chemistry see C. M. Geise, C. M. Hadad, F. Zheng, and P. B. Shevlin, ‘‘An Experimental and Computational Evaluation of the Energetics of the Isomeric Methoxyphenylcarbenes Generated in Carbon Atom Reactions,’’ J. Amer. Chem. Soc. 2002, 124, 355.

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2.R. Wolfgang, Prog. React. Kinet. 1965, 3, 97.

3.R. Wolfgang, Adv. High. Temp. Chem. 1971, 4, 43.

4.P. S. Skell, J. Havel, and M. J. McGlinchey, Acc. Chem. Res. 1973, 6, 97.

5.C. MacKay, in Carbenes, Vol. II, R. A. Moss, M. Jones, Jr., Eds., Wiley-Interscience, New York, 1975, pp. 1–42.

6.P. B. Shevlin, in Reactive Intermediates, Vol. I, R. A. Abramovitch, Ed., Plenum Press, New York, 1980, pp. 1–36.

7.F. P. Israel and F. Baas, Astron. Astrophys. 2002, 383, 82.