Reactive Intermediate Chemistry

exothermic ( 47 kcal/mol for the formation of H2C O O), carbonyl oxides are generated in vibrationally excited states. Their stabilization occurs either through relaxation to the thermal ground state or by extrusion of an O atom and subsequent formation of a carbonyl compound. The ratio of carbonyl oxide to ketone is strongly dependent on the substituents at the carbene center. Especially in the case of hydrogen atom substituents, the extrusion of oxygen atoms predominates. Some of the oxygen atoms react with O2 to give O3, especially in matrices with high O2 concentration (Scheme 9.21).

The conversion of triplet carbenes and O2 to the corresponding oxides and the following reactions are easily followed spectroscopically, and therefore are used to characterize the reaction of triplet carbenes.

Singlet carbenes generated in an O2-doped inert gas matrix also react with O2 to give the corresponding oxides (80b, Scheme 9.22). For example, chlorophenyl-

O

Scheme 9.22

less efficient than similar reactions of triplet carbene. Under conditions where the triplet carbenes are completely converted into carbonyl oxide, 67 does not react perceptibly.112 The origin of this spin-forbidden process is not clear. A complex formation between singlet carbene and O2 might facilitate intersystem crossing before addition. From the fact that the more electron-deficient p-nitrophenylcarbene reacts markedly faster than the parent chlorocarbenephenyl both in matrix and in solution (see below), it is suggested that the reaction is dominated by the philicity of the carbene. The electrophilic attack of the carbene on the O2 molecule is the ratedetermining step. On the other hand, decrease in EST by an electron-withdrawing nitro group may facilitate the reaction from the upper lying triplet state. It is inter-

esting to note that apparent spin-forbidden reactions of matrix-isolated triplet carbenes with (singlet) CO2, CO, and N2 have been observed.109,113 Further work to

clarify these points is required.

6.3. Dimesityl ketone Oxide

Carbonyl oxides are usually highly unstable and can be detected only by matrix isolation spectroscopy at low temperature or by LFP techniques. However, they can be stabilized through steric protection. Dimesityl ketone oxide (80c, lmax ¼ 398 nm) generated by the reaction of triplet dimesitylcarbene (19c) with O2 at

426 TRIPLET CARBENES

77 K is found to be stable even at 79 C for many hours in the dark and is characterized by 13C and 1H NMR spectroscopy (in CFCl3/(CF2Br)2).114 A 13C reso-

nance at d 211.1 ppm is in good agreement with the chemical shift calculated (using the IGLO method) for the parent carbonyl O-oxide H2C O O (d 227 ppm)115 and is assigned to the carbonyl carbon of dimesitylketone oxide (80c). The number of methyl groups observed in the 1H NMR spectrum indicates that the C O O moiety is configurationally stable at 70 C. Exposure to daylight or irradiation with l>455 nm rapidly causes complete bleaching of the yellow solution of the oxide. After the solution is warmed to room temperature, the ketone, ester (83c), and dioxirane (82c) are isolated. Dimesityldioxirane (82c) is also stable and can be obtained as a colorless crystalline material, completely stable at 20 C. Irradiation of the dioxirane with l > 420 nm light results in the formation of the ester (83c), which is not formed from the oxide either thermally or photochemically (Scheme 9.23). These results confirm the photochemistry observed in solid argon at low temperature.

Scheme 9.23

6.4. Emission

The emission of a strong chemiluminescence was observed during warm-up from 77 K of organic glasses containing triplet DPC and a traces of O2.116 The chemiluminescence was identified as the phosphorescence of benzophenone. Since chemiluminescence decay was faster than consumption of the carbene, it was proposed that oxygen transfer from carbonyl oxides to triplet carbenes results in the formation of the ketones in electronically excited states. In O2-doped Ar matrices, chemiluminescence starts at temperatures as low as 15 K. Chemiluminescence decay was faster than the oxidation of the carbenes, but IR spectroscopy excluded carbonyl oxides as oxygen-transfer reagents. A direct reaction of the oxygen atom, produced by decomposition of carbonyl oxides, with triplet carbenes, producing a C O bond in a very exothermic reaction, was proposed as the chemiluminescence step (Scheme 9.24).117

Scheme 9.24

6.5. Laser Flash Photolysis Studies

Carbonyl oxides are also easily detected by LFP with fast TRUV–vis spectroscopy. For example, LFP of a-diazophenylacetate in deaerated Freon-113 generates a transient absorption band at l < 270 nm ascribable to triplet methoxycarbonylphenylcarbene (53), which shows a pseudo-first-order decay with lifetime of 460 ns. When LFP is carried out in aerated solvents, a new transient band appears at 410 nm at the expense of the transient band due to triplet carbene. The decay rate of triplet carbene increases as the concentration of oxygen increases. This correspondence indicates that the triplet carbene is trapped by oxygen to form the carbonyl oxides (53-O2). This result confirms that the transient absorption quenched by oxygen

Scheme 9.25

belongs to the triplet carbene (Scheme 9.25). The apparent built-up rate constant, kobs, of the carbonyl oxide is essentially identical to that of the carbene, and kobs is expressed as given in Eq. 24,84

where k0 represents the rate of decay of triplet carbene in the absence of oxygen and kO2 is the quenching rate constant of triplet carbene with oxygen. The plot of the observed pseudo-first-order rate constant of the formation of the carbonyl oxide is linear. From the slope of this plot, the rate constant for the quenching of triplet carbene by oxygen is determined to be 8:6 108 M 1 s 1.

Absolute rate constants for the representative triplet carbene–O2 reaction in solution at room temperature are listed in Table 9.12.118–123 The second-order rate con-

stants are close to 109 M 1s 1, near the diffusion-controlled rate. Thus, the reaction with O2 is taken as evidence for the presence of the triplet state of the carbene.

Singlet carbenes usually do not react with O2. For example, LFP of 3-chloro-3- phenyldiazirine has shown that the reactions of chlorophenylcarbene (67) with numerous substrates are insensitive to the presence of oxygen, indicating that the rate constant for chlorophenylcarbene with O2 must be < 104 M 1s 1. However, singlet chloro p-nitrophenylcarbene (67b) is readily scavenged by O2, as evidenced by the decrease in lifetime in the presence of increasing O2 concentration and the appearance of a new broad transient absorption in the 350–500-nm range (lmax ¼ 400 nm). A plot of the reciprocal lifetimes for the carbene decay versus oxygen concentration yields the quenching rate constant, kO2 ¼ 2:24 107M 1 s 1. This value is two orders of magnitude smaller than that for the reaction of triplet carbenes with O2. Continuous irradiation of the diazirine with O2 in isooctane gave several oxidation products including p-NO2C6H4CHO and p- NO2C6H4COCl. However, when the photolysis is carried out in the presence of (Z)-4-methyl-2-pentene at 25 C under O2, all additions take place stereospecifically with no discernible effect by O2, again suggesting that the reaction of singlet carbene with O2 is very inefficient.123 The reason why only chloro-p-nitrophenylcarbene reacts with O2, while unsubstituted 67 does not, is perplexing.

Carbonyl oxide formation can also be used to monitor carbenes that are transparent in the most useful UV region. For example, most alkylcarbenes are spectroscopically invisible. The ground-state singlet alkylcarbenes are generally monitored by trapping with pyridine to form ylides that show an intense absorption band400 nm. However, this technique cannot be used to monitor the ground-state triplet dialkylcarbenes bearing rather bulky groups. For example, LFP of diadamantyldiazomethane in degassed cyclohexane does not produce a UV–vis active transient intermediate. However, LFP in aerated solution produces a transient spectrum exhibiting a maximum at 307 nm with a rise time of 200 ns. The transient band is attributable to carbonyl oxide (7-O2). The absorption maximum of the oxide is shifted by roughly 100 nm relative to benzophenone oxide and other aryl carbonyl oxides because of the lack of conjugation with an aromatic ring. Since diadamantylcarbene (7) has been shown to react with methanol in its singlet state, it is expected that the yield of 7-O2 will be reduced while its rate of formation will be increased upon addition of methanol. Stern–Volmer analysis of the methanol quenching data and the slope of the observed rate constant of 7-O2 formation versus

107 M 1 s 1, respectively. Thus, diadamantylcarbene (7) and DPC (14) react with the singlet carbene quencher methanol with about the same rate constant, probably at spin equilibrium.7b

Compared to the time scale of their formation, the carbonyl oxides are quite long lived (10 5–10 3 s), and so their subsequent reactions can be monitored kinetically. For most of the carbonyl oxides, the decay is best fit to a second-order rate law, indicating a bimolecular decomposition pathway. For benzophenone oxide, the ketone is the major product at room temperature, and no dimer can be detected. A bimolecular process involving O2 extrusion from two molecules of the oxides is suggested under these conditions.

430 TRIPLET CARBENES

6.6. Reaction with Tetramethylpiperidine N-Oxide

The reaction of triplet carbenes with a persistent nitroxide such as 2,2,6,6-tetra- methylpiperidine N-oxide (TEMPO, 84) to form benzophenone would be spin allowed and >100-kcal/mol exothermic (Scheme 9.26). The reaction has a few parallels in free radical chemistry, such as the reaction of tert-butoxyl with carbon monoxide (to yield CO2) or with phosphorus (III) substrates to yield P(V) products.124

Ph

O + H-N

85

Scheme 9.26

Photolysis of diphenyldiazomethane in MeCN solution in the presence of sufficient 84 yields benzophenone in >90% yield and tetramethylpiperidine in equimolar amounts. A similar photolysis with 4-hydroxy-TEMPO (84, X ¼ OH) gives benzophenone and ether (85) in 16:1 ratio, indicating that attack at the nitroxide center predominated even though it is more hindered than the OH group. In the case of cyclohexanol, which is used as a reference substrate for 4-hydroxy-TEMPO, the reaction with DPC gives >90% yield of the expected ether.125

Triplet DPC is readily detectable at 315 nm and is quenched by TEMPOs. The bimolecular rate constant (kq) for carbene scavenging is determined by plotting the pseudo-first-order rate constant for carbene decay as a function of substrate concentration. The values of kq (2–3 108 M 1 s 1) are practically the same for the reaction of all TEMPOs employed and are approximately one-tenth of that for carbene scavenging by O2. One expects one-third of the encounters to have the appropriate spin configuration for the reaction. Thus the process is, while fast, well below the diffusion limit.

The reaction with 4-hydroxy-TEMPO leading to the ether is particularly interesting as the reaction could be spin allowed (i.e., doublet þ triplet ! doublet) if sufficient interaction between the O H and nitroxide centers takes place. However, the EPR parameters for 4-hydroxy-TEMPO suggest that the interaction between the two sites is small.126 The magnitude of the relaxation of spin conservation rules seems unclear, but the kinetic results show virtually no effect. The rate constant for insertion at the O H bond is 2 107 M 1 s 1, which is essentially the

same in the case of cyclohexanol. This result may suggest that the dominant factor controlling differences in reactivity between singlet and triplet carbenes appears to be their orbital occupancy, rather than the overall multiplicity.125

7. ADDITION TO DOUBLE BONDS

A singlet carbene adds in concerted fashion to an alkene to form a cyclopropane stereospecifically, while the reaction of a triplet is stepwise and nonstereospecific (Skell–Woodworth rule). This difference is because the intermediate triplet 1,3- diradical undergoes bond rotation before it intersystem crosses to the singlet. The intervention of a diradical intermediate can be proven by using an alkene bearing a substituent susceptible to radical rearrangement. For example, the reaction of a triplet carbene with dicyclopropylethylene (86) results in formation of a product (88) resulting from cyclopropane ring opening of the intermediate diradical (870) (Scheme 9.27).127

Scheme 9.27

Triplet carbenes show enhanced reactivity toward alkenes that can stabilize the intermediate radical center. For example, the reactivity of 1,3-butadiene toward 3DPC is shown to be some a 100 times larger than that of 1-hexene.128

The bimolecular rate constant for the reaction of 3DPC with butadiene is determined to be 6:5 105 M 1 s 1. Isoprene can be employed as a selective trap for triplet carbenes. Styrene is also shown to be an efficient trap for triplet carbene. (E)- b-Deutero-a-methylstyrene (89) is a very convenient reagent to diagnose the multplicity of the reacting carbene because it reacts with both singlet and triplet carbenes with different stereochemical outcomes. The stereochemistry of the adduct cyclopropane (90) can be easily judged by 1H NMR (Scheme 9.28). For example, BA (22) reacts with styrene with total loss of stereochemistry, while in the reaction with dimethoxy FL (23a), the expected cyclopropane is obtained with complete retention of stereochemistry. The rate constants for the additions are ð1:2 0:2Þ 107 and