Lifetime of carbene

Further studies were carried out with halocarbene amides 34 and 357 Although again no direct spectroscopic signatures for specifically solvated carbenes were found, compelling evidence for such solvation was obtained with a combination of laser flash photolysis (LFP) with UV-VIS detection via pyridine ylides, TRIR spectroscopy, density functional theory (DFT) calculations, and kinetic simulations. Carbenes 34 and 35 were generated by photolysis of indan-based precursors (Scheme 4.7) and were directly observed by TRIR spectroscopy in Freon-113 at 1635 and 1650 cm , respectively. The addition of small amounts of dioxane or THF significantly retarded the rate of biomolecular reaction with both pyridine and TME in Freon-113. Also, the addition of dioxane increased the observed lifetime of carbene 34 in Freon-113. These are both unprecedented observations. 

As compared to 8,4 a and 4 a show a bathochromic shift. This is obviously due to a larger delocalization of the fx electron. There is practically no difference between the UV spectra of 4a and 4a. The flash photolysis experiments demonstrate clearly that the spectroscopic information obtained at 77 °K also appUes at ordinary temperature i9-2i) lifetime of carbene 4 a at ambient temperature has been shown to be of the order of 10 /r sec 2i). 

The kinetic data in Figure 2.2 indicate that carbene 10, under the conditions of our TRIR experiments, does not form ketene 11, in contrast to the observed reactivity of acyclic carbene 7. Platz and co-workers [94] determined that carbene 7 is separated from ketene 8 by a 3.4 kcal/mol barrier in hexafluorobenzene. It is likely that the higher singlet/triplet gap for carbene 10 relative to that of 7 raises the effective barrier to rearrangement. We find, by monitoring the lifetime of carbene 10 as a function of concentration of diazoester 9, that 10 is effectively quenched by 9 with /tdiazo=(5.0 0.5) x 10 M s in Freon-113. This observation suggests that a major decay route of the carbene under the conditions of our experiment is formation of azine 16. 

Because most carbenes are so reactive, it is often difficult to prove that they are actually present in a given reaction. The lifetime of formylcarbene was measured by transient absorption and transient grating spectroscopy to be 0.15-0.73 ns in dichloromethane. In many instances where a carbene is apparently produced by an a elimination or by disintegration of a double-bond compound, there is evidence that no free carbene is actually involved. The neutral term carbenoid is used where it is known that a free carbene is not present or in cases where there is doubt. a-Halo organometallic compounds (R2CXM) are often called carbenoids because they readily give a elimination reactions (e.g., see 12-37). ° ... 

In either neat dioxane or THF, carbene-ether ylides are observed as a broad IR absorption band between 1560 and 1610 cm , distinct from the IR bands of the free carbenes. With discrete spectroscopic signatures for the free carbene and its corresponding ether ylides, TRIR spectroscopy was used to confirm that the effects described above with dilute ether in Freon-113 were due to specific solvation of the carbene (Scheme 4.6, Reaction 2) rather than a pre-equilibration with the coordinating solvent (Scheme 4.6, Reaction 3) or reactivity of the ylide itself (Scheme 6, Reaction 4). In Freon-113 containing 0.095M THF simultaneous TRIR observation of both the free carbene (x = ca. 500 ns) and the carbene-THF ylide (x = ca. 5ps) was possible7 The observation that lifetimes of these species were observed to be so different conclusively demonstrates that the free carbene and the carbene-THF ylide are not in rapid equilibrium and that Reaction 3 of Scheme 4.6 is not operative. By examining the kinetics of the carbene 34 at 1635 cm directly in Freon-113 with small amounts of added dioxane, it was observed that the rate of reaction with TME was reduced, consistent with Reaction 2 (and not Reaction 4) of Scheme 4.6. 

Owing to the low barriers to bond formation, reactant conformation often plays a decisive role in the outcome of these reactions. Carbocations, carbene, and radicals frequently undergo very efficient intramolecular reactions that depend on the proximity of the reaction centers. Conversely, because of the short lifetimes of the intermediates, reactions through unfavorable conformations are unusual. Mechanistic analyses and synthetic designs that involve carbocations, carbenes, and radicals must pay particularly close attention to conformational factors. 

There are a number of ways of generating carbenes that will be discussed shortly. In some cases, the reactions involve complexes or precursors of carbenes rather than the carbene per se. For example, carbenes can be generated by a-elimination reactions. Under some circumstances the question arises as to whether the carbene has a finite lifetime, and in some cases a completely free carbene structure is never attained. 

Substitution of a measured or estimated value of ifcpyr (e.g., 109 for alkylcar-benes), affords the value of k0 for those processes that destroy the carbene other than ylide formation. The inverse of <, can be taken as r0, the lifetime of the... 

In the same vein is the observation that the lifetime of dipropylcarbene (59) in CH2C12 or cyclohexane is 0.3 ns,84 which, after statistical correction is 48 times less than the lifetime ( 21 ns) of Me2C in pentane.22 This reflects promotion by the propyl bystander groups of 59 of the 1,2-H shift to Z- and E-3-heptene.84 (Dipropylcarbene can be photolytically generated from either an oxadiazoline (diazoalkane)84 or diazirine85 precursor, but RIES lowers the efficiency of carbene production in either case.) Recently reported LFP lifetimes for Et2C and MeCEt in cyclohexane or benzene are 0.6-3 ns (cyclohexane) or 1-5 ns (benzene),14 in accord with the lifetimes of S822 and S9.84 The rate constants for carbene disappearance in cyclohexane ( 3 x 108 to 2 x 109 s 1) are presumably limited by 1,2-H shifts.14... 

Generation of 78 by thermolysis or photolysis of a diazoalkane or diazirine precursor, however, affords the singlet carbene, whose 1,2-H shift to ethene is opposed by a barrier of only 0.678 to 1.298 kcal/mol. Consequently, even in cryogenic matrices, singlet 78 rearranges more rapidly than it intersystem crosses to the triplet, which has therefore not been detected by UV or ESR in either an Ar matrix at 8 K or a Xe matrix at 15 K." The lifetime of singlet 78 at ambient temperature has been estimated at <0.5 ns.89,98b (Note the enormous spectator substituent effect of Cl the lifetime of MeCCl is 740 ns,60 at least 1500 times longer than that of MeCH.)... 

Substitution of a second Me substituent at the carbenic center of 78 furnishes dimethylcarbene (36), the simplest dialkylcarbene. A significant spectator effect attends the change of H to Me the lifetime of 36 is 21 ns (Jfcj 5x 107 s-1),... 

Pyridine ylide/LFP studies of 83-85 in pentane or isooctane afforded carbene lifetimes of 21-24 ns (k 4 to 5 x 107 s 1), similar to the lifetime of dimethylcarbene under these conditions. Unfortunately, these lifetimes are limited by reactions with the hydrocarbon solvents the lifetime of 83 is 1.5 times longer in cyclohexane-d12 than in cyclohexane. The observation that the lifetimes of 55-CI ( 1000 ns) and 55-F (—7000 ns) are considerably longer than those of 83 and 84 could reflect the superior stabilization provided by the halogen spectator substituents of 55, but this conclusion is tentative in the absence of definitive intramolecularly controlled lifetimes for 83-85. 

Surprising is the absence of evidence for additional stability of 85 over 83. Electron donation from the electron-rich a bonds of the cyclopropyl ring to the carbene s vacant p orbital is widely believed to stabilize cyclopropylcarbenes.4 One would therefore expect 85, with an additional cyclopropyl substituent, to react more slowly than either parent carbene 83 or dimethylcarbene, but all three lifetimes are comparable. The lifetimes of 83-85 need to be redetermined in inert (fluorocarbon) solvents in order to reveal their innate differences. Note, however, that the effect of cyclopropyl substitution is apparent upon comparison of 83 (r 24 ns) to MeCH (r < 0.5 ns).89110... 

In contrast, 1,2-H shift to olefin 106 is the dominant reaction of carbene 104, and this process is slow enough to be measured by LFP r = 300 ns in cyclohexane and 560 ns in pentane at 25°C.117 There is a polar solvent effect the lifetime decreases to 52 ns in acetonitrile. However, at least in the case of cyclohexane, the lifetime is solvent limited, with a KIE of 1.5 on the lifetime in cyclohexane- (460 ns). Carbene 104 is much longer-lived than dimethylcarbene (r 21 ns in pentane) or methylcarbene (<1 ns).22,89... 

Laser flash photolysis (LFP) of quinone diazide 2d in Freon-113 at room temperature produces carbene Id, which could be monitored indirectly by addition of trapping reagents.25 At 2.0 xs the lifetime of Id is slightly longer than that of la (1.65 xs), otherwise the reactivities of these carbenes are very similar. The Id —> 11 rearrangement is not observed in the LFP experiments. All trapping products with a variety of reagents (O2, acetonitrile, pyridine etc.) are derived from carbene Id. 

Xanthylidene also does not react measurably with 02. The lifetime of XA is the same in 02-saturated cyclohexane as it is in solutions which have been deoxygenated. Bearing in mind that triplet carbenes react with 02 at nearly the diffusion limited rate, if 3XA were in rapid equilibrium with XA, then 02 should shorten the apparent lifetime of the singlet by reacting with the triplet. 

Irradiation of DMDAF in cyclohexane solution gives mainly insertion and only very minor amounts of the coupling and disproportionation products expected if hydrogen-atom abstraction were a major process. The lifetime of the carbene in cyclohexane is ca 11 ns and increases to 19 ns in C6D12. When the irradiation is performed in a 1 1 mixture of C6H12 and C6D12 the insertion product shows no crossover (Table 5). These chemical properties are those normally associated with a singlet carbene. 

Moritani 81,82) showed that the absorption spectra of dibenzo[a, djcyclohep-tatrienylidene and 10, 11-dihydro dibenzo[a, djcycloheptadienylidene can be recorded both at 77 °K and at room temperature. This proves that the same species is observed in the matrix and in solution, and is the triplet carbene. The lifetime of this carbene at room temperature was determined to be about 10 sec. 

With the exception of thermodynamically stabilized [64] or sterically protected [65] carbenes, these species and their hetero-analogs, nitrenes, are very reactive and therefore special conditions are required for their direct observation. Fast spectroscopic techniques capable of characterizing species with lifetimes of a few picoseconds have been used [1-3]. More recently, time-resolved IR (TRIR) experiments have been used to characterize species with lifetimes of microseconds and even nanoseconds [4-6]. 

Laser flash photolysis of 18 (Ar = -N02C6H4) led to the formation of the aryl chlorocarbene detected at 320 nm. In the presence of acetone, a new species was observed at X = 590 nm that was assigned the structure of carbonyl ylide 19. The ylide, formed by attack of acetone on the carbene, was shown to be irreversible, where the lifetime of the ylide (1.35 ps, fecyciization = 7.40 x 10 s ) was controlled by cyclization to the aryl epoxide 22. The rate constant for the cycloaddition of substituted benzaldehydes to produce dehydrodioxolane (21) was determined experimentally (e.g., p-ClPhCHO=6.16 x 10 M s ). 

The lifetime of 56 in hydrocarbon solvents at 25 °C is only several nanoseconds, so that its intermolecular chemistry should be difficult to observe. Indeed, intermol-ecular C H insertion reactions of 56 are inefficient, although the carbene can be captured (competitively with 1,2-H shift) by insertions into O H or N H bonds or by addition to isobutene. ... 

By plotting 1/AOD as a function of [Q], the ratio of fcq/ y[Y ] can be derived. This ratio corresoponds to the k x term of the Stern-Volmer equation. Here r is the lifetime of the carbene in the absence of quenchers. 

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