Carbenes, TRIR spectroscopy

As demonstrated in the two previous sections, TRIR spectroscopy can be used to provide direct structural information concerning organic reactive intermediates in solution as well as kinetic insight into mechanisms of prodnct formation. TRIR spectroscopy can also be used to examine solvent effects by revealing the inflnence of solvent on IR band positions and intensities. For example, TRIR spectroscopy has been used to examine the solvent dependence of some carbonylcarbene singlet-triplet energy gaps. Here, we will focns on TRIR stndies of specific solvation of carbenes. 

Since the most direct evidence for specihc solvation of a carbene would be a spectroscopic signature distinct from that of the free carbene and also from that of a fully formed ylide, TRIR spectroscopy has been used to search for such car-bene-solvent interactions. Chlorophenylcarbene (32) and fluorophenylcarbene (33) were recently examined by TRIR spectroscopy in the absence and presence of tetrahydrofuran (THF) or benzene. These carbenes possess IR bands near 1225 cm that largely involve stretching of the partial double bond between the carbene carbon and the aromatic ring. It was anticipated that electron pair donation from a coordinating solvent such as THF or benzene into the empty carbene p-orbital might reduce the partial double bond character to the carbene center, shifting this vibrational frequency to a lower value. However, such shifts were not observed, perhaps because these halophenylcarbenes are so well stabilized that interactions with solvent are too weak to be observed. The bimolecular rate constant for the reaction of carbenes 32 and 33 with tetramethylethylene (TME) was also unaffected by THF or benzene, consistent with the lack of solvent coordination in these cases. °... 

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. 

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. 

The photoinduced Wolff rearrangement of 5-diazo-2,2-dimethyl-l,3-dioxan-4,6-dione has also been examined by TRIR methods [116], These ultrafast measurements, conducted in a PMMA matrix, revealed that the formation of the ketene rearrangement product was complete within 20 ps a carbonyl carbene was not detected in this study. Other applications of TRIR spectroscopy to the study of carbene-related chemistry include investigations of diazirine to diazo rearrangements [117] and of oxygen and sulfur atom transfer reactions [118]. 

The lack of excited state involvement for 21 is in contrast to the behavior in systems that have substantial equilibrium concentrations of the syn conformer. To examine the effect that conformation has on ketene growth kinetics, 4-diazo-3-isochromanone (22), ° a cyclic analogue (phenyl version) of 21 that is locked in the syn conformation, was also recently studied by TRIR spectroscopy. In this case, a carbene IR band is observed at 1686 cm that decays with a lifetime of 526 50 ns in Freon-113. The ketene IR band at 2116 cm, however, in dramatic contrast to the data observed with acyclic diazocarbonyl 21, is produced faster than the experimental time resolution. Ketene 24, therefore, is formed entirely from a non-carbene source, presumably the excited state of 22 (Scheme 8). In agreement with this hypothesis, oxygen and methanol quench carbene 23, but they leave the initial intensity of the ketene IR band unaffected. These... 

Time-Resolved IR Spectroscopy. More recently, time-resolved IR (TRIR) experiments have been used to characterize species with lifetimes of micro-and even nanoseconds. Since IR spectra provide structural information in more detail than UV, this technique will be more powerful than TRUV-vis if one can find a carbene that can be detected and studied by this technique. To date, however, only one carbene has been studied by using TRIR. The matrix IR study shows that the planar triplet and twisted singlet states of 2-naphthyl(methoxycartbonyl) carbenes (NMC, 17) show distinctly different IR bands (see Section 3.1.4). Both NMC and NMC are detected by TRIR in solution and their kinetics have been studied. Such experiments provide clear cut data for the reaction kinetics as well as energetics of both states (see Sections 4.2 and 4.3... 

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