Carbene, generation by photolysis

A detailed reinvestigation of the products and kinetics of reactions of benzylchloro-carbene, generated by photolysis of the corresponding diazirine, indicated that the... 

Take the reaction of carbenes generated by photolysis of diazo compounds (Scheme 9.1), for example. Direct irradiation of a diazo compound (12) is believed to generate the carbene initially in singlet state ( 3) via the singlet excited state of the precursor. Triplet sensization, on the other hand, is presumed to give the triplet... 

More attention has been paid to photoelimination of nitrogen from cyclic azoalkanes. Diazirine is a special case and undergoes decomposition via an identifiable carbene, with evidence in some cases for the intermediacy of a linear diazoalkane. The infrared spectra of phenylbromocarbene and phenyltrifluoromethy1-carbene, generated by photolysis of the corresponding diazirines... 

The intramolecular O—H insertion reaction of a carbene has been successful in the synthesis of the oxygen-containing heterocycle 216 in quantitative yield (85JOC5223). Carbene generated by photolysis of diazo precursor 217 in protic solvents gives 2-phenyl-2,5-dihydroisobenzofuran competitively with insertion into the O —H bonds of the solvent (90JOC2325). 

By Cu-catalyzed decomposition of ortho-seleno- and -telluro-substituted oj-diazoacetophenones, Lohner and Praefcke 482 synthesized seleno- and telluro-3-coumaranones 483. This reaction with the participation of a free carbene generated by photolysis of 482 (Z = Te) gives unsatisfactory results (80JOM173). 

The carbene generated by photolysis of diazo compound 499 rearranges via phenyl migration to a short-lived phosphene 500, which with aldehydes or ketones gives [2-l-2]-cycloadducts (A -l,2-oxaphosphetanes (501 (80CB3303 81T1039). 

Carbenes, generated by photolysis of di- and tetrachloro-o-quinone diazides, react with oxetane in a 1 3 ratio to afford 15-membered crown ethers. Benzocrown ether 675 was obtained in 16% yield (91CB1865). Derivatives of macrocyclic crown ethers with four or five oxygen atoms in a ring were synthesized by Cu(acac)2-catalyzed cyclization of a,polyethylene glycols. 20-26-Membered crown-4(5) ethers 676 were prepared from the above-mentioned diazo ketones with tri- or tetra-ethylene glycols in 7-26% yields. Treatment of l,8-bis(diazoacetyl)octane with dodecane-l,12-diol under the same conditions results in a mixture of 52-membered tetraether 646 (40%) and compound 645 (81CC616). 

Transient phosphoryl-substituted carbenes, generated by photolysis or by copper catalysis, react selectively with one double bond of a 1,3-diene affording mixtures of trans- and c -isomers which, in some cases, can be separated by column chromatography on silica gel or by distillation (Table3). ... 

Three major classes of photoreactive compounds have been used to label specific sites of proteins nitrenes, carbenes, and free radicals. Carbenes, generated by photolysis of diazo derivatives, are highly reactive and relatively nonselective in their target sites. Nitrenes, formed by photoactivation of azido derivatives, are less reactive than carbenes and more electrophilic, exhibiting a preference... 

Triplet carbene generated by photolysis of bis[9-(10-phenyl)anthryl]-diazomethane (56) in MTHF glass at 77 K gave EPR signals very similar to those... 

The carbene generated by photolysis of the tosylhydrazone salt (117) adds to the intramolecular triple bond to provide the second example of a bicyclo[4,l,0]hepta-l(7),2,4-triene, which can be trapped by butadiene to give the norcaradiene (118) (Scheme H). " ... 

A final method of /3-lactam 3,4-bond formation which has found fairly wide application is based on carbenlc insertion (78T1731 p. 1739). The carbenic centre can be generated by photolysis of a diazo compound as in the case of (158) (72JA1629, 79CC846) or from organometalllc precursors, for example (159) (71ACS1927). 

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. 

It is suggested that the real carbene, generated by thermolysis of the diazo or diazirine precursors or photolysis of 40, gives mostly 1,3-insertion, whereas photolysis of either the diazoalkane or diazirine yields much 1,2-Me migration directly from precursor excited states.15 1 An analogous intervention of 1,2-Me migration via RIES was also observed in the photolytic decomposition of f-butylchlorodiazirine (24) to f-butylchlorocarbene (18) cf. Eq. 14.27... 

Much also depends on how the carbene is generated. Duncan et al showed that carbene generated by the photolysis of ketone adds to the above substrate in a non-stereospecific manner. 

The absolute rate constants for reaction of /t-tolyl(trifluoromethyl)carbene, generated by laser flash photolysis of the corresponding diazirine, with pyridine (4 x lO lmor s ... 

Laser flash photolysis of phenylchlorodiazirine was used to measure the absolute rate constants for intermolecular insertion of phenylchlorocarbene into CH bonds of a variety of co-reactants. Selective stabilization of the carbene ground state by r-complexation to benzene was proposed to explain the slower insertions observed in this solvent in comparison with those in pentane. Insertion into the secondary CH bond of cyclohexane showed a primary kinetic isotope effect k ikY) of 3.8. l-Hydroxymethyl-9-fluorenylidene (79), generated by photolysis of the corresponding diazo compound, gave aldehyde (80) in benzene or acetonitrile via intramolecular H-transfer. In methanol, the major product was the ether, formed by insertion of the carbene into the MeO-H bond, and the aldehyde (80) was formed in minor amounts through H-transfer from the triplet carbene to give a triplet diradical which can relax to the enol. 

Insertion of the carbenes, via a low-lying singlet state close to the ground-state triplet, generated by photolysis of the diazo compounds (128), into the CO bond of cyclic ether solvents (THF, THP, dioxane) gave rise to polyether-bridged azulenes such as (129). ... 

We first investigated the synthesis of a (phosphino)carbene featuring an electron-rich aryl group. The (phosphino)(mesityl)carbene (XlVa) was generated by photolysis at —50 °C of a THF solution of the corresponding diazo compound. The and NMR chemical shifts of XlVa ( P 8 -23.5 5 151.1) compare... 

Most carbenes are relatively easily generated by photolysis of nitrogeneous precursors such as diazo compoundsor diazirines. The reaction is clean, as nitrogen is the only byproduct and it is also very efficient as nitrogen evolution is a highly exothermic reaction. Therefore carbenes can be easily generated even under very inert conditions, such as in a noble gas matrix at very low temperatures. 

Bis(2,4,6-tribromophenyl)carbene (106f) was easily generated by photolysis of the precursor diazomethane (105f) and was characterized by EPR spectroscopy (Scheme 9.14). The triplet carbene generated in a degassed solution at room temperature decayed very slowly, persisting for at least 30 s. The decay was found to be second order Ik/el = 8.9 s ). The parameter fj/2, is estimated to be Is (Table 9.14). 

CHi generated by photolysis of CH2N2 in the liquid phase is indiscriminate—totally non-selective—in its reactivity (p. 199). CH, generated in other ways and other carbenes are less reactive and insert in the order tertiary > secondary > primary.232 Halocarbenes insert much less readily, though a number of instances have been reported.233 Nevertheless, even for less reactive carbenes, the insertion reaction has seldom been used for synthetic purposes.234 The carbenes can be generated in any of the ways mentioned in Chapter 5 (p. 198). For the similar insertion of nitrenes, see 2-12. 

The dihydrothiazol-2-ylidene (4) was generated by photolysis of matrix-isolated thiazol-2-carboxylic acid.12 Calculations suggested that the barrier to isomerization to thiazole is about 42.3 kcal mol 1 and that the carbene resembles the related imidazol-2-ylidene in structure. An ab initio study of hydroxyoxiranone predicted that the decarboxylation of the zwitterion (5) to form hydroxycarbene (6) would be favourable in vacuo but not in water.13 A theoretical study showed that dihalosulfenes (X2C=S02) are best viewed as dihalocarbenc-SO complexes with a carbon-sulfur bond order of approximately zero.14 hi a study directed at the elusive thionformic acid (7), tandem mass spectrometric methods were applied to isomeric ethyl thioformates.15 The results suggest that the radical cations generated have the carbene structure [(HS)C(OH)]+ ... 

Non-stereospecific cyclopropanation reactions of the diazafluorenylidene (10), generated by photolysis of the diazo compound, indicated a triplet carbene.18 Competition experiments suggested a singlet-triplet equilibrium at room temperature and a Hammett study of additions to substituted styrenes indicated that the carbene reacts as an electrophile (p = —0.65). 

When (trimethylsilyl)carbene (3a) is generated by photolysis of the diazo precursor 19 in an alcohol solution at room temperature, the rearrangement to the silene is so fast that only the addition products of the latter, but not the 0,H insertion product of the carbene, can be isolated (equation 4)34. The same holds true for photolysis in diethylamine38. 

In this context, it is worth mentioning that there is only one other, clear-cut example for the simultaneous occurrence of the acyl(silyl)carbene-to-acylsilene and the acylcarbene-to-silylketene rearrangement of an acylcarbene bearing a Si—Si substituent. Carbene 57, generated by photolysis of diazoketone 56 in benzene, isomerized to both 58 and 59 in about equal amounts44. While the acylsilene cyclized to 1,2-silaoxetene 60, the ketene was isolated and structurally characterized by X-ray diffraction analysis of the derived... 

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