Irradiation aryl carbenes

C-H carbene insertion reactions. Hydrogen abstraction products were formed at the expense of insertion products on irradiation of -nitrophenyldiazomethane under the same conditions. Possible explanations for this difference in reactivity have been offered. Photochemically generated aryl carbenes have been intercepted with carbon monoxide and with oxygen. Benzophenone 0-oxide has been detected spectroscopically on irradiation (515 nm) of diphenyl-... 

Figure 11.3 Identification of protein targets using photo-cross-linking beads, (a) Upon UV Irradiation, aryl-diazirine groups covalently Introduced onto a solid matrix (e.g., agarose beads) are transformed into highly reactive carbenes, which in turn bind to or insert irreversibly into a proximal small molecule in a functionality independent manner, (b) Pull-down assays are performed by mixing...

Methoxy(aryl)carbene]iron complexes are obtained by reaction of the appropriate benzoic chlorides with [Fe(C5Mes)(CO)2]K to give the acyliron complexes, followed by methylation with methyl mesylate. Reduction with sodium borohydride and removal of the methoxy group by treatment with trimethylsilyl triflate affords the defunctionalized (arylcarbene)iron complexes. The presence of ortAo-methoxy or ortho-chloro substituents at the arene moiety gives rise to (T) -C,OMe) or (T1 -C,C1) chelate (arylcarbene)iron complexes upon irradiation. Disodium tetracarbonylferrate reacts with aromatic (V,A -diallylamides followed by treatment with trimethylsilyl chloride with direct formation of chelated a-aminocarbene complexes (Scheme 4-59). ... 

Matrix isolated aryl carbenes have yielded routes to a number of strained polyenic systems, including strained cyclopropenes and cychc cumulenes. For example, photolysis of phenyldiazomethane (65) with visible hght (A, > 478 nm) gave phenyhnethylene (66), which was characterized by its highly structured UV absorptions and its IR spectrum (Scheme 15). Further irradiation of 66 at shorter wavelengths (A > 416 nm) prod need a new species with a broad UV absorption with 271 nm and IR absorptions... 

Scheme 78) [89]. Aryl chlorides with activating as well as deactivating substituents could also be coupled under the same conditions in high yields, ranging from 60% to 95%, within 30-60 min of microwave irradiation. The process does not require an inert atmosphere. The increased conversion observed with the addition of the ionic liquid reveals that it might have an additional function besides simply acting as a molecular irradiator . It cannot be excluded for instance that carbene palladium complexes are formed in situ and implicated in the catalytic cycle. 

The first examples of microwave-assisted cross-couplings with organozinc compounds were recently reported [47]. In addition, the first high-speed synthesis of aryl boronates (Suzuki coupling reactants) has been performed under the action of single-mode irradiation with an in-situ-generated palladium carbene catalyst [48],... 

Three types of photoextrusion reactions have been identified in the irradiation of aryl-substituted 1,3,2-dioxathiolane 2-oxides [5 1-1-2 4-2] cycloelimination to produce a carbonyl compound, a carbene and sulfur dioxide extrusion of sulfur dioxide accompanied by a pinacol-like rearrangement to yield an aldehyde or ketone and extrusion of sulfur trioxide to give an alkene <72JOC2589>. Sensitization and quenching experiments indicate that a singlet state is responsible for the cycloelimination reaction, whereas the rearrangement and sulfur trioxide extrusion reactions arise from a triplet state <82JCR(S)175>. 

Thermolysis of aryl chloro diazirine (18) in the presence of acetone and a trapping agent such as A -phenylmaleimide gave rise to cycloadducts such as 41. The unstable adduct hydrolyzed during purification resulting in synthesis of bicyclic hemiacetals 42 and 43 as a mixture of endo and exo adducts in 37 and 8% yield, respectively. The exclusive generation of the singlet carbene was confirmed by low-temperature electron spin resonance (ESR) study of the irradiated diazirine. 

Surface-introduced photoreactive group, aryl diazirine, can generate carbenes by 365 nm UV exposure. However, gold-sulfiir bonds, which support photolinker on gold, can be easily broken by UV light below 300 nm. Therefore, UV transmission filter is essential in UV irradiation. 

Like diazo compounds and aryl azides, diazirines give rise in most instances to unwanted photolysis products besides the desired carbenes, including long-lived electrophilic intermediates. The most serious problem is the generation of diazo compounds (e.g. Fig. 3.13). For example, 3-H-3-aryl-diazirines form diazo isomers to the extent of 30 to 70% when irradiated. These isomers are themselves photolysed to form carbenes, but relatively slowly at wavelengths at which the diazirines absorb (Smith and Knowles, 1973, 1975). 

Studies of the electron transfer photochemistry of l,2-diarylethanes and of aryl glycopyranosides have been reptorted. The wavelength-dependent photochemistry of 2,3-dimethylbut-2-ene and of oct-2-ene has been described by Inoue et al Direct irradiation (185 nm) of the bicycloalkenes (42) in pentane solution affords the corresponding cycloalkadiene (43) as well as the fragmentation products acetylene and (44), produced via the Intermediate carbene (45). ... 

A review has discussed the photochemical cyclization of a variety of aryl- and heteroaryl-prop-2-enoic acids. Irradiation (A,>340nm) of the cyclohexenone (119) in argon purged benzene solution affords the cyclized derivatives (120) as a 3 1 mixture of diastereoisomers. The formation of the cyclopropyl group arises via the carbene (121) and insertion into a C-H bond of a neighbouring methyl group. This carbene is formed, presumably, from the biradical (122) which arises by addition of the alkene to the excited state of the enone. Further evidence for the carbene intermediate comes from a reaction in methanol when the diastereo-isomeric mixture of the ethers (123) and (124) is obtained." ... 

Although arylcarbenes have been generated from aryl-substituted tetrazoles, the necessary reaction conditions (flash-vacuum pyrolysis) are such that cyclopropane formation does not occur. When the corresponding anion is used as the precursor, however, carbene generation and cyclopropane formation can be achieved under mild conditions. Thus, irradiation of tetra-butylammonium 5-phenyltetrazolide dissolved in a hydrocarbon containing 2-methylbut-2-ene yielded, conceivably via phenyldiazomethane, a mixture of cis- and fra 5-2,2,3-trimethyl-l-phenylcyclopropane in a 0.8 1.0 ratio. 

UV Irradiation of an 3-aryl-2-azidoacrylic ester (2-azidocinnamate) results in elimination of nitrogen and formation of a 2/f-azirine, which undergoes spontaneous electrocyclic ring opening to give a reactive intermediate which can react as an azomethine ylide dipole or an alk-oxycarbonyl(benzylidenimino)carbene. In favorable cases, the carbene form can be trapped by intramolecular [l-t-2] cycloaddition to an alkene double bond to give a dihydro-cyclopropa[c]isoquinoline, e.g.l, and 2 (Houben-Weyl, Vol. E19b, pi 166). 

Several examples have appeared of diversion of the normal photochemistry of ortho-alkyl aromatic ketones due to apparent interception of the intermediate biradical by an unsaturated group present elsewhere in the molecule.Thus the vinyl aryl ketones (376) are converted to (377) in reasonable yields upon irradiation with ultra-violet light,while irradiation of the ortho-alkynyl acetophenone (378) in methanol yields diastereomers of (379)The latter reaction is thought to proceed by coupling of the initially formed ketyl radical onto the alkyne to give a carbene (380). The photochemistry of the jS-diketone (381) gives a mixture of the benzocyclobutenol (382) and the tetralone (383), and the proportions depend on the identity of the R-substituent in (381). The formation of (383) can be rationalised in terms of interception of the ketyl radical in the initially formed 1,4-biradical by the remote carbonyl group. 

Aryl chlorides are more reluctant to undergo amination than most other aryl halides/pseudohalides. To address this problem, Caddick and coworkers investigated the outcome of palladium-N-heterocyclic carbenes as catalysts in rapid microwave promoted reactions [121]. para-Tolyl and para-anisyl chloride were coupled with aromatic and aliphatic amines in generally good yields within 6 min at 160 °C. Reactions between tolyl/anisyl/phenyl chlorides and aliphatic amines have also been reported by Maes et al. using a more classic reaction system with a phosphine ligand and a strong base the reaction afforded the desired products after 10 min irradiation at 110-200 °C [122]. 

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