2-Methylpyrrole, irradiation

Perfluoroalkylation of substituted benzenes and heterocyclic substrates has been accomplished through thermolysis of perfluoroalkyl iodides in the presence of the appropriate aromatic compound Isomeric mixtures are often obtained W-Methylpyrrole [143] and furan [148] yield only the a-substituted products (equation 128) Imidazoles are perfluoroalkylated under LTV irradiation [149] (equation 129). 4-Perfluoroalkylimidazoles are obtained regioselectively by SET reactions of an imidazole anion with fluoroalkyl iodides or bromides under mild conditions [150] (equation 130) (for the SET mechanism, see equation 57)... 

In contrast, when the irradiation is performed on 2-cyanopyrrole, the isomeric products are observed. In fact, in this case, the corresponding Dewar pyrrole shows a lower energy than in the previous case, allowing the formation of the isomeric products (Fig. 6). When 2-methylpyrrole is used as substrate, the formation of the triplet state is favored, but this triplet state cannot evolve through the formation of the biradical intermediate. 

The principal products of irradiation of 2-cyano-l-methylpyrrole (166) in methanol, however, are the corresponding 3-cyanopyrrole 167 and the methanol adduct 168.129 These products appear to arise by a 2,5-bonding process followed by a thermal walk rearrangement as shown in Scheme 5. 

A review of aromatic substitution by the 5 rnI reaction has been published. The reactions of enolate ions of 2-acetyl-(147) and 3-acetyl-1-methylpyrroles (148) with aryl iodides and neopentyl iodides under irradiation conditions afforded good yields of substitution products by 5rn1 mechanisms, without the need for initiator. These... 

Decomposition of l-methyl-2-pyrrolidinone (67) was studied by vapor-phase photolysis (72JA8281). Irradiation (Hg sensitized) led, in addition to extensive polymer formation, to the following products carbon monoxide (31%), ethene (24%), water (24%), l,3,5-trimethyl-hexahydro-l,3,5-triazine (8%), 1-methylazetidine (6%), 1-methylpyrrole, and methane (<1%). The mechanism of formation of most of these products involves... 

The reactions of t with pyrrole and 1-methylpyrrole have been investigated by Kubota and Sakurai (117,135). The formation of adducts 71 and 72 upon irradiation of t-1 in pyrrole solvent is proposed to occur via a nonfluorescent singlet ex-ciplex, which undergoes N-H atom transfer from pyrrole to t-1 to yield a radical pair (117). Radical pair combination occurs at C-2 or C-3 of the pyrrole radical to yield adducts 71 and 72, which isomerize to yield the isolated products, 73 and 74... 

The quenching of - -t by 1-methylpyrrole in nonpolar solvents results in the formation of a fluorescent exciplex, but no adduct formation (117). Irradiation in acetonitrile (anhydrous or aqueous) solution produces 1,2,3,4-tetraphenylbutane (65) in low yield. Formation of adducts 75 and 76 and 1,2-diphenylethane (64) is observed upon irradiation in acetic acid solution and attributed to protonation of t-lT followed by coupling of the 1,2-diphenylethyl radical and 1-methylpyrrole cation radical and deprotonation of the coupling product ... 

The reaction of t with 1-methylimidazole is reported by Kaupp and Gunter (136) to yield the addition product 77. Unlike the reaction of It with 1-methylpyrrole, the formation of 77 does not require acid catalysis. The formation of 77 is proposed to occur via the 1,2-biradical intermediate 78, which undergoes a 1,3-hydrogen atom migration to yield 77 (eq. 27). The reaction of It with benzothiazole also yields an acyclic adduct 79 and a second adduct 80 as the major products. Both 79 and 80 are proposed to arise via a 1,4-biradical intermediate similar to 78. Irradiation of t-1 with caffeine (81) yields a [2+2] cycloadduct 82 and an acyclic adduct 83, analogous to 80, as the major products. 

Only a few additional examples of intermolecular photochemical vinylations of (hetero)aromatic compounds have been forthcoming. Coupling products are formed in the irradiation of dichloro- and dibromo-A-methylmaleimide in the presence of 1,3-dimethyluracils341 and of 3-bromocoumarin in the presence of naphthalene, phenanthrene, 1-methylpyrrole and other aromatic compounds342. The former reaction is accompanied by cyclobutane adduct formation, which is the mode of reaction of A-methylmaleimide itself. The mechanism of these vinylation reactions is not clear, but most probably an exci-plex (cf equation 20a) or a charge-transfer complex (cf equation 20b) is involved. 

Photochemical reaction in acetonitrile of 2 -deoxyuridine 5 -phosphate with the halo-heteroarenes 2-iodothiophene, 2-iodofuran, l-methyl-2-iodopyrrole and 3-iodothiophene affords the C-5 heteroaryl substituted nucleotides518.6-Aryluridines have been prepared by irradiation of 6-iodouridines in benzene, anisole, thiophene, Af-methylpyrrole or 2-methyl-furan in the presence of triethylamine519. 

A number of substituted benzenes, naphthalenes, indans, pyridmes, and indoles form arene(tricarbonyl)chromium complexes upon thermolysis under an inert atmosphere, usually in a high boiling ether, or by irradiation of the arenes in the presence of chromium hexacarbonyl. The complexes are relatively air-stable and can usually be stored for long periods in the absence of light. Somewhat milder conditions can be used by transfer of the chromium tricarbonyl group from preformed naphthalene(tricarbonyl)chromium, tris(L)tricarbonyl chromium (L = acetonitrile, ammonia, pyridine), or tricarbonyl( -l-methylpyrrole)chromium. Enan-tiomerically pure arene(tricarbonyl)chromium complexes having two different substituents, either ortho or meta can be prepared conveniently by classical resolution of racemic... 

The formation of pyrroles by the photolysis of furan or thiophene in the presence of a primary amine was considered to involve valence-bond isomers of pyrroles but the latter were not isolated. However, the photoreaction of cyanopyrrole was examined at the same time as that of cyanofuran (Section I,A), and much earlier than that of cyanothiophenes (Section I,B). Irradiation of 2-cyano- 1-methylpyrrole in methanol was reported to give l-cyano-3-methoxy-5-methyl-5-azabicyclo[2.1.0]pentane, which may have been formed from the Dewar compound (Eq. 14).4 The structure of the... 

Pyrrole and iV-methylpyrrole form exciplexes with a variety of arylalkenes and arylalkynes. When pyrrole is irradiated in the presence of styrene the adduct (27) is formed in 67% yield. The reaction is brought about by an electron-transfer process with the amine as the donor and the alkene as the acceptor. Ultimately coupling affords the final product. An analogous addition is seen with 1-methylstyrene when the adduct (28, 52%) is produced. Other examples of such additions were also described. ... 

In contrast, N-methylpyrrole underwent direct arylations atthe 3-position [93], and a comparable result was obtained in direct arylations of N-phenylpyrrole employing the electron-deficient rhodium complex 114, in combination with Ag2C03 under microwave irradiation [74b]. Selected examples of regioselective arylations of substituted five-membered heteroarenes are summarized in Table 9.3. 

A related study led to a reactivity test for HBTU-activated carboxylic acids involving N-methylpyrrole derivatives as building blocks for oligopyrrolamides to bind DNA duplexes [47]. Polymer-bound DNA was decorated with Fmoc-protected N-methylpyrrole-2-carboxylic acid (Scheme 16.26) under the action of microwave irradiation for 10 min at 80 °C subsequent deprotection furnished the desired modified oligonucleotide in moderate yields. 

Irradiation of 1 in the presence of A -methylpyrrole furnished the bicyclic product 9 in almost quantitative yield. On longer heating at 100°C the bicyclic product 9 underwent an electrocyclic rearrangement to afford the cyclohexadiene derivative 10. Renewed attack of 1 (2) on 9 finally led to the seven-membered heterocyclic product 11 which was characterized by X-ray crystallography [7]. This example again illustrates that exchange of the heteroatom in these five-membered ring compounds can result in a eompletely different product spectrum. 

W-2-pyridyIcarboxamido) -2-pyridone, 644 2-Acetyl-5-methylpyrrole, from irradiation of 2,6-lutidine -1-oxide, 725... 

Surprisingly, the reaction of CF2I2 with pyrrole (lb) and A -methylpyrrole (la) gave the trifluoromethylated products 38b and 38a in moderate yields under UV irradiation in DMF (Table 1, entries 5 and 6) [33]. In contrast the related reaction with 2,5-dimethylpyrrole 67 gave the trifluoromethylated dimer 68 instead of the desired 2,5-dimethyl-3-trifluoromethylpyrrole. 

Irradiation of stilbene in N-methyHmidazole results in the formation of the a-CH adduct 5a. Adduct formation is proposed to occur via a 1,4-biradical intermediate (Scheme 6). Similarly, irradiation of stilbene in pyrrole yields two CH adducts, 6a and 6b.Irradiation of stilbene with indole or carbazole in acetonitrile solution results in the formation of both NH and CH adducts in low yield. However, irradiation of polycrystalline mixtures of stilbene and these heterocycles yields predominantly the NH adducts in moderate yield (ca. 50%). The role of electron transfer in these reactions has not been established however, exciplex fluorescence is observed upon quenching of stilbene by N-methylpyrrole in cyclohexane solution. 

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