Pentachloro-3//-pyrrole

Reduction of indolenines with sodium and ethanol gives indolines. The pentachloro-a-pyrrolenine (205) is in equilibrium with small but finite amounts of the isomeric (3-pyrrolenine form (3//-pyrrole 206), since it forms cycloadduct (207) with styrene (80JOC435). Pentachloropyrrole acts as a dienophile in its reaction with cyclopentadiene via its ene moiety (81JOC3036). 

As with the alkyl and aryl derivatives of the pyrrolenines and indolenines, a tautomeric equilibrium has also been noted between the pentachloro-2//- and -3H- pyrroles, such that when the 2//-pyrrole, produced by chlorination of 2,3,4,5-tetrachloropyrrole or of 3,4-dichloromaleimide, is allowed to react with dienophiles, the adducts are those formed by cycloaddition with the 3H-pyrrole tautomer (Scheme 84) (80JOC435, 80JA7862, 81JOC3036). Cycloaddition with cyclopentadiene occurs on the 2H-pyrrole, which behaves as the dienophile. 

Pyrrole is readily halogenated. Chlorination with one equivalent of sulfuryl chloride in ether at OC gives 2-chloropyr-role further chlorination with this reagent yields di-, tri-, and tetra-chloro derivatives and ultimately pentachloro-2//-pyrrole 75. 

Reduction of 3//-indoles with sodium and ethanol gives 2,3-dihydroindoles. 2,2,3,4,5-Pentachloro-2//-pyrrole 250 is in equilibrium with small but finite amounts of an isomeric 3//-pyrrole 251, since the latter can be trapped as 252 with styrene. 

There are a number of examples of substituted pyrroles participating as 4tt components of all-carbon Diels-Alder reactions.63 However, their reported participation as a 1- or 2-azadiene system in [4 + 2] cycloadditions are limited to selected pyrroles.64,65 Pentachloro-2//-pyrrole has been shown to participate in inverse electron demand Diels-Alder reactions with electron-rich dienophiles exclusively in the form of the 2-azadiene system, pentachloro-S/Z-pyrrole,64 and one example of an intramolecular Diels-Alder reaction of a substituted, in situ generated 3//-pyrrole has been described (Chapter 9, Section 2).65... 

Treatment of pyrrole or 2,3,4,5-tetrachloropyrrole with sulfuryl chloride gave pentachloro-2fl-pyrrole (21, Scheme 8).35 The same product had been obtained earlier from succinimide and phosphorus pentachloride (Section H,B,l,a).36... 

Pentachloro-2//-pyrrole (21) was converted to the trifluoro derivative 143 by excess silver(I) fluoride at 135°C57 and to the salts 144 with N,N-dimethyl(trimethyl)silylamine (R = Cl or NMe2, depending on the amount... 

Pentachloro-2//-pyrrole (21) is reduced by sodium amalgam and acetic acid to 2,3,4,5-tetrachloropyrrole.36 The spiro compound 55 is hydrolyzed to 153 (69%) by concentrated hydrochloric acid,56 and the adduct 105 is cleaved by base to the 1//-pyrrole 154.88... 

The use of ozonolysis to establish the structure of pentamethyl-2//-pyrrole (1) was described earlier (Section II,A,2).16 Treatment of pentachloro-2f/-pyrrole with silver(I) fluoride gave in addition to the fluoride (143), the bis(acid fluoride) of dichloromaleic acid.57 Oxidation of the diamino ester 155 with lead tetraacetate led to the cyanoimine 156, which in turn was cyclized to the pyrimidine 157 (Scheme 54).137... 

Section IV,A,2,a Pentachloro-2H-pyrrole (21) behaves as a diene via the rearranged 3f/-pyrrole form 133 in its reaction with trans-1,3-pentadiene, giving a product 132 having R = trans-l-propenyl. However, the mode of cycloaddition is reversed with cyclopentadiene, the 2i/-pyrrole (21) reacting directly as a dienophile at the 3-4 C=C bond. With 1,3-cyclohexadiene, both modes of cycloaddition occur simultaneously. Steric factors rather than relative HOMO and LUMO energies are thought to determine the reaction course.147... 

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