Furan and Pyrrole

Bromination of pyrrole with bromine gives mainly the thermodynamically more stable 3-bromo derivative, which is produced from the 2-isomer, formed initially under kinetic control, through isomerization catalyzed by the reaction product HBr. If N-bromosuccinimide is used for bromination, no HBr is produced and the 2-isomer dominates (8IJOC222I). Bromination of furan gave cis- and frans-2,5,-dibromo-2,5-dihydrofurans (isolated as the dimethoxy derivatives) in a ratio of 3 I (67BCJ130). A spectroscopic study carried out at low temperature (- 50°C) confirmed this ratio and also showed that 20% of trans-2,3-dibro-mo-2,3-dihydrofuran was formed (75CC875). It is not known whether these adducts (which demonstrate the competition between 1,4- and 1,2-addition) are intermediates along the pathway for bromine substitution, or whether they are produced in a separate equilibrium as is the case for bromination of benzo[6]furan. 

Bromination of 1-benzylpyrrole gives mainly the 3-substituted product (as in nitration) although polysubstitution occurs very easily (67CJC2227). 

---

The disadvantages associated with the Clemmensen reduction of carbonyl compounds (see 3 above), viz., (a) the production of small amounts of carbinols and unsaturated compounds as by-products, (h) the poor results obtained with many compounds of high molecular weight, (c) the non-appUcability to furan and pyrrole compounds (owing to their sensitivity to acids), and (d) the sensitivity to steric hindrance, are absent in the modified Wolff-Kishner reduction. 

Pd(II) salts promote the carbonylation of organomercury compounds. Reaction of phenylmercury chloride and PdCh under CO pressure affords benzophenone (429)[387]. Both esters and ketones are obtained by the carbonylation of furylmercury(Il) chloride in alcohol[388]. Although the yields are not satisfactory, esters are obtained by the carbonylation of aryl- and alkylmercuryfll) chlorides[389,390]. One-pot catalytic carbonylation of thiophene, furan, and pyrrole (430) takes place at the 2-position via mercuration and transmetallation by the use of PdCb, Hg(N03), and CuCl2[391]. 

The MCD spectra of furan and pyrrole are similar and quite different to those of the foregoing heterocycles. Both are soft chromophores and give rise to negative absorptions. 

Stoichiometric closure of furan and pyrrole cycles on McMurry reaction induced by low-valent transition metals 98PAC1071. 

Into Other Ring-Contracted Systems To Furan and Pyrrole Derivatives... 

Values of Resonance Energy of Derivatives of Furan and Pyrrole... 

The thiophene ring can be elaborated using standard electrophilic, nucleophilic, and organometallic chemistry. A variety of methods have been developed to exploit the tendency for the thiophene ring (analogous to that of furan and pyrrole) to favor electrophilic substitution and metallation at its a-carbons. Substitution at the p-carbons is more challenging, but this problem can also be solved by utilizing relative reactivity differences. 

Although the easy dehydrations experienced with the tetrahydroxybutyl derivatives of furan and pyrrole seem to be similar in kind to those of the 2-(aMo-polyhydroxyalkyl)benzimidazoles,31 a notable difference is observed... 

Donohoe reports a novel and unprecedented reductive aldol process involving the Birch reduction of furans and pyrroles (e.g., 62) which presumably generates a dianion 63 and subsequently (after protonation at C-5 by ammonia) an enolate 64. After quenching excess... 

Fig. 4.24. Reaction of electrophilic carbene complexes with furans and pyrroles leading to ring-opening of the heterocycle [1429].

Table 4.26. Ring fission of furans and pyrroles by acceptor-substituted carbene complexes. Starting Material Reagents, Conditions Product Yield Ref.

NICS(O) value (—11.0) is lower than that for the corresponding azadiphosphole (—12.8 see section III.C.2.1), similar to the differences in the furan and pyrrole NICS values (—12.3 and —14.7, respectively, at the same level of theory).The high reactivity of the oxadiphosphole in cycloaddition reactions is in accord with its low aromaticity. 

Heteroaromatics very reactive toward electrophilic species, such as furan and pyrrole, are not suitable for homolytic aminations owing to their low stability under the reaction conditions. Thiophene, however, can be aminated, leading to 2-dialkylamino derivatives. ... 

In an extension of traditional Stetter methodology, Miiller and co-workers have used the Stetter reaction in a one-pot multicomponent reaction for the synthesis of furans and pyrroles (Scheme 19) [85,86], The ot,P-unsaturated ketone XXVI is formed in situ and undergoes a Stetter reaction followed by a Paal-Knorr condensation. 

Radical cyclization of oxime ethers having allylic substiffients to five-membered rings including furan and pyrrole derivatives was described in reviews A thiophenol-promoted radical cyclization of oxime ethers into tetrahydrofurans was recently described. For example, oxime derivative 36 in the presence of thiophenol and azobisisobuty-ronitrile (AIBN) afforded substimted tetrahydrofurans 37 and 38 in a ratio 1.2-3 1 (equation 17) . Radical cyclization of oxime ethers 39 to tetrahydrofurans 40 was successfully realized in the presence of alkyl iodides and EtsB in refluxing toluene (equation 18) . ... 

The following important conclusions can be drawn from the above results [88JST(163)173]. First, the values of [2A ]n are nearly equal for furan and pyrrole hence the correct aromaticity trend can be ascertained only if the [XAE], contributions are also taken into account. Thus, the relative aromatic character of the compounds under discussion is determined by the sum of the stabilizing effects of the two electron interactions. These are the stabilization energy AE, referring to the interaction be-... 

The results of dipole moment and Kerr constant elaboration indicated (84JST(116)377) that in the 2-pivaloyl derivatives of furan and pyrrole the... 

Rate and Equilibrium Constants for the Formation of ct-Adducts by the Reaction of Some Furan and Pyrrole Derivatives with MeO" at 25°C... 

The parent heterocycles (Table 13) display a strong band near 220 nm with one additional band at longer wavelengths for thiophene and selenophene, and two for tellurophene. Analogous weak bands reported in the older literature for furan and pyrrole are now generally accepted as arising from autoxidation products. 

Pyridine iV-sulfides are known only in the form of their derivatives. Thus, 1-arylthiopyridinium cations (from pyridine and sulfenyl chloride) react with KCN to form ArSCN and pyridine (81CC703). Pyridine-sulfur trioxide is a mild sulfonating reagent, used for sulfonation of furan and pyrrole. 

---