Furan, Dewar

In the case of the direct irradiation, the singlet excited state is populated, hence the formation of the Dewar furan is energetically possible (Fig. 1). This result is in agreement both with the evidence for the formation of the Dewar furan in the direct irradiation and with the formation of isomeric furans. 

CONTENTS List of Contributors. Introduction to the Series An Editor s Forward, Albert Padwa. Preface, Randolph P. Thummel. Cyclooctatetraenes Conformational and ii-Elec-tronic Dynamics Within Polyolefinic [8] Annulene Frameworks, Leo A. Paquette. A Compilation and Analysis of Structural Data of Distorted Bridgehead Olefins and Amides, Timothy G. Lease and Kenneth J. Shea. Nonplanarity and Aromaticity in Polycyclic Benzenoid Hydrocarbons, William C. Herndon and Paul C. Nowak. The Dewar Furan Story, Ronald N. Warrener. Author Index. Subject Index. 

Dewar furan and Dewar thiophene (162, X = 0,S) have been obtained only under conditions of low-temperature photolysis along with... 

To conclude this section two compound specific reactions are worthy of mention. Firstly, the interaction of atomic carbon with benzene has been found to provide 1 albeit in 11% yield and as one of eleven hydrocarbon products. Secondly, 4,5,7-tri-/-butylisobenzofuran has been reported to undergo furan-cyclopropenylaldehyde photorearrangement rather than An electrocyclization to the Dewar furan Steric factors undoubtedly dominate in yielding 2,3,5-tri-r-butylcyclopropabenzene-l-carbaldehyde the product was obtained in solution and characterized by H NMR only. 

No Dewar furan has been isolated or observed spectrometrically. Van Tamelen, who first isolated Dewar benzene substituted with f-butyl groups, examined the photolysis of di- and tri-t-butylfurans to compare results with those for the benzene. In the photoreaction of 2,5-di-r-butylfuran, three isomers were isolated (Eq. 2), including a cyclopropenyl ketone but not a Dewar furan.3... 

Thus, furan derivatives seem to isomerize to cyclopropenylcarbonyl compounds in photoreactions. However, recently Lemal informed us that Dewar furan was derived from tetrakis(trifluoromethyl)Dewar thiophene (II B). 

The research of Warrener et al. in this area has yielded a number of interesting oxabicyclopentane derivatives. In their epoxidation of the Cookson ester (30) a mixture of the two monoepoxides (31) and (32) was isolated (Equation (1)) <94TL1075>. Epoxidation of Nenitzescu s ester (33) with MCPBA, contrary to an earlier report <83JOU1390, 83ZOR1543), delivers a mixture of the three epoxides (34), (35), and (36) (Equation (2)). Several research groups have directed interest toward the synthesis of Dewar furan (5-oxabicyclo[2.1,0]pent-2-ene) and its derivatives these have been... 

The Dewar furan (43), prepared by debromination of (42), proved to be too unstable <84CC1466>. Its formation was inferred from the trapping products (44a,b) with 2,5-dimethylfuran (Scheme 5). Initial attempts to generate alkyl substituted Dewar furans by photochemical breakdown of... 

The parent Dewar furan (49) was generated in a similar fashion from the corresponding propellane precursor (48) and trapped with isobenzofuran to give the cycloadducts (51) and (52) (Scheme 7) <85JA7176, 86PACI61). The proportion of isobenzofuran-Dewar furan adducts (51) and (52) was increased at the cost of the cyclopropene aldehyde adducts (53) and (54) as the photolysis temperature was lowered, indicating that (49) is indeed the primary photolysis product. 

Thermal decomposition of suitable precursors can afford Dewar furan as well (Schemes 8 <83ZOR1543> and 9 <94JCS(P1)1983 . Under these conditions, however, (34) is too unstable for detection and gives rise to (50) and/or furan. 

Dewar tetrakis(trifluoromethyl)thiophene, (59), has served as a dieneophile (dipolarophile) in several cycloadditions. Diels-Alder reaction of (59) with butadiene <8icci289> and pyrrole (82JA847) has served as the starting point for the synthesis of 1,2,3,8-tetrakis(trifluoromethyl)cyclooctatetraene (60) and tetrakis(trifluoromethyl)Dewar furan (61), respectively, as illustrated in Scheme 20. It is noteworthy that (61) was unattainable by direct photolysis of the substituted furan. Moreover, (59) served as a common precursor for the preparation of tetrakis- (80JA6633) and penta-<83JA3563> (trifluoromethyl)cyclopentadienides (64) and (65), respectively (Scheme 21). Accordingly, photolysis... 

After the isolation of a Dewar benzene substituted by tert-butyl groups, van Tamelen tried to isolate a Dewar furan stabilized by terr-butyl groups. However, the photolysis of di- or tri-fe/7-butylfuran did not give any Dewar compound but only cyclopropenyl ketones and its ring-opened products I04>. The reaction of di-fert-butylfuran is described by the following equation (104). 

Another attempt to synthesize a Dewar furan by the photolysis of tetrakis(tri-fluoromethyl)furan was made since the photolysis of tetrakis(trifluoromethyl)thio-phene gives a stable Dewar compound (see Eq. 109). However, even this furan only yields the corresponding cyclopropenyl ketone 105) ... 

So far, the formation of valence-bond isomers by photolysis has mainly been discussed. Tetrakis(trifluoromethyl) Dewar thiophene is stable at room temperature and was used for the synthesis of Dewar pyrroles, a Dewar furan, and other interesting compounds from the standpoint of the structural chemistry. In this section, some of these reactions will be discussed. 

The Diels-Alder adduct formed from the Dewar thiophene and pyrrole was used for the synthesis of a Dewar furan (121)125). 

The Dewar furan isomerizes not to the furan but to the corresponding cyclo-propenyl ketone. This may be due to the weak oxygen-carbon bond of Dewar furan... 

E C omaticity in Polycyclic Benzenoid Hydrocarbons, William C. Herndon and Paul C. Nowak. The Dewar Furan Story, Ronald N. Warrener. Author Index. Subject Index. Volume 3,1995, 316 pp. 97.50 ISBN 1-55938-698-3... 

It is interesting that the systems related to 11—pertrifluoromethyl Dewar pyrrole (19) and pertrifluoromethyl Dewar furan (20)-do not appear to undergo comparable walk rearrangements. Upon heating, these compounds isomerize to the corresponding substituted pyrrole and cyclopropenyltrifluoromethylketone, respectively. 

Flash vacuum thermolysis of (109) at 420 °C and 2 x 10 Torr gave (110) (95%). The Dewar-furan (111 X = 0) has been prepared in six steps from the corresponding Dewar-thiophen (111 X = S) because the direct route via photolysis of furan proved unsuccessful/ ... 

Perfluorotetramethylcyclobutadiene oxide (43) rearranges with a half-life of about 20 min at 95°C." As a Dewar furan, it might have been expected to aromatize to perfluorotetramethylfuran (44). That process is orbital topology forbidden," however, and instead 43 is transformed cleanly into cyclopropenyl ketone 45, apparently... 

There are several reactions, proceeding with transformation of the fluorine-containing furan cycle. Photochemical transformations of perfluoro-tri and tetra-methyl furans under irradiation by high pressure Hg lamp lead to cyclopropenylcarbonyl derivatives 235. ° No Dewar furan derivatives 236 were detected. 

Tetrakis(trifluoromethyl) Dewar thiophene has been converted to the Dewar furan in several steps <82JA847>. The first step in the sequence is the cycloaddition to pyrrole, followed by PhjP-induced desulfurization. 

---