Some Observations on the Reaction Route of the Clemmensen Reduction in Fluorenone Derivatives

Some Observations on the Reaction Route of the Clemmensen Reduction in Fluorenone Derivatives

Creators Suzuki Kazuo

Creators Fujimoto Minoru

The reduction of fluorenone (I) has been found in a few examples for the preparation of fluorenes in the presence of dilute alcohol or hydrocarbons^<1)>. But I has been best reduced to fluorene (II) by the Wolff-Kischner method with or without an alkaline catalyst, as has been observed by Weisburgers and Grantham^<2)>, except fluorenone-1-carboxylic acid. This acid is unsatisfactory because of the formation of phthalazine derivatives^<3)>. In connection with this studies on the reaction of II and 2-bromofluorene (IV) from their corresponding fluorenones by the modification of the Clemmensen-Martin reduction we had occasion to investigate the substitution effect in 2-bromofluorenone (III). A great part of I does not normally proceed by way of fluorenol (V) in the Clemmensen reduction and also many related products were obtained by an abnormal reaction. Moreover, some of them do not appear to have been previously reported as direct products from I in this reduction. The present authors carried out the reduction by the procedure of 'Organic Reactions'^<4)> : I was dissolved in solvents such as toluene, benzene and xylene, which were mixed with the aqueous hydrochloric acid. The mixture was heated under reflux for a period of twenty-four hours, concentrated hydrochloric acid being added at intervals of about six hours. In this study, V, II and dimeric reaction products, namely, 9,9'-dihydroxy-9,9'-bifluorenyl (fluorenopinacol) (VI), 10,10-diphenylene-9-phenanthrone (VII), 10,10-diphenylene-9,10-dihydro-9-phenanthrol (VIII), 9,10-diphenylenephenanthrene or tetrabenzonaphthalene (IX), 9-hydroxy-9,9'-bifluorenyl (X), 9,9'-bifluorenyl (XI) and mixed crystals of dibiphenylene-ethylene (XII) and IX were isolated by means of a combination of recrystallization and column chromatography. I was converted to II and the other intermediates, V, VI, VII, VIII, X and XII, after a short reduction time (2 hr.) in toluene. Furthermore, XII, X and XI were increased, but VI was decreased and VI remained almost constant when the reaction was continued for twenty-four hours (red colored toluene). Three kinds of end-products, IX (10%), XI (55.6%) and (II, 8.7%) were obtained by extending the refluxing periods for seventy-two hours (almost colorless). IX and XI were then obtained in approximately a ratio of 1 : 5.6. It may, therefore, be assumed that VII and X were also formed from VI through the positively charged ion VI' by the action of the acidic medium in the Clemmensen reduction. From these results, the mechanism of the formation of IX via VIII from VII by the retropinacolone rearrangement involves the intramolecular transfer of the electron pair by formation of a carbonium ion. Thus, the reaction sequences can be written as in Chart 1. On the other hand, there is only one reference in the literature to the Clemmensen-Martin figure reduction of III, that by Miller and Bachman^<5)>, who reported a nearly quantitative yield of IV. However, the present study deals with the effect on this reduction of an electoron-attracting group such as bromine at the 2-position of I. This is of interest because the substituent influences may be transmitted to the carbonyl at the 9-position, both directly through the substituted ring and indirectly through the unsubstituted ring. III was submitted to same procedure as was I. From this reduction, II, 2-bromofluorenol (XIII) and several dimeric reaction products, namely 2,2'-dibromo-9,9-dihydroxy-9,9'-bifluorenyl(XIV), 2,2'-dibromodibiphenylene-ehtylene(XV)^<6)>, -ethane(XVI)^<6)> and a trace of spiro-(2-bromofluorene-9,9'-(10')-2'-bromophenanthrone) (XVII) were isolated by means of a combination of recrystallization and column chromatograpy. But the reduction of III is very slow

[NCID]AN00244217