Fluorenylidene ylides

Methylene triphenylarsorane (//, 31, 34, 39) has been prepared in situ from the salt in ether (34, 39), benzene (34, 39, 60), tetrahydrofuran (31), or dimethyl sulfoxide (II) by reaction with butyl- or phenyllithium (34, 39), sodium hydride (31), methylsulfinylcarbanion (II), or sodamide (102). Ethylidene triphenylarsorane has been prepared by reaction with methylsulfinylcarbanion (II) whereas fluorenylidene-(9)-trimethyl- and fluor-enylidene-(9)-dimethylbenzylarsorane (101) have been made using phenyllithium in ether. Sodium methoxide in methanol has been used successfully (41, 47, 48, 94) to generate ylides from their corresponding salts when R is an electron-withdrawing group (e.g., COOR, COR, or CN). 

A number of minima corresponding to oxonium ylides and H-bonded structures were found on the potential-energy surface for reaction of singlet carbenes with water and alcohols." Laser flash photolysis revealed that the rates of reaction between cyclopentadienylidene or fluorenylidene and alcohols increased with alcohol acidity and had linear Bronsted plots with slopes of 0.061 and 0.082, respectively.100 These results point to protonation with a very early transition state or to concerted OH insertion. For tetrachlorocyclopentadienylidene, the results showed that ylide formation (100) is predominant. 

The potential of laser flash photolysis in the study of carbene reactions with heteroatoms has come to be recognized in recent years. A number of kinetic studies using this technique have been carried out with carbene precursors in nitrile solvents.122-127 An absorption band at 470 nm was observed in the laser flash photolysis of diazofluorene (246) in inert solvents. This band was assigned to triplet fluorenylidene (247). In acetonitrile, however, a second band was also detected at 400 nm and whose buildup is concurrent with the decay at 470 nm.122 Laser flash experiments in other nitrile solvents (i.e., benzonitrile and pivalonitrile) also produced a transient absorption band which is very similar to that observed in acetonitrile. The band at 400 nm was assigned to an intermediate nitrile ylide (248). This absorption could be quenched on addition of an electron-deficient olefin providing good support for its... 

Rate constants have been measured by flash-photolysis techniques for the reactions of cyclopentadienylidene and fluorenylidene with alcohols and other quenchers. The rates increase with increasing acidity of the alcohol, and alcohols react faster than likely ylide-forming reagents, such as pyridine or THF. In contrast, tetrachlorocyclopentadienylidene reacts most rapidly with the least acidic alcohol studied, and reacts more rapidly with pyridine and THF than with methanol. 

Ylides of singlet 1-naphthylcarbene ( 11) with acetone, triethylamine and pyridine were soon observed. [21] Fluorenylidene was also found to react with coordinating solvents to form ylides. [22]... 

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