Benzylfluorocarbene: Reactions and kinetics

✍ Scribed by Robert A. Moss; Ljiljana Maksimovic; Dina C. Merrer

• Book ID: 104258018
• Publisher: Elsevier Science
• Year: 1997
• Tongue: French
• Weight: 200 KB
• Volume: 38
• Category: Article
• ISSN: 0040-4039
• DOI: 10.1016/s0040-4039(97)01709-7

✦ Synopsis

Among the problems connected with the 1,2-H shift of benzylchiorocarbene (1, BCC) to cis-and trans-[l-chlorostyrene is the nonlinear Arrhenius correlation associated with this rearrangement in isooctane. ~ Curvature is apparent below -15 °C, followed by leveling of the correlation at -40 °C.~a These phenomena have

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1 (BCC) Me 2 3 (BFC) 4

been attributed to competitive reactions of BCC with the solvent or to quantum mechanical tunneling, l~ The latter explanation is particularly intriguing because tunneling is strongly implicated in the 1,2-H rearrangement of BCC at cryogenic temperatures, 2 and could be important in the analogous reaction of methylchlorocarbene at near-ambient temperatures. 3'4

Our recent examination of the 1,2-H rearrangement of mesitylmethylchlorocarbene (2), a close relative of BCC, revealed similar Arrhenius curvature commencing at -40 °C in isooctane. 5 We found, however, that increasingly competitive azine-forming reactions of 2 with its diazirine precursor at temperatures below ~ -35 °C sufficed to explain the nonlinearity. Indeed, dilution of the diazirine (to ~ 10 mM) afforded a linear Arrhenius correlation for 2 down to -70 °C. 5 Subsequent reinvestigation of the BCC reactions also revealed significant azine formation at temperatures where Arrhenius leveling was apparent, suggesting that tunneling may not be required to rationalize this phenomenon. ~

If competitive intermolecular carbene/diazirine reactions that afford azine are important with carbenes 1 and 2, then we might expect them to be even more contributory with benzylfluorocarbene, BFC (3).

Replacement of CI in BCC by F in BFC should stabilize the carbene, 6 retard the intramolecular t ,2-H shift, 6 and provide greater opportunity for competitive intermolecular azine formation. Here, we describe initial studies of the generation, reactions, absolute kinetics, and Arrhenius behavior of BFC that conform to these predictions.