J. Org. Chem. 1987,52, 2196-2201
2196
The Cycloadditions of Nitrones with Fluoroallene William R. Dolbier, Jr.,* Gene E. Wicks, and Conrad R. Burkholder Department of Chemistry, University of Florida, Gainesville, Florida 3261 1
Received October 27, 1986
Cycloadditions of nitrones with fluoroallene proceed regiospecifically and with a remarkable stereochemical preference for addition syn to the fluorine substituent. The effects of solvent polarity on the rates and stereochemistry of these cycloadditionsare reported and discussed. In general these effects are consistent with those reported for other nitrone cycloadditions,and the activation parameters are also similar. The cycloadditions of fluoroallene (MFA) and 1,l-difluoroallene (DFA) may, on the basiq of reaction characteristics, be clearly broken down into two broad categories, which also seem to be mechanistic demarcations. First, those reactions which are orbital-symmetry allowed, such as Diels-Alder' and 1,3-dipolar cycloadditions,2 are regiospecific with respect to the allene, with reactions occurring only at the C&3 bond. In contrast, the [2 + 21 cycloadditions of MFA and DFA have been bound to be nonregio~pecific,'~~ with an excess of C2-C3 cyclizations for MFA and an excess of C1-C2 cyclization for DFA being observed. These results have been rationalized as being characteristic of concerted mechanisms for the former and of a multistep, diradical mechanism for the latter reactions. In the cycloadditions of fluoroallene one must also consider the stereochemistry of the reactions, since products which derive from C2-C3 addition will have exhibited either a net syn or anti addition (eq 1). While a slight H
F
H
Table I. Fluoroallene Cycloadditions with N -Alkyl-C-phenylnitrones reacn product % R time, h ratio 2:3" yieldb k , s-lC k..i 95 (9.58 0.19)X lob 1.0 CH3 72 82:18 10 85:15 99 (1.11 f 0.04) X loa 11.6 Ph 2-naphthyl 6 84:16 90 (1.15 f 0.07) X lo-* 12.0
*
OError, fl%. *Error, f3%; determined by 19FNMR using mbromobenzotrifluoride as an internal standard. e Pseudo-first-order rate constants.
Table 11. I3C NMR Fluorine-Carbon Coupling Constants (Hz)for MFA-Nitrone Adducts adducts 2
C CHPh CH2
a
