Stereochemical and Reactivity Studies of the [2 2], [2 4] and 1,3-dipolar Cycloadditions of Partially Fluorinated Allenes

Abstract:
Cycloadditions involving fluoroallenes are extremely useful mechanistic probes. Due to the high reactivity of these allenes and the small size of their fluorine substituents, cycloadditions of fluoroallenes are free from many of the ambiguities surrounding analogous studies by previous workers.
Styrene and 1,1-difluoroallene, DFA, undergo clean reactions between 60o and 100oC to give good yields (approx. 60%) of two cylcloadducts: 2,2-difluoro-3-phenyl-1-methylenecyclobutane (major) and 3-phenyl-1-(difluoromethylene)cyclobutane (minor). Similar experiments comparing the reactions of p-nitro and p-methoxystyrene with the above results give relative reactivities of 2.1 and 1.3, respectively. The [2+2] cycloaddition of DFA with Z-¿-deuteriostyrene gives the expected products in an 82:18 ratio with stereochemical ratios of 58:42 and 79:21, respectively, at 80oC. A mechanism involving two kinetically distinguishable diradicals is proposed to rationalize the results. The reaction of DFA with 1-cyanovinyl acetate, a "capto-dative" reagent is also studied and found to give results similar to the styrene reactions.
N-Methyl, N-phenyl, and N-(2-naphthyl)-C-phenylnitrones undergo regiospecific C2-C3 dipolar cycloadditions with monofluoroallene, MFA, in nearly quantitative yields at 25oC. The average stereochemical ratio for the two 4-substituted isoxazolidines produced is 84:16 ±2% with the product from syn addition of MFA in excess. The stereochemical assignments of the products are made with the aid of difference nuclear Overhauser effect (NOE) experiments and an X-ray crystal structure. Extensive solvent and kinetic studies show that these are well-behaved second-order kinetic processes consistent with previously well-studied 1,3-dipolar cycloadditions. The reaction of MFA with diazomethane-d2 strongly favors the product from the syn addition (88:12 ratio), also. Consideration of transition-state dipole-dipole interactions and solvation phenomena show that the factors affecting the stereochemical outcome of all these 1,3-dipolar cycloadditions are extremely subtle.
Finally, MFA undergoes Diels-Alder reactions with cyclopentadiene, butadiene, and furan to give a modest preference for the syn adduct (51:49, 59:41, and 63:37, respectively). The results are discussed in terms of a theoretical study done for MFA using INDO calculations.
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