Carbanionic Rearrangements of Halomethylenecyclobutanes. The Role of the Halogen

Abstract

The carbanionic ring enlargement of (halomethylene)cyclobutanes to 1-halocyclopentenes has been extended to the fluoro analogues. At 180 °C, 3-hexyl-1-(fluoromethylene)cyclobutane provides better yields of rearranged product than the corresponding chloride, bromide, or iodide. At temperatures < 100 °C, the yield of ring-enlarged product follows the order I > Br > Cl, and the fluoride does not react. Experiments with labeled substrates show that, in general, the larger the halide and the higher the reaction temperature, the greater the preference for double migration over single migration as a mechanistic pathway. The trifluormethyl group is ineffective in promoting anionic rearrangement.

Alkylidenecarbenes, Alkylidenecarbenoids, and Competing Species: Which Is Responsible for Vinylic Nucleophilic Substitution, [1 + 2] Cycloadditions, 1,5-CH Insertions, and the Fritsch−Buttenberg−Wiechell Rearrangement?

“…The label distribution 2.6:1 in 280a/b proved the predominant [1,2]-migration of the Me 2 C group (anti to Br), accompanied by a formal 1,2-shift of the bromide along the double bond, leading to 280a. This formally "dyotropic" process 278 was dubbed the "Beckmann mechanism" by the authors 275 and later 279 called "double migration", but its detailed course is uncertain. It appears to resemble the corresponding carbene rearrangement depicted in the transition state model 13 inasmuch as development of a certain degree of contact-ion-pair character 278 by partial C-Br heterolysis in (E)-279 would provide for an emptying atom orbital at C-R (*) into which the bond electron pair of the migrating carbon atom can be delivered.…”

Synthetic approaches to monofluoroalkenes

“…54 It remains a reactant of choice nowadays for the synthesis of terminal trisubstituted fluoroalkenes (Scheme 34). 97,98 Similarly, terminal bromofluoroalkenes can be synthesized by a zinc-promoted Wittig reaction which proceeds with the in situ formation of the phosphonium salt. 99 Pannecoucke and coworkers later replaced activated zinc powder with Et 2 Zn to improve yields for unactivated aldehydes or ketones although the bromofluoroalkenes are obtained with moderate E : Z selectivity (Scheme 35).…”

Synthesis of terminal monofluoro olefins

“…12 Heating terminal epoxides 3 with a 1 : 1 ethyldiisopropylamine-HF complex led to regioselective formation of fluorohydrins 4, with the ratio of isomers 4 : 5 shown to be > 9 : 1 for a number of epoxides. 13 The alcohol function was removed through tosylation and treatment with base, 13 or via chlorination in pyridine 14 to give fluoro olefins in overall yields up to 86% (Scheme 2).…”

“…72 Deprotonation of phosphonium salt 51 69-71 or 53 73 with nbutyllithium to form ylide 56, followed by addition of the carbonyl compound gave fluoro olefins with (E ) : (Z) ratios invariably near unity. Yields were improved by addition of KOBu t after betaine formation (Scheme 27), 70, 74 or by a cunning simultaneous generation of benzaldehyde and ylide 56. 75 Yields of up to 91% have been obtained.…”