Butatriene formation via CuBr-induced self coupling of an α-bromo vinyl zinc reagent: synthesis and X-ray structure of Z-1,1,1,6,6,6-hexafluoro-2,5-diphenylhexa-2,3,4-triene

Abstract

Title cumulenes 2a and 2b were prepared by reaction of CF3(Ph)C=C(Br)ZnX [X = Br, CF3(Ph)C=CBr-] 1 with CuBr, or by oxidative coupling of 1 with FeCI3 to form 2a,b and [CF3(Ph)C=CBr-I2 3, which can be dehalogenated with zinc to afford 2a,b; the cumulene isomers are readily separated by silica-gel chromatography, and may be prepared in good yield on a multi-gram scale.

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 latter was avoided for Ph 2 CCBrLi ( 239 ) in THF at −90 °C by use of certain Cu(I) catalysts or other reagents , furnishing aryl-substituted butatrienes. In striking contrast, the carbenoid α-bromo-β-phenyl-β-trifluoromethylvinyllithium ( 247 ) did not exhibit any proclivity to FBW rearrangement in diethyl ether but afforded only the butatriene 245d . Thus, trifluoromethyl as an electron-withdrawing stationary β-substituent (σ Ι = 0.40) 27,168 obviously prevented the expected phenyl migration, supposedly due to the electronic destabilization of the transition state.…”

“…Vice versa, the relatively slow FBW shift of PhSO 2 might become slower if a neighboring alkyl group were exchanged for a more electron-withdrawing β-substituent: Indeed, the small portion of “FBW” product from PhSO 2 C(R 1 )C: obtained with R 1 = alkyl (entry 9) 134 dropped to zero with acyl substituents 175 R 1 = AlkC(O)− (entry 5). There are almost no systematic quantum chemical studies 16,21,31,33 of such substitutent effects, but quite a few experimental data collected in this article seem to support this conjecture: The surprising refusal of phenyl to migrate in the presence of the inductively electron-withdrawing substituents I, Br, or Cl (entry 1), isopropyloxy (entry 15), , azido (entry 19), dimethylamino 364 (in Me 2 NCPhCLiCl, 412 in section 3.4.3), trifluoromethyl (entry 20), and so forth does not appear to have been recognized 374 previously. This phenomenon is perhaps at least partly responsible for extending the realm of stability up to room temperature for i -PrO(Ph)CCLiI ( 379 , entry 15) in THF.…”

“…This phenomenon is perhaps at least partly responsible for extending the realm of stability up to room temperature for i -PrO(Ph)CCLiI ( 379 , entry 15) in THF. Some uncertainty may remain here because of the absence of butatrienes R 1 R 2 CCCCR 1 R 2 ( 245 ) that should arise as the products of carbenoid dimerization and that were properly produced from F 3 C(Ph)CCLiBr ( 247 , entry 20) . Even β-hydrogen migration seems to become retarded with β-ethoxy as the stationary substituent in the ( E )-isomer 343 of EtOCHCLiBr (section 3.4.2).…”

Fluorinated Organozinc Reagents

“…Fluorinated α-halovinylzinc reagents (halogen = Cl, Br, I), when treated with a catalytic amount of CuBr, provides a useful entry to fluorinated 1,2,3-cumulenes (equation 60) 53,54 . The E-and Z-cumulenes were readily separated by silica gel chromatography and pure multigram quantities of pure cumulenes were easily synthesized by this methodology.…”

Diverse Reactivity of Zirconacyclocumulenes Derived from Coupling of Benzynezirconocenes with 1,3‐Butadiynes towards Acyl Cyanides: Synthesis of Indeno[2,1‐b]pyrroles or [3]Cumulenones

“…Butadiyne 1 g with an OPh group was also compatible with this reaction, furnishing 11 k in 72 % yield (Table 2, entry 11). It should be noted that cis-trans isomerization [13] easily occurred during the workup of 11 h and 11 k. It turned out that if all of the operations were carried out at room temperature, isomerization could be minimized, and cis/trans ratios of 91:9 (11 h) and 90:10 (11 k) were obtained.…”