A Facile Synthesis of 1,1-Difluoroallenes from Commercially Available 1,1,1-Trifluoro-2-iodoethane

Abstract

1,1-Difluoroallenes are synthesized in good yield via zinc-promoted 1,2-elimination of 3,3-difluoro-2-iodoallylic acetates, which are prepared by the reaction of aldehydes or ketones with 1-iodo-2,2-difluorovinyllithium, generated from commercially available 1,1,1-trifluoro-2-iodoethane.

Key words

fluorinated allenes - metalation - carbanions - elimination - difluorovinylidenation

References

• 1a Dolbier WR. Burkholder CR. Piedrahita CA.
  J. Fluorine Chem.  1982,  20:  637 
• 1b Dolbier WR. Burkholder CR. Winchester WR. J. Org. Chem.  1984,  49:  1518 
• 1c Dolbier WR. Burkholder CR. Israel J. Chem.  1985,  26:  115 
• 1d Dolbier WR. Wicks GE. Burkholder CR. J. Org. Chem.  1987,  52:  2196 
• 1e Dolbier WR. Burkholder CR. Wicks GE. Palenik GJ. Gawron M. J. Am. Chem. Soc.  1985,  107:  7183 
• 1f Dolbier WR. Acc. Chem. Res.  1991,  24:  63 
• 2a Dolbier WR. Wicks GE. J. Am. Chem. Soc.  1985,  107:  3626 
• 2b Shen Q. Hammond GB. J. Am. Chem. Soc.  2002,  124:  6534 
• 3a Mae M. Hong JA. Xu B. Hammond GB. Org. Lett.  2006,  8:  479 
• 3b Xu Y.-Y. Jin F.-Q. Huang W.-Y. J. Fluorine Chem.  1995,  70:  5 
• 4a Blomquist AT. Longone DT. J. Am. Chem. Soc.  1957,  79:  4981 
• 4b Knoth WH. Coffman DD. J. Am. Chem. Soc.  1960,  82:  3873 
• 5a Shi G. Xu Y. J. Fluorine Chem.  1989,  44:  161 
• 5b Wang ZG. Hammond GB. J. Org. Chem.  2000,  65:  6547 
• 5c Shen Q. Hammond GB. Org. Lett.  2001,  3:  2213 
• 5d Xu B. Hammond GB. Angew. Chem. Int. Ed.  2008,  47:  689 
• 6 Yokota M. Fuchibe K. Ueda M. Mayumi Y. Ichikawa J. Org. Lett.  2009,  11:  3994 
• For the generation of 2,2-difluoro-1-tosyloxyvinyllithium [F₂C=C(OTs)Li], see:
• 7a Tanaka K. Nakai T. Ishikawa N. Tetrahedron Lett.  1978,  19:  4809 
• 7b Ichikawa J. Hamada S. Sonoda T. Kobayashi H. Tetrahedron Lett.  1992,  33:  337 
• For the generation of 2,2-difluoro-1-halovinyllithium (F₂C=CXLi), see:
• 8a (X = F): Burdon J. Coe PL. Haslock IB. Powell RL. Chem. Commun.  1996,  49 
• 8b (X = F): Burdon J. Coe PL. Haslock IB. Powell RL. J. Fluorine Chem.  1999,  99:  127 
• 8c (X = Cl): Burdon J. Coe PL. Haslock IB. Powell RL. J. Fluorine Chem.  1997,  85:  151 
• 8d (X = F or Cl): Coe PL. Burdon J. Haslock IB. J. Fluorine Chem.  2000,  102:  43 
• For the generation of 2,2-difluoro-1-halovinylzinc(II) chloride (F₂C=CXZnCl) at r.t., see:
• 9a (X = F): Anilkumar R. Burton DJ. Tetrahedron Lett.  2002,  43:  2731 
• 9b (X = Cl): Anilkumar R. Burton DJ. Tetrahedron Lett.  2002,  43:  6979 
• 9c (X = Br): Anilkumar R. Burton DJ.
  J. Fluorine Chem.  2004,  125:  561 
• 9d (X = I): Anilkumar R. Burton DJ. J. Fluorine Chem.  2005,  126:  455 

10

The lithiation of CF₃CH₂I with BuLi instead of LDA at
-93 to -85 ˚C led to a low yield of 2,2-difluoro-1-iodovinyl-lithium, probably due to I-Li exchange reaction.

11

For 2-bromo-3,3-difluoroallylic acetates, zinc-promoted 1,2-elimination also took place readily at r.t. and led to the formation of the corresponding 1,1-difluoroallenes.

12

Mg was employed in THF to promote the 1,2-elimination of acetate 3d in vain.

13

When THF was used as a solvent, only a trace amount of 1g was observed by the ^¹9F NMR measurement in spite of a large excess amount of Zn and an extended reaction time. A similar behavior was exhibited by some other acetates 3 without a heteroaromatic ring.