[594-02-5] · C2H4I2 · 1,1-Diiodoethane · (MW 281.86)
(reagent for methylcyclopropanation of alkenes and ethylidenation of carbonyl compounds)
Alternate Name: ethylidene iodide.
Physical Data: bp 179-180 °C; d 2.84 g cm-3.
Solubility: sol most organic solvents.
Preparative Methods: mix 39.6 g (0.40 mol) of 1,1-dichloroethane, 187 g (1.2 mol) of ethyl iodide, and 2.0 g of Aluminum Chloride and heat on a steam bath for 3 h; wash the mixture with H2O and then NaHSO3, and dry over MgSO4; distillation provides 67.3 g (60%) of the product boiling at 75-76 °C/25 mmHg.1 An alternative procedure utilizes the reaction of Iodine, Triethylamine, and the hydrazone of Acetaldehyde: from 1 mol of acetaldehyde, 95 g (34% from acetaldehyde) of 1,1-diiodoethane can be isolated; this procedure is a little more involved, but provides a viable alternative if 1,1-dichloroethane is not available.2
Handling, Storage, and Precautions: corrosive and irritant; emits toxic fumes if heated to decomposition or comes in contact with acids. Use in a fume hood; store over copper powder and protect from light.
Various reagents will react with 1,1-diiodoethane to effect the methylcyclopropanation of alkenes. These reagents include Diethylzinc,3 Ethylzinc Iodide,4 Copper powder,5 Zinc/Copper Couple,4 Zinc/Silver Couple,4 trialkylaluminum reagents,6 and Samarium(0).7 For simple alkenes, the diethylzinc (eq 1) and trialkylaluminum methods (eq 2) appear to be the ones of choice. The various zinc couples and copper powder provided lower yields of methylcyclopropane products. For cyclohexene (eq 1), diethylzinc afforded predominantly the endo-isomer.3 The use of copper powder gave the exo-isomer as the major product.5
Reactive alkenes, such as enol ethers, can be methylcyclopropanated effectively using Zn/Cu couple. Trimethylsilyloxycyclohexene (eq 3) produces a 3.1:1.0 mixture of isomers, with the exo-isomer being the major product.8 The reaction of 1-methoxy-1,4-cyclohexadiene was regioselective as well as stereoselective, yielding exclusively the endo-isomer (eq 4).9 A cyclohexenediol bis-silyl ether has been methylcyclopropanated using either diethylzinc or Zn/Cu couple.10 A ketene acetal was cyclopropanated using the diethylzinc procedure.11
Allylic alcohols may be methylcyclopropanated using a number of different conditions. In Table 1 are shown the results for some cyclic allylic alcohols under different reaction conditions.3b,4a,4b,7 For 2-cyclopenten-1-ol and 2-cyclohexen-1-ol, the yields as well as the ratios of the isomeric products formed were found to be very similar for different zinc reagents. The samarium-promoted reaction for cyclohexenol provided the cyclopropane with a higher exo:endo diastereoselectivity (83:17).7
The allylic alcohols derived from the bislactim ether of cyclo(-L-Val-Gly-) have been methylcyclopropanated (eq 5) using the diethylzinc method in good yields with good diastereoselectivities.12
The reduction of 1,1-diiodoethane by Chromium(II) Chloride produces a gem-dichromium reagent which will react with aldehydes, selectively affording (E)-alkenes in excellent yield. The reaction conditions also work well for enolizable ketones. The reaction of a-tetralone produced the alkene in 85% yield as a 16:84 mixture of (E):(Z) isomers, respectively.13 The lactol (eq 6) on reaction with CrCl2 and 1,1-diiodoethane delivered the alkene with >95:5 (E):(Z) selectivity.14 The corresponding reactions with the ethylidene triphenyl- or triethylphosphorane were less selective.
The a-alkoxyaldehyde (eq 7) was ethylidenated in excellent yield under these conditions. Again, the reaction was highly (E) selective (>99%); more importantly, no epimerization was observed.15
A deuterated vinylcyclobutane (eq 8) was synthesized using 1,1-diiodoethane-1-d. The (E):(Z) ratio in this case was 19.5:1. The deuterated reagent was prepared using the hydrazone method. The reaction was accompanied by a small amount of epimerization (1.7%).16
1,1-Diiodoethane has been used to prepare ethylidene ditriflate by reaction with Silver(I) Trifluoromethanesulfonate.17 It has also been utilized to prepare 1,1-bis(bistrifluoromethyl)phosphano)ethane18 and octacarbonyl (m-ethylidene)diiron.19
Michael J. Taschner
The University of Akron, OH, USA