Zinc Iodide as a rearrangement catalyst?
(Rated as: excellent)
I was browsing through some articles I found on my hard drive the other day, and came across the following one:
A concise synthesis of 1-substituted-2-tetralones by selective diol dehydration leading to ketone transposition
Jensen, B. L. and Slobodzian, S. V. "Tetrahedron Letters" 41:6029-6033 (2000)
Dehydration of 1-substituted-1,2-tetralindiols with zinc iodide afforded the corresponding 2-tetralones
in excellent yields. This procedure was found to be superior to the more conventional BF3-catalyzed
rearrangement of 1-substituted-1,2-epoxytetralins.
In this article, it discusses, obviously, the synthesis of tetralones, which they achieve by an oxidation of an alkene using m-chloroperbenzoic acid (m-CPBA), forming either a 1,2-epoxide or an alpha-hydroxy ester (just like the peracid oxidations used commonly here). They then use BF3 or ZnI2 to rearrange these intermediates to the ketone; the important fact is that the ketones take the 2 position (ie. on the second carbon from the aromatic ring) in over 90% - and up to 99% - yeilds - and in all listed cases, the ZnI2 actually achieves better conversion than does the BF3. Anyways, I feel that it may, perhaps, be worth trying - if nothing else, Zinc Iodide should be easier to obtain or make than Lithium Iodide, which is Rhodium's current favourite for rearrangements. I'm going on a hunt now to try and find some more articles using Zinc Iodide as a rearrangement catalyst, but for now, here's the procedure they give for the rearrangement in the article:
Solid zinc iodide (100 mg, 0.30 mmol) was heated under aspirator pressure at 120oC for a period of 1 h. After cooling, a solution of epoxide 3c (210 mg, 0.89 mmol) in benzene (5 mL) was added to the zinc iodide. The mixture was stirred under refux for 1 h, cooled to room temperature, and washed with water. After drying over magnesium sulfate, the solvent was removed affording 6c (208 mg, 99%)
Boron trifluoride etherate (2 drops) was added to a solution of diol 5c (333 mg, 1.31 mmol) in ether (3 mL). The solution was stirred at 30oC for 20 min, washed with water and dried over magnesium sulfate. Evaporation of the solvent afforded a colorless oil which distilled at 127oC (750 mm) giving 6c (301 mg, 97%)
NOTE: One would assume solvents other than benzene could be used (eg. toluene or xylene, perhaps). Also, I only included the Boron Triflouride method for completeness - they say that Zinc Iodide works for both diol and epoxide.
As a backup to my comment about synthesizing Zinc Iodide, here's an article:
Synthesis and Decomposition of Zinc Iodide: Model Reactions for Investigating Chemical Change in the Introductory Laboratory
DeMeo, S "Journal of Chemical Education", 72(9):836 (1995)
The procedure in this article involves reaction between Zinc powder and Iodine crystals in slightly acidified water, which gives a yeild in the order of 98% ZnI2.
|Thank you, looks interesting.|
Thank you, looks interesting.
These might be related:
Isomerisation of Epoxides to Carbonyl Compounds by Iodides in DMSO (../rhodium/chemistry /epox-i
Phenyl acetones by electrolytic oxidation (../rhodium/chemistry /guest.
In the second reference, electrolytically prepared epoxides are rearranged to carbonyl functional groups with LiI or LiBr. Maybe ZnBr2 will work as well?
Dirty old man
|Synthesis of Zinc Iodide Revisited|
Here is a follow-up to the J. Chem. Educ. article mentioned above. I'll upload the first part later.
Synthesis of Zinc Iodide Revisited
J. Chem. Educ. 80, 796 (2003) (../rhodium/pdf /zinc.iodide.
Two inquiry-based labs that complement a previously published activity in this Journal, "The Synthesis and Decomposition of Zinc Iodide: Model Reactions for Investigating Chemical Change in the Introductory Laboratory", are described. These two experiments could be of interest to introductory chemistry instructors at the college or high school level who teach their students about limiting and excess stoichiometry as well as acid–base chemistry. The inquiry-based experiments center on alternate reaction pathways involving a second synthesis of zinc iodide and a side reaction that produces zinc hydroxide. In the first experiment, students draw upon their understanding of solubility and molarity to propose a synthesis of zinc iodide from a double replacement reaction involving zinc sulfate and barium iodide. Students compare the double replacement reaction with the elemental synthesis in terms of percentage yield, efficiency, safety, and cost. In the second experiment, students are asked to identify a white precipitate that forms during a synthesis of zinc iodide from its elements when a specific reagent, acetic acid, is not used. By referring to the literature and conducting qualitative tests, students determine that the white product is zinc hydroxide, a base produced from the hydrolysis of zinc ion.