Zinc Iodide cycloaddition reactions

In the above cycloaddition reactions, carbene is generated in situ. A more convenient way is to use Simmons-Smith reagent ", which transfers methylene from methylene iodide and zinc-copper couple to a carbon-carbon double bond (Scheme 2.56). In the reaction in Scheme 2.56, free carbene is not generated. The intermediate is believed to be ICH2ZnI, which behaves as an electrophile known as carbenoid. 

Furan also undergoes cycloadditions with allenes," " with benzyne" and with simpler dienophiles, like acrylonitrile and acrylate various Lewis acidic catalysts can assist" in some cases, zinc iodide is one such, hafnium tetrachloride another, and improved endo exo ratios are obtained in an ionic liquid as reaction solvent. Maleate and fumarate esters react if the addition is conducted under high pressure. This device can also be used to increase markedly the reactivity of 2-methoxyfuran and 2-acetoxyfuran towards dienophiles. At higher reaction temperatures alkynes and even electron-rich alkenes will add to furan. 3- or 5-Halo-furans react faster in these cycloadditions. ... 

General Procedure for the Enantioselective Co(I)-Catalyzed [6+2] Cycloadditions In a 10-mL, flame-dried Schlenck, phosphoramidite (0.10 equiv) was added to a solution of C0I2 (13 mg, 0.042 mmol, 5 mol%) in 1,2-dichloethane (1 mL) and stirred for 10 min. Then, powdered zinc (8.3 mg, 0.127mmol, 0.15 equiv), a solution of 1,3,5-cycloheptatriene (93 mg, Immol, 1.20 equiv) in 1,2-dichloroethane (ImL), a solution of acetylenic compound (0.85 mmol, 1 equiv) in 1,2-dichoroethane (1 mL), and zinc iodide (27 mg, 0.085 mmol, 0.1 equiv) were added. The resulting mixture was heated at 40 C for 20 h. After cooling to room temperature, the reaction... 

The earlier examples of [2 + 1] cycloaddition of a carbene (or carbenoid) on the double bond of alkylidenecyelopropanes to yield spiropentane derivatives were observed as undesired side reactions in the synthesis of alkylidenecyelopropanes through the addition of a carbene to a substituted allene [161]. In some cases the spiropentane derivative was obtained as the major product [161a, c] especially when a large excess of the carbene reagent was used. For example, when methyl 3,4-pentadienoate (610) was treated with a ten-fold excess of methylene iodide and zinc-copper couple the two products 611 and 612 were isolated in 1 4.5 ratio (Scheme 86) [161a]. 

With an acceptor-substituted alkene moiety tethered to the molecule, the intermediate silyl enol ether may undergo an intramolecular [2-I-2] cycloaddition.The silyl-assisted addition of hydrogen halides to cyclopropanes is not restricted to ketones with carbonyl groups as activating function or iodide as nucleophile. Esters and other acid derivatives underwent similar reactions when treated with iodotrimethylsilane alone or in the presence of an additional catalyst such as mercury(II) or zinc(II) chloride.Subsequent treatment of the y-iodo ester with potassium carbonate in tetrahydrofuran gave the respective y-butyrolactones in good yield. 

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