Zinc Compounds Simmons-Smith reagent

A branched-chain iodo sugar derivative, l,5-anhydro-4,6-0-benzyl-idene-2,3-dideoxy-3-C-(iodomethyl)-D-rifoo-hex-l-enitol [4,6-O-ben-zylidene-3-deoxy-3-C-(iodomethyl)-D-allal] (200), is one of the products formed on treatment of methyl 4,6-0-benzylidene-2,3-dideoxy-a-D-en/thro-hex-2-enopyranoside (77) with the Simmons-Smith reagent (diiodomethane and zinc-copper couple).123,212 Compound 200 displays high solvolytic reactivity, an observation that has been rationalized by supposing the formation of the highly stabilized carbonium ion213 (201). Thus, under conditions wherein methyl 2,3,4-tri-0-acetyl-6-deoxy-6-iodo-a-D-glucopyranoside required more than 24 hours to react appreciably with an excess of silver nitrate in 50% aqueous p-dioxane buffered with silver carbonate, the iodide 200 was hydrolyzed completely in less than 1 minute the product of hydrolysis of 200 is the cyclopropyl aldehyde 202. Methanolysis of... 

CyclopropanationJ The title compound, prepared from ArOH, Et2Zn, and CH2I2, is a modified Simmons-Smith reagent, with which alkenes are transformed into cyclopropanes in excellent yields (6 examples, 90-98%). In terms of reactivity, the zinc phenoxide is comparable to bis(chloromethyl)zinc, but more reactive than bis(iodomethyl)zinc and Furukawa s reagent. 

The Simmons-Smith Reaction (Section 15.3C) Treatment of CHjIj with a zinc-copper couple generates an organozinc compound, known as the Simmons-Smith reagent, which reacts with alkenes to give cyclopropanes. 

Organozinc compounds, XZnCRR (RR = H, alkyl, X halogen), are widely used for the synthesis of cyclopropanes as Simmon-Smith reagents [12,36,37,79- 93]. Simmon-Smith reagents are prepared by two processes as shown in eqs. (5.32) and (5.53). The first one is the reaction of diazomethane with zinc halides. The second is the reaction of methylene iodide with an activated zinc. Zinc containing 10% of... 

Methylene ( CH2), the simplest carbene, can be prepared by decomposition of the highly toxic and explosive reagent diazomethane (CH Nj). However, more easily used reagents have been developed that, while they do not produce methylene directly, function as methylene transfer agents. They are called carbenoid species because they react like carbenes. lodomethylzinc iodide, known as the Simmons-Smith reagent, is a carbenoid. In the Simmons-Smith method, diiodomethane reacts with a zinc-copper alloy to produce an intermediate I—CH Zn—I compound. 

Lack of cooling during preparation of the Simmons-Smith organozinc reagent caused the reaction to erupt. The possibly pyrophoric nature of organozinc compounds and the presence of ether presents a severe fire hazard [1]. An alternative, safer method of activating the zinc for the reaction involves use of ultrasonic irradiation rather than the copper-zinc couple [2]. 

Among methods of preparing optically active cyclopropane compounds, the Simmons-Smith reaction, first reported in 1958, is of significance. This reaction refers to the cyclopropanation of alkene with a reagent prepared in situ from a zinc-copper alloy and diiodomethane. The reaction is stereospecific with respect to the geometry of the alkene and is generally free from side reactions in contrast to reactions involving free carbenes. 

Synthesis of model compounds and structural units are being investigated. A double Simmons-Smith reaction on the l,3-dioxolane-4,5-diylbis(alkene) 107 afforded the product 108 with excellent stereoselectivity. The required asymmetry in the double cyclopropanation was the result of coordination of the zinc carbenoid reagent by the Lewis basic dioxolane ring oxygen prior to each cyclopropanation event. The cyclopropanated product was converted to ( )-l,2-bis[(l 5,25)-2-methylcyclopropyl]ethene, a relevant model for the complete structural assignment of FR-900848. 

Methyleuation. Methylenation of aldehydes (but not ketones) has been reported with CH2I2 and zinc (large excess), but yields are not satisfactory. However, addition of trimethylaluminum gives a reagent that converts both aldehydes and ketones into terminal alkenes in 60-85% yield. No Simmons-Smith products are formed from unsaturated carbonyl compounds. 

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