Zinc-copper carbenoid

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]. 

The Simmons-Smith cyclopropanation utilizes methylene diiodide and a zinc-copper couple to produce a carbenoid intermediate. This intermediate reacts with olefins to give cyclopropanes. The geometry of the double bond is preserved in file cyclopropane. 

The Simmons-Smith reagent, named for the two DuPont chemists who discovered it, is made by adding methylene iodide to the zinc-copper couple (zinc dust that has been activated with an impurity of copper). The reagent probably resembles iodomethyl zinc iodide, ICH2ZnI. This kind of reagent is called a carbenoid because it reacts much like a carbene, but it does not actually contain a divalent carbon atom. 

A cyclopropanation of an alkene using the carbenoid reagent generated from diiodomethane and the zinc-copper couple, (p. 358)... 

The intermediate formation of a zinc carbenoid tert-BuCOCH(ZnCl)2 was postulated for the reaction of a,a -dichloropinacoline with zinc-copper couple which produces tranr-tripivalylcyclopropane 78). 

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. 

Finally, a fairly convenient protocol for carbene transfer reactions utilizes species like ICH(R)ZnI (a carbenoid), generated in situ by the interaction of 1,1 -di-iodides, RCHI2, with a zinc-copper pair (Simmons-Smith procedure, reaction... 

Reaction of CH2I2 with zinc-copper couple forms ICH2Znl, the Simmons-Smith reagent Since the CH2 group is bonded to the metal and does not exist as a free carbene, this intermediate is called a carbenoid. 

When diiodomethane is treated with zinc-copper couple, a carbenoid is formed. 

Reactions of halogen-stabilized carbenoids with imines have been carried out using preformed lithium species e.g. equation or via a carbenoid generated from diiodomethane utilizing zinc-copper... 

Reactions of halogen-stabilized carbenoids with imines have been carried out using prefoimed lithium species (e.g. equation 36), or via a carbenoid generated from diiodomethane utilizing zinc-copper couple (Simmons-Smith conditions). A stereospecific ring closure is observed after the addition of lithiodichloromethane to a benzaldimine (equation 37).The addition of lithiochloro(phenylsulfon-yl)methane to aromatic imines affords 2-phenylsulfonyl-substituted aziridines, which can be deproton-ated and alkylated in excellent yield (Scheme 20). 

The electron-deficient double bond in 2-phenylsulfonyI-l,3-dienes underwent regioselective cyclopropanation with sulfur ylides in very good yield, e.g. cyclopropanation of 13, 14 and 15 with sulfur ylides.On the other hand, the use of zinc-copper couple and diiodomethane afforded a 83 17 mixture of vinylcyclopropanes 16 and 17 from 2-(phenylsulfonyl)cyclohexa-1,3-diene (15) in moderate yield. The reaction at the electron-deficient double bond rather than at the expected electron-rich double bond shows that the Simmons-Smith carbenoid is fairly nucleophilic in character. 

Most chemical syntheses of compounds containing cyclopropyl groups make use of the addition of a carhene, or carbene equivalent, to an alkene. What do we mean by carbene equivalent Usually, this is a molecule that has the potential to form a carbene, although it may not actually react via a carbene intermediate. One such example is the zinc carbenoid formed when diiodomethane reacts with zinc metal (most conveniently as a mixture with copper—a zinc—copper couple ). It reacts with alkenes just as a carbene would—it undergoes addition to the jc bond and produces a cyclopropane. 

Cyclopropanation of alkenes, in both inter- and intramolecular modes, constitutes an attractive route to polycycloalkanes. These cyclopropanations of alkenes are carried out through car bene or carbenoid intermediates . For the direct cyclopropanation of alkenes, the methylene iodide zinc-copper couple (Simmons-Smith reaction)reagent is commonly used. Several modifications of this procedure, including acceleration with ultrasound, are known °°. A somewhat less frequently used procedure for cyclopropanation is through methylene addition from diazomethane, which can be carried out either thermally or photochemically or in the presence of metal salts, e.g. Pd(OAc)2 or Rh2(OAc)4. In Table some examples of the preparation of cyclopropane bearing... 

A useful cyclopropane synthesis was developed by H. E. Simmons and R. D. Smith of the DuPont Company. In this synthesis diiodomethane and a zinc-copper couple are stirred together with an alkene. The diiodomethane and zinc react to produce a carbene-like species called a carbenoid ... 

Carbenoid (Section 8.14C) A carbene-like species. A species such as the reagent formed when diiodomethane reacts with a zinc-copper couple. This reagent, called the Simmons—Smith reagent, reacts with alkenes to add methylene to the double bond in a stereospecific way. 

However, in 1958, yet another powerful synthetic tool in which zinc plays a major role, was introduced to organic chemists i.e. the Simmons-Smith reaction [44, 45]. In this procedure, use is made of a carbenoid intermediate, formed by interaction of the metal (most commonly a zinc/copper alloy is used for this reaction) with diiodomethane ... 

Singlet carbenes are formed by treatment of CHCI3 with t-BuOK (results in loss of HCl to give iCCy, by loss of N2 from diazo compounds, such as diazomethane (CH2N2), or by reaction of CH2I2 with zinc-copper amalgam (cyclo-propanation with the resulting CH2 synthetic equivalent, called a carbenoid, is the Simmons-Smith reaction). 

Less suited for introduction of carbon No. 1 is the carbenoid from the Simmons-Smith-reaction of diiodoacetic-ester and zinc-copper [16], and the radical addition of chloroacetic ester using cuprous chloride and bipyridyl as a radical starter [17]. 

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