Schlenk equilibrium structures

In addition dimeric species are formed, being in equilibrium with the monomeric RMgX. The Schlenk equilibrium is influenced by substrate structure, the nature of the solvent, concentration and temperature. 

By reaction of zinc-copper couple with diiodomethane 2 an organozinc species 4 is formed, similar to a Grignard reagent. Its structure cannot be fully described by a single structural formula. The actual structure depends on the reaction conditions—e.g. the solvent used this corresponds to the Schlenk equilibrium as it is observed with the Grignard reaction ... 

Although the simple alkylzinc halides RZnX (R = Me or Et, X = Cl, Br, I) were among the first known organozinc compounds, for a long time nothing was known about the actual structures of such compounds in solution or in the solid state. The constitution of these compounds in solution seemed to depend on the particular solvent employed " . It should be noted that in ethereal solutions, the possibility of the presence of a Schlenk equilibrium should always be considered. In these solvents the organozinc halides most... 

The solution structure of halomethylzinc derivatives has been the subject of several debates. Among those, the possible Schlenk equilibrium between IZnCH2l and Zn(CH2l)2/ Znl2 has been invoked to account for the stereochemical outcome of some cyclopropanation reactions, but little has been established unequivocally. Recently, in-depth low temperature spectroscopic studies of iodomethylzinc complexes have clearly established that the equilibrium between these species lies heavily on the side of IZnCH2l (equation 11) . ... 

The role of titanium salt is to activate the carbonyl compounds as Lewis acid. As described above, bis(iodozincio)methane (3) is nucleophilic enough to attack the carbonyl group of aldehydes or ce-alkoxyketones. In the reaction with simple ketones or esters, however, the addition of titanium salt is necessary to facilitate the nucleophilic attack. Instead of this Lewis acid activator, simple heating may induce the nucleophilic attack. Treatment of 2-dodecanone with 3 without titanium salt at higher temperature, however, does not improve the yield of alkene (Scheme 13). The reason for the low reactivity of 3 at higher temperature comes from the structural change of 3 into the polymeric methylene zinc 4 through the Schlenk equilibrium shown in equation 740. 

A mixture of 40 and 41 provides the active zinc species 42 in a reaction similar to the Schlenk equilibrium (see Chapter l).22 This presumably alkylates alkene 16 in a bimolecular process via the carbenoid transition structure 44 No free carbenes arise. 

Fig. 10.5. Calculated structures of Grignard compounds in the gas phase (numbers to stability given in decreasing order) or experimentally proven structures of Grignard compounds in solution (shaded pink). The Schlenk equilibrium ensures that not only RMgHal (shaded grey) occurs as a monomeric organometallic compound, but some I Mg as well.

Although the Simmons-Smith cyclopropanation has attracted increased attention during recent years, the exact structure of the cyclopropanating reagent is still uncertain. NMR-spectroscopic investigations revealed a Schlenk equilibrium between IZnCH2l and ICH2ZnCH2l [Eq. (2)] [15. ... 

Denmark et al. studied the effect of zinc iodide on the catalytic, enantioselective cyclopropanation of allylic alcohols with bis(iodomethyl)-zinc as the reagent and a bismethanesulfonamide as the catalyst 17]. They found significant rate enhancement and an increased enantiomeric excess of the product cyclopropane upon addition of 1 equivalent zinc iodide. Their studies and spectroscopic investigations showed that the Schlenk equilibrium appears to lie far on the left (IZnCHjI). Charette et al. used low temperature - C-NMR spectroscopy to differentiate several zinc-carbenoid species [18]. They also found evidence that in the presence of zinc iodide, bis(iodomethyl)zinc is rapidly converted to (io-domethyOzinc iodide. Solid-state structures of (halomethyl)zinc species have been described by Denmark for a bis(iodomethyl)zinc ether complex (6a) [19] and Charette for an (iodo-methyl)zinc iodide as a complex with 18-crown-6 (6b) [20] (Fig. 2). 

The role of Z11I2 is that an equimolar quantity of the compound drives the Schlenk equilibrium from the reagent bis(iodomethyl)zinc to (iodomethyl)zinc iodide, which is the actual cyclopropanation catalyst and has high reactivity and stereoselectivity [50c,52], The structure of the active catalyst, Zn-bis(sulfonamide) complex XXIV, was characterized by NMR analysis and X-ray study of the structure of its bipyri-dyl complex 66 (Sch. 28) [53]. The Zn-bis(sulfonamide) complex XXIV aggregates in solution and functions as a divalent Lewis acid. 

It is generally accepted that the structure of RMgX can be represented by the Schlenk equilibrium shown below. " ... 

---