Of dimethylzinc

We came up with the idea of using a dummy ligand, as shown in Scheme 1.23 [34]. Reaction of dimethylzinc with our chiral modifier (amino-alcohol) 46 provided the methylzinc complex 62, which was subsequently reacted with 1 equiv of MeOH, to form chiral zinc alkoxide 63, generating a total of 2 moles of methane. Addition of lithium acetylide to 63 would generate an ate complex 64. The ate complex 64 should exist in equilibrium with the monomeric zincate 65 and the dimer 66. However, we expected that the monomer ate complex 64 and the mono-... 

Figure 9 Adducts of dimethylzinc with (a) trimethyltriazine (b) 1,4-dioxan (c) 1,4-thioxan.

Even the addition of 4 equiv. of dimethylzinc to a bis(dicarbollide)tantalum dichloride, Scheme 23, produced only the monomethyl complex 26 in an isolated yield of 75%.63 In these reactions, the oxidation state of the transition metal and the steric bulk of its ligands obviously play a role in the degree of alkylation. 

In their outcomes, similar, but mechanistically different, coupling reactions were observed in the zincation of (tert-butyldimethylsilyl)(2-pyridinemethyl)amine (Scheme 75), which produced methylzinc(/ r/-butyldimethylsilyl) (2-pyridylmethyl)amide 113. Thermolysis of 113, or its treatment with one additional equivalent of dimethylzinc, yielded the C-G coupled dimer [l,2-bis(/ / -butyldimethylsilylamido)-l,2-dipyridylethane]bis(methylzinc) 114.170... 

Another useful method for generating various IV-allylhydroxylamines is the reaction between vinyl boronic ester of pinacol and nitrone in the presence of dimethylzinc (655). 

Feringa and co-workers applied chiral phosphoramidite ligand (S,R,R)-67 in the conjugate addition of dimethylzinc to acyclic unsaturated malonates 68 and obtained up to 98% ee (Scheme 22).71... 

Carbon dioxide instead of aldehydes can be involved in Ni(0)-promoted reductive coupling reactions (Equations (76) and (77) Scheme 90).434,434a 434c A stoichiometric amount of Ni(COD)2/DBU reacts with C02 and dienes, alkynes, or allenes to afford a metallacycle intermediate. This metallacycle reacts with organozinc compounds or aldehydes in one-pot to give carboxylic acid derivatives. As shown in Scheme 90, double carboxylation occurs in the presence of dimethylzinc, where the stereochemical outcome is opposite to that of the reaction with diphenylzinc. 

As shown in Scheme 7-21, an equimolar mixture of dimethylzinc and the coside chain vinyl lithium is treated sequentially with siloxyl-2-cyclopentenone and the propargylic iodide in the presence of HMPA. The desired product 79 is formed with 71% yield.12 The acetylenic product is a common intermediate for the general synthesis of naturally occurring PGs. 

Previously published methods for the synthesis of dimethylzinc, a useful alkylating agent, include the reaction of dimethylmercury with metallic zinc,1 the reaction of a zinc-copper couple with methyl iodide,2 and the Grignard method.3 The reaction of trimethylaluminum with zinc(II) halides or alkoxides can be used,4 but it is more convenient to use zinc(ll) acetate, which is very readily obtained by dehydrating the commercial dihydrate with boiling acetic anhydride or by the reaction5 ... 

A variation of this strategy involves conjugate addition of the reagent formed from a 1 1 mixture of dimethylzinc and a vinyllithium reagent. The presence of dimethylzinc suppresses... 

A study of the catalytic use of diastereomeric mixtures of catalysts in additions of dimethylzinc to benzaldehyde, in which combinations of ferrocenes 9 and 23 were applied, showed a strong kinetic preference for 9 over 23. Thus, even with 23 in large excess, the stereochemical result was mainly determined by ferrocene 9 [28]. 

Until fairly recently, little was known of the stmcmres and properties of the organozinc compounds occurring as intermediates in varions reactions. Interestingly, the complexforming ability of organozinc componnds had already been recognized very early. In 1858, Wanklyn reported the formation of the ionic sodium triethylzinc complex. One year later, Frankland observed that the formation of dimethylzinc from methyl iodide and zinc was accelerated by the addition of dimethyl ether or diethyl ether. It appeared that separation of the dimethylzinc from the ether was impossible, bnt it lasted nntil 1962 when it was established that dimethylzinc and dimethyl ether form a 1 1 complex in solntion, which is appreciably dissociated in the vaponr phase. ... 

The complex of dimethylzinc with 2-(l-methoxypropyl)ferrocenylaldimine, in which the 2-(l-methoxypropyl)ferrocenylaldimine ligand is Al,0-chelate-bonded to zinc [Zn—N 2.213(2), Zn—O 2.381(2) A], is the only example of a diorganozinc coordination complex in which in one complex both Zn—N and Zn—O coordination occurs . The coordination geometry at zinc is distorted tetrahedral with a larger C—Zn—C (141.6°) and a smaller N-Zn-0 (74.9°) bond angle. 

A recent study combined quantum chemical calculations and electron diffrac-tion/photoelectron spectroscopy to derive the following dialkylzinc gas phase enthalpies of formation ethyl, 57 8 n-propyl, 10 8 isopropyl, 32 8 f-butyl, —17 8 neopentyl, — 117 8 kJmoF. The benchmark value of 53 1 kJmol was chosen for the gas phase enthalpy of formation of dimethylzinc. Compared to the experimental values, the diethyl and dineopentyl values are very close, but the w-propyl enthalpy of formation is just barely within the combined large error bars. The methylene increment from the theoretically derived values of diethylzinc and di-n-propylzinc is —23.5 kJ mol , a value that is consistent with other gas phase homologous series. Using this increment, the enthalpies of formation of gaseous di-w-butylzinc and di-n-pentylzinc are calculated to be —37 and —84 kJmoU, respectively. 

TABLE 2. Relative intensities of the principal ions observed in the El mass spectrum (70 eV) of dimethylzinc ... 

The photochemistry of Zn+—(CH4) and Zn —(CH3OH) complexes has been studied in detail In that context, the possibility of the formation of a metal-hydroxo insertion complex [HO—Zn—CH3]+ (the isomer of [Zn—(CH30H)]+) has also been discussed . In other series of studies, the mechanism of dimethylzinc zinc monoethyl cation, diethylzinc diethylzinc dimer and dipropylzinc photolysis has been investigated by photoionization techniques. It was the study of Borsella and Larciprete that first observed different gas phase photodissociation mechanisms for Mc2Zn and Et2Zn by using one- and two-color multiphoton ionization combined with TOE MS. 

Another protocol was reported by Wipf and Xu. The hydrozirconation of terminal alkynes followed by the addition of dimethylzinc forms (E)-alkenylmethylzinc (equation 18)58. The chiral amino thiol 38 has been employed as a catalyst for this reaction59. Li and Walsh found that ketones as well as aldehydes are alkenylated by alkenylzinc using the chiral catalyst 3460. 

Traditionally, the adducts have been prepared by adding stoichiometric amounts of the ligand to a solution of dimethylzinc (in diethyl ether or petroleum spirit) followed by removal of the solvent in vacuo. The compounds presented here were synthesized by direct reaction of the ligand and the electronic grade metal alkyl in the absence of any solvent. 

The procedures and apparatus for the preparation of the diethyl ether solution of dimethylzinc and the dissociation of the adduct are essentially the... 

Unlike the situation with trimethylindium, there is no need to distill off the free diethyl ether from the dimethylzinc solution as the adduct of dimethylzinc with l,3-bis(4-pyridine)propane is readily precipitated from diethyl ether (follow procedure s). 

Purchased dimethylzinc ( dry but unpurified) can be used in the preparation of the adduct in exactly the same way as described above. Thus, 50 g of dimethylzinc (0.524 mol) is dissolved in 300 mL of diethyl ether and this solution is added over a period of 30 min, by cannula, to a rapidly stirred and gently refluxing solution of 103 g of l,3-bis(4-pyridyl)propane (0.520 mol) in 300 mL of diethyl ether. The crystalline product is worked up as above. Yield 150g (98% based on amine or dimethylzinc). 

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