Organozincs oxidation

Organozinc Halides by the Oxidative Addition of Organohalides to Zinc 329... 

The general utility of the oxidative addition of functionalized organic halides to zinc was demonstrated by the formation of organozinc iodides 28 from protected (3- and 7-amino acids (Scheme 26).73 The organozinc iodides prepared in this manner were neither sufficiently stable nor sufficiently reactive in THF, but excellent yields were obtained in more polar aprotic solvents, such as DMF and DMSO. 

Organozinc Compounds Bearing Alkoxides, Siloxides, Boryloxides, Complex Oxides, and Related Ligands... 

The report of the first zinc compound with a Zn-Zn core elicited a number of critical comments on the structure and bonding of decamethyldizincocene, and the interpretation of the results.236,237 None of the authors of these commentaries questioned the data or their interpretation. Parkin, however, has pointed out that the formal oxidation state of +1 for zinc in this compound is merely due to the convention that metals are assigned an oxidation state of 0 when they form bonds with like atoms.237 If the conventional definition of valence, namely the capacity of atoms to form bonds to other atoms is used, then the zinc atoms in decamethyldizincocene are not monovalent, but divalent. The synthesis of a paramagnetic organozinc compound in which zinc uses only one of its two 4s electrons will remain an interesting challenge to many synthetic organometallic chemists. 

Although the copolymerization of propylene oxide with C02 takes place effectively with organozinc additives or the (tetraphenyl) porphyrin-AlCl system [61], the copolymerization of epichlorohydrin with C02 seldom occurs with these catalysts. Shen et al. [62] showed that a rare earth metal catalyst such as the Nd(2-EP)3/AliBu3 (Al/Nd = 8) system was very effective for the copolymerization of epichlorohydrin with C02 (30-40 atm) at 60 °C (Scheme 16). The content of C02 in the copolymer reached 23-24 mol % when 1,4-dioxane was used as solvent. 

Catalytic properties of external chiral additives such as (2S,3/ )-4-dimethyl-amino-l,2-diphenyl-3- methyl-2-butoxide (A 16) (574, 575) and 2-magnesium-3-zinc salts of dialkyl (f ,f )-tartrate (A17) were employed in the highly stereoselective addition of organozinc reagents to derivatives of 3,4-dihydro-isoquinoline-A-oxide (Scheme 2.147) (576). 

The mechanism of the ZnBr2-assisted, nickel-catalyzed Reformatsky reaction has been discussed [540]. The reaction involves the electroreduction of a Ni(II) complex to a Ni(0) complex, oxidative addition of the a-chloroester to the Ni(0) complex, and Zn(II)/Ni(II) transmetallation, leading to an organozinc Reformatsky reagent. Most recently, the Reformatsky reaction... 

The Reformatsky reaction can also be performed electrochemically either directly or using a mediator. Ni-catalysis has proven to be an efficient way to prepare j3-hydroxy ester or nitrile from the corresponding a-chlorocompounds (Table 14) [94]. Here again the first step is the oxidative addition of the cathodically generated Ni°bpy to the halocompound. The nature of the sacrificial anode also plays a crucial role in this reaction, though the formation of an organozinc intermediate has not been fully demonstrated. 

The first asymmetric procedure consists of the addition of R2Zn to a mixture of aldehyde and enone in the presence of the chiral copper catalyst (Scheme 7.14) [38, 52]. For instance, the tandem addition of Me2Zn and propanal to 2-cyclohexenone in the presence of 1.2 mol% chiral catalyst (S, R, R)-1S gave, after oxidation of the alcohol 51, the diketone 52 in 81% yield and with an ee of 97%. The formation of erythro and threo isomers is due to poor stereocontrol in the aldol step. A variety of trans-2,3-disubstituted cyclohexanones are obtained in this regioselective and enantioselective three-component organozinc reagent coupling. 

Oxidative nickel-catalyzed coupling of aldehydes and alkynes to generate allylic alcohols. Intermolecular and intramolecular examples are both effective, and the transmetalating agent (MR" ) may be an organosilane, organoborane, organozinc, or alkenylzirconium. ... 

A-Mono- and iV,iV-disubstituted 0-benzoylhydroxylamines 2a-i recently developed for amination of organozinc and Grignard reagents and 0-acylhydroxylamines 2j-l for amination of enolates have been listed in Table 2. 0-Benzoylhydroxylamines were prepared by Berman and Johnson " by oxidation of primary and secondary amines with... 

SCHEME 3. Functionalized organozinc compounds prepared by oxidative addition... 

RX, thus affording the corresponding RNiX compounds. In the presence of zinc(II) ions, this intermediate would undergo a transmetallation reaction to produce the organozinc compound RZnX. Note that zinc(II) ions can be brought either by addition of a zinc salt (ZnBr2 or ZnCl2) in the solution or by oxidation of a zinc anode. 

However, the mechanisms involved in these reactions have not been elucidated yet. Nevertheless, we can advance the formation of an organozinc compound able to reduce oxide-type impurities being at the zinc surface. 

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