Zincates reactions

The palladation of propargylic mesylates is known to occur with inversion of configuration [113]. The predominant formation of anti products strongly suggests a cyclic transition state for the addition. It can therefore be surmised that the zincation reaction proceeds with retention of configuration. A possible catalytic cycle is shown in Scheme 9.28 [110]. 

In these reactions one equivalent of chiral azaenolate remains unused. This can be overcome when a mixed azaenol cuprate 9 is employed with an acetylide as nontransferable ligand or when a mixed azaenol dimethylzincate 10 is used. Higher diastereoselectivities are achieved with zincates than with copper azaenolates and with (li ,2S )-2-methoxy-1,2-diphenylethanamine as auxiliary231. 

In 0-level qualitative analysis, unknown cations are identified by reacting them with aqueous sodium hydroxide and/or aqueous ammonia. For example, zinc salt solutions react with aqueous sodium hydroxide to form a white precipitate, zinc hydroxide, which in turn will react with excess aqueous sodium hydroxide to form a colourless solution of sodium zincate, a complex salt. However, 25% of the 915 students thought that when sodium hydroxide solution was added to a solution of a zinc salt, a displacement reaction resulted leading to the formation of a precipitate... 

When the phosphonium ylide 81 is reacted with zinc amide, the corresponding a-zincated phosphorus yUde is formed. Thermally unstable, it evolves almost quantitatively to zincatacyclobutane 82 which in presence of pyridine leads to the formation of the zincataphosphoniaindane 83. In order to explain this unprecedented cyclometallation reaction, a mechanism is proposed involving a low coordinated zinc center. The new product, reacted with benzaldehyde leads to the diphenylallene 84 (Scheme 27) [106-108]. 

Alkenylzinc reagents can also be made from alkynes by (Cp)2TiCl2-catalyzed hydro-zincation (see Section 4.6).139 The reaction proceeds with high syn stereoselectivity, and the regioselectivity corresponds to relative carbanion stability. 

In zinc chloride, zinc is present as the positive cation, Zn2+, whereas in sodium zincate, zinc is present as part of the negative zincate anion, (Zn02)2-. In the present context, reference must be drawn to water. It can accept a proton and can act as a base similarly, it can part with a proton and can act as an acid. Thus, it has an amphoteric (dual) nature. Water can function as a base in the presence of acids stronger than itself and as an acid in the presence of bases stronger than itself. As an example in the former category reference may be drawn to the reaction,... 

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-... 

The reaction of diorganozincs bearing bulky trimethylsilyl-substituted methyl groups and methyl- or phenyllithium in the presence of the 1,3,5-trimethyl-1,3,5-triazacyclohexane (TAGH), Scheme 53, afforded the corresponding lithium zincates as poorly soluble ion pairs.124... 

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... 

Methylenetriphenylphosphorane reacted with Zn N(SiMe3)2 2 in a multistep reaction (Scheme 78) to furnish initially the 1,3-dizincatacyclobutane 119, featuring two three-coordinate zinc atoms, and finally the cr-zincated phosphorus ylide 120.176... 

The oxyanion containing Zn2+, Zn022-, is known as a zincate, and it equivalent to Zn(OII)42, which is named as the tetrahydroxozincate(II) anion (see Chapter 16). From these reactions, it is clear that ZnO can react as either an acidic or basic oxide and it is therefore known as an amphoteric oxide. In essence, there are some oxides that are clearly acidic, some that are dearly basic, and some that are in between. There is, in fact, a continuum of acid-base character for the oxides of elements that is shown in Figure 9.2. 

A divergent protocol for a solid-phase synthesis of 3-substituted 2,5-biarylfurans was reported. Thus, reaction of furan zincate A with polymer bound aryl bromide or iodide provides resin intermediates 61. Subsequent bromination-Suzuki coupling reaction followed by further transformations gives rise to structurally diverse 2,3,5-trisubstituted furans 68 in good overall yields and chemical purities <00TL5447>. 

---