A-zincation

Additional metal layers can create bimetallic corrosion cells if discontinuities appear in service. The layer of copper beneath cadmium plate on aluminium (using a zincate plus cuprocyanide deposit technique) can cause corrosion troubles. When aluminium is plated with nickel and chromium, rapid service corrosion in the zinc layer causes exfoliation. 

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]. 

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. 

Formally, a zincate is equivalent to a hydroxo complex that is dehydrated ... 

Alkylation of a 1,1-dibromoalkene with a zincate synthesis of 1-methoxy-4,4-dimethyl-2-phenyl-2-pentene (Structure 8)5b... 

C/s-selective alkylation of a gem-dibromocyclopropane via a zincate carbenoid synthesis of r-1 -acetyl-1 -ethyl-f-2-phenylcyclopropane (Structure IS)8 ... 

Vinylmagnesium bromide reacts as a Michael acceptor with a zincated hydrazone, yielding a 1,1-bimetallic species, which in some cases can be sequentially trapped with two different electrophiles.246 2463 The reaction proceeds via metalla-aza-Claisen rearrangement. The dimethyl hydrazone anion behaves as an azaallylic system (Scheme 81).247 Vinylphosphonium salts add Grignard reagents, forming alkylphosphonium ylides. These ylides... 

The reaction of ZnO with a base can be shown in different ways depending on whether the product is written as a zincate (Zn022-) or a zinc hydroxide complex. If the reaction occurs between ZnO and a basic oxide such as Na20, the equation can be shown as... 

Heck reaction has also been applied. It is also possible to terminate such a sequence after transmetallation from a zincate [106a,c], either added to the reaction mixture [106a] or generated in situ from an iodoalkene [106cJ. 

Lithium halides (bromide or Iodide) may well modify the Lewis character of the zinc atom, probably via a zincate species [53], and prevent the efficient coordination of the zinc atom to the double bond, coordination which is required for the carbocyclization. Thus, in the Rieke method, it is essential to wash the active zinc thoroughly since the lithium naphthalenide reduction of zinc bromide also generates lithium bromide, which is detrimental to the success of the reaction. Indeed, the insertion of Rieke s zinc in the presence of LiBr leads to the linear organozinc iodide but not to the cyclic product [52]. 

Nowack and Habermehl [64] recorded a significant drop in cathodic efficiency, from 90% to 20%, as the current density in a silent system was changed from 100 to 200 Am-2 for bright nickel. Ultrasound was highly beneficial and the efficiency remained very high (70%) at the much higher current density of800 A m-2. Similar improvements were also recorded for copper from acidic sulfate baths [65], and for zinc from a zincate solution [66]. 

Typically, before Ni coating on Al, a zincation pretreatment of the A1 is essential to enhance the Al-Ni interfacial contact, acting as a sacrificial layer during the autocatalytic electroless nickel plating process [3], This paper focuses on the zincation treatments for electroless nickel plating by analyzing the surface morphology and the deposited Ni properties. 

Fig. 11. SEM of electroless Ni bumps on A1 bond pads formed from (a) zincation solution 2 and (b) zincation solution 5 by double zincation treatments.

A simple two-step synthesis of l,3-oxazin-6-ones (e.g. 185) requires the formation of intermediate A-zincates (184) from a-bromoacetates and nitriles in contact with zinc powder. When the zincates... 

Another important application for 4-/< octylphenol is in the production of phenoHc resins. Novolak resins based on 4-/( l -octylphenol are widely used in the tire industry as tacldfiers. The tackiness of these resins binds the many parts of an automobile tire prior to final vulcanization. A specialty use for novolak resins based on 4-/< oct5iphenol is the production of a zincated resin, which is formulated as a dispersion in water and coated onto paper in combination with encapsulated leuco dyes to yield carbonless copy paper (see Microencapsulation). Pressure from writing bursts the encapsulated leuco dye, which is converted from its colorless form to its colored form by the i ncated resin (53). Novolak resins based on 4-/< oct5dphenol are also used in the production of specialty printing inks. 

Zinc Reagents. Negishi coupling of a-zincated vinyltrimethylsilanes with 2-bromopyridine yields 2-vinylpyridine functionalized by a silyl substituent at the a-carbon of the vinyl group 214 (Scheme 81). Similarly, the trifluoroethenyl group has been substituted into the 2-position in pyridine 215 nsing zincated trifluoroethene and 2-iodopyridine. ... 

---