Zinc halo complexes

The most common oxidation state of niobium is +5, although many anhydrous compounds have been made with lower oxidation states, notably +4 and +3, and Nb can be reduced in aqueous solution to Nb by zinc. The aqueous chemistry primarily involves halo- and organic acid anionic complexes. Virtually no cationic chemistry exists because of the irreversible hydrolysis of the cation in dilute solutions. Metal—metal bonding is common. Extensive polymeric anions form. Niobium resembles tantalum and titanium in its chemistry, and separation from these elements is difficult. In the soHd state, niobium has the same atomic radius as tantalum and essentially the same ionic radius as well, ie, Nb Ta = 68 pm. This is the same size as Ti ... 

Although chlorination, bromination and iodination of thiophenes by polyhalide salts require forcing conditions with the addition of zinc chloride [52], halogenation of acridine and acridone has been recorded to yield both 3-halo and 3,7-dihalo derivatives under relatively mild reaction conditions [53], However, whereas chloro-, bromo- and iodo-compounds are readily obtained from acridone, acridine only forms the bromo derivatives, as it produces stable complexes with the dichloroiodate and tetrachloroiodate salts [53]. 

Before preparation of any a-halo boronic ester is undertaken, it should be noted that two sets of model conditions are outlined in what follows. If the group R1 of the boronic ester 1 is a typical alkyl group, then the rearrangement of the derived borate complex 2 requires a number of hours at 20 C, but if R1 is aryl or alkenyl, then shorter times and lower temperatures are required in order to avoid epimerization of the product 3 catalyzed by zinc chloride and lithium... 

REFORMATSKY REACTION. Dating back to 1887, this reaction depends on interaction between a carbonyl compound, an a-halo ester, and activated zinc in the presence of anhydrous ether or ether-benzene, followed by hydrolysis. The halogen component for example ethyl bromoacetate, combines with zinc bo form an organozinc bromide that adds to the carbonyl group of the second component to give a complex readily hydrolyzed to carbinol. The reaction... 

The insight that zinc ester enolates can be prepared prior to the addition of the electrophile has largely expanded the scope of the Reformatsky reaction.1-3 Substrates such as azomethines that quaternize in the presence of a-halo-esters do react without incident under these two-step conditions.23 The same holds true for acyl halides which readily decompose on exposure to zinc dust, but react properly with preformed zinc ester enolates in the presence of catalytic amounts of Pd(0) complexes.24 Alkylations of Reformatsky reagents are usually difficult to achieve and proceed only with the most reactive agents such as methyl iodide or benzyl halides.25 However, zinc ester enolates can be cross-coupled with aryl- and alkenyl halides or -triflates, respectively, in the presence of transition metal catalysts in a Negishi-type reaction.26 Table 14.2 compiles a few selected examples of Reformatsky reactions with electrophiles other than aldehydes or ketones.27... 

Reaction (2) illustrates the elegant synthetic potential of the bromo supra-Cp s. In addition, bulky halo- and in particular the bromocyclopen-tadienes are also good sources of long-lived radicals which could react with low-valent metal complexes [Eq. (4)]. The radicals are generated by treatment of the substituted bromo-Cp s with zinc (46) or silver (33). So far, radicals have been only employed once, namely, in the formation of decaphenylnickelocene (78). 

Properties M.w. 184.31 sp.gr. 0.993-0.997 b.p. 95-97 C (13 mm Hg) ref. index 1.579 Toxicology TSCA listed Uses Reducing agent for prep, of silyl-substituted alkylidene complexes of tantalum, in ionic reduction of enones to sat. ketones, in reductive cyclization of unsat. ketones, for esters in presence of zinc hydride catalyst, for o-halo ketones in presence of Mo(0), reduction of thio esters to ethers, esters to alcohols with Rh catalysis, in assym. reduction of methyl ketones, reductive cleaving of allyl acetates Manuf./Distrib. ABCR http //www.abcr.de, Alfa Aesar http //www.aifa.com-. Digital Spec. Chems. http //www.digHaichem.com, Fluorochem USA... 

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