Bromide zinc

Copytighie 1999 Jotm VWgy4 Sons Rafrtwed from www.kiKWBl.eotn 

Purification heat to 300 °C under vacuum (2 x 10 mmHg) for 1 h, then sublime. 

Handling, Storage, and Precautions very hygroscopic store under anhydrous conditions. Irritant. 

Concerted Ring-Forming Reactions. The mild Lewis acid character of ZnBr sometime imparts a catalytic effect on thermally allowed pericyclic reactions. The rate and stereoselectivity of cycloaddition reactions (eq 8), including dipolar cycloadditions (eq 9), are significantly improved by the presence of this zinc salt. 

Some intramolecular ene reactions benefit from ZnBr catalysis to afford the cyclic products under milder conditions, in higher yields and selectivities (eqs 10 and 11). Generally, the use of ZnBr is preferred over Zinc Chloride or Zinc Iodide in this type of reaction.  

---

Technical grade zinc cyanide was used as supplied by Matheson, Coleman and Bell. Other Lewis acids, notably aluminum chloride, zinc bromide, and zinc iodide may be used as catalysts for the reaction. 

Brom-wasserstoffsaure, /. hydrobromic acid, -wismut, tn. n. bismuth bromide, -zahl, /. bromine number, -zink, n. zinc bromide, -zinn, n. tin bromide. 

Kitazume and Kasai [55] have investigated the Reformatsky reaction in three ionic liquids. This reaction involves treatment of an a-bromo ester with zinc to give an a-zinc bromide ester, which in turn reacts with an aldehyde to give an addition product. An example is given in Scheme 5.1-26. Moderate to good yields (45-95 %) were obtained in ionic liquids such as [EDBU][OTf] for the reactions between ethyl bro-moacetate or ethyl bromodifluoroacetate and benzaldehyde [55]. 

Zinc-Bromide. Unlike sodium-sulfur batteries, zinc-bromide batteries operate at ordinary temperatures. Although they use low-cost, readily available... 

At 03C, a solution of 120 mg (1.2 mmol) of phenylacetylene in 2 mL of dry Et20 is treated with 0.45 mL (1.2 mmol) of 2.6 M BuLi in hexane, The mixture is stirred for 30 min, then a solution of 290 mg (1.3 mmol) of anhyd zinc bromide in 2 mL of Et20 is added. After cooling to —78 C, 100 mg (0.6 mmol) of 2-(ben-zyloxy)propanal (7) are added and the reaction mixture is allowed to warm to 0 C over a 2-h period. The reaction is quenched with sat. aq NH4C1. The organic layer is separated, dried over Na2S04 and concentrated. The residual oil is purified by column chromatography on silica gel yield 154 mg (95%) d.r. (syn/anti) 95 5 (determined by HPLC). 

An analogous solvent effect was observed upon treatment of the chiral a-alkoxy aldehyde 11 with 2-lithio-4-methylfuran in the presence of zinc bromide. This highly diastereoselective addition reaction was the key step in a synthesis of the enantiomcrically pure C-10-C-20 fragment of the immunosuppressant KK 506139. 

On the contrary, in the latter case, a total loss of stereoselectivity occurs68. TV-Bis-benzyl-a-amino aldehydes 1 (R = R3 = Bn) under the assistance of boron trifluoride, zinc bromide or tin(lV) chloride lead to the nonchclation-controlled adducts preferentially, whereas titanium(IV) chloride or magnesium bromide result in chelation control70. In some cases, the O-trimcthylsilyl cyanohydrins arc the primary products, but the workup procedure usually provides the desily-lated products. 

In the course of a synthesis of /V-acetylneuraminic acid55, the [2-(Av7-butoxycarbonyl)-2-propenyl]zinc bromide reagent, preformed from a zinc/silver couple on graphite36, gave the best result. Several other methods of generation resulted either in lower yields or stereoselectivities 55. 

The diastereoselectivity of the zinc iodide catalyzed reaction of the azetidinone I with the trimethylsilyl enolate derivatives of the chiral 3-(l-oxopropyI)oxazolidinones 6 was considerably lower (about 60 40), although independent generation of the zinc enolate, via exchange of the lithium enolate with zinc bromide, afforded the /9-Iactam carboximide derivatives in a ratio (RIS) 80 20177. 

Zinc bromide promoted addition of a functionalized benzyl Grignard reagent to the a,/ -unsat-urated y-lactone, 3-(4-methylphenylsulfinyl)-2(5//)-furanone, via a zinc-chelated intermediate, gave, after several synthetic steps, (-)-podorhizon with 95% ee18. 

Note. With silyl ketene acetals, anhydrous zinc bromide (lmol%) is the preferred catalyst. 

Alkylzinc halides have also been prepared under microwave irradiation. The Reformatsky reagents (2-t-butoxy-2-oxoethyl)zinc bromide and [(2-dibenzylamino)-2-oxoethyl]zinc bromide were synthesized from the corresponding bromides via reaction with zinc in THF (Scheme 5) [24], The oxidative addition was executed at 100 °C in 5 min. The obtained reagents were subsequently used in Negishi reactions on 2-bromopyridine, 3-bromopyridine, 2-bromo-5-nitropyridine, and 2-bromo-5-trifluoromethyl-pyridine using Pd(PPh3)4 as a catalyst (Scheme 5). 

Allylic zinc bromides add to vinylic Grignard and lithium reagents to give the cm-dimetallo compounds 65. The two metallo groups can be separately reacted with various nucleophiles. ... 

---