Zinc amide

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 trigonal planar zinc phenoxide complex [K(THF)6][Zn(0-2,6-tBu2C6H3)3] is formed by the reaction of a zinc amide complex, via a bis phenoxide, which is then further reacted with potassium phenoxide. TheoX-ray structure shows a nearly perfect planar arrangement of the three ligands with zinc only 0.04 A out of the least squares plane defined by the three oxygen atoms.15 Unlike the bisphenoxide complexes of zinc with coordinated THF molecules, these complexes are not cataly-tically active in the copolymerization of epoxides with C02. The bisphenoxide complex has also been structurally characterized and shown to be an effective polymerization catalyst. 43... 

Bidentate ligand systems are also known with selenium and nitrogen donors. For example, Se/N bidentate phosphinochalcogenoic ligand and zinc amides react to form a distorted tetrahedral complex with two di-t-butylphosphinoselenoic isopropylamidate ligands.590... 

A related enantiomerically pure zinc amide initiator, (340), has also been described.966 This complex catalyzes the alternating copolymerization of CHO and C02 to yield isotactic material (RR SS = 86 14). Similar enantiomeric excesses have been achieved using a mixture of Et2Zn and the chiral amino alcohol (341).967 Molecular weight distributions are much broader than using catalyst (340), but this protocol is still a convenient way to prepare optically pure diols (Scheme 23). 

Prasad and Joshi121 presented a conceptually different catalyst system—zinc amides of oxazolidine. Because the addition of dialkylzinc to aldehyde is known to involve a chiral zinc alkoxide with a coordinately unsaturated tricoordinated center, they anticipated that a zinc amide with dicoordinate zinc should be a better Lewis acid. Examining three different zinc species 128-130, zinc amide derived from the corresponding oxazolidine 130 was found to lead to a very fast reaction (4 hours, 0°C) and 100% ee (Scheme 2-50). The reaction proceeds even faster at room temperature (completed within 1 hour) without significant loss of stereoselectivity. This reaction can provide excellent ee for aromatic aldehydes,... 

Both the aldehyde and diethylzinc are activated by the zinc amide, and the ethyl group transformation from diethylzinc to aldehyde furnishes the highly enantioselective alkylation of aromatic aldehydes. 

Schiff base ligands are a particularly suitable alternative in ROP of lactide because of their ease of preparation and the possibility to easily tunable steric and electronic properties. Chisholm and coworkers reported Schiff base-supported zinc amide and phenoxide complexes 51a, b (Fig. 10) [75], which catalyze the polymerization of L-lactide in benzene at room temperature yielding 90% conversion in 3 h for 51a and 72 h for 51b. The significant difference in reactivity between the catalysts can be taken from the rate of initiation, which is slower for 51a owing to presence of bulky 2,6-ferf-butylphenoxide leaving group. 

Table 7.1 Selected X-ray structural data for some two-coordinate monomeric zinc amides...

Amphoteric amides, imides, and nitrides. Zinc amide. . Zn(NHa),... 

E R. E. Mulvey, s-Block metal inverse crowns synthetic and structural synergism in mixed alkali metal-magnesium (or zinc) amide chemistry. Chem. Commun. 1049-56 (2001). 

The preparation of alkylzinc iodides is facilitated if the reaction is performed in THF and NMP mixtures. Such solutions of Me02C(CH2)4ZnI add to benzaldehyde in the presence of TMSCI (2 equiv.) in 70% yield.33 The use of ultrasound also promotes the formation of organozinc compounds.34>34a 34e This procedure proved to be especially useful for the preparation of Jackson reagent 6 derived from serine. In the presence of a copper(l) or palladium(O) catalyst, this zinc derivative reacts with various electrophiles (Scheme 1) 3S,3Sa-3Sj 0f z nc reagent 7 leading to methyl but-3-enoate and the zinc amide... 

Amides and imides. These salts, formed only by the more electropositive metals, contain respectively the NHJ and NH " ions. The alkali, alkaline-earth, and zinc amides are colourless crystalline compounds formed directly by the action of ammonia on the molten metal or in solution in liquid ammonia. A solution of NaNH2 in liquid ammonia is a good conductor of electricity, indicating that the salt is ionized in this solvent. 

The chiral amino thioester 9, a thioester analog of DBNE, does not form a zinc amide or a zinc thioate because it does not possess an acidic hydrogen atom. Simple coordination of nitrogen and sulfur atoms to the zinc atom of diethylzinc may generate an efficient chiral catalyst. 

Using the dry Wislicenus process, N-doped zinc azide was made by reacting zinc amide with nitrous oxide at 180°C for 96 hr [85]. The product can be considered pure Zn(N3)2, but an equimolar amount of zinc hydroxide is admixed which was not separated. 

Fig. 281. Preparation of zinc amide. a storage vessel for diethylzinc f glass slug.

Zinc amides 57 [78, 79], 58 [80], and 59 [81] (Figure 4.2), prepared from the corresponding lithium or magnesium amide and zinc chloride, can be used to... 

Zinc amide 59 shows better chemoselectivity (but lower reactivity) and leads to the formation of aryl- and heteroarylzinc chlorides. Notably, it can be used to achieve [82] directed ortho- and meta-zincation of functionalized aryl nonaflates, thus providing suitable substrates for further Negishi cross-coupling reactions. 

Amphoteric behaviour is observed in liquid ammonia just as in water. For example, Zn(OH)2 is amphoteric in water, and similarly zinc amide Zn(NH2)2 is amphoteric in ammonia. In both the cases, complex formation is observed. 

Chisholm, M. H. Phomphrai, K. Conformational effects in -diiminate ligated magnesium and zinc amides. Solution dynamics and lactide polymerization. Inorg. Chim. Acta. 2003, 350, 121-125. 

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