Zirconium complexes azides

Along with the development of chiral Lewis acid catalysts, a chiral trialkanolamine (42) has been used to prepare the catalyst (43) (Eq. 19). By use of this zirconium complex as a catalyst, enantioselective addition of the azide to meso epoxides was achieved [20a]. Thus, the oxirane ring was opened by /-PrMe2SiN3 to give the adduct (44) with high enantioselectivity (Eq. 20). In another example, a diamide ligand (45), which behaves as a tetradentate ligand, was used to achieve a similar reaction (Eqs 21 and 22) [20b]. 

Another related zirconium complex 32 containing redox active tridentate NNN pincer ligand catalyzes nitrene transfer reactions from organic azides to f rf-butyl isocyanide to form non-symmetrical carbodiimides where the metal maintains the Zr(IV)... 

In 1992, Nugent [12] reported a significant advance in the Lewis acid-promoted opening of meso epoxides (Scheme 2). A novel chiral zirconium complex catalyses epoxide opening with hindered silyl azides iii excellent ee. The catalyst is derived from zirconium tert-h xi-oxide and the tetradentate C3-symmetric ligand 1, available by combination of (5)-pro-pene oxide with (S)-l-aminopropan-2-ol. Al-... 

Highly enantioselective catalytic desymmetrization of meso-epoxides through nucleophihc ring opening was first effectively demonstrated by Nugent, who found that a zirconium trialkanolamine complex catalyzed the addition of azi-dosilanes to meso-epoxides (Scheme 1) [3]. Azide has been the most widely explored nitrogen nucleophile [4,5,6,7], in part due to its utiHty as an amine sur-... 

Nugent adapted the zirconium trialkanolamine complex developed for the enantioselective addition of TMSN3 (Scheme 1) to catalyze the addition of bromide, where substitution of bromide for azide at the metal center was proposed to account for the epoxide halogenation product (Scheme 15) [29]. When a large excess of the bromide source was used to suppress azido alcohol formation, a wide range of cyclic epoxides reacted with 5 mol % of the catalyst to afford good yields of the bromohydrins in 84-96% ee. 

Carhon nucleophiles of pXa 10-20 have heen most studied and often work hest with these substrates (eq 1), hut enamines, cyclopentadiene anions, enolates (eq 2), organotin, -thaUium, -zinc, -aluminum, -lithium, and -zirconiums, as weU as Grignards (eq 3) and horates, also add to r-allylpaUadium complexes. Heteroatom nucleophiles also add successfully these include amines (eq 4), amides, azides, magnesium amides, sulfonamides, alcohols, acids (eq 5), nitrites, sulhnic acids, thiols, phosphines, and phosphites. Limited use of transition metal nucleophiles is also known. ... 

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