Hydride scandium

Related to the nitrile oxide cycloadditions presented in Scheme 6.206 are 1,3-dipolar cycloaddition reactions of nitrones with alkenes leading to isoxazolidines. The group of Comes-Franchini has described cycloadditions of (Z)-a-phenyl-N-methylnitrone with allylic fluorides leading to enantiopure fluorine-containing isoxazolidines, and ultimately to amino polyols (Scheme 6.207) [374]. The reactions were carried out under solvent-free conditions in the presence of 5 mol% of either scandium(III) or indium(III) triflate. In the racemic series, an optimized 74% yield of an exo/endo mixture of cycloadducts was obtained within 15 min at 100 °C. In the case of the enantiopure allyl fluoride, a similar product distribution was achieved after 25 min at 100 °C. Reduction of the isoxazolidine cycloadducts with lithium aluminum hydride provided fluorinated enantiopure polyols of pharmaceutical interest possessing four stereocenters. 

In this equilibrium (a a bond metathesis reaction ), the scandium-hydrogen bond in to(pentamethylcyclopentadienyl)scandium hydride is replaced by a scandium-carbon bond in phenylto(pentamethylcyclopendienyl) scandium... 

Hydrogen reacts at elevated temperatures with many transition metals and their alloys to form hydrides. The electropositive elements are the most reactive, that is, scandium, yttrium, the lanthanides, the actinides and members of the titanium and vanadium groups (Figure 5.20). 

A well-defined, single-component Ziegler-Natta catalyst is the amido-cyclo-pentadienyl scandium hydride [(CsMe4SiMe2N-t-Bu)Sc(PMe3)(/r-H)]2 which was described by Bercaw et al. [60],... 

Oxasilacyclopentenes were shown to be competent substrates for a scandium triflate-catalyzed Mukaiyama aldol process (Scheme 7.35).104 Exposure of silacy-clopentene 121 and benzaldehyde to substoichiometric amounts of scandium triflate produced ketone 122 diastereoselectively.105 This ketone was proposed to form by addition of enolate 123, resulting from desilylation of 121,106 to benzaldehyde. A 1,3-Brook rearrangement then afforded 122 from 124.107 This ketone could be further functionalized through lithium aluminum hydride reduction followed by deprotection to afford triol 125 containing four contiguous stereocenters. Thus, the molecular complexity of silyloxyalkynes can be increased dramatically in just three operations. 

Two notable points from the aforegoing discussion are as follows. (1) By far the majority of the known monohalogenoalkyl compounds are of the group VTII transition elements there are very few early transition metal halogenoalkyl compounds known. (2) Very few monofluoroalkyl metal complexes have been prepared. The lack of early transition metal halogenoalkyl compounds may be in part due to the high electropositivity of these metals, which facilitates a- and / -elimination reactions. Related hydride elimination reactions almost certainly occur more easily for early transition metal alkyl compounds than for later transition metal compounds. In this regard it is particularly noteworthy that one of the only early transition metal haloalkyl compounds mentioned is the fluoroethyl scandium com-... 

Novel and efficient [2 - - 3] cycloaddition reactions of NADH analogues with Q derivatives rather than the hydride-transfer reactions occur in the presence of scandium triflate, Sc(OTf)3, in MeCN to afford the cycloadducts (142). When 1 -benzyl-4-tert-butyl-1,4-dihydronicotinamide (t-BuBNAH) is used as an NADH analogue in the Sc +-catalyzed reaction with Q, the crystal structure of the cycloadduct was determined successfully, as shown in Fig. 53 (142). 

Both scandium and yttrium form borohydride complexes [M(BH4)3(thf) ] with thf coordinated to the metal.53 In each compound two borohydride groups are bound through three hydride bridges and one borohydride is bound through two hydride bridges. The scandium compound is... 

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