Transition-metal derivatives catalytic reactions involving

Formal hydration of the double bond appeared by the hydroboration-oxidation sequence. Desymmetrization reactions with catalytic asymmetric hydroboration are not restricted to norbornene or nonfunctionalized substrates and can be successfully applied to meso bicyclic hydrazines. In the case of 157, hydroxy derivative 158 is formed with only moderate enantioselectivity both using Rh or Ir precatalysts. Interestingly, a reversal of enantioselectivity is observed for the catalytic desymmetrization reaction by exchanging these two transition metals. Rh-catalyzed hydroboration involves a metal-H insertion, and a boryl migration is involved when using an Ir precatalyst (Equation 17) <2002JA12098, 2002JOC3522>. 

Metal-catalyzed substitution reactions involving propargylic derivatives have not been studied in much detail until recently [311, 312]. In this context, the ability shown by transition-metal allenylidenes to undergo nucleophilic additions at the Cy atom of the cumulenic chain has allowed the development of efficient catalytic processes for the direct substitution of the hydroxyl group in propargylic alcohols [313]. These transformations represent an appealing alternative to the well-known and extensively investigated Nicholas reaction, in which stoichiometric amounts of [Co2(CO)g] are employed [314-317]. 

Organometallic methods, with the possible exception of those involving the stoichiometric generation of enolates and other stabilized carbanionic species 140], have seldom been used in carbohydrate chemistry for the synthesis of cyclohexane and cyclopentane derivatives. The present discussion will not cover these areas. The earliest of the examples using a catalytic transition metal appears in the work of Trost and Runge [41], who reported the Pd-catalyzed transformation of the mannose-derived intermediate 22 to the functionalized cyclopentane 23 in 98% yield (Scheme 10). Under a different set of conditions, the same substrate gives a cycloheptenone 24. Other related reactions are the catalytic versions of the Ferrier protocol for the conversion of methylene sugars to cyclohexanones (see Chap. 26) [40,42,43]. 

As mentioned earlier, acyl derivatives of transition metals are probably intermediates in some catalytic carbonylation reactions involving molecular hydrogen. Significantly, tertiary phosphine-substituted acyl derivatives of cobalt(I) react with Hj at elevated pressure and ambient temperature in pentane to give aldehydes ... 

In Sect. 2.3, generation of silylene complexes of transition metals was discussed on the basis of the reactivity of disilanyl-transition-metal complexes. The formation of silylene species in the presence of a catalytic amount of transition metals is also involved in the reactions of hydrodisilanes, which may readily form disilanyl complexes through oxidative addition of the Si-H bond prior to the activation of the Si-Si bond. Platinum-catalyzed disproportionation of hydrodisilanes affords a mixture of oligosilanes 89 up to hexasilane (Eq.45) [83]. The involvement of silylene-platinum intermediate was proven by the formation of a l,4-disila-2,5-cyclohexadiene derivative in the reaction of the hydrodisilane in the presence of diphenylacetylene. 

Reaction of pentafluoroacetophenone with hexamethyldisilane for 20 h in toluene in a 130°C oil bath in the presence of 10 mol% Rh(cod)2BF4gave a 79-88% yield of 2,3,4,5-tetrafluoro-6-trimethylsilylacetophenone. Not surprisingly, 2,6-difluoroacetophenone affords the mono-Me3Si-F exchange product in somewhat lower yields (33-48%). In the case of a related oxazoline derivative some disilylation accompanied the mono-substituted product. The authors propose a chelate assisted mechanism for the initial C-F activation step [62]. It is interesting to note that these catalytic reactions all involve the later transition metal rhodium with a relatively labile Rh-F bond removed as R3Si-F or HF. However, related catalytic reactions of aromatic C-F bonds have also been discovered for early transition metals and even in the very electropositive lanthanide series. 

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