Cerium ammonium nitrate decomplexation

Methyldithiane lithium adds to [Cr(CO)3(l,4-dimethoxynaphthalene)] irreversibly at the (3-position. Protonation/decomplexation in this case has been carried out with cerium ammonium nitrate in THF/H2O (9 1) to give a daunomyci-none precursor (Scheme 8) [28]. 

The cyclohexadiene complex 29 has been further elaborated to afford either the cydo-hexenone 34 or the cyclohexene 36 in moderate yields (Scheme 1) [21]. The addition of HOTf to 29 generates the oxonium species 33, which can be hydrolyzed and treated with cerium(IV) ammonium nitrate (CAN) to release the cyclohexanone 34 in 43 % yield from 29. Alternatively, hydride reduction of 33 followed by treatment with acid eliminates methanol to generate the r 3-allyl complex 35. This species can be trapped by the conjugate base of dimethyl malonate to afford a cyclohexene complex. Oxidative decomplexation of this species using silver trifluoromethanesulfonate liberates the cyclohexene 36 in 57 % overall yield (based on 29). 

In a nice illustration of the impact of metal coordination upon the reactivity of phospholes, a methodology for the functionalization of these heterocycles in the /3-position has been described (see also Scheme 22) <2001JOM105>. Here, coordination of both the P-lone pair and the cyclic diene system was undertaken. The resulting multimetallic complex 79 was treated with lithium diisopropylamide (LDA) to afford the lithium salt 350 (Scheme 118). This readily undergoes nucleophilic substitution with a variety of electrophiles to afford the corresponding substituted phosphole complexes 351-353. The free phospholes can be isolated following decomplexation with cerium(iv) ammonium nitrate (CAN). 

A diastereoisomeric mixture of dicobalt hexacarbonyl complexes 33 reacted with trifluoroborane etherate at -20 °C to give the reduced product 3.3 (minus a benzyl ether) as a single diastereoisomer after decomplexation of the metal with cerium(IV) ammonium nitrate. Suggest a mechanism for the formation of 33 which accounts for the stereochemistry of the product. 

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