Allylic copper alkoxide

Scheme 18 Allylic substitution reactions between copper alkoxides and allylic carbonates...

Disubstituted dihydrofurans and dihydropyrans were prepared via allylic etherification [68] in a similar manner to dihydropyrroles (cf Section 9.4.6). Thus, diaste-reoisomeric ethers were generated by the reaction of cinnamyl tert-butyl carbonate with the copper alkoxide prepared from (Rj-l-octen-3-ol, depending on which enantiomer of the phosphoramidite ligand was used (Scheme 9.39). Good yields and excellent selectivities were obtained. RCM in a standard manner gave cis- and trans-dihydrofuran derivatives in good yield, and the same method was used for the preparation of dihydropyrans. 

Similar to rhodium, copper mediates retro-allylation when it is complexed with an N-heterocyclic carbene ligand [31]. The allyl transfer takes place not only to aromatic aldehydes but also to aromatic imines (Scheme 5.43). Notably, secondary homoallylic alcohols transfer their allyl groups, retro-allylation dominating over P-hydride elimination from the copper alkoxide intermediates. 

Tab. 10.8 summarizes the application of rhodium-catalyzed allylic etherification to a variety of racemic secondary allylic carbonates, using the copper(I) alkoxide derived from 2,4-dimethyl-3-pentanol vide intro). Although the allyhc etherification is tolerant of linear alkyl substituents (entries 1-4), branched derivatives proved more challenging in terms of selectivity and turnover, the y-position being the first point at which branching does not appear to interfere with the substitution (entry 5). The allylic etherification also proved feasible for hydroxymethyl, alkene, and aryl substituents, albeit with lower selectivity (entries 6-9). This transformation is remarkably tolerant, given that the classical alkylation of a hindered metal alkoxide with a secondary alkyl halide would undoubtedly lead to elimination. Hence, regioselective rhodium-catalyzed allylic etherification with a secondary copper(l) alkoxide provides an important method for the synthesis of allylic ethers. 

Tab. 10.10 Probing the role of lithium iodide and the copper(l) alkoxide in allylic etherification.

More recently, a copper-bifluoride complex has been described, starting once again from the alkoxide species. Subsequently, [Cu(NHC)2] [HF2] complexes were developed (Figure 8.2) [91]. Both complexes were highly air stable in the solid state and moderately stable in solution compared to their analog [Cu(F) (IPr)] [92]. Remarkably, catalytic activity was observed in several transformations such as the reduction of ketones, the 1,4-conjugated borylation and silylation, and the diastereoselective allylation. 

The 1,2-adducts (32) of 2-litho-l,3-dithians and cyclohex-2-enone can be isomerized to the more stable 1,4-adducts (33) by conversion into the potassium alkoxides (but not the less dissociated sodium or lithium salts).Direct formation of enone 1,4-adducts occurs when HMPA-THF is used as the solvent, and enolate trapping can be effected when the initial enone 1,4-adduct is treated with methyl iodide.The allylic anion derived from (34) undergoes cr-1,4-addition to cyclohexenone in the presence of Cul (Scheme 2). In the absence of the copper salt, both y-1,4- and cr-1,4-addition occur, with the former predominating. In contrast, allylation of (34) in the presence of Cul occurs primarily at the y-position, while cr-allylation is observed otherwise.The lithio-anion of 2-(yff-styryl)-... 

The stereochemical courses of the copper-catalysed allyl-allq l coupling between enantioenriched chiral allylic phosphates (60) and allqrlboranes (61) have been described as switchable between 1,3-anti and 1,3-syn selectivities by the choice of solvents and achiral alkoxide bases with different steric demands (Scheme 20). Moreover, the synthetic protocols allowed the stereoselective conversion of silicon-substituted allylic phosphates into enantioenriched chiral allylsilanes with tertiary or quaternary carbon stereogenic centers. Thus, both enantiomers of the allylsilanes with high enantiomeric purities were readily available from one substrate enantiomer. ... 

---