INDOLPhosphole and INDOLPhos Palladium−Allyl Complexes in Asymmetric Allylic Alkylations

Abstract

The new INDOLPhosphole ligands 3a,b are obtained in good yield in a two-step synthetic sequence from 3-methylindole, (S)-BINOL, and the corresponding cyanophosphole. Palladium-allyl complexes have been prepared from INDOLPhosphole (3b) and INDOLPhos (1a) of the type [Pd(INDOLPhos(phole))(η 3 -C 3 H 5 )]PF 6 and studied by multidimensional NMR spectroscopy and X-ray crystallography. The allyl ligand undergoes a η 3 -η 1 -η 3 isomerization in these complexes, which is selective when 1a is the ligand. A tetrameric, boxlike structure, encapsulating a PF 6 counteranion, is formed in the solid state in the case of complex 5 ([Pd(3b)(η 3 -C 3 H 5 )]PF 6 ). INDOLPhosphole ligands 3a,b and a small library of INDOLPhos ligands were screened in the Pd-catalyzed asymmetric allylic alkylation of mono-and disubstituted allylic acetates. The catalysts derived from these ligands were highly active, and enantioselectivities were obtained for 1,3-diphenylprop-2-enyl acetate up to 90% ee. Cinnamyl acetate was converted quantitatively with low regioselectivity (b/l ) 14/86) and good enantioselectivity up to 81% ee. In the case of disubstituted substrates, the absolute configuration of the product could be rationalized by a model of selective attack on one of the enantiotopic termini in the Pd-allyl intermediate.

Better Performance of Monodentate P‐Stereogenic Phosphanes Compared to Bidentate Analogues in Pd‐Catalyzed Asymmetric Allylic Alkylations

“…Complexes of type 3 with two monodentate phosphanes showed higher activity than those we observed with dithioethers or bis(oxazolines), and similar activity to that reported for phosphane-oxazolines [37] or phosphane-phosphoramidite ligands [38] (entries 1-6, Table 5). Complexes 4 and 5 containing bidentate phosphanes showed lower rates of reaction, which is consistent with the findings of Imamoto [14] and Mezzetti.…”

Diphosphorus Ligands Containing a P‐Stereogenic Phosphane and a Chiral Phosphite or Phosphorodiamidite – Evaluation in Pd‐Catalysed Asymmetric Allylic Substitution Reactions

“…The distances and angles are similar to those for comparable complexes. [14d] The Pd–C allyl distance trans to the phosphorodiamidite fragment is longer than the Pd–C allyl distance trans to the phosphane moiety, as observed for complexes containing comparable phosphane–phosphoramidite ligands . The two P–N bond lengths are different and in the range found for similar compounds.…”

“…The same trend has been found for comparable complexes. [27] The X-ray structure of the phosphane-phosphorodiamidite complex Pd13 is depicted in Figure 3. terminal atoms of the methallyl fragment.…”

Palladium-Catalyzed Enantioselective Allylic Substitution

“…6), both versions of L25 with X ¼ C(O) or X ¼ CH 2 gave the same results, probably due to the distance between the binding sites and the site of variation. In the case of IndolPHOS L26 [53], the enantioselectivity increased with increasing steric hindrance of both the phosphine and the phosphoramidite units. With IndolPhospholes L27, the two reported examples did not allow for trends in the influence of the bulkiness of the phosphole unit to be identified (Table 15).…”