Helically chiral polymers: a class of ligands for asymmetric catalysis

Abstract

Helically chiral polymers from achiral monomers containing N and P atoms have been shown to be ligands for transition metals such as Pd and Rh. The Rh complex of the phosphane-containing polyisocyanate p(18-co-17) was an active albeit hardly enantioselective catalyst in the asymmetric hydrogenation of the dehydro amino acid N-acetamidocinnamic acid (15% enantiomeric excess). The most active catalyst obtained until now was the Pd-complexed polymethacrylate Pd-p12, which catalyzes the allylic substitution reaction of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate even at -20 degrees C in quantitative yield, although again the enantioselectivity was unsatisfactory. The most successful application of a helically chiral polymer in asymmetric catalysis with respect to both reactivity and enantioselectivity is the polymethacrylate p(5-co-8). Its palladium complex catalyzes the above-mentioned reaction at 0 degrees C with quantitative yield and 60% enantiomeric excess.

Scheme 1.

Complexation of poly(3PyDBSMA) (p1) with allyl palladium chloride (15). r.t., room temperature.

Scheme 2.

Allylic substitution of 1,3-diphenylprop-2-enyl acetate (DPPA; 2) catalyzed by palladium complexes of p1, yielding 1,3-diphenylprop-2-enyl dimethyl malonate (DPPMM; 4) (15).

Scheme 4.

Synthesis, helix sense selective polymerization, and subsequent complexation of methacrylate 12 with palladium (p12: n = 19, M_w/M_n = 1.10, >99% isotactic, ) (see supporting information for details).

Scheme 5.

Synthesis of 3DPBPIC (16; see supporting information for details). Boc, t-butoxycarbonyl.

Fig. 1.

Formula (Left) and x-ray structure (Right) of the PB2PyMA complex with palladium (Pd-5) (counterion not shown).

Scheme 3.

Helix sense selective anionic polymerization to poly(PB2PyMA) (p5) (30% yield, n = x + y = 40, M_w/M_n = 1.42, 73% isotactic, ) and poly(TrMA-co-PB2PyMA) p(5-co-8) (77% yield, (x + y) = 65, x/y = 1:1, M_w/M_n = 1.26, 99% isotactic, ) (see ref. ; see also supporting information for details).

Fig. 2.

(A–C) ³¹P-NMR spectra (CD₂Cl₂, 202 MHz, 300 K) of p12 (A), (Pd-p12)_a, 1 hr after deprotection and complexation (B, compare Scheme 4), and (Pd-p12)_b, 12 hr after deprotection and complexation (C, compare Scheme 4). (D–F) ¹H-NMR spectra (CD₂Cl₂, 500 MHz, 300 K) of [Pd(η³-C₃H₅)Cl]₂ (D), (Pd-p12)_a (E), and (Pd-p12)_b (F).

Scheme 6.

Copolymerization of 16 and 17 followed by deprotection with DABCO. DMF, dimethylformamide.

Scheme 7.

Rhodium-catalyzed asymmetric hydrogenation of 19 with a Rh-p(18-co-17) complex generated in situ as catalyst. COD, cyclooctadiene; OTf, trifluoromethanesulfonate.