Info

35 36

Scheme 6. Polymer-supported organotin reagents as useful intermediates for C-C bond-forming reactions.

electrophiles, e.g., acid chloride 34. Product 37 was obtained in good yield, although the reaction time was considerably longer than in solution.

Procedure

Hydrostannation of a terminal alkyne [30]: 2 equiv. of the alkyne 34 and 0.08 equiv. of AIBN were added to the tin hydride resin 32 in dry toluene under argon.The mixture was slowly stirred for 20 h at 60° C. Every 2.5 h, portions of AIBN (4 ( 0.08 equiv.)) were added. The polymer was separated from the solvent in an argon frit, washed several times with dry toluene and ether and dried.

Grignard reaction [30]:The dry tin halide resin 31 was swollen under argon in dry ether at 0° C for 30 min.The Grignard reagent (5 equiv.) was dissolved in dry ether and added to the suspension of resin 31 over 1 h. After 15 h at ambient temperature, the polymer was filtered off and washed with ether and water.The polymer was then extracted with reflux-ingTHF:water (2:1) for 8 h, with refluxing ether for 4 h and then dried in vacuo. Stille reaction [30]:Jo phenylethyltin resin 33 (1.33 g, 2.0 mmol) in dry toluene (8 mL) were added Pd(PPh3)4 (23 mg, 0.02 mmol) and propionyl chloride 4 (0.28 g, 2.9 mmol).The mixture was stirred and heated to 80° C for 1 h. Filtration and washing of the resin gave the clean product 37 (83 %).

4.2.1.2 PSR Prepared by Polymerization

The covalent linkage between the support and the reactive moiety can be built up not only by solid-phase chemistry (as described in Section 4.2.1.1), but also by co-polymerization of, e.g., divinylbenzene and a co-monomer carrying the reactive moiety used for the chemical transformation of a molecule in solution (see Fig. 2). The latter method has often been used for the immobilization of chiral catalysts. Three groups of supported catalysts are described in the following section: borane amino alcohol complexes for asymmetric Diels-Alder reactions; quinine-derived systems for the asymmetric dihydroxylation reaction; and polymer-bound (salen)-complexes for the enantioselective epoxidation of olefins.

Supported Borane Amino Alcohol Complexes and other supported Lewis Acids

Itsuno et al. have reported the use of polymer-supported oxazaborolidinone 38 as chiral catalysts for asymmetric Diels-Alder reactions (Scheme 7) [31]. The catalyst was synthesized from borane dimethylsulfide and a chiral polymer bearing N-sulfonylamino acid 39. The latter was prepared by co-polymerization of chiral sulfonamide 40 with styrene 41 and the crosslinking agent 42 in the presence of benzoyl peroxide as radical initiator.

benzoyl peroxide | Crosslinkage

Was this article helpful?

0 0

Post a comment