Zinc Acetate-Promoted Buchwald−Hartwig Couplings of Heteroaromatic Amines
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Regioselective manipulation of hydroxyl groups in polyols has been critical to carbohydrate chemistry.
A robust selective allylation of 1-methyl-L-α-rhamnose was developed using n-Bu2SnO as the catalyst and proton sponge as optimal base for high regioselective control. The optimized condition afforded the 3-O-allylated rhamnose in excellent regioselectivity (> 20:1) and 82 % isolated yield on a 50 g scale (scheme).
A scalable isolation/purification process was developed which afforded the desired product in 72 % yield with 1530 ppm of Sn. The residual Sn level can be further reduced to < 20 ppm by treatment with SiliaMetS Cysteine & TAAcONa (Table 1).
| Residual Sn (ppm) | |||
|---|---|---|---|
| Experimental Step | Starting Level | End Level | Recovery (%) |
| Extractive Work Up | - | 1,530 | 72 |
| 1st treatment 20 wt % w/ SiliaMetS Cysteine | 1,530 | 273 | 92 |
| 2nd treatment 20 wt % w/ SiliaMetS Cysteine | 273 | 31.9 | 93 |
| 1st treatment 20 wt % w/ SiliaMetS TAAcONa | 1,530 | 267 | 91 |
| 2nd treatment 20 wt % w/ SiliaMetS TAAcONa | 267 | 19.5 | 95 |
This catalytic system also showed outstanding potential in the selective benzylation of the same substrate.
The isomeric purity remained unchanged after the scavenger treatment.
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SiliaMetS TAAcONa nec (R69230B)
SiliaMetS TAAcONa or (or SiliaMetS Triaminetetraacetate, sodium salt) is a silica bound metal scavenger for Pd(II), Ni(II) and Cu. It is the supported version of EDTA in its sodium salt form. It is an effective scavenger for metals in higher oxidation state, 2+ or higher. -
SiliaMetS Cysteine (CYS) (R80530B)
SiliaMetS Cysteine (Si-Cys) is the silica bound equivalent of the amino acid Cysteine. It is a versatile metal scavenger for a variety of metals including Pd, Sn, Ru, Pt, Cu, Rh, Cd and Sc under a wide range of conditions and the preferred metal scavenger for tin residues.
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