Silver(I) Phosphate


[7784-09-0]  · Ag3O4P  · Silver(I) Phosphate  · (MW 418.58)

(basic halophilic salt capable of accelerating the rate of palladium-catalyzed Heck reactions,1,2 exhibiting large effects on enantio- or diastereoselectivity, and suppressing product alkene isomerization3,4)

Physical Data: mp 849 °C; d 6.37 g cm-3.

Solubility: slightly sol H2O, acetic acid; freely sol nitric acid; ammonia.

Form Supplied in: yellow, odorless powder.

Handling, Storage, and Precautions: protect from light.

Palladium-Catalyzed Heck Reactions.

Rate Enhancement.

The rate of Heck reactions of aryl or alkenyl halides is typically increased upon addition of silver salts with nonnucleophilic or weakly nucleophilic counterions, including NO3-, OTf-, CO32-, OAc-, and PO43-. Silver phosphate is among the most effective of these salts, and its relative efficiency has been attributed to the basicity of the anion. Its role is likely two-fold. First, the Ag+ likely abstracts the halide from the PdII complex which is generated by oxidative addition of the aryl or alkenyl halide. Halide abstraction produces a cationic PdII complex which binds more efficiently to the alkene which is inserted in the next step of the process.1,2 Secondly, the PO43- is an efficient base for scavenging the proton generated by the b-hydride elimination which terminates the process.

Suppression of Product Isomerization.

Alkene isomerization is frequently a problem in Heck reactions and is undoubtedly caused by readdition-elimination of a palladium hydride species to the product alkene. The removal of the hydrohalic acid by silver phosphate prevents such a readdition and thus effectively suppresses alkene isomerization (eq 1). Silver phosphate is typically the most effective silver salt in the suppression of alkene isomerization.3,4

Effect on Diastereoselectivity.

The diastereoselectivity of Heck reactions can be markedly affected by the presence of silver phosphate (eqs 2 and 3). Such an effect was attributed to coordination of a remote alkene to the cationic palladium complex generated in a silver phosphate-promoted cyclization.5

Effect on Enantioselectivity.

Silver phosphate has been used in the catalytic asymmetric synthesis of cis-hydrindanes6 and cis-decalins7,8 by Heck reactions (eqs 4 and 5). Enantioselectivities with silver phosphate generally exceed those of other silver salts and of non-silver-catalyzed reactions. In one study (eq 1), the addition of silver phosphate completely reversed the absolute stereochemistry of the products with respect to the enantioselectivity observed in the non-silver-catalyzed reaction. Less basic silver salts afforded virtually no asymmetric induction in this study.4

Related Reagents.

Silver(I) Acetate; Silver(I) Carbonate; Silver(I) Nitrate; Silver(I) Trifluoromethanesulfonate.

1. Karabelas, K.; Westerlund, C.; Hallberg, A. JOC 1985, 50, 3896.
2. Karabelas, K.; Hallberg, A. JOC 1986, 51, 5286.
3. Abelman, M. M.; Oh, T.; Overman, L. E. JOC 1987, 52, 4130.
4. Ashimori, A.; Overman, L. E. JOC 1992, 57, 4571.
5. Madin, A.; Overman, L. E. TL 1992, 33, 4859.
6. Sato, Y.; Honda, T.; Shibasaki, M. TL 1992, 33, 2593.
7. Sato, Y.; Sodeoka, M.; Shibasaki, M. CL 1990, 1953.
8. Sato, Y.; Watanabe, S.; Shibasaki, M. TL 1992, 33, 2589.

John Montgomery

Wayne State University, Detroit, MI, USA

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