[zarhym]

This is my first time doing Buchwald coupling. The procedure says uses Xantphos but there is only Xphos available. I looked into some literature which suggest Xphos should be good enough for my coupling reaction. (J. Am. Chem. Soc. 125 (22): 6653–6655.) However, the result of using Xphos gives me ~25% conversion over 72hr. The reaction is good since there are only two starting material and a product.

Since there is a big difference between the structures Xphos and Xantphos, I am wonding whether there is a significant difference of the reactivity between these two ligands.

Thanks.

[Dan]

Yes it's significantly different, Xantphos is a constrained, wide bite angle, bidentate ligand. That translates to better catalyst stability (chelate effect) and also favours equatorial coordination.

Xphos can also work, but tBuXPhos may be better (works very well for carbamate arylation, for example). XPhos was the beginning of the Buchwald ligands, which have come quite a long way since then. Xantphos is a good, cheap ligand for many Buchwald-Hartwig reactions, and has the advantage of forming relatively stable complexes, meaning less sensitivity to oxygen. For more difficult systems on small scale, I've had very nice results using Buchwald's palladium pre-catalysts (e.g. tBuXPhox-Pd-G3, Sigma-Aldrich sell them, but they are quite expensive [they're not too difficult to make yourself if you need them in large quantities]).

Tri-t-butylphosphine is another relativley cheap option (see Hartwig's work). For convenience, I'd recommend tri-t-butylphosphonium tetrafluoroborate, which is a stable solid from which the phosphine can be generated in situ.