The abiotic, regioselective conjugation of peptides and proteins with non-proteinogenic structural elements requires mild and fast coupling reactions which are compatible with an aqueous reaction medium and orthogonal in their reactivity with all other functional groups in the protein. Sonogashira coupling with a palladium-guanidinophosphane catalyst that is prepared in situ (see reaction on the right) complies with these demands.

Cationic phosphine ligands containing m-guanidinium phenyl substituents {Ph(3-n)P[C(6)H(4)-m-NHC(NH(2))(NMe(2))](n)}(n+) nCl(-) (n = 1-3) (17a-c) have been obtained by addition of dimethylcyanamide to the amino groups of tertiary (m-aminophenyl)phosphines in acidic medium. The tertiary (m-aminophenyl)phosphines Ph(3-n)P(C(6)H(4)-m-NH(2))(n) (4a-c) were prepared by reaction of (3-[N,N-bis(trimethylsilyl)amino]phenyl)magnesium chloride (1) with chlorophosphines Ph(3-n)PCl(n) followed by deprotection of the bis(trimethylsilyl)amino groups with methanol. Using a similar protected group synthesis as above, the secondary (m-aminophenyl)phosphine Ph(H)PC(6)H(4)-m-NH(2) (7) could be prepared as well. It may be employed as a building block for the syntheses of chiral bidentate phosphine ligands (11, 14, and 15) bearing m-aminophenyl substituents. The guanidinium phosphines 17b and 17c are readily soluble in water. A comparative study of 17b and 17c, the aryl alkyl guanidinium phosphines 18 and 19, and TPPTS (P(C(6)H(4)-m-SO(3)Na)(3)) in the aqueous phase palladium-catalyzed C-C coupling reaction between p-iodobenzoate and (trifluoroacetyl)propargylamine shows 17b to be of surmounting activity.

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