Complementary binding between U2 snRNA (in an area lying towards the 5' end but 3' to hairpin I) and the branchpoint sequence (BPS) of the intron results in the bulging out of an unpaired adenosine, on the BPS, which initiates a nucleophilic attack at the intronic 5' splice site, thus starting the first of two transesterification reactions that mediate splicing.
Tyr 324 acts as a nucleophile to form a covalent 3’-phosphotyrosine linkage to the DNA substrate.
Ferrocene reacts readily with butyl lithium to give 1,1'-dilithioferrocene, which in turn is a versatile nucleophile.
Usage of a Ni(CO)4 catalyst with CO and water as a nucleophile is known as the Reppe carbonylation, and there are many variations on this type of metal-mediated carbonylation used in industry, particularly those used by Monsanto and the Cativa processes, which convert methanol to acetic acid using acid catalysts and carbon monoxide in the presence of metal catalysts.