Question: is tosylate a good nucleophile?
Answer: A nucleophile is a species that donates a pair of electrons to form a new covalent bond. A good nucleophile, such as an alkoxide RO⁻, will have a high negative charge on the nucleophilic atom. ... This makes mesylate, triflate, and tosylate poor nucleophiles, but (for the same reason) they are excellent leaving groups.