conditional insert with values; insert into .. values(...) where not exist (....);

This is what I wish I could to do.

insert into table_x (a, b, c)
  values(1, 2, 3)
    where not exist (select * from table_x where a = 2);

Thank you.

additional info:
I've seen conditional inserts without values(....), as in

INSERT INTO clients

            (client_id, client_name, client_type)    SELECT supplier_id, supplier_name, 'advertising'      FROM suppliers
    WHERE NOT EXISTS (SELECT *

                        FROM clients
                       WHERE clients.client_id =