Is there a database on chemical reactions, similar to NIST, but far more complete?
The best known databases are Beilstein and Chemical Abstracts with around 22 millions reactions each since the end of the 19th century but they are commercial.

Organic Syntheses has made available its database for free and it covers 6000 reactions. One of the greatest advantages is that each step is described in depth and each reaction has been thoroughly tested prior to publication.

Have a look here for a list of databases.