Is it correct to say Not to leave him alone, we took him with us too?
I think it's OK to say to put reason to the end: We took him with us too not to leave him alone.
What are other options?
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
It is called "fronting" in English grammar when you want to change a normal or common word order in order to emphasize adjuncts or complements.
Some elements like adjuncts or complements do not typically belong at the beginning of a clause. When we want to focus on them, we bring them to the front or beginning of the clause. We often find this in written literary or formal contexts.
To infinitive in your example indicates "purpose" of the action "to take" and it modifies the action. It is grammatically correct to front the to-infinitive.
Whether it should be "not to leave him alone" or "to not leave him alone" is controversial and both seem to be okay. However, for some idiomatic expressions such as "not to mention", it is not okay to use "to not mention".
As MorganFR mentioned, change the "to infinitive" to "so that clause" is another option:
So that he wouldn't be left alone (So that we didn't leave him alone), we took him with us too.
We took him with us too so that he wouldn't be left alone (so that we didn't leave him alone).