Can I claim a loss (over $100K) on my 2018 tax return if I was divorced in 2015 and as part of my agreement had to transfer 50% of my rollover IRA which I did in 2018?

[TomD8]

No.  Generally, no gain or loss is recognized on a transfer of property from you to (or in trust for the benefit of):

• Your spouse, or
• Your former spouse, but only if the transfer is incident to your divorce.

This rule applies even if the transfer was in exchange for cash, the release of marital rights, the assumption of liabilities, or other consideration.

https://taxmap.irs.gov/taxmap/pubs/p504-005.htm