You cannot deduct it as alimony, unfortunately.
To be deductible as alimony, it must be made in cash and must be classified as alimony in your divorce. An allocation of 401(k) plan is considered property settlement, and property settlement is not alimony.
Below link, page 18 discusses the situation in detail:
https://www.irs.gov/pub/irs-pdf/p504.pdf
