You also can book first class on any length D/H if your combined duty time with an operational leg exceeds 11:30, or the combined block time of an operational trip and a D/H, exceeds 8 total block hours.
So, say you operate an ANC-MEM leg with 8 hrs of duty, sit in MEM for 3 hrs, and then get on a MEM-ATL 1 hr D/H for a total of 12 hrs from your initial report time in ANC until your land time in ATL, then you would qualify for a first class seat on the 1 hr D/H.
You would also qualify for first class in this example because the combined block time is over 8 hrs.
CBA 8.4.A
b. A class of service higher than coach shall be authorized if:
i. a deadhead exceeds 5 scheduled block hours; or
ii. a deadhead is included in a single duty period exceeding 11:30; or
iii. the scheduled block hours of a revenue flight plus the scheduled block hours of a deadhead in a single duty period exceed 8 hours.
When you call Corp Travel to book the first class ticket, just tell them it qualifies under one of these contract provisions: they know about them and will book it if you ask them to.