Sorry about the issues...hopefully it will recover and its just from transplant shock...I do know that they like water expecially right after planting (you should never let the soil dry out when you first transplant a new fruit tree and Longans and Lychee are more water loving fruit trees than some...

Did you water the tree everyday since you planted it? Especially since its been SO hot lately...it might even be necessary to water it 2x a day if the temps are above 90+ and the humidity is low and you have it in direct sun?

Best of luck and I'm betting it will recover...just keep the soil "moist" till the tree shows some stability...You "might" lose all the leaves but if the tree's roots are okay and not damaged it should "regrow" some new leaves in a few weeks or so...

Good luck!!

I've read online that longan goes dormant after flowering/fruiting.

I've noticed the old leaves on my longan were yellowing, but the new growth wasn't.

I do not know what Vitamin B-1 liquid fertilizer is, but I do know that I once killed a young Emperor Lychee when it was freshly planted in the ground by placing a fertilizer spike next to it. I think for Lychees especially and probably also Longans, they may not like too much fertilizers especially when young. Now, I only give my lychees organic fertilizers, but sparingly ... I guess I'm too afraid that I might hurt them again.

Then again, it may not be the fert you use ... I think the lack of water in the hot weather makes a lot of sense to me. That said, most plants do have a strong will to survive. If there are any dried off/dead leaves or branches, it's best to remove them so as to rejuvenate new growth. I hope your longan will come around for you soon.