It is? I´m confused

or with: olacağı ?

"kaptan olacak birini" is an adjective phrase. So it is correct. It means "someone who is going to be a captain"

Think of it like this:

Büyükelçi düzeyinde görevden almalar olacağı açıklandı.

It has been announced that there will be dismissals from duties at the level of ambassador.

Since this is a passive structure, the entire part "Büyükelçi düzeyinde görevden almalar olacağı" is the subject of the verb "açıklanmak".

I hope this helps!

Then why are there ´-i´ in the following examples (from "TURKISH: A COMPREHENSIVE
GRAMMAR - Aslı Göksel and Celia Kerslake" page 384)

(23) [gönderecekleri] temsilci (cf. Bir temsilci gönderecekler.)
‘the representative [(whom) they will send]’

(26) [Erol-un] (kendi-si-yle) konuş-abil-eceğ-i birisi (cf. Erol öyle birisiyle
konuşabilir.)
Erol-GEN kendi-3SG.POSS-COM talk-PSB-PART-3SG.POSS someone
‘someone [with whom Erol can/could/would be able to talk]’

gönderecekleri temsilci:"The representative who they will send" - "gönder-ecek-leri"  this is an adjective verb. An adjective verb can have possesive suffix "ex: -leri" in Turkish because it can function as a noun like all adjectives. "-ecek" here is not only a tense it is also adjective verb suffix.

konuşabileceği birisi : "Someone who he can talk to" "konuş-abil-ecek-i" this is an adjective verb too. -ecek has the same function here too. The adjective verb has a possesive suffix -i here .

If you want detailed information, you must study "fiilimsiler".

http://www.turkdilbilgisi.com/fiilimsiler/fiilimsiler.html

So in the sentences above there are possesive suffixes not the definitive (?) suffixes.

(?) I dont know how to call them in English

.