Would it not of made more sense to redirect the vast resources from an eastern front to an existing war in Africa with the aim to capture the Iranian oil fields which produced 8 million tons of oil and was vital for the Anglo war effort. It would of resulted in a triple win, direct control over their own supply of oil, denial of a key supply to the allies and change the tide of an active engagement on the African continent.
After Germany declared war on the Soviet Union (SU), the SU and UK declared war on Iran defeating the country in days and taking direct control over the oil and the supply route to the SU. so I am making an assumption Germany would of had similar ease in taking Iraq and then Iran.
I hope the question doesn't revert too much to "Why didn't Hitler redirect the resources to Africa" so perhaps more simply: do we know if it was ever a choice between the middle east and the SU and if so why was the SU prioritised?
The short answer would be that the oilfields of Grosny and Maikop, later on Baku were easier accessable than Irans. For this there are two reasons, logistical prakticality and availibilty.
On the one side the it is the logistical side. We can see in the North African campaign that the Germans were already running on last the reserves. This logistic issues were caused by the Royal Navy dominance of the Medditerreain Sea. Even if the Germans would focus on Middle East they would not have the naval resources to contest British. So the supply situation would be terrible, even worse than the German standard. An option would to go through Turkey. Yet this but Germany in the problem that Turkey might not allow it. They accept the invasion of Greece through Bulgaria but had thier demands and sceptism about the Germans at the borders. So, if the Turks joins axis they have to face Allies (they saw that the axis werent the winning team) and all thier trade, so joing would cost them making it unlikely. If the Turks resited the germans have to fight them through Anatolia and Allies could re-enforce them. Furthermore, Anatolia is hill and mountainous country, not ideal for armoured warfare. If the Germans broke through they had fight further through the middle east to Iran, supplies had to come from Beirut or Palestina, and the infrastructure of the region is worst than that of the soviet union. Taking not away that they had to run thier supplies from Germany over the Balkans, Turkey etc. To supply the front. Than we come the Mountainous region of Iran. Even the British and Soviet with abbanous supplies and logistics system had thier hardships. The Germans haf terrible logistics, not speaking about the lack of material. So, at the end it was not worth the effort extending the fronts to a large extent and weaking thier defense against the soviets in the east and uk/us in the west. Yes, the germans would invade the soviets in this senario but it does not take away that the soviets would invade the germans after 1942/1943.
On the other side, if they manage it, they would be surrounded by UK territory and airfields given the RAF a field day in targetting German oilsupplies. In theory they could do that too with Maikop and Baku, but the caucasus mountains formed a natural barier that limited its effectiveness.
However the bigger issue is that for the Germans, the Soviet were not just another target, they were the big enemy, the large show down of the war. The germans would invade the soviet union, independent of its resources. The invasion was ideologicaly motivated, the lack of resource of the Germans and soviet re-armment forced the Germans to invade earilier. However in the mind of Hitler and rest of the German leadership every thing, every campaign, every invasion had the purpose of making germany ready to invade the soviets. So taking away troops from the east was not an option. Especially as the Germans were already spread thing. Sicily and Normandy, took away little but to much troops from the east weaking the Germans tremendous. The invasion of the middle east would need a large mount of troop, not only to conquer it, but to hold and defend it.
It was simple impossible to carry out and even it was successful the costs to hold it to much for thier gains