After Operation Barbarossa the Soviets retained a significant number of troops in the Far East to deter against Japanese attack.
What if Hitler and the Japanese had become more trusting of each other and communicated more? What if Hitler had told the Japanese that when he had defeated the USSR, then Germany would take control of the Middle East before then helping the Japanese in a joint German-Japanese invasion of India? But he makes clear to the Japanese that he needs them to act threateningly (not attack) the USSR for his plan to succeed. This then results in the Japanese acting threateningly towards the USSR in the Far East from June 1941 onwards, which makes Stalin convinved the Japanese will attack Siberia. Therefore, during the Battle of Moscow, there can be no reinforcements from the Far East, due to the threat of Japanese invasion.
Stalin has already decided that he will defend Moscow and has told the Politburo this. With no Soviet reinforcements, the Germans reach central Moscow.
What does Stalin do then - does he kill himself? Allow himself to be captured by the Germans? Or flee to Central Asia or Siberia?