I was rather surprised to read that Stalin had his representative in Stockholm offer separate peace negotiations in December 1942 and again in summer 1943 culminating in definite proposal in September 1943. Source is claimed as Goebbels Tagebucher 1942-43 p 392 and Zwischen London und Moskau p 265. The basic offer from Russia was restoration of 1914 borders and economic ties. It is beleived that Stalin was concerned that the West was waiting in the wings for Russia and Germany to exhaust each other and then to step in scoop up the spoils. Hitler apparently turned down the offers flat - told Goebbels he was only prepared to negotiated after an emphatic victory.