The seventh son of Munetake TOKUGAWA of the Tayasu family, Masamaru (later Sadanobu MATSUDAIRA), was famous for his intelligence since childhood and considered to be a successor, but roju Okitsugu TANUMA, an influential person in those times, and Harusada TOKUGAWA (the head of Hitotsubashi family) who backed up his son Ienari TOKUGAWA made him being adopted as a son of Sadakuni MATSUDAIRA, the lord of Shirakawa Domain in Mutsu Province.
田安家徳川宗武の七男・松平定信（後の松平定信）が幼少の頃から英明を謳われ、後継と目されたが、時の権力者・老中田沼意次や、子の徳川家斉（後の家斉）を推す徳川治済（一橋家当主）らの画策で、陸奥国白河藩主・松平定邦の養子にさせられたとも言われる。