Complex sentence: Iemochi was first the head of the Tokugawa family of Kii, under the name Tokugawa Yoshitomi, but was named as the successor to the main Tokugawa house due to the 13th shogun, Iesada, being heirless.
Split into simpler sentences: Iemochi was first the head of the Tokugawa family of Kii, under the name Tokugawa Yoshitomi. However, he was named as the successor to the main Tokugawa house due to the 13th shogun, Iesada, being heirless.