Complex sentence: Michelangelo fell out of favour with the young Alessandro Medici who had been installed as the first Duke of Florence, and fearing for his life, he fled to Rome, leaving assistants to complete the Medici chapel and the Laurentian Library.
Split into simpler sentences: Michelangelo fell out of favour with the young Alessandro Medici, who had been installed as the first Duke of Florence. Fearing for his life, he fled to Rome, leaving assistants to complete the Medici chapel and the Laurentian Library.