In a food processor, pulse the tomatoes with their juices until finely chopped. In a large saucepan, heat the olive oil over moderately high heat. Add the basil leaves and cook until they begin to wilt, 10 seconds. Add the pinch of crushed red pepper and season with salt and pepper. Add the tomatoes, bring to a boil and cook, stirring, until slightly thickened, about 6 minutes. Remove from the heat and stir in the unsalted butter. Season the sauce with salt and pepper and toss with pasta.