Push the first sentence as the post title

With this in your theme’s functions.php you can show a custom field with a key “title” as the title (if there is no title):

add_filter( 'the_title', 'meta_title', 10, 2 );
function meta_title( $title, $id ) {
	if ( trim( $title ) == '' ) {
		$meta_title = get_post_meta( $id, 'title', true );
		if ( $meta_title ) {
			$title = $meta_title;
		}
	}
	return $title;
}

Another (less reliable) way is to use a regular expression to try and get all the characters of the post content before the first “.?!” followed by a space.

add_filter( 'the_title', 'first_sentence', 10, 2 );
function first_sentence( $title, $id ) {
	if ( trim( $title ) == '' ) {
		$post = get_post( $id );
		if ( isset( $post->post_content ) && $post->post_content ) {
			if ( preg_match( '/(.*?[?!.](?=\s|$)).*/', $post->post_content ) ) {
				$title  = preg_replace( '/(.*?[?!.](?=\s|$)).*/', '\\1', $post->post_content );

			}
		}
	}
	return $title;
}

Don’t use both code samples at the same time.