Two things.
The backslash character is a special character in Java, so you have to
escape it with a backslash, if you don't you end up using the octal
escape syntax.
In Java the "capturing group n" syntax can only be used in the pattern,
not in the replacement text (and remember to escape the backslash).

You can accomplish what you are trying to do
with look-behind / look-ahead

public class ToPhrase {
    public static void main(String[] args) {
        String s = "ToPhrase".replaceAll("(?<=.)(?=[A-Z])", " ");
        System.out.println(s);
    }
}