SBUX at the bottom of the street you are on if you come out of the GHT and turn left is open from 7am and there one in Azabu-Juban, turn right at the bottom of the hill where the 7am SBUX is opens at 6. No need for anything before that really. 5am no offices are open anyway, and if they are there is coffee there. Japanese firms all tend to build their schedules around the transportation links, i.e. when public transportation is available. People, in general, do not drive to work here. This is the same reason most gyms do not pen before 7am as the workers need to get there.