Why didn't Doctor Strange chop Thanos' arm off?

There was a scene in Infinity War where one of the Black Order (the Hulk wannabe) was about to kill Iron Man. Before it happens, Wong opens up a portal and he falls through, when he attempts to get back out though Wong closes it and it chops his arm off.

So why when they had restrained Thanos did Doctor Strange not open up a portal around Thanos and then close it around his arm to sever it and get the Infinity Gauntlet? Or he could have sent him into the sun perhaps.