The expression "doubtful as to whether" is correct and can be used in written English.
It is used to express doubt or uncertainty about something. For example: I am doubtful as to whether Jane will be able to complete the project by the deadline.
On being doubtful as to whether the fire engines are really going.
Pay consultants are doubtful as to whether this will do much good.
"But I think he was always doubtful as to whether the economics department had hired him because of his Afro-American connections.
They insist that the issue is irrelevant to how they cast their vote, though they are doubtful as to whether the US is ready to elect its first atheist president.
Many in Singapore are also doubtful as to whether Dan Tan really is quite as important as he has been made out to be.
It would take a strong leader to move and people are really doubtful as to whether the current Australian Prime Minister is a strong leader.
My part-robotness is down to the wisdom and expertise of surgeon Simon Holmes, who fought against much doubtful speculation as to whether this was really worthy surgery to undertake.
Whether Wondoloski scores as many again is doubtful though, and there must be a question as to whether Gordon can contribute as much again after another year throwing his battered frame around.
As to whether the Green Dam program works, we are doubtful.
Bardhan et al. suggested that it is doubtful that GLUT transporters also exist in the large bowel and speculated as to whether they can also transport in the opposite direction.
As to whether he was gaining an early-mover advantage in offering such content for sale before many other Web sites, Mr. Tosto said he was doubtful.
