Can pretax dollars be used for purchase of S corporation shares?

All employment earnings are subject to taxation. The exception is where the tax laws allow pre-tax dollars to be paid, for example, for certain employment benefits. I know of no exception for purchase of shares. However, there may be way in which the physician clients can purchase shares in a more tax-efficient manner, but this would require review of the whole transaction to determine. You should talk with experienced corporate and tax counsel to determine this before proceeding.