Use Conditional Formatting in Google Sheets to highlight cell

Problem

I want to be able to highlight a cell in google sheets if a cell on the same row is TRUE and the value does not exist in a named range (or a sheet).

e.g.

Based on the scenario below in sheet1, if column a is TRUE, and the value in column b does not match anything in sheet2, highlight the cell red.

sheet1
column a     column b
TRUE         value1
FALSE        value2

sheet2 
column a 
value2

I would like to do this in google sheets using conditional formatting custom formulas and named ranges (if required).