solved:
[status,result] = system(['"C:\Program Files\7za920\7za.exe" -y x ' '"' filename{f} '"' ' -o' '"' outputDir '"']);
Also, just in case anyone finds it useful, compressing a file with winrar is done by:
[status,result] = system(['"C:\Program Files\WinRAR\Rar.exe" a ' 'C:\Users\user\Desktop\8736_test.rar ' ' C:\Users\user\Desktop\8736.mat' ' >null']);