Arduino Leonardo programming with AVRdude

Hello,

I am new in Arduino, and I would like to program with bootloader my newest leonardo board (CDC-ACM serial).

I can program it with arduino IDE, but I would like to use my own hex files.

I press the button, flashing the led with boot mode, In the device manager I see the proper COM port. But I receive this error:

avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0x3f

my command:
avrdude -F -cstk500v1 -pm32u4 -P COM15 -b19200 -v

I know that the flashing is missing, but it doesn't connect.

Any idea?