can bipolar surround speaker on backwall with 5.1

Yes, but get ones intended for wall placement. Most intended for HT use should work. Most two channel speakers are designed to be used out into the room and the sound is affected if placed too close to wall. You are going to have trouble with the window side of your room, will be very reflective. Have you consideded an omni speaker like the Shanian Compass? Not 5.1 but would give you a large non directional sound that might work well in your room.

I'd really like to know your entire layout, the other speakers you are using (crucial), and overall acoustics of your space. Not knowing all of that, my first question would be to know what L/R/C speakers you're using? I'd then match to those. In your case, I'd default to maximizing the main front speaker setup - and integration of the subwoofer (all considering good acoustic fundamental practices) - and strongly consider doing in-ceiling speakers for rears.
I would be doing in-ceiling because the mains are most important in anchoring the soundstage, most of the dynamics, imaging, etc. You really want just some steering and ambience from the rears, regarless. If you put speakers behind you, you'll be having a big gap with transitions from front to back. Overhead is less localizable, you have more flexibilty, and will transition from front to back better than back wall only.
A better option for the money would be matching dipoler speakers mounted above and to the sides of your seating on the ceiling, just like in-ceiling. However, care and caution must be used in mounting on ceiling. So anchor wisely.
Yes, my final is to find MATCHING rears to your mains, and put em on the ceiling. I'd alway recommend here over behind with no sides...better envelopement, better stearing, less localization, more placement flexiblity for best sound, out of the way, etc. (if a little more work in installing)
Good luck

Thanks for everybody feedback. I plan to use PSB synchrony one as the L and R. Synchrony C as the center. Original idea was to use PSB imagine S as the surround speakers at the backwall, but some suggest that I can get platinum S2 with similar price in Audiogon.
In-ceiling speaker was my option. My wife, the CFO, says abolutely no no. She can't stand two "monsters" (to me, they are babies) mounted besides her lovely chandelier

If you haven't already purchased the receiver and speakers I highly recommend a 7.1 HDMI receiver and seven matching Sync One B's or smaller speakers and a sub. If your couch backs up to the back wall the rears would mount in the upper rear corners on the side wall pointing down at the listening position. Mount the the sides at ear level next to the couch. The L and R at ear level and on the same plane with no toe in as close to the display as possible with the C either over or under the display on plane with the L and R. With the calibration mic at the listening position let room correction do the rest.

No room is too small for 7.1 and matrixed 7.1 sounds better than 5.1.

You can mount a Dipole speaker for rear surround just fine its all up to you. The comment about not being able to use a dipole is silly, dipole surrounds radiate from both sides not front rear.