HDMI to ThunderBolt adapters are actually Mini DisplayPort to HDMI adapter adapters. They are for connecting HDMI displays to computers with Mini DisplayPort or Thunderbolt ports. Computer Thunderbolt ports also support Mini DisplayPort for output to displays. The adapters won't work in reverse and convert HDMI to Thunderbolt.
The 2011 iMac can output Thunderbolt or HDMI, but for Target Display Mode it requires a Thunderbolt signal, not Mini DisplayPort. There are some adapters that can convert HDMI to Mini DisplayPort, but not to Thunderbolt, so they would not help.
There is no way to use a 2011 iMac in Target Display Node from a computer that has no Thunderbolt port.