well; in short an adapter no.. not going to happen; you can get devices that modulate a signal over coax for ethernet networking but they are not affordable and feature rich as ethernet, not a good option unless replacing RG59 with CAT6 is extremely expensive, there made for a niche market of renters in high density apartments with crappy WiFi..
Using adapters is just a band-aid as once you go digital your not going back to analogue.. best option is just to tie some cat6 cable to the end of the coax and start pulling
http://www.amazon.com/Ethernet-Exte..._sbs_e_4?ie=UTF8&refRID=154YXYFX4R03K5KW BZ2J