Connect iPhone Lightning Headphones to Mac 2014 (standard headphone jack)
I'd say one does not exist.
The reasoning behind you being able to find an adaptor from USB-C to lightning is that both are digital connections. I'm not sure what, if any, data translation would need to be done, but if it was necessary USB-C can carry the power to do it. (Similarly for the lightning to audio cable supplied with the iPhones).
To do the same from a regular headphone socket would need the adaptor to carry two-way DACs (digital analogues converters) to &amp; from the earphones &amp; mic, &amp; would also have to be battery-powered, as it can't take power from the headphone socket on the Mac.
It's not impossible to do that, but it would be costly. It seems so far, no-one is willing to see if the consumer would be prepared to pay the price.