Your mistake is getting hung up on specific rhythms. Forget rhythm for a second -- in fact, forget music altogether -- and just focus in on basic principles of leading and following. Think in terms of speed rather than timing.
If your partner is tuned into your movement, and is responsive and well-connected, she should be able to follow not only what you're leading but also the speed at which you're leading it. As a simple exercise to illustrate this concept, turn off the music and pick a simple movement, such as New York (Crossovers). Play around with different speeds from excruciatingly slow to blazingly fast, and everything in between. It only takes a few minutes of experimenting with this to get a feel of the timing and degree of intensity required in your own body in order to get the desired results from your partner.
Realize that a syncopated action is nothing more than a fast version of something slow. For example, a syncopated underarm turn is nothing more than a basic underarm turn. The only thing that's diiferent about the syncopated version is that it's danced at a faster speed... fast enough to cause the steps to align with a specific rhythm. Therefore, to lead the underarm turn at any speed, syncopated or not, is simply a matter of producing the exact level of speed through your body and/or intensity through the connection to achieve the desired rhythm. I hope this makes sense.
Regards,
Jonathan Atkinson
www.ballroomdancers.com