I'm trying to do a reaction with phenyl piperazine, triphosgene, and 4-bromophenol. However, every time I do this reaction I seem to get the carbonyl di-4-phenylpiperazine as a major product (based on the LCMS masses). I tried to reduce the triphosgene step to 5min, which seemed to yield trace amounts of the intended product, but I still can't get my intended product as the major product. This should be such a simple reaction - what am I doing wrong?
You are forming the urea because there is always an excess of phenylpiperazine present to react with the carbamoyl chloride you are forming. You need to add the phenylpiperazine to the triphosgene. The paper by Varjosaari et al.  has a procedure (described on Organic Chemistry Portal) that should work.
@Mathew Mahindaratne's suggestion will also work:
Try reversing the order. First, add 1: 1 triphosgene to 4-bromophenol (with stronger base) and then add phenyl piperazine.
- Varjosaari, S. E.; Suating, P.; Adler, M. J. One-Pot Synthesis of O-Aryl Carbamates. Synthesis 2016, 48 (01), 43–47. https://doi.org/10/gf4wh2.