Traceless Protection for More Broadly Applicable Olefin Metathesis

An operationally simple in situ protection/deprotection strategy that significantly expands the scope of kinetically controlled catalytic Z‐ and E‐selective olefin metathesis is introduced. Prior to the addition of a sensitive Mo‐ or Ru‐based complex, treatment of a hydroxy‐ or a carboxylic‐acid‐containing olefin with commercially available HB(pin) or readily accessible HB(trip)2 (pin=pinacolato, trip=2,4,6‐tri(isopropyl)phenyl) for 15 min is sufficient for efficient generation of a desired product. Routine workup leads to quantitative deprotection. A range of stereochemically defined Z‐ and E‐alkenyl chlorides, bromides, fluorides, and boronates or Z‐trifluoromethyl‐substituted alkenes with a hydroxy or carboxylic acid group were thus prepared in 51–97 % yield with 93 to >98 % stereoselectivity. We also show that, regardless of whether a polar functional unit is present or not, a small amount of HB(pin) may be used to remove residual water, significantly enhancing efficiency. Without a trace: Alkenes containing a hydroxy and/or a carboxylic acid group were converted into Z or E alkenes through one‐pot operations consisting of in situ protection, catalytic cross‐metathesis, and deprotection by mild workup/purification. Prior to the addition of a sensitive Mo or Ru complex, treatment of the olefin with HB(pin) or readily available HB(trip)2 for 15 min was sufficient for the efficient generation of a desired product (see scheme).