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. We demonstrate that, 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(iso-propyl)phenyl) for 15 minutes is sufficient for efficient generation of a desired product. Routine workup leads to quantitative deprotection. A range of stereochemically defined Z- or E-alkenyl chlorides, bromides, fluorides, and boronates or Z-trifluoromethyl-substituted alkenes with a hydroxy- or a carboxylic acid group were thus prepared in 51–97% yield and 93% to >98% stereoselectivity. The substrates, HB(pin), and cross-partners were used as received. We also show that, regardless of whether a polar functional unit is present or not, a small amount of HB(pin) (e.g., 10 mol %) may be used to remove residual water, significantly enhancing efficiency (i.e., lower catalyst loading).