Science of Synthesis

Liu, J.; Johnson, J. A., Science of Synthesis: N-Heterocyclic Carbenes in Catalytic Organic Synthesis, (2016) 2, 113.

Both the tacticity (syndiotactic, isotactic, atactic) and the alkene configuration (Z versus E) of polymers synthesized via ring-opening metathesis polymerization (ROMP) can impact their physical and mechanical properties.[‌87‌,‌88‌] ROMP using complexes ‌3‌–‌6‌ generally provides atactic polymers with a majority of E-alkenes. Molybdenum and tungsten initiators for ROMP have been developed that enable outstanding control over the tacticity and Z/E content of the polymer backbone.[‌89‌,‌90‌] However, these complexes do not possess N-heterocyclic carbene (NHC) ligands. In addition, a number of examples of stereoregular polymers prepared from ruthenium initiators have appeared. ‌Scheme 13‌ shows NHC-based ruthenium initiators that have enabled stereoselective ROMP of simple norbornene monomers including (i) a highly Z-selective backbone (catalyst ‌23‌);[‌91‌] (ii) a Z/E switchable backbone via modulation of the NHC electron-donation properties (catalyst ‌24‌);[‌92‌] (iii) a highly Z, syndiotactic backbone (catalyst ‌25‌).[‌93‌]

‌Meeeee 88‌ Meeeeeeee Meeeeeeee eeee Meeeeeee M-Meeeeeeeeeee Meeeeeee eee Meeeeeeeeeeeeee Meee-Meeeeee Meeeeeeeee Meeeeeeeeeeeee[‌88‌–‌88‌]

Meeeeeeeeee, M-eeeeeeeeeeee eeeeeeee eeee eeee eeeeeeee ee eeeeee MMMM eeeee eeeeeee eeeeeeeeee, eee eeeeeeeee-eeeee MMMM eeeeeeeeee eeee eeee eeeeeeee ee eeeee eeeeeeee eee eee M-eeeeeeeeeeee eeeeeee eeeeee. ‌Meeeee 88‌ eeeee eeeeeeee eeeeeeeee eeeeeeeee eeee M-eeeeeeeeeeee eeeeeee eeeeeee (‌88‌ eee ‌88‌) eeeeeeeee eee eeeeeee MMMM ee eeeee-eeeeeee eeeeeeee.[‌88‌,‌88‌] MMMM eeeeeeeeee eeeeeeeeeee ee eeeee eeeeeeee eee eee M-eeeeeeeeeeee eeeeeee eeeeee (e.e., ‌88‌) eeeeeee eee eeeeeeeee ee eeee eeeeeee ee eeeeeeeee eeee eee eeeeeee eeeeeee (Me eeeeeee eeeee ee eee eeeee eeeeeee), eee eee eeee ee eeeeeeeeeeeee ee eeeeee eeeeeeeeee eee eeeee eee eeeeeeeeee.[‌88‌–‌888‌] Meeeeeeeeeeee, eeeeeeee eeeee ee eeeeee eeee eeeeeeee eee ee eeeeeeee ee eee eeeeeeeee ee eeeeeeeee–eeeeeeeeeee eeeee eee eeeeeeeeee eeeeee ee eeeeeeee ee eee eeee-eeeeeeee ee ee M-eeeeeeeeeeee-8-eeeeeee–eeee eeeeee (e.e., ‌88‌).[‌888‌]

‌Meeeee 88‌ Meeeeeeee Meeeeeeee eee Meeeeee eee Meeee-Meeee Meee-Meeeeee Meeeeeeeee Meeeeeeeeeeeee[‌88‌–‌88‌,‌888‌]

References