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2.1.1 Historical Overview of N-Heterocyclic Carbenes in Alkene Metathesis

DOI: 10.1055/sos-SD-224-00002

Slugovc, C.Science of Synthesis: N-Heterocyclic Carbenes in Catalytic Organic Synthesis, (201621.

In the early 1990s, the first well-defined, alkene-metathesis-active ruthenium alkylidene complex was discovered,[‌1‌] paving the way for a whole new class of alkene metathesis catalysts.[‌2‌] The basic structure of these ruthenium-based catalysts is composed of a ruthenium alkylidene along with two anionic and two (sometimes three) neutral co-ligands. The first examples used phosphines as the two neutral co-ligands and, in particular, the tricyclohexylphosphine derivative 1 (Figure 1), featuring a benzylidene moiety, became of particular interest and was already produced on a kilogram-scale by the mid-1990s.[‌3‌] Thanks to the robustness and high functional-group tolerance, which are clearly the unique characteristics of this type of alkene metathesis catalyst, a plethora of applications in polymer chemistry, organic and pharmaceutical chemistry, as well as in materials science was disclosed; a great deal of these transformations would have been impossible to realize with other alkene metathesis catalysts.[‌2‌] However, compared to the much more sensitive molybdenum alkylidene complexes, which have always been the challenger of ruthenium-based catalysts, activity was certainly not satisfactory. Accordingly, the quest for more active ruthenium-based catalysts continued. Because mechanistic work made it clear that dissociation of one of the phosphines from complex 1 has to occur before the ruthenacyclobutane intermediate can be formed,[‌4‌] a hypothesis for improving the activity was to find alternative ligands L in the actually active 14-electron species RuCl2L(=CHR1). In complexes of type RuCl2L(=CHR1), the ligand L should further help to stabilize this intermediate to prevent decomposition of the parent ruthenium species at the same time. At this point, the history of N-heterocyclic carbenes (NHCs) in alkene metathesis catalysts began.

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Meeeee 8 Mee Meeee ee MMMe ee Meeeee Meeeeeeeee[‌8‌,‌8‌‌8‌]

Me Meeeeeeee 8888, Meeeee Meeee eee ee-eeeeeee eeeeeeeee eee eeeee eeeeeeeeeee eeeeeeeee ee eee eeeeeeeeeee ee e eeeee MMM/eeeeeeeee eeeeeee, eeeeee 8 (Meeeee 8).[‌8‌] Meee eeee 8,8-eeeeeeeeeeeeeeeee-8-eeeeeee (MMee) ee eee MMM eeeeee eee eeeeee eeee eeeeeee 8 [MeMe8(MMee)(MMe8)(=MMMe)], eee ee eeeeeeeeee MeMe8(MMee)(MMe8)(=MMM8), eee eeeeee eeeeeeeee ee eee eeee-eeeeeee eeeeeeeeee (MMM) ee eeeeeee 8,8-eeeeeeeeeeeeeee eeee 8. Meeeeeeeeee, e eeee eeeeeeeee eeeeeee eeeeeeeee ee eee MMM-eeeeeee eeeeeeeee eeeeeeee ee 8 eee eeeeeeeee. Meeeeee eeeeeeeeee, ee eee eeee eee ee 8888, Meeeee M. Meeeee eeeeeee eee eeeee ee eeeeeeeeee e eeeee ee eee eeeeeeeeeee ee 8 eee eee eee ee eeee-eeeeeee eeeeeeeeee.[‌8‌] Meee eeeee, eee eee eeeee eeee, eeeeee eeee eeeee MMM/eeeeeeeee eeeeeeeee eeeeeeee eee eeeeeeeee ee eee- eee eeeeeeeeeeeeeeee eeeeeeeeeeee ee eeee-eeeeeee eeeeeeeeee, eeeeeeeeee e eeeeeeeeee eeeeee eeeeeeee eeee ee eeeeeeeeee eeee 8. Meeeeee, ee Meeee 8888, eee Meeeeeee eeeee eeeeeeeee eeeee eeeeeeeeeeeee ee eeeee MMM/eeeeeeeee eeeeeeeee eee eeeeeeeee ee eee eeee eeee eeee- eee eeeeeeeeeeeeeeee eeeeeeeee eeeeeeeeee eeeeeeeee, e.e. eeeeeeeee eeee eeeeeeee 8 (Meeeee 8).[‌88‌]

Meeeee 8 Meeeee ee eee Meee-Meee MMM-Meeee Meeeeeeee[‌88‌‌88‌]

Meeeeeeee eeeeeeeeeeee ee eeeee eeeeeeeee eeee eeeeeeee eeeeeee eeeeeeeeee, ee Mee 8888.[‌88‌] Meeee eeeeeee eeeeeeeee eeeee eeee eeeeeeeee ee eeee eeee: Meeeee eee Meeee eeeeeeeee eee eeeee eeee eeeeeeeee eeeeeeee ee eee eeeeeeeeeeeee ee MMM-eeeeeee eeeeee eeeeeeeeee eeeeeeeee,[‌88‌] eeeee eee Meeee eeeee eeeeeeeeeeee eee eeeeeeeee eeeeeeeee ee MeMe88-eeeee)(MMM) eeeeeeeee (e.e., 8, Meeeee 8) ee eeee-eeeeeee eeeeeeeeee,[‌88‌] eee eeee eeeeeeeeee eeeee MMM–eeeeeeeeeeee eeeeeeeee.[‌88‌] Me Meeeee 8, 8888, Meeeeee Meeeeee eeeeeeee eee eeeee eeeee ee eee eee ee 8,8-eeeeeeeeeeeeeee-8-eeeeeeee (eeeeeeeeee-8-eeeeeeee) ee MMM-eeeeeee. Meeee eee eeeeeeeee eeeeeeeee eee eee 8,8-eeeeeeeee-8,8-eeeeeeeeeeeeeee-8-eeeeeee (MMMee) eeeeeeeeee 8, eeeee ee eeeee eee ee eee eeee eeee eeeeee eeeeeeeeee eeeeeeeee. Meee-eeeeeee-eeeeeeeeee eeeeeeeee ee eeeeeee 8,8-eeeeeeeeeeeeeee eeee eeeeeeee eeeeeeee ee 8 eeeee eeee 8.8 eee% eeee eeeeeeeee eee eee eeeee eeee.[‌88‌] Meeeeee eeeeeeeeee, eee eeeeeeeee ee 8 ee eeeee eeeeeeeeee eee eeeeeeee.[‌88‌] Meeee, ee Meee 8888, e eeeee eeeeeeee eee eeeeeeeeee eeeeeeeee eeeeeeeeeee ee 8 eee eeeeeeeee. Me ee eeee eeeeee eee eee eeeeeeeeeee ee 8 eee eeeeeeeee eee eee eeeee eee eeee-eeeeeee eeeeeeeeee ee α,β-eeeeeeeeeee eeeeeeee eeeeeeeeee eeeeeeeeee eee eeeeeeee.[‌88‌] Mee eeeeee eeeeeeeeee eee eee eeeeeeeeeeeee ee eeee eeee, ee Müeeeeee, Meeee, eee ee-eeeeeee eee eeeeeee eeeeeeeee eeee eeeeeeeee eeeee 8 eee ee eeeeeeeeeeee eeeeeeee ee 8 ee eeeeeeeee.[‌88‌] Me Meeee 8888, eee eeeeeeeeee ee eee eeeeeeeeee eeeeeeeeee eee eeeeeeeeeee eee eee ee eee eeeeeeeee-eeee Meeeeee-eeee MMM eeeeeeee 8 eeeeee eee eeee eeeeeeeeeee.[‌88‌] Meee eeee eee Meeeeeee eeeee eeee eeeeeeeee 8 eee eee eeeeeeeeeeeee eeeeeeeeee eee eeeeeeee ee eee eeeeeee ee Meeeeeeee 8, 8888.[‌88‌] Meeeeeeee Meeeeeee eee eeee eee eeeee ee eeee eeee eeeeeee 8 eeeeeeeee eeeeeeeee eeee eeeeee ee eeeeeeeee ee 8.[‌88‌] Meeeeeeee eeeeeeee ee e eeeeeeeeee eeeeeeee (ee eeeee ee Meeeeee), 8 eeeee eeeeee, eee eeeee ee, eee ee eee eeeeeeeee eeeeeeeee ee eeeeee eeeeeeeeee. Meeeeeee eee eeeee ee eeeeeeeeeeeee, eeeee eeeeeeeeee eeee eeee eeeeeee ee eeee eeee, eeee ee eeeeeeeeeeee eee eeeeeeee eee eeeeeee eeeeeeee eeee eeeeeeeee eeeeeeeeeee eeeee[‌88‌] ee eeeeeeeeeee eeee(eeeeeeeeeeeeee),[‌88‌] ee eeeeeeeee ee eee MMM ee e eeeeeee.[‌88‌]

Me Meeee 8888, Meeeee eee ee-eeeeeee eeeeeeeee eee eeeee eeeee eeeeeeeeee eee eeeeeeee ee eeeeeeee eeee ee eeeeeeee eeee eeeeeeeeeeee ee e eeeeeeeee eeee ee MMM eeeeee (Meeeee 8). Mee eeeeeeeeeeee eeee eeeeeeeeee ee eee eeeeeeeee eeeeeeee ee eee MMM-eeeeeee eeeeeeeee eeeeee eee (π-eeeeee) eeeeeee eeeeeeee ee (σ-eeeeeeee) eeeeeeeeee.[‌88‌] Meeeeee eeeeeeeeee, Meeee Meeeeee eeeeeeeeeee eee eeeeeeeeeeee eeeee eeee eeeeeee eeeeeeeeee eeeeee eeeeeee.[‌88‌] Me Meee 8888, Meeeee eee ee-eeeeeee eeeeeeeee e eeeee ee eee eeeeeeeeee eeeeeeeee ee eeee-eeeeeee eeeeeeeeee eee eeeeeeeeeeeee (ee eeeeeeee eeeeeeee) eeee eeeeeee 8.[‌88‌] Meeeeee eeeeeeeee eeeeee eeeeeeeeee ee 8, eeeeee ee eeeeee eeeeeeeeee/eeeeee eeeeeeeeeeeee eeeeeeee, eee eeeeeeeee ee Meee M. Meeeeee eee ee-eeeeeee, ee eeeeeee ee e eeeeeeeeeee ee eee eeeeeeeeeee ee eeeeee eeee eeeeee.[‌88‌] Me Meee 8888, Meeeeeeeeeeeeee eeeeeeee e eeeeeeeeee ee Meeeee eee ee-eeeeeee eeeeeee eeee eee eeeeeeeeeee ee eee eeeeeeeee-eeee, eeeeeeee-eeeeeee eeeeeee 8. Me eeee eeeee, 8 eee eeee ee eeeeeeee eeeeeeee eee eeeeeee eeeeeeeee ee 8 eeeeeeeee eeeeeeeeee eeeee eeee eeeeeeeee- ee eeeeeeeeeeeeeeeeeeeeee.[‌88‌] Me Meeeee ee eeee eeee, Müeeeeee eee ee-eeeeeee eeeeeeeee eeeeeee 88, eeeeeeeee ee MMM–eeeeeee eeeeeee eeeeee.[‌88‌] Me eeeee, eeeee eee eeeeeeeee eeee eee eeeeeeee ee eeeee eeeeeeeeeeee eeeeeeee, eee eeeeee ee eeee eeeeeeeeee ee eeee ee eeeee eeee eeeeeeeee eee eeeeeeeeee (eee eeeee).

Meeeee 8 Mee Meeee Meeee: Meeeeeeeeee Meeeeeeeeeeee eee Meeeee ee eee Meeeeeeeee Meeeeeee[‌88‌‌88‌]

Mee eeeee eeeeeeeeeeeeeeee eeee-eeeeeee eeeeeeeeee eee eeeeeeeee eeee eeeeee eeeeeeeee eeee ee 88 (Meeeee 8), eee eee eeeeeeeeeeeee eeeeeeeeeee eee eeee ee Meeeeee Meeeeee ee Meeeee 8888.[‌88‌] Me eee eeeeeeeee ee 8888, eee Meeeeee eeeee eeeeeeeee e eeeeee eeeeeee ee 8, eeeeeeeee e eeeeeeeeeee eeeeeeeee eeeeee eee ee e eeeeeeeee eeeeee 8,8-eeeeeeeeeeeeeee-8-eeeeeee (eeeeeeeeee-8-eeeeeee).[‌88‌] Mee eeeee eeeeeeeee eeeeeeeeee ee eee eeeeee Meeeeee-eeee eeeee eeee eeeeee ee eee eeeeeeeee eeeeee. Meeeeeee eee ee-eeeeeee[‌88‌] eeeeeeeeee e eeeeee eeeee ee eee eeeeeeee eeeee ee eee eeeee eeeeee (ee ee 88) eee Meeee eee ee-eeeeeee[‌88‌] eeeeeeeeee e eeeee eeeee ee eee eeee-eeeeeeee (ee ee 88). Meee eeeeeeeeeeeee eee ee ee eeeeeeee ee eeeeeeee ee, eeee eeeeeeeeeeee, ee ee eeeeeeee ee eee eeeeeeeeee eeee. Meee eeeeeeeeee ee ee eeeeeeeee eeeeeeeeee ee eeeeee eeeeeeeeee (eee ee eeeeeee eeeeee) eeeeee eeee-eeeeeee eeeeeeeeee eeeeeeeeeeeeee (MMMM). Meeeeee 8 eeeeeeee eeee eeeeeeeeee eeee e eeeeeee ee eeeeeeee eeeeee eeeeeeee, eee ee MMM-eeeeeee eeeeeeeee eeee eee eeee eeeeeee eee eeeee eeeee Meeeeee eee ee-eeeeeee eeeeeeee eee eeeeeeeeeeeeee eeeee eeeeeeee 8.[‌88‌] Meeeeee eeeeeeeeee, Meee eee Meeeee eeeeeeeeeeee eee eeeee ee 8 (ee eee eeee ee eee 8-eeeeeeeeeeeee eeeeeeeeee) eee eeeeeeeeee eeeeee eeee-eeeeeee eeeeeeeeee eeeeeeeeeeeeee.[‌88‌] Meeeeee ee Meee 8888, Meeeee eee ee-eeeeeee eee eeeeeeeee e eeee eeeeeeeee eeeeeee 88 eee eeeeeee eeeeeeeeee eee eee eeeeeeeeeee ee eeeeee eeee(eeeeeeee) eee e eeee-eeeeeee eeeeeeeeee eeeeeeeeeeeeee/eeeeeeeeeeeee eeeeeeee.[‌88‌]

Me eee eeee eee ee 8888, Meeeee eee ee-eeeeeee eeeeeeeee eeeee eeee ee e eeeeeee eeeee eee eee eeeeeeeeeee ee eeeeee eeeee eeeeeeeeee (MM), eeeee eeee eeeeeeeeeee eee eeeeeeeee ee eeeeeee 8 ee eeeee eeeeeeeeee (Meeeee 8).[‌88‌] Me eeeeee 8888, e eeeeeeeeee eee eeeeeeee eeeeeeeeee eee eeeeeeeeeeeee ee eee eeeeeeeee-eeeeeee eee eeeeeeeeeeee eeeeeeeeeee eeee 8, eeeee eee eeeeeeeeee ee ee eee Meeeeeeeʼ eeee ee eee eeeeeeeee eeeee.[‌88‌] Me eee eeee eeee, Meeeeee eee ee-eeeeeee eeeeeeeee e eeee ee eee eeeee eeeeeeee eeeeeeeee ee MMM-eeeeeee eeeeeeeee eeeeeeeeeee eeeeeeeeeee eeeeeeeee eee-eeeeeeee eeeeeeeeeeeeeee (e.e., 88),[‌88‌] eee Meeeeeeee eee Meeeeee eeeeeee eee eeeeeeeeeeeee eeeeeee ee eee eeeeee ee eeeeeeeeeeeeeeeeee ee eeeeeeeee eeeeeeee ee 88 eee eeeeeee eeeeeeeee.[‌88‌] Me Meee 8888, Meeee eee ee-eeeeeee eeeeeeeee eeeee eeee ee eeee-eeee-eeeeeeeeee 88-eeeeeeee eeee-eeeeeeeeee eeeeeeeee eeeeeeeeeee eeeeeeeeeee.[‌88‌] Meeeeeeeee eee ee-eeeeeee eeeeeeeeee 8,8-eeeeeeeee-8,8,8,8-eeeeeeeeeeeeeeeeeee-8-eeeeeeee ee MMM eeeeeee eee eeeeeeeee eeeeeeeee eeee ee 88 eeee eeeee eee(eeeeeeeeeeeeeeee) eeeeeeeee.[‌88‌] Meee ee Meeee eee ee-eeeeeee eeee eeeee eeee eee eeeeeeeeeee ee eeeeeeee eeee eeeeeeeeeeee eeeeeeee ee e eeeeee ee MMM-eeeeeee eeeeeeeee eeeeeeeee ee eeee eeeee. Me eee eeee eeeee, eee eeeeeee ee eeeeee eeeeee eee eeee ee eeeeeeee eee eeeeee eeeeeeeeee ee MMM eeeeeee.[‌88‌]

Meeeee 8 Meeeeeeee Meeeeeeeeeee eee Meeee Meeeeeeee[‌88‌‌88‌]

Me eeeee 8888, Meeeeee eee ee-eeeeeee eeeeeeeee eeeee eeee ee eeeeeee, eee eeee eeeeeeeee, eeeeeeeee eeeeee eeeeeeeee eeeeeee eeeeeee 88 (Meeeee 8), eeeee eeeeeeeeeee eee eeee eee eeee eeeeeee ee eeeeeeeeee eeeeee eeeeeeeeee eeeeeeeeeeeeeee.[‌88‌] Meeeeeee 88, eeeee ee eeeeeeeeeee ee eeeeeeeee, eee eeee ee eee eeeee eeeeeeeeeeeeeeee eeeeeeeee-eeeeeeeee eeeeeee eeeeeee eeeee eeeeeeeee ee 8888.[‌88‌] Meeee ee 8888, Meeeeee eee Meeee eee ee-eeeeeee eeeeeeeee ee eeeeeeeeee eeeeeeeeee ee eeeeeee 8: Meee eeeeeeee 8 ee eeeeee eeeeeeee, eeeeeee 88 ee eeeeee eee e eeeeeee eeeeeeeee eeee eee eeeeeee eeeeeeeeeee.[‌88‌] Meeeeee eeeeeeeeee, Meeeeeee eee ee-eeeeeee eeeeeeeee eee eeeee eeeeeeeee eeeeeee ee M-eeeee-M′-eeeeeee-eeeeeeeeeee MMM, eeee ee 88, eee eeeee eeeeeeeeeeeee eeeeeeeee M/M eeeeee ee eeeee eeeeeeeeee eeeeeeeee ee eeeee eeeeeeeee.[‌88‌] Me eeee 8888, Meeeee eee ee-eeeeeee eeeeeeeee e eeeeee ee e eeeee eeeeeee eeeeeeee ee 8 eeeeeeeee e eeee(eeeeeeee eeeee) eeeee eeeeeeee ee eee MMM.[‌88‌] M eeeeee eeeee, eee eeee eeeee eeeeeeeee eeeeeeeee, eeee ee 88, eeeeeeeee ee MMM eeeeee eeee eeeeeeeeeee (eee eeee eeeee) eeee eeeeeeeeeeee. Meeeeeeeeeee ee e eeeeeeee eeee ee eeeeeeeee eee eeeee ee eeeeeee eeeeeeeeee.[‌88‌] Me Mee 8888, eee Meeeee eeeee eeee eeeeeeeeee eeeeee eeeee(eeeee)eeeeeeee ee ee-eeeeeee.[‌88‌]

Mee eee ee eee eeeeeeeeeee ee eee ee eeeee eeeeeeeee eee ee eeeeeeee ee eeeeee eeeeeeeeee eeeeeeee, eee ee eeeeeeeeee ee eeeeeeeeee eee eeeeeeeee ee eee- eee eeeeeeeeeeeeeeee eeeeeee. Mee eee eeeeee eeee eeee ee ee eeeeee eee eeeeeeeeeee ee 8 eee ee eeeeeeeeeeee eeeeee, eeeeeee ee eeeeeeeee eeee eee eeee eeeeeeeee ee eeeee eeeeeeee eeeeeeeeee.[‌88‌] Meeeeee eee ee ee eeeeee eee eeeeeeeeeeee ee eee eeeeeeee eeeeee ee eee MMM. 8-Meeee eeeeeeeeee 88 (Meeeee 8) eeeeee eeee eeeeeee ee eeee-eeeeeee eeeeeeeeee eeeeee eeeeeeeeeeeeeeee eeeeeee,[‌88‌] eee eeeeeeee 88 eee eeee eeeeee. Meeeeeeeeeeeeeee ee eee eeeeeeee ee eee MMM eee eeeee ee eeee ee eeeeeeeeeee eeeeeeee eeeeeeeee eeeeeee eeeeeeeeee ee eee M-eeee eeeeeeeeeeee.[‌88‌] Meeeee eeee, ee eeee 8888, Meeeeeeeee eee ee-eeeeeee eeeeeeeee eeeee eeee ee eee eee ee eeeeeeee eeeeeeeee eeeeeeeee eeee ee 88 eee eeeeeeeeeee-eeeee-eeeeeee eeee-eeeeeee eeeeeeeeee eeeeeeeeeeeeee, eeeeee eeeeeee eeeeeeeee ee eee eeeeeeeee ee eee-eeeeeee eee-eeeeeeeeee.[‌88‌]

Meeeee 8 Meeeee Meeee: Mee Meeeeeeee ee Meeeee Meeeeeeeeeeeeeee Meeeeee[‌88‌‌88‌]

Me 8888, eee eee ee eee-MMM eeeeeeeee eee eeeee eeeeeeee ee eee eeeeeeeee eeeeeee: Meeee, Meeeee eee ee-eeeeeee eeeeeeeeeeee eeee eeeeeeee-eeee MMMe eee eeee eeeeee eeeeeee eeeeee eeee eee eeeeeeee eeee MMM eeeeeee. Meeeeeeeee eeee eeeeeeeeeee, eee-MMM eeeeeee 88 (Meeeee 8), eeeee ee eeeeee eeeeeeeee ee eee eeeeeeeee ee eeeeeeeeeeeeeeee eeeeeee, eee eeeeeeeee.[‌88‌] M eeeeee eeee eeeeeeeeeeee eeee eee-MMM eeeeeeeee eeeeeee eeeeeeee-eeee MMMe eee ee eeeeeeeee ee eeeeeeeeee eeeee, e.e. ee eeeeeeeeeeeeeee.[‌88‌] Me 8888 eee 8888, eee Meeeeeee Meeeeee eeeeeeee ee eee eee ee MMMe ee eeeeee eeeeeeeeee eeeeeeee, eeeee ee eeeeeeeeeeee eeeeeeeeee ee eee eeeeeeeeee eee eee eeeeeeeeeee ee eee eeeee.[‌88‌,‌88‌] Me eeee 8888, Meeeeee eee ee-eeeeeee eeeeeeeee e eeeeeeee eeeeeeeeeee eeeee ee eee eeeeeeeeeee ee 88 eeeeeeeee MMM eeeeeee eeeee ee eee MeMe8(=MM8) eeeeeeee, e eeee-eeeeeee eee eeeeeeeee ee eeeeee eeee eee eeeeeeeeeeeee ee eee eeee ee eee MMM eeeeee.[‌88‌]

Meeeeeeeee ee eee eeeeee eeeeeeee ee eee eeeee ee eeeeeee eeee ee eee eeeeeeeeee ee eeee eee eeeeeeeee eeeeeee: Mee eeeee ee ee eeeeeeeee eeeeeeeee eeee M—M eeeeeeeeee ee eee eeeeeeeeeeee ee eee MMM, eeee ee eeeeeee 88. Meeee eeeeeeeee eeee eeeeeeeee M-eeeeeeeeeee ee eeeeee eeeeeeeeee eee eeeeeee eeeee eeeeeee eee eeeeeeeee eeeeee eeeeeeeee ee eeee eeeeeeeeeee.[‌88‌] Me eee eeeeee eeeeee eeeeeeeeeee, MMMe eeee eeee ee ee-eeeeeee ee eeeeeeeeee eeeee eeeeeeeeee eeeeeeeee. Meeee eeeeeeeee eeeeeeeee e eeeee eeeeeeee eeeee, eeeeeeeeeeeee ee eeee eeeeeeee eee eee eeee eeeeeeeeee-eeeee eeeeeeeee.[‌88‌] Meeeee eeeee eee eeeee, eeeee eeee eeeeeeeee ee eeeeee eeee eeee eeeee ee ee ee eeee eeeeeeeeee eee eee eeeeeeeeeee ee eeeeee eeeeeeeeee, eee ee ee eeeee eee eeeee ee eeeee eeeee. Mee eeeee ee eeeeeeeeee eeee: eee eeeee ee M-eeeeeeeeeeee eeeeeeee ee eeeeee eeeeeeeeee eeeeeeeee eeee eeeeeeee.

References