Science of Synthesis

Collins, S. K., Science of Synthesis: Stereoselective Synthesis, (2011) 1, 863.

Z-Selective metathesis protocols are perhaps some of the most desirable transformations in the field of alkene metathesis. In almost all cases, the ruthenium or molybdenum complexes catalyze cross-metathesis reactions with E selectivity. Despite the interest in Z-selective alkene metathesis reactions, there are no general Z-selective metathesis reactions that routinely produce Z/E ratios of >25:1. Some rare cases of E or Z selectivity in macrocyclic ring closings via catalyst control have been reported.[‌75‌,‌76‌] In some cases when one of the alkene substrates bears an sp-hybridized substituent (alkynes or nitriles), some Z selectivity is observed.[‌77‌–‌79‌] Some progress has also been made in ene–yne metathesis reactions using both ruthenium- and molybdenum-based catalysts. For ruthenium-based catalysts, the commercially available Grubbs-type catalysts have been exploited in cross ene–yne metathesis reactions to produce either 1,3-dienes or 1,3-enynes.[‌80‌] While both processes are Z selective, it should be noted that the selectivity is still heavily dependent upon the nature of the substrate. For example, cross-metathesis reactions involving acrylonitrile have long been known to afford product mixtures in which the alkene is predominantly Z (‌Schemes 18‌ and ‌19‌).[‌77‌,‌78‌] Recently, some cross ene–yne metathesis reactions utilizing alkynylboronates have been shown to be Z selective (‌Scheme 20‌).[‌79‌] The groups of Schrock and Hoveyda have reported the use of new chiral-at-molybdenum complexes that can promote highly Z-selective polymerizations of norbornadienes to form highly syndiotactic polymers. In addition, they reported enantioselective ring-opening/cross-metathesis reactions using the chiral-at-molybdenum catalysts which were highly Z selective in the formation of the alkene during the cross metathesis step (‌Scheme 21‌).[‌81‌] Although these are still the only reports of Z-selective processes, the research does suggest that further Z-selective metathesis processes may be possible using the new chiral-at-molybdenum catalysts.

‌Meeeee 88‌ Meeeeeeee-Meeeeeeee Meeee Meeeeeeeee ee Meeeeeeeeeeee[‌88‌]

‌Meeeeeeeeee 88‌

M8	Meeee (M/M)	Meeee (%)	Mee
(MM8)8MMe	8:8	88	[‌88‌]
(MM8)8MM	8:8	88	[‌88‌]
(MM8)8MMM	8:8	88	[‌88‌]
(MM8)8MM8M	8:8	88	[‌88‌]
MM8MM(MM8Me)8	8:8	88	[‌88‌]
	8:8	88	[‌88‌]

‌Meeeee 88‌ Meeeeeeeee-Meeeeeeee Meeee-Meeeeeeeee ee Meeeeeeeeeeee[‌88‌]

‌Meeeeeeeeee 88‌

‌Meeeee 88‌ Meeeeeeee-Meeeeeeee Mee–Mee Meeeeeeeee ee Meeeeeeeeeeeeeee Me Meeeee 8,8-Meeeee[‌88‌]

‌Meeeeeeeeee 88‌

M8	M8	Meeee (M/M)	Meeee (%)	Mee
Me	(MM8)8MMMMMM	88:88	88	[‌88‌]
MM8MMe	(MM8)8Me	88:88	88	[‌88‌]
MM8MMe	MM8MMM	8:88	88	[‌88‌]
	(MM8)8MMe	88:88	88	[‌88‌]
	(MM8)8Me	88:8	88	[‌88‌]
	(MM8)8Me	>88:8	88	[‌88‌]
		>88:8	88	[‌88‌]
	(MM8)8Me	88:8	88	[‌88‌]

‌Meeeee 88‌ Meeeeeeeee- eee Meeeeeee-Meeeeeeee M-Meeeeeeee Meeeeeeeee ee Meeeeeee Meeeeee[‌88‌]

‌Meeeeeeeeee 88‌

M8	Meeeeeee	Meee (e)	Meeee (M/M)	Meeeeeeeee (%)	Mee
MM8Me		8	88:8	88	[‌88‌]
MM8Me		88	88:8	88	[‌88‌]
MM8Me		8	88:8	88	[‌88‌]
MM8MMM		8	88:8	88	[‌88‌]
(MM8)8MM8Me		8.8	88:8	88	[‌88‌]
(MM8)8MM8Me		8	88:88	88	[‌88‌]
		8	88:8	88	[‌88‌]
		8.8	88:8	88	[‌88‌]
		8	88:8	88	[‌88‌]
MM8MMe		8.8	88:8	88	[‌88‌]
MM8MMMe		8	88:8	88	[‌88‌]

Meeeeeeeeeee Meeeeeeee

(M)-8-Meeeeeeee-8-eeee Meeeeeee [‌88‌, M8 = (MM8)8MMe]; Meeeeee Meeeeeeee:[‌88‌]

MMMMMMM: Meeeeeeeeeeee ee eeeeee eeeeeeeee, ee eee, eeee, eee eeeeeeeeeee eeeee eeeeeeee, eee e eeeeeeee eeeee eeeeeeeeee.

Me e eeeeeee ee eeee-8-eeee eeeeeeee (888 ee, 8.88 eeee) eee eeeeeeeeeeeee (888 ee, 8.88 eeee) ee MM8Me8 (88 eM) eee eeeee Me eeeeeeee ‌88‌ (88 ee, 8 eee%). Mee eeeeeeeee eeeeeee eee eeeeeee ee 88 °M eee 8 e eee eeee eee eeeeeee eee eeeeeee eeeee eeeeeee eeeeeeee. Meeeeeeeeeee ee eeeeeeeeeeeeee ee eee eeeee eeeeeee eeee eee eeeeeee; eeeee: 888 ee (88%).

(M)-Mee-8-eeeeeeeeee [‌88‌, M8 = (MM8)8Me]; Meeeeee Meeeeeeee:[‌88‌]

MMMMMMM: Meeeeeeeeeeee ee eeeeee eeeeeeeee, ee eee, eeee, eee eeeeeeeeeee eeeee eeeeeeee, eee e eeeeeeee eeeee eeeeeeeeee.

Me e eeeeeee ee eee-8-eee (888 ee, 8.8 eeee) eee eeeeeeeeeeeee (88.8 ee, 8.8 eeee) ee MM8Me8 (8 eM) eee eeeee Me eeeeeeee ‌88‌ (88 ee, 8 eee%). Mee eeeeeeeee eeeeeee eee eeeeeee ee ee eee 8 e eee eeee eeeeee eeeeeee e eee ee eeeeee eee eee eeeeee eeee MM8Me8/MeMMe (8:8). Mee eeeeeee eee eeeeeee eeeee eeeeeee eeeeeeee, eee eee eeeee eeeeeee eee eeeeeeee ee eeeeeeeeeeeeee (eeeeee eee, eeeeee/MeMMe 88:8) ee eeee e eeeee eeeeee eee; eeeee: 88.8 ee (88%); eeeee (M/M) 8:8.

8-[(M)-8-(eeee-Meeeeeeeeeeeeeeeeee)-8-eeeeeeeeeeeeee-8-ee-8-ee]-8,8,8,8-eeeeeeeeeee-8,8,8-eeeeeeeeeeeee [‌88‌, M8 = 8-(eeee-Meeeeeeeeeeeeeeeeee)eeeee; M8 = (MM8)8Me]; Meeeeee Meeeeeeee:[‌88‌]

8-[8-(eeee-Meeeeeeeeeeeeeeeeee)eee-8-eeee]-8,8,8,8-eeeeeeeeeee-8,8,8-eeeeeeeeeeeee (88 ee, 8.888 eeee) eee eee-8-eee (88 ee, 8.888 eeee) eeee eeeeeeeee ee eeeeeee eeeeeeeee MM8Me8 (8.88 M). Meeeee eeeeeeee ‌88‌ (8.8 ee, 8.888 eeee, 8 eee%) eee eeeee ee eee eeeeeee. Meeee eeeeeeee eeeee eeeeee eee 8 e, eee eeeeeee eee eeeeeeeeeeee eeeee eeeeeee eeeeeeee. Mee eeeeeee eee eeeeeeee ee eeeee eeeeee eeeeeeeeeeeeee (eeeeeee/Me8M, eeeeeeee eeeeeee) ee eeeee ee eee; eeeee: 88 ee (88%).

M-Meeeeee ‌88‌; Meeeeee Meeeeeeee:[‌88‌]

Me e M8-eeeeee eeeeeeee, e 8-eM eeee eee eeeeeee eeee eee eeeeee eeeeeeeee (8.88 eeee) eee eeeeeee (888 µM) (MMMMMMM: eeeeeeeeee). M eeee ee eee eeeeeeee ee eeeeeee (88 µM, 8 eee%) eee eeeee ee eee eeee ee eee eeeeeee. Mee eeeeeee eee eeeeeee ee eeee ee 88 °M eee ee eeeeeee ee eee eeeeeeee eee eeee eeeeeeeeeee ee ee MMM eeee eee eeeee eee ee eee eee. Mee eeeeeee eee eeeeeeee ee eeeeeeee ee eee, eee eeee eeeeeee eeee MMMe8. Mee eeeeeeeeee eee eeeeeeeeeee ee eee eeeeeeeee eee eeeeeeeee ee 8M MMM eeeeeeeeeeee.

References