Stereoselective Synthesis of High-Value Alkenes through Catalytic Olefin Metathesis

Abstract

Chapter 1. Development of Ru-Based Catechothiolate Complexes for Z-selective Ring-Opening/Cross-Metathesis and Cross-Metathesis. We have developed a broadly applicable Ru-catalyzed protocol for Z-selective ring-opening/cross-metathesis (ROCM). Transformations are promoted by 2.0–5.0 mol % of a Ru-based catechothiolate complex, furnishing products in up to 97 % yield and >98:2 Z:E ratio. The Z-selective ROCM processes are found to be compatible with terminal alkenes of different sizes that include the first examples involving heteroaryl olefins, 1,3-dienes, and O- and S-substituted alkenes as well as allylic and homoallylic alcohols. Reactions with an enantioenriched α-substituted allylic alcohol are shown to afford congested Z-olefins with high diastereoselectivity. The insights gained from these investigations provided the impetus to develop electronically modified Ru catechothiolate catalysts that are readily accessible from a commercially available dichloro-Ru carbene and an easily generated air-stable zinc catechothiolate. The new complex is effective in catalyzing Z-selective cross-metathesis (CM) of terminal alkenes and inexpensive Z-2-butene-1,4-diol to directly generate linear Z-allylic alcohols, including those that bear a hindered neighboring substituent or reactive functionalities such as a phenol, an aldehyde or a carboxylic acid. Transformations typically proceed with 5.0 mol % of the catalyst within 4–8 hours under ambient conditions, and products are obtained in up to 80% yield and 98:2 Z:E selectivity. Utility is highlighted through synthesis of a molecular fragment en route to anti-tumor agent neopeltolide and in a single-step stereoselective gram-scale conversion of renewable feedstock to synthetically valuable Z-allylic alcohols. Chapter 2. Kinetically Controlled Z- and E-Selective Cross-Metathesis to Access 1,2- Disubstituted Alkenyl Halides. We have discovered that previously unknown halo-substituted molybdenum alkylidenes are capable of participating in highly efficient olefin metathesis reactions that afford linear 1,2-disubstituted Z-alkenyl halides. Transformations are promoted by 1.0–10.0 mol % of a Mo-based pentafluorophenylimido monoaryloxide pyrrolide (MAP) complex that is generated in situ and used with unpurified, commercially available and easy-to-handle liquid 1,2-dihaloethene reagents, delivering a myriad of alkenyl chlorides, bromides and fluorides in up to 91% yield and >98:2 Z:E ratio. Through mechanism-based modification of the aryloxide ligand, a newly synthesized Mo-based MAP complex was shown to be effective in promoting kinetically controlled E-selective CM to access the corresponding thermodynamically less favored E-isomers of alkenyl chlorides and fluorides. Reactions typically proceed within 4 hours at ambient temperature with 1.0–5.0 mol % of the catalyst, which may be utilized in the form of air- and moisture-stable paraffin pellets. Utility of the aforementioned protocols is demonstrated through preparation of biologically active compounds and related analogues as well as late-stage site- and stereoselective fluorination of complex organic molecules. Chapter 3. Molybdenum-Based Chloride Catalysts for Z-Selective Olefin Metathesis. A new class of Mo-based monoaryloxide chloride (MAC) complexes for Z-selective olefin metathesis has been developed. The MAC catalysts are capable of promoting CM with commercially available, inexpensive and typically inert Z-1,1,1,4,4,4-hexafluoro-2-butene to furnish the higher-energy Z-isomers of trifluoromethyl-substituted alkenes in up to 95% yield and >98:2 Z:E selectivity. Furthermore, otherwise inefficient and non-stereoselective transformations with Z-1,2-dichloroethene and 1,2-dibromoethene can be accomplished with appreciably improved efficiency and Z-selectivity. The method enables synthesis of biologically active compounds and CF3-analogues of medicinally relevant molecules. Density functional theory (DFT) calculations shed light on the origins of the activity and selectivity levels observed in these transformations. Chapter 4. Stereoselective Synthesis of Z- and E-Trisubstituted Alkenes by Merging Cross-Coupling with Cross-Metathesis. We have discovered that challenging acyclic E- and Z-trisubstituted alkenes, particularly alkenyl chlorides and bromides, can be accessed efficiently and in high stereoisomeric purity (up to >98% E and 95% Z) through a sequence involving catalytic cross-coupling followed by stereoretentive CM promoted by Mo-based catalysts. Initial exploratory studies with 1,1-disubstiuted alkenes revealed crucial mechanistic features of the transformations that led us to utilize readily accessible trisubstituted olefins as substrates, in combination with commercially available 1,2-dihaloethenes as cross-partners for CM. Applications to synthesis of biologically active compounds and synthetic precursors underscore utility. The stereoretentive transformations may be extended to trisubstituted non-halogenated alkenes such as aliphatic olefins.