Ethenolysis: A Green Catalytic Tool to Cleave Carbon–Carbon Double Bonds

Remarkable innovations have been made in the field of olefin metathesis due to the design and preparation of new catalysts. Ethenolysis, which is cross‐metathesis with ethylene, represents one catalytic transformation that has been used with the purpose of cleaving internal carbon–carbon double bonds. The objectives were either the ring opening of cyclic olefins to produce dienes or the shortening of unsaturated hydrocarbon chains to degrade polymers or generate valuable shorter terminal olefins in a controlled manner. This Review summarizes several aspects of this reaction: the catalysts, their degradation in the presence of ethylene, some parameters driving their productivity, the side reactions, and the applications of ethenolysis in organic synthesis and in potential industrial applications.     

Highly selective ruthenium metathesis catalysts for ethenolysis

N-Aryl,N-alkyl N-heterocyclic carbene (NHC) ruthenium metathesis catalysts are highly selective toward the ethenolysis of methyl oleate, giving selectivity as high as 95% for the kinetic ethenolysis products over the thermodynamic self-metathesis products. The examples described herein represent some of the most selective NHC-based ruthenium catalysts for ethenolysis reactions to date. Furthermore, many of these catalysts show unusual preference and stability toward propagation as a methylidene species and provide good yields and turnover numbers at relatively low catalyst loading (<500 ppm). A catalyst comparison showed that ruthenium complexes bearing sterically hindered NHC substituents afforded greater selectivity and stability and exhibited longer catalyst lifetime during reactions. Comparative analysis of the catalyst preference for kinetic versus thermodynamic product formation was achieved via evaluation of their steady-state conversion in the cross-metathesis reaction of terminal olefins. These results coincided with the observed ethenolysis selectivities, in which the more selective catalysts reach a steady state characterized by lower conversion to cross-metathesis products compared to less selective catalysts, which show higher conversion to cross-metathesis products.     

Aromatic-amide-derived olefins as a springboard: isomerization-initiated palladium-catalyzed hydrogenation of olefins and reductive decarbonylation of acyl chlorides with hydrosilane

A highly efficient catalytic protocol for the isomerization of substituted amide-derived olefins is presented that successfully uses a hydride palladium catalyst system generated from [PdCl(2)(PPh(3))(2)] and HSi(OEt)(3). The Z to E isomerization was carried out smoothly and resulted in geometrically pure substituted olefins. Apart from the cis-trans isomerization of double bonds, the selective reduction of terminal olefins and activated alkenes was performed with excellent functional group tolerance in the presence of an amide-derived olefin ligand, and the products were obtained in high isolated yields (up to >99?%). Furthermore, the palladium/hydrosilane system was able to promote the reductive decarbonylation of benzoyl chloride when a (Z)-olefin with an aromatic amide moiety was used as a ligand.     

Cyclic Alkyl Amino Carbene (CAAC) Ruthenium Complexes as Remarkably Active Catalysts for Ethenolysis

An expanded family of ruthenium‐based metathesis catalysts bearing cyclic alkyl amino carbene (CAAC) ligands was prepared. These catalysts exhibited exceptional activity in the ethenolysis of the seed‐oil derivative methyl oleate. In many cases, catalyst turnover numbers (TONs) of more than 100 000 were achieved, at a catalyst loading of only 3 ppm. Remarkably, the most active catalyst system was able to achieve a TON of 340 000, at a catalyst loading of only 1 ppm. This is the first time a series of metathesis catalysts has exhibited such high performance in cross‐metathesis reactions employing ethylene gas, with activities sufficient to render ethenolysis applicable to the industrial‐scale production of linear α‐olefins (LAOs) and other terminal‐olefin products.     

In a Quest for Selectivity Paired with Activity: A Ruthenium Olefin Metathesis Catalyst Bearing an Unsymmetrical Phenanthrene-Based N-Heterocyclic Carbene

Robust, selective, and stable in the presence of ethylene, ruthenium olefin metathesis pre-catalyst, {[3-benzyl-1-(10-phenyl-9-phenanthryl)]-2-imidazolidinylidene}dichloro(o-isopropoxyphenylmethylene)ruthenium(II), Ru-3 , bearing an unsymetrical N-heterocyclic carbene (uNHC) ligand, has been synthesized. The initiation rate of Ru-3 was examined by ring-closing metathesis and cross-metathesis reactions with a broad spectrum of olefins, showing an unprecendented selectivity. It was also tested in industrially relevant ethenolysis reactions of olefinic substrates from renewable feedstock with very good yields and selectivities.     

Synthèses à l'aide de sulfones (XXIV) Synthèse stéréosélective d'oléfines par hydrogénolyse des vinylsulfones.

The now readily available E or Z vinylsulphones can be reduced stereospecifically to the corresponding olefins with sodium dithionite.     

Oxidative fragmentation of γ-hydroxyalkyl stannanes stereospecific formation of (e) and (z)-keto olefins

Treatment of γ-hydroxyalkyl stannanes with lead tetraacetate in refluxing benzene leads to the stereospecific formation of (E) and (Z)-keto olefins according to the stereochemistry of the starting materials in excellent yield.     

Photochemistry of 2-Cyclohexene-imines and 2,3,4,4a,5,6-Hexahydroquinolines. Preliminary communication

The 2-cyclohexene-imines 2b–2d and the hexahydroquinolines 5a, b are synthesized. n,π*-Excitation of these α,β-unsaturated imines leads to (E/Z) isomerization for compounds 2 while compounds 5 are unreactive. No cyclobutanes are formed from 2 or 5 under these conditions in the presence of olefins, and only 2d adds to 2,3-dimethyl-2-butene via the C?N bond to give an azetidine. On π,π* excitation 2 and 5 rearrange to the corresponding β,γ-unsaturated imines 8 and 9 with low efficiency. It is concluded that the failure of such imines to undergo [2+2]-photocycloadditions with olefins is not mainly due to radiationless decay via (E/Z) isomerization.     

The behavior of 2-chloroquinoline-3-carbaldehyde and 2-oxoquinoline-3-carbaldehyde toward stabilized methylenetriphenylphosphoranes and secondary amines

In this study use was made of the Wittig carbonyl olefination reaction and stereo-identification of the resulting alkenes. Condensation of 2-chloroquinoline-3-carbaldehyde with some selected stabilized phosphonium ylides yielded a mixture of the corresponding E and Z olefins in each case. On the other hand reaction of 2-oxoquinoline-3-carbaldehyde with the selected ylides afforded the respective olefins only in one of the possible stereoisomers. The reaction of 2-chloroquinoline-3-carbaldehyde with acetylmethylenetriphenylphosphorane produced the respective olefine together with acridin-3-ol. Heating of (E) and/or (Z)-methyl 3-(2-chloroquinolin-3-yl)acrylate with hydrazine hydrate yielded the corresponding propan-1-ol derivative. Dechlorination of (E)-ethyl 3-(2-chloroquinolin-3-yl)acrylate and/or (E)-4-(2-chloroquinolin-3-yl)but-3-en-2-one was effected upon treatment with morpholine or piperidine in absolute ethanol to give the respective enone derivatives.     

Stereoselective synthesis of trisubstituted olefins by a directed allylic substitution strategy

New methodology for the stereoselective synthesis of trisubstituted olefins is presented. The use of ortho-diphenylphosphanyl benzoate (o-DPPB) as a directing leaving group for copper-mediated allylic substitution with Grignard reagents allowed for the stereoselective construction of a wide range of E olefins, without the need for an adjacent electron-withdrawing group. Our modular three-step approach toward trisubstituted alkenes commenced with geminal α-methylene aldehydes. Addition of an organometallic reagent and introduction of the o-DPPB group by esterification was followed by the o-DPPB-directed copper-mediated allylic substitution with a Grignard reagent to furnish stereodefined trisubstituted olefins. Additionally, incorporation of a stereocenter from the chiral pool allowed the preparation of an enantiomerically pure olefin that bore three alkyl substituents in high E/Z selectivity.     

Ph3P/CF3CCl3作为一个新的羰基烯烃化试剂的研究—一锅法合成烯烃RCH=CClCF3

Ph3P/CF3CCl3与醛在室温下一锅反应生成三氟甲基取代的烯烃RCH=CClCF3(3). 芳香醛及α,β-不饱和醛的反应显示高度立体选择性, 给出Z式异构体. 而饱和脂肪醛的反应立体选择性较低, Z与E式异构体的比例为2:1至3:1. 反应可能是经由叶立德Ph3P-CClCF3的Wittig型反应. 本工作表明, Ph3P/CF3CCl3可以作为在十分温和的条件下向有机分子中引入=CClCF3结构单元的方便试剂.     

Efficient ruthenium carbenoid-catalyzed cross-metathesis of allyl halides with olefins

[reaction: see text] Cross-metatheses of allyl halides and terminal olefins mediated by catalyst 2 are reported and showed good yield and excellent E/Z selectivity. The application of these compounds as alkylating reagents is also demonstrated.     

Highly sterically hindered olefins: a case of E- and Z-di-tert-butyl alpha,beta-unsaturated acids

Use of a superbase in the Favorskii rearrangement of 12 resulted in the synthesis of highly sterically hindered olefins, (E)-2- tert-butyl-4,4-dimethyl-pent-2-enoic acid (4) and (Z)-2- tert-butyl-4,4-dimethyl-pent-2-enoic acid (3).     

Highly enantioselective hydrogenation of styrenes directed by 2'-hydroxyl groups

A new synthetic strategy that turns styrene-type olefins into excellent substrates for Rh-catalyzed asymmetric hydrogenation by installing a 2'-hydroxyl substituent is described. This methodology accommodates trisubstituted olefinic substrates in various E/Z mixtures, leading to valuable benzylic chiral compounds including (R)-tolterodine. It is also demonstrated that the 2'-hydroxyl groups could be readily removed in high yield without loss of ee from the products. Thus, this technology represents an attractive alternative to the Ir(P-N) catalyst system for the asymmetric hydrogenation of unfunctionalized olefins.     

Carbolithiation of diphenylacetylene as a stereoselective route to (Z)-tamoxifen and related tetrasubstituted olefins

Carbolithiation of diphenylacetylene can be exploited to generate (E)-1-lithio-1,2-diphenylalkyl-1-enes which can be reacted in situ with triisopropylborate to stereoselectively provide (E)-1,2-diphenyl-1-alkylene boronic acids. These tetrasubstituted vinylboronic acids served as versatile intermediates for the generation of tetrasubstituted olefins with retention of stereochemistry. The application of this method for the stereoselective synthesis of (Z)-tamoxifen and related analogues is described.     

Stereoselective Synthesis of （Z）-5-（Trideca-4-enyl）resorcinol and Gibbilimbols A-D

(Z)-5-(Trideca-4-enyl)resorcinol (1) and gibbilimbols A-D(2-5) were synthesized in 47%-60% yields over 6 steps from commercially available starting materials. The Wittig reaction of various alkyl phosphonium bromides with appropriate aldehydes in the presence of potassium tert-butoxide (t-BuOK) in anhydrous THF solution at room temperature served as the key step, and the result showed that only (Z)-configuration olefins were formed by this procedure. The synthesis of the (Z)-5-(trideca-4-enyl)resorcinol (1) was reported for the first time.     

Construction of multiporphyrin arrays via selective cross-metathesis

Selective cross-metathesis of type I and type II meso-functionalized porphyrin olefins afforded alkenyl-coupled dimeric and trimeric porphyrin systems in good yield with excellent E/Z selectivity. The synthetic utility of the method is demonstrated through the preparation of mixed metalated (M = 2H, Zn) porphyrin dimer and trimer. [reaction: see text]     

Synthesis of trifluoro- or difluoromethylated olefins by regio- and stereocontrolled S(N)2' reactions of gem-difluorinated vinyloxiranes

This paper presents a highly stereoselective synthesis of trifluoro- or difluoromethylated olefins via an S(N)2' type fluorination or reductions of gem-difluorinated vinyloxiranes. Their fluorination with HF-Py furnished trifluoromethylated allylic alcohols with exclusive E selectivity. On the other hand, their reduction with DIBAL-H afforded difluoromethylated E-allylic alcohols predominantly, whereas the corresponding Z isomers were formed exclusively by treatment with BH3.THF.     

Direct and Tandem Routes for the Copolymerization of Ethylene with Polar Functionalized Internal Olefins

Transition metal catalyzed ethylene copolymerization with polar monomers is a highly challenging reaction. After decades of research, the scope of suitable comonomer substrates has expanded from special to fundamental polar monomers and, recently, to 1,1‐disubstituted ethylenes. Described in this contribution is a direct and tandem strategy to realize ethylene copolymerization with various 1,2‐disubstituted ethylenes. The direct route is sensitive to sterics of both the comonomers and the catalyst. In the tandem route, ruthenium‐catalyzed ethenolysis can convert 1,2‐disubstituted ethylenes into terminal olefins, which can be subsequently copolymerized with ethylene to afford polar functionalized polyolefins. The one‐pot, two‐step tandem route is highly versatile and efficient in dealing with challenging substrates. This work is a step forward in terms of expanding the substrate scope for transition metal catalyzed ethylene copolymerization with polar‐functionalized comonomers.     

Heteroatom-guided torquoselective olefination of alpha-oxy and alpha-amino ketones via ynolates

Ynolates were found to react with alpha-alkoxy-, alpha-siloxy-, and alpha-aryloxyketones at room temperature to afford tetrasubstituted olefins with high Z selectivity. Since the geometrical selectivity was determined in the ring opening of the beta-lactone enolate intermediates, the torquoselectivity was controlled by the ethereal oxygen atoms. From experimental and theoretical studies, the high Z selectivity is induced by orbital and steric interactions rather than by chelation. In a similar manner, alpha-dialkylamino ketones provided olefins with excellent Z selectivity. These products can be easily converted into multisubstituted butenolides and gamma-butyrolactams in good yield.