Propargylated aromatic compounds

Use of cationic thiolate-bridged diruthenium complexes 84 promotes the catalytic propargylation of aromatic compounds with propargylic alcohols bearing not only a terminal alkyne but also an internal alkyne unit (Equation (34)). " A variety of propargylated aromatic compounds are isolated in high to excellent yields. Although... 

Propargylic alcohols bearing a terminal triple bond react with electron-rich aromatic compounds in the presence of thiolate-bridged diruthenium complexes to give the propargylated aromatic compounds.30 l-Phenylprop-2-yn-l-ol, for example, reacts with 2-methylfuran to form (15). Intramolecular examples of the reaction were also reported. The process is believed to involve electrophilic attack by the ruthenium-stabilized propargyl cation. 

Proparg lation of Aromatic Compounds with Propargylic Alcohols 233 7.3... 

As described in the previous sections, a variety of nucleophiles attack the Cy atom of ruthenium-allenylidene intermediates. Aromatic compounds should also be suitable candidates and this was found to be the case [30]. Thus, reactions of propargylic alcohols with heteroaromatic compounds such as furans, thiophenes, pyrroles, and indoles in the presence of a diruthenium catalyst such as la proceeded smoothly to afford the corresponding propargylated heteroaromatic compounds in high yields with complete regioselectivity (Scheme 7.25). The reaction is considered to be an electrophilic aromatic substitution if viewed from the side of aromatic compounds. 

Scheme 7.26 Propargylation of aromatic compounds with propargylic alcohols.

Quite recently, some mononuclear ruthenium complexes such as [(p-cymene)RuX-(CO)(PR3)]OTf (X = Cl, OTf, R = Ph, Cy) have been found to work as catalysts for the propargylation of aromatic compounds such as furans, where some ruthenium complexes were isolated as catalytically active species from the stoichiometric reactions of propargylic alcohols (Scheme 7.27) [31]. The produced active species promoted the propargylation of furans vdth propargylic alcohols bearing not only a terminal alkyne moiety but also an internal alkyne moiety, indicating that this propargylation does not proceed via allenylidene complexes as key intermediates. 

In 2003, Torre and his co-workers prepared new terpene-aromatic hybrids 47 in high yields in the reactions of cobalt-coordinated propargylic compounds, derived from commercial (7iJ)-myrtenal, with a variety of aromatic compounds (Equation (23)). Interestingly, any terpene-aromatic hybrids are not produced from propargylic compounds without cobalt coordination. 

Thiolate-bridged diruthenium complexes such as Cp RuCl(p2-SR)2RuCp Cl catalyze the propargylic substitution reaction of propargylic alcohol derivatives with various carbon-centered nucleophiles [118-120]. Ketones [119] (Eq. 88), aromatic compounds [120] (Eq. 89), or alkenes thus selectively afford the corresponding propargylated products with C-C bond formation. An allenylidene intermediate is proposed in these reactions. They are detailed in the chapter Ruthenium Vinylidenes and Allenylidenes in Catalysis of this volume. 

Electron-rich aromatic componnds are readily alkylated at room temperature with propargyl complexes to form, after demetallation, 2-propyne-snbstituted aromatic compounds. Depending on the snbstitution pattern of the alkyne complex and the aromatic substrate, alkylation occurs at times with very high regioselectivity. This type of annulation was used in a synthesis of psendoipterosin... 

In addition to their reactions with trlmethylsilyl enol ethers, (propargyl1um)Co2(C0)g complexes react with a variety of other mild carbon nucleophiles including activated aromatic compounds, g-dicarbonyl compounds, other enol derivatives (enol acetates and ketones directly), allylsilanes, and alkyl- and alkynyl-aluminum reagents. These reactions provide a flexible means to introduce the synthetically versatile propargyl function. Key features of propargylations using these complexes are 1) ready... 

The reaction of propargyl alcohols with dicobalt octacarbonyl to give the complex salts 148 (X = BF4 or PF6) and synthetic uses of the latter have been reviewed. The salts react with electron-rich aromatic compounds ArH, such as anisole, phenol or N,N-dimethylaniline, to yield substitution products 149 after oxidative demetallation with an iron(III) or cerium(I V) salt with j5-diketones or j -keto esters the corresponding propargyl-substituted compounds 150 are obtained k Acetone reacts in an analogous fashion to give 151. The action of the cobalt complexes 148 on allylsilanes 152 leads to enynes 153. Indole reacts with the complex 148 (R = H R = R = Me) in the presence of boron trifluoride etherate to give 154, which was converted into 155 by the action of iron(III) nitrate " ... 

Propargylation. Propargylic alcohols can be converted into stabilized cations by reaction with Co2(CO)s and then with tetralluoroboric acid. The resulting cation propargylates activated aromatic compounds. The —Co2(CO)g unit is removed by treatment with ferric nitrate. An example is shown in equation (1). The steps can be conducted in sequence without purification of intermediates. The complex of propargyl alcohol itself reacts at both the ortho- andpara-positions of anisole. 

In general, unsaturated heterocyclic compounds are generated from the reactions of allyl-type 1,3-dipole precursors with either alkynes or alkenes as well as the reactions of propargyl/allenyl-type 1,3-dipoles with alkenes, while aromatic compounds are often accessible by the cycloaddition reactions of propargyl/allenyl type 1,3-dipoles and alkynes (Scheme 16.2). To obtain aromatic compounds from the former type of cycloaddition reactions, the resulting unsaturated cycloadducts are required to undergo further aromatization, such as oxidation and elimination of small molecules (i.e., H2O, CH3COOH, CO2, etc.). 

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