Ethers reactions with alkene

In addition to insertion into p-C—H bonds, cyclopropylidenes can undergo other reactions such as alkylation (c/. Section 4.7.3.2), dimerization, insertion into C—H bonds of the ether solvent (equation 60)183 or reaction with alkenes to afford spirocyclopropanes (equation 61).184 Addition of stoichiometric amounts of Bu OK has been shown to promote the reactions of lithium carbenoids, even at -83 C, with THF to give the insertion product (equation 62).185 Addition to alkenes is also promoted under these conditions. Intramolecular addition of the carbenoid to double bonds has been exploited in the synthesis of spirotricyclic compounds (equation 63).186... 

Dicarbonyl(ri5-cyclopentadienyl)iron-alky] complexes represent useful precursors for iron-carbene complexes [47]. For example, iron-carbene complexes are intermediates in the acid-promoted reaction of Fp-alkyl ether derivatives with alkenes to provide cyclopropanes via a [2 + l]-cycloaddition (Scheme 1.16). 

Iodonium salts 46 have been proposed as the reactive intermediates in several synthetically useful carbon-carbon bond forming reactions [1,40]. Reactions of adducts 46 with various silyl enol ethers selectively afford 1,4-diones, while the reactions with alkenes lead to the products of alkylation at the allylic position (Scheme 22) [40]. 

DOT CLASSIFICATION 5.1 Label Oxidizer SAFETY PROFILE Moderately toxic by intraperitoneal route. Severe skin and eye irritant. A powerful oxidizer which has caused many explosions in industry. Potentially explosive reactions with alkenes (above 220°C), ammonia, arjl hydrazine + ether, dimethyl sulfoxide + heat, ethylene oxide, fluorobutane + water, organic materials, phosphorus, trimethyl phosphate. Reacts to form explosive products with ethanol (forms ethyl perchlorate), cellulose + dinitrogen tetraoxide + oxygen (forms cellulose nitrate). Avoid contact with mineral acids, butyl fluorides, hydrocarbons. A drying agent. When heated to decomposition it emits toxic fumes of MgO and Cr. See also MAGNESIUM COMPOUNDS and PERCHLORATES. 

The early literature associated with monochloroborane hydroborations contains errors and it is probably appropriate to ignore publications prior to about 1970. Monochloroborane complexes THE more strongly than does borane and this slows down its reactions with alkenes in this solvent. Thus, other reactions may compete and mixtures may result. The diethyl ether complex is weaker and hydroborations... 

If alkyldichloroboranes are specifically required, dichloroborane-dimethyl sulfide is the reagent of choice. It is more stable and more convenient than the dichloroborane-diethyl ether complex, but its hydroborating properties are very similar." Dichloroborane complexes ethers even more strongly than monochloroborane, and its reactions with alkenes in this solvent are slow and lead to mixtures. Therefore, it is generally used in pentane and trichloroborane is added to liberate uncomplexed dichloroborane. Under these conditions it readily gives alkyldichloroboranes on reaction with alkenes or alkenyldichloro-boranes on reaction with alkynes. " " The latter reaction has been applied to alkynylsilanes (equation 44)." ... 

Susceptible positions of organic compounds can be directly acyloxylated " by ferf-butyl peroxyesters, the most frequently used being acetic and benzoic (R = Me or Ph). " The reaction requires a catalyst (cuprous ion is the actual catalyst, but a trace is all that is necessary, and such traces are usually present in cupric compounds, so that these are often used) and without it is not selective. Susceptible positions are similar to those in 14-6 benzylic, allylic, and the a position of ethers and sulfides. Terminal alkenes are substituted almost entirely in the 3 position, that is, with only a small amount of allylic rearrangement, but internal alkenes generally give mixtures containing a large amount of allylic-shift product. If the reaction with alkenes is carried out in an excess of... 

The reaction between CHCI3 and HO- is often carried out under phase transfer conditions. " It has been shown that the reaction between PhCHCl2 and t-BuOK produces a carbenoid, but when the reaction is run in the presence of a crown ether, the free Ph(Cl)C is formed instead. The reaction of iodoform and CrCl2 leads to iodocyclopropanes upon reaction with alkenes. Dihalocyclopropanes are very useful compounds that can be reduced to cyclopropanes, treated with magnesium or sodium to give allenes (18-3), or converted to a number of other products. 

Halofurans and halobenzofurans readily participate in Heck reactions with alkenes, and it is possible to synthesize benzofurans via intramolecular Heck reactions of aryl vinyl ethers. The heteroaromatic behavior of furans enable them to undergo heteroaryl Heck reactions to provide substituted furans. 

Returning to a reaction we met right at the start of this chapter will illustrate that the regio-and stereochemistry of many different electrophilic reactions with alkenes can be controlled by intramolecular nucleophiles. The mercuration of the cis alkene Z-184 leads to a 6 1 ratio of diastereoisomers of a cyclic ether 185 by a related trapping of the intermediate by the internal OH group. 

The ene and Prins reactions are not mechanistically distinct. Coverage will therefore be organized by the nature of the carbonyl compound, with intermolecular reactions presented first, followed by intramolecular reactions. The emphasis will be on material published since the field has been reviewed " and on examples demonstrating the stereo-, regio- and chemo-selectivity of these reactions. Coverage is restricted to the addition of carbonyl and thiocarbonyl compounds to simple alkenes. Addition of carbonyl compounds to vinylsilanes, allylsilanes and enol ethers is covered in the following chapters. Addition of imines and iminium compounds to alkenes is presented in Part 4 of this volume. Ene reactions with alkenes and alkynes as enophiles are covered in Volume 5, Chapter 1.1. Use of aldehydes and acetals as initiators for polyene cyclizations is covered in Volume 3, Chapter 1.6. 

Synthetically useful precursors of oxygen-substituted carbenes (see Houben-Weyl Vol. E19b. pp 1628-1682) are diazirines (photolysis or thermolysis), a-halogen ethers (base treatment), a-dihalogen ethers (treatment with alkyllithium compounds), and stable carbene complexes of the Fischer type (thermolysis). Only the mechanism-based stereoselectivity and the simple diastcreoselectivity of their reactions with alkenes have been studied to date. 

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