Ethers alkyl dehydrohalogenation

This procedure gives typical reaction conditions for the alkylation and hydroxy-alkylation of alkyl vinyl ethers via the 1-potassio- and 1-lithio derivatives, respectively. The alkylation of the potassium compounds with alkyl bromides proceeds smoothly at temperatures in the region of —40 °C. Alkylation is often accompanied by dehydrohalogenation. In the experiment described below the ratio alkylation dehydrohalogenation is high. Under similar conditions vinylpotassium H2C=CH—K gave a much lower yield of the alkylation product (1-decene) in its... 

Polystyrene-bound alkylating agents can react with phenols under basic conditions to yield aryl ethers. The reaction conditions must be carefully adjusted for halides bearing (3-hydrogens so as to prevent dehydrohalogenation. This is, of course, no prob-... 

Dehydrohalogenation. This base is superior to DBU for dehydrohalogenation of secondary alkyl bromides or iodides. The reaction can be conducted at 0° in DME, ether, benzene, or diglyme. Secondary alkyl chlorides undergo slow dehydrohalogenation under the same conditions, and primary alkyl halides are completely stable. 

Dehydrohalogenatioiu Alkyl halides can be converted into alkenes by solid potassium 1-butoxide and a catalytic amount of 18-crown-6. Petroleum ether with a suitable boiling point range is used as solvent. No reaction is observed in the absence of a crown ether other bases (KOH, K2CO3) are ineffective. The method is particularly useful for dehydrohalogenation of primary alkyl halides, which are converted into ethers by potassium f-butoxide under usual conditions. ... 

Dehydrohalogenation by oxygen bases from vinyl halides proceeds readily by trans elimination via an 2-type of mechanism. c/s-Elimination is sluggish or does not occur at all. Thus trans elimination from bromovinyl ethers 2 (R = alkyl) furnishes the acetylenes 3 in a fast reaction in about 90% yield, whereas elimination from 4 (R = alkyl) is very sluggish and yields a mixture of the acetylene 3 and allene 5 (equations 17 and 18). Similar observations were shown in Schemes 1 and... 

Dehydrohalogenation of Alkyl Halides Dehalogenation of 12-Dihalides Dehydration of Alcohols Alkenes from Ethers... 

If the two halogens are on the same or adjacent carbons, two consecutive E2 dehydrohalogenations can result in the formation of a triple bond. The Williamson ether synthesis involves the reaction of an alkyl halide with an alkoxide ion. If the two functional groups of a bifunctional molecule can react with each other, both intermolecular and intramolecular reactions can occur. The reaction that is more likely to occur depends on the concentration of the bifunctional molecule and the size of the ring that will be formed in the intramolecular reaction. 

The most general and well-developed procedures for the preparation of acetylenic ethers are based on dehydrohalogenation or dehalogenation of various haloacetals (for example, acetals of chloroacetaldehyde 52) or haloalkenyl ethers 53 (equation 34) . The first product in these eliminations usually is alkoxyacetylide 54 which can be subsequently quenched with a variety of electrophiles such as water, alkyl halides, ketones, etc to give the final product 55. 

Alkylation of Amides, Phenols, Alcohols, and Adds. A variety of carboxamides were alkylated with primary alkyl halides using KOH in DMSO to give the fYalkyl amides (eq 1) in 54—90% yield. Most reactions were carried out at rt, but in some cases heating to 90 °C was required. Similar conditions were applied to alcohols, phenols, and acids to form ethers and esters. The procedure applies to Mel and all primary halides. Secondary alkyl halides show evidence of competitive dehydrohalogenation, while tertiary halides do not give any alkylation products. The procedure was applied to the N- and O-permethylation of peptides. It was also applied to the methylation of hydroxypyridines in 39-78% yield. In all the above cases, the substrate and alkyl halide were added to powdered KOH in DMSO and stirred at rt. It was unnecessary to use especially dry DMSO or to protect the reaction mixture from atmospheric moisture. 

Dehydrohalogenation of alkyl halides in the presence of strong base (E2) is often accompanied by the formation of substitution (Sn2) products. The extent of the competitive substitution reaction depends on the structure of the alkyl halide. Primary alkyl halides give predominantly substitution products (the corresponding ether), secondary alkyl halides give predominantly elimination products, and tertiary alkyl halides give exclusively elimination products. For example, the reaction of 2-bromopropane with sodium ethoxide proceeds as follows ... 

The most reliable and most general method of alkyl enol ether formation is the halogenation of an alkene in alcohol solvent and the subsequent elimination of HX (dehydrohalogenation) (Scheme 8.94). 

Polymeric amines can be proton acceptors, acyl transfer agents, or ligands for metal ions. The 2- and 4-isomers of poly(vinylpyridine) (11) and (12) and the weakly basic ion exchange resins, p-dimethylaminomethylated PS (2) and poly(2-dimethylaminoethyl acrylate), are commercial. The ion exchange resins are catalysts for aldol condensations, Knoevenagel condensations, Perkin reactions, cyanohydrin formation and redistributions of chlorosilanes. " The poly(vinylpyridine)s have been used in stoichiometric amounts for preparation of esters from acid chlorides and alcohols, and for preparation of trimethylsilyl ethers and trimethylsilylamines from chlorotrimethylsilane and alcohols or amines. Polymer-suppored DBU (l,8-diazabicyclo[5.4.0]undec-7-ene) (52) in stoichiometric amounts promotes dehydrohalogenation of alkyl bromides and esterification of carboxylic acids with alkyl halides. The protonated tertiary amine resins are converted to free base form by treatment with aqueous sodium hydroxide. 

In addition to the cross-coupling reactions of alkyl halides or sulfonates with organometallic nucleophiles, the former can also directly undergo C-C bond formation with nonactivated C-H bonds of alkenes or arenes in a Heck-type reaction. Iron-catalyzed intramolecular dehydrohalogenation of 2-iodoethanal alkenyl acetals with phenyl-magnesium bromide in the presence of a catalytic amount of iron(II) chloride provides tetrahydrofliran derivatives in moderate yields (Scheme 4-240). Allyl 2-halophenyl ethers provide dihydrobenzofiiran derivatives under these conditions. ... 

---