Reactions to form alkenes

Quaternary ammonium salts as we have seen are useful m synthetic organic chem istry as phase transfer catalysts In another more direct application quaternary ammo mum hydroxides are used as substrates m an elimination reaction to form alkenes... 

Alcohols, like alkyl halides, can undergo elimination reactions to form alkenes (Following fig.). Since water is eliminated, the reaction is also called a dehydration. 

Sulfonyl carbanions undergo aldol-type reactions with aldehydes and ketones these are very important owing to the ability of the resultant p-hydroxysulfones to participate in elimination reactions to form alkenes (the Julia reaction, see p. 197). Early experiments used Grignard reagents to form the magnesium sulfonyl carbanions. By this procedure, methyl phenyl sulfone (104) is transformed into the p-hydroxysulfone (105) which subsequently can be either oxidised to the ketone (106) or dehydrated to the alkene (107) (Scheme 45). 

New reactions involve the cleavage of a C-C bond inside the ring to form biradical intermediates, which quickly rearrange themselves into molecular products initiation, in this case, acts as an isomerization reaction to form alkenes. On the basis of available thermochemical evaluations (Benson, 1976), the suggested reference kinetic parameters are... 

Monocyclic Phosphoranes. - Further studies of the reaction of P-fluoro-ylids (53) with carbonyl compounds (54) reveals formation of P-fluoro-oxaphosphetans (55) as the initial addition products. These may decompose in a conventional Wittig reaction to form alkenes (56)21 ajgo lose HF to form alkyl phosphonates (57).22... 

Step 3 IS new to us It is an acid-base reachon m which the carbocation acts as a Br0n sted acid transferrmg a proton to a Brpnsted base (water) This is the property of carbo cations that is of the most significance to elimination reactions Carbocations are strong acids they are the conjugate acids of alkenes and readily lose a proton to form alkenes Even weak bases such as water are sufficiently basic to abstract a proton from a carbocation... 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

When applied to the synthesis of ethers, the reaction is effective only with primary alcohols. Elimination to form alkenes predominates with secondary and tertiary alcohols. 

The addition, therefore, follows Markovnikov s rule. Primary alcohols give better results than secondary, and tertiary alcohols are very inactive. This is a convenient method for the preparation of tertiary ethers by the use of a suitable alkene such as Me2C=CH2. Alcohols add intramolecularly to alkenes to generate cyclic ethers, often bearing a hydroxyl unit as well. This addition can be promoted by a palladium catalyst, with migration of the double bond in the final product. Rhenium compounds also facilitate this cyclization reaction to form functionalized tetrahydrofurans. 

Carbanions derived from phosphine oxides also add to carbonyl compounds. The adducts are stable but undergo elimination to form alkene on heating with a base such as sodium hydride. This reaction is known as the Horner-Wittig reaction.268... 

In 1996, the Knochel group reported their observations that primary alkyl iodides with a C=C bond and three or four carbons between the halogen and the sp2)C atom 300 reacted with dialkylzincs to form alkene 301 (Scheme 152).405 The reaction took place in the presence of a catalytic amount of Ni(acac)2. 

Electrophilic substitution of the ring hydrogen atom in 1,3,4-oxadiazoles is uncommon. In contrast, several reactions of electrophiles with C-linked substituents of 1,3,4-oxadiazole have been reported. 2,5-Diaryl-l,3,4-oxadiazoles are bromi-nated and nitrated on aryl substituents. Oxidation of 2,5-ditolyl-l,3,4-oxadiazole afforded the corresponding dialdehydes or dicarboxylic acids. 2-Methyl-5-phenyl-l,3,4-oxadiazole treated with butyllithium and then with isoamyl nitrite yielded the oxime of 5-phenyl-l,3,4-oxadiazol-2-carbaldehyde. 2-Chloromethyl-5-phenyl-l,3,4-oxadiazole under the action of sulfur and methyl iodide followed by amines affords the respective thioamides. 2-Chloromethyl-5-methyl-l,3,4-oxadia-zole and triethyl phosphite gave a product, which underwent a Wittig reation with aromatic aldehydes to form alkenes. Alkyl l,3,4-oxadiazole-2-carboxylates undergo typical reactions with ammonia, amines, and hydrazines to afford amides or hydrazides. It has been shown that 5-amino-l,3,4-oxadiazole-2-carboxylic acids and their esters decarboxylate. 

Among all the reactions of nitroso compounds the most widely studied has been homolysis to generate NO [189]. However, other reactions are known for example, N-nitrosoaziridenes lose N2O to form alkenes in a cheletropic fragmentation [190]. In contrast, N-nitrosimines (Scheme 3.21) spontaneously lose nitrogen rather than... 

Good yields of 1° amines are obtained by using large excess of ammonia. The ammonolysis proceeds best by using primary alkyl halides. The tertiary alkyl halides tend to form alkenes due to the dominance of the competing elimination reaction. 

Oxidation is the first step for producing molecules with a very wide range of functional groups because oxygenated compounds are precursors to many other products. For example, alcohols may be converted to ethers, esters, alkenes, and, via nucleophilic substitution, to halogenated or amine products. Ketones and aldehydes may be used in condensation reactions to form new C-C double bonds, epoxides may be ring opened to form diols and polymers, and, finally, carboxylic acids are routinely converted to esters, amides, acid chlorides and acid anhydrides. Oxidation reactions are some of the largest scale industrial processes in synthetic chemistry, and the production of alcohols, ketones, aldehydes, epoxides and carboxylic acids is performed on a mammoth scale. For example, world production of ethylene oxide is estimated at 58 million tonnes, 2 million tonnes of adipic acid are made, mainly as a precursor in the synthesis of nylons, and 8 million tonnes of terephthalic acid are produced each year, mainly for the production of polyethylene terephthalate) [1]. 

Cobalt, as its CpCo(CO)2 complex, has proven to be especially suited to catalyze [2 + 2 + 2] cycloadditions of two alkyne units with an alkyne or alkene. These cobalt-mediated [2 + 2 + 2] cycloaddition reactions have been studied in great detail by Vollhardt337. The generally accepted mechanism for these cobalt mediated cycloadditions, and similar transition metal mediated cycloadditions in general, has been depicted in equation 166. Consecutive co-ordination of two triple bonds to CpCo(CO)2 with concomitant extrusion of two molecules of carbon monoxide leads to intermediates 578 and 579 via monoalkyne complex 577. These react with another multiple bond to form intermediate 580. The conversion of 578 to 580 is said to be kinetically favored over that of 579 to 580. Because intermediates like 580 have never been isolated, it is still unclear whether the next step is a Diels-Alder reaction to form the final product or an insertion to form 581. The exact circumstances might determine which pathway is followed. 

Alcohols can react in several ways, depending on the reactants and on the conditions of the reaction. For example, alcohols can undergo substitution with halogen acids, elimination to form alkenes, and oxidation to form aldehydes, ketones, or carboxylic acids. 

Condensation of ester (44e) with the anion of malononitrile gave the alkene (44g) <91JPR35>. Oxadiazole (43e) and triethyl phosphite gave a methane phosphate which underwent a Wittig reation with aldehydes ArCHO to form alkenes (43f). When the alkyl side chain contained an active methylene group, as in (43g), reaction with arenediazonium salts ArN2X yielded arylhydrazones (43h) <88LA909>. 

In sharp contrast to simple alkenes that undergo silylcarbonylation, genuine silylformylation of an olefinic moiety is attained only by an intramolecular technique (Equation (33)). Since the most serious competitive reaetion is an intermolecular hydrosilylation reaction to form 192, the key for the successful transformation depends on an appropriate combination of substituents R and R. ... 

During an elimination reaction, atoms are removed, or eliminated, from adjacent carbons on a carbon chain, producing a small molecule, which is often water. A double bond forms between the adjacent carbons, producing an alkene. For example, ethyl alcohol (ethanol) can undergo an elimination reaction to form ethylene by the loss of H and OH, which form H2O (water). 

The reduction of alkynes occurs in two steps addition of one mole of hydrogen atoms to form alkenes, and then addition of the second mole of hydrogen to alkenes to form alkanes. This reaction proceeds through a cis-alkene intermediate, hut cannot he stopped at this stage except with the use of a special catalyst. 

The most common method to prepare cyclopropenyl derivatives is the reaction between an electrophilic carbenoid and an alkene. On the other hand, sp3-geminated organodimetal compounds possess two nucleophilic sites on the same carbon, so should lead to nucleophilic [2+ 1] reaction with 1,2-diketones. Indeed, the reaction of bis(iodozincio)methane (3) with 1,2-diketones shows a novel [2+1] reaction to form c -cyclopropanediol diastereoselectively as shown in Scheme 3467. 

Atkenes from sic-dihaiides. The reaction of /c-dihalides with NaSeCH3 or NaSeC6H5 in ethanol or THF-HMPT (3 1) results in alkenes. The reaction involves a formal a rr-elimination in the case of wc-dibromides or uie-chloroiodidcs, but a formal xyn-elimination in the case of w c-dichlorides. Elimination to form alkenes also occurs with /K-haloalkyl phenyl selenides. 

Interception of the anion intermediate in the Shapiro Reaction to form trisubstituted alkenes... 

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