Alkenes Sodium amide

Alkyne Sodium Ammonia Trans alkene Sodium amide ... 

Acetylene and terminal alkynes are more acidic than other hydrocarbons They have s of approximately 26 compared with about 45 for alkenes and about 60 for alkanes Sodium amide is a strong enough base to remove a proton from acetylene or a terminal alkyne but sodium hydroxide is not... 

The most striking difference between alkenes and alkynes is that terminal alkynes are weakly acidic. When a terminal alkyne is treated with a strong base, such as sodium amide, Na+ -NH2, the terminal hydrogen is removed and an acetylide anion is formed. 

The overall sequence of three steps may be called the Wittig reaction, or only the final step. Phosphonium salts are also prepared by addition of phosphines to Michael alkenes (hke 15-8) and in other ways. The phosphonium salts are most often converted to the ylids by treatment with a strong base such as butyllithium, sodium amide, sodium hydride, or a sodium alkoxide, though weaker bases can be used if... 

By Wittig and related reactions (3-Dimethylaminopropyl)-triphenylphosphorane, 119 Sodium amide, 278 Vinyl(triphenyl)phosphonium bromide, 343 (E)-Alkenes By elimination reactions Arylselenocarboxamides, 22 Dichlorobis(cyclopentadienyl)-titanium, 102 Hydrogen peroxide, 145 From three-membered heterocycles Tributyltinlithium-Trimethylalu-minum, 320 Trisubstituted alkenes Chloromethyldiphenylsilane, 74 Organocopper reagents, 207 Alkenes (Methods to form alkenes)... 

Triple bonds are linear and the carbons are sp-hybridized (Figure 3.16). Alkynes, like alkenes, undergo addition reactions. A hydrogen connected to a triply bonded carbon is weakly acidic and can be removed by a very strong base such as sodium amide, NaNH2, to give acetylides. 

Horner-Wittig modification Alternatively, phosphine oxide reacts with aldehydes in the presence of a base (sodium amide, sodium hydride or potassium t-butoxide) to give an alkene. The phosphine oxide can be prepared by the thermal decomposition of alkyl-triphenylphosphonium hydroxide. Deprotonation of phosphine oxide with a base followed by addition to aldehyde yields salt of (3-hydroxy phosphineoxide, which undergoes further syn-elimination of the anion Ph2P02. The lithium salt of (3-hydroxy phosphineoxide can be isolated, but Na and K salt of (3-hydroxy phosphine oxide undergoes in situ elimination to give alkene (Scheme 4.26). 

Initial studies of solvent effects, on the reactions of triarylarsonium benzoylylides with p-nitrobenzaldehyde in N, A-dimethylformamide, dimethyl sulphoxide or methanol, indicated little solvent effect in these cases" ", but later studies of the more finely balanced reactions of semi-stabilized ylides have provided examples of strong influences due to the effect of different base and solvent when the ylide is generated in the presence of a carbonyl compound ". Thus, when benzyltriphenylarsonium bromide or p-chloroben-zyltriphenylarsonium bromide were treated with sodium hydride in benzene in the presence of a variety of p-substituted benzaldehydes the products were alkenes, but if sodium ethoxide in ethanol was used the isolated products were epoxides ". Likewise, when triphenylarsonium benzylylide was generated by phenyllithium in the presence of either benzaldehyde or acetaldehyde, the preponderant product was the epoxide whereas use of sodium amide as base provided mostly the alkene . Similar results were obtained when an allyltriphenylarsonium salt was deprotonated using different hexamethyldisilaz-... 

Typically, nonstabilized ylides are utilized for the synthesis of (Z)-alkenes. In 1986, Schlosser published a paper summarizing the factors that enhance (Z)-selectivity. Salt effects have historically been defined as the response to the presence of soluble lithium salts. Any soluble salt will compromise the (Z)-selectivity of the reaction, and typically this issue has been resolved by the use of sodium amide or sodium or potassium hexamethyldisilazane (NaHMDS or KHMDS) as the base. Solvent effects are also vital to the stereoselectivity. In general, ethereal solvents such as THF, diethyl ether, DME and t-butyl methyl ether are the solvents of choice." In cases where competitive enolate fomnation is problematic, toluene may be utilized. Protic solvents, such as alcohols, as well as DMSO, should be avoided in attempts to maximize (Z)-selectivity. Finally, the dropwise addition of the carbonyl to the ylide should be carried out at low temperature (-78 C). Recent applications of phosphonium ylides in natural product synthesis have been extensively reviewed by Maryanoff and Reitz. 

With some very strong bases, such as alkyl-sodium or -lithium compounds or sodium amide, ethers can be converted to alkenes (Scheme 23). The reaction is supported by electron-withdrawing groups in the p-position thus, Et0CH2CH(C02R)2 can be converted to H2C==C(C02R)2 (retro-Michael-type reaction). 

In dehydrohalogenation reactions, hydrogen and halogen are the atoms eliminated from adjacent carbons. Bases such as potassium hydroxide and sodium amide are the reagents. Both alkenes and alkynes can be synthesized by dehydrohalogenation. 

Dehydrohalogenation [1, 1035-1037, at end]. In a general procedure for the conversion of alkenes into alkynes by bromination and dehydrobromination, Ward and van Dorp recommend sodium amide and liquid ammonia for the dehydrobromination step.103 The preparation of propargyl aldehyde diethyl acetal is an example ,cb... 

Terminal alkynes contain an acidic proton, which can be deprotonated by sodium amide (NaNH2). The negative charge of the alkynyl (or acetylide) anion resides in an sp-orbital. This is more stable than a vinyl anion (produced on deprotonation of an alkene), because these are sp2 hybridised. The greater the s character, the more closely the anion is held to the positively charged nucleus (which stabilises it). 

In anionic polymerization, the initiator is a nucleophile that reacts with the alkene to form a propagating site that is an anion. Nucleophilic attack on an alkene does not occur readily because alkenes are themselves electron rich. Therefore, the initiator must be a very good nucleophile, such as sodium amide or butyllithium, and the alkene must contain an electron-withdrawing substituent to decrease its electron density. Some alkenes that imdergo polymerization by an anionic mechanism are shown in Table 28.5. 

Elimination reactions can also be used to prepare alkynes. Dihalides can be converted into alkynes by treatment with base, provided that the two halogen atoms are on the same, or adjacent, carbon atoms. In general, alkyne formation requires more drastic reaction conditions than alkene formation, and a strong base, such as sodium amide, is usually employed (Reactions 5.4 and 5.5). The product of Reaction 5.5, phenylethyne, is shown in Figure 5.1 as a ball and stick model. 

The relative amount of base has a significant effect on the product distribution. When the tosyl hydrazone of pinacolone was treated with > 1.8 equivalents of n-butyllithium, 3,3-dimethyl-l-butene was formed. When only 1 equivalent of n-butyllithium was used, however, a 57% yield of 3,3-dimethyl-l-butene, 40% of 1,1,2-trimethylcyclopropane, and 3% of the rearranged alkene (2,3-dimethyl-1-butene) were isolated.264 jhe type of base employed is also important. The product distribution from the hydrazone of cyclopentanone, for example, gave a mixture of cyclopentene/( )-2-pentene/(Z)-2-pentene.265 When sodium methoxide was used, a 2 84 14 mixture of these products was obtained whereas sodium hydride led to a 83 14 3 mixture and sodium amide a 60 35 3 mixture.265... 

Alkenes containing electron-withdrawing groups polymerize in the presence of strong bases. Acrylonitrile, for example, polymerizes when it is treated with sodium amide (NaNH2) in liquid ammonia. The growing chains in this polymerization are anions ... 

One of the major differences between the chemistry of alkynes and that of alkenes and alkanes is that a hydrogen bonded to a carbon atom of a terminal alkyne is sufRciendy acidic (pA 25) that it can be removed by a strong base, such as sodium amide, NaNHg, to give an acetylide anion. 

In this equilibrium, acetylene is the stronger acid and sodium amide is the stronger base, and the position of equilibrium lies considerably toward the right and favors formation of the acetylide anion and ammonia (Section 2.4). Table 4.1 gives pA values for an alkane, alkene, and an alkyne hydrogen. Also given for comparison is the value for water. 

The pA values for alkene hydrogens (pA approximately 44) and alkane hydrogens (pA4 approximately 51) are so large (they are so weakly acidic) that neither the commonly used alkali metal hydroxides nor sodium amide are strong enough bases to remove a proton from an alkene or an alkane. 

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