Alkene 1.2- dihalides from

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

The Wittig reaction of phosphorus ylides derived from phosphonium salts with carbonyl compounds to form alkenes is widely used in synthetic chemistry. The alkylation of phosphorus ylides offers a route to a variety of phosphonium salts which can be further manipulated. Bestman and coworkers - have been responsible for a large number of reports based upon this and other aspects of phosphorus chemistry. Their work includes many examples of the intramolecular alkylation of ylides derived from phosphonium salts to form cyclic compounds. The salts can be prepared from dihalides either with triphenylphos-... 

Alkenes from sic-dihaiides. The reaction of n c-dihalides with NaSeCH3 or NaSeC6H5 in ethanol or THF-HMPT (3 1) results in alkenes. The reaction involves a formal anti-elimination in the case of wc-dibromides or uic-chloroiodides, but a formal syn-elimination in the case of w c-dichlorides. Elimination to form alkenes also occurs with /1-haloalkyl phenyl selenides. 

Dihalogenation of alkynes gives a dihalogenated alkene, which is also susceptible to reaction with bromine, chlorine, or iodine. Tetrahalo derivatives are available from dihalogenated alkenes (vinyl dihalides). When 1-pentyne reacts with one molar equivalent of diatomic bromine. 111 is the product. Because alkenes are also subject to reaction with halogens. 111 can react with a second molar equivalent of bromine to give 1,1,2,2-tetrabromopentane, 112. 

The preparation of heteroatom-substituted alkenes from the corresponding gem-dihalides and aldehydes was also mediated with chromium(II) chloride. In Scheme 8.46, representative examples of the preparation of alkenylborane, [51] -silane, [52] and -stannane [53] are shown. In each case, the high -selectivity is observed. As these compounds are very important substrates for Suzuki-, Hiyama-, and Stille coupling, the stereoselective formation of these compounds heightens the value of the chromium(II)-chloride-mediated reactions. 

Alkynes can be prepared by the elimination of HX from alkyl halides in much the same manner as alkenes (Section 7.1). Treatment of a 1,2-dihaloaJkane (a vicinal dihalide) with excess strong base such as KOH or NaNH2 results in a twofold elimination of HX and formation of an alkyne. As with the elimination of HX to form an alkene, we ll defer a discussion of the mechanism until Chapter 11. 

The necessary vicinal dihalides are themselves readily available by addition of Br2 or Cl2 to alkenes. Thus, the overall halogenation/dehvdrohalogenation sequence makes it possible to go from an alkene to an alkyne. for example, diphenylethylene is converted into diphenylacetylene by reaction with Br2 and subsequent base treatment. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Monodehydrohalogenation of allylic halides is another classical method for diene synthesis24. This method is complementary to double dehydrohalogenation as both the 1,2-dihalides and allylic halides are readily accessed from alkenes. The commonly employed protocol for diene synthesis, particularly for cyclic 1,3-dienes, is through the allylic monobromination of the alkene with A-bromosuccinimide or related reagents followed by dehydrobromination with hindered bases such as DBN or DBU (equation l)25. 

From a mechanistic point of view, the titanocene(II)-promoted intramolecular cyclopro-panation of gem-dihalides possessing a terminal double bond is interesting. Although the products of ring-closing metathesis, i. e. cycloalkenes, are produced in certain cases, the treatment of 6,6- and 7,7-dihalo-l-alkenes (e. g. 39 and 40) with titanocene(II) species affords bicyclo[3.1. OJhexane and bicyclo[4.1.0]heptane derivatives 41 and 42, respectively (Scheme 14.19) [34],... 

Alkenes are obtained by the transformation of various functional groups, e.g. dehydration of alcohols (see Section 5.4.3), dehalogenation of alkyl halides (see Section 5.4.5) and dehalogenation or reduction of alkyl dihalides (see Section 5.4.5). These reactions are known as elimination reactions. An elimination reaction results when a proton and a leaving group are removed from adjacent carbon atoms, giving rise to a tt bond between the two carbon atoms. 

A simple method for introducing a triple bond into an organic compound is to treat an appropriate dihalide with a strong base. Since vicinal dihalides (usually the bromide) are readily formed by reaction of bromine with an alkene, and geminal dihalides from aldehydes or ketones with phosphorus pentachloride, the method is a useful general procedure for the preparation of terminal and non-terminal alkynes from readily available starting materials. 

The reaction of an alkene with a halogen like bromine and chlorine generally gives a vicinal dihalide. However, if the reaction is carried out in water as solvent, the product obtained is a halohydrin where the halogen adds to one end of the double bond and a hydroxyl group from water adds to the other. 

Using Hydrohalic Acids When an epoxide reacts with a hydrohalic acid (HC1, HBr, or HI), a halide ion attacks the protonated epoxide. This reaction is analogous to the cleavage of ethers by HBr or HI. The halohydrin initially formed reacts further with HX to give a 1,2-dihalide. This is rarely a useful synthetic reaction, because the 1,2-dihalide can be made directly from the alkene by electrophilic addition of X2. 

Thus, for example, the direct conversion of an ether into an acetal or ketal is difficult to achieve whereas the oxidation of an alcohol to an aldehyde or ketone (or the reverse process) is a trivial transformation. Similarly, the transition from an oxidation level of 2 to level 1 is problematic in the case when one tries to convert dihalides into monohalides while the transformation of alkynes into alkenes may be safely considered a viable route to carry out this transition. 

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