Alkenes preparation from alcohols

Considering the problem retrosynthetically, we can see that a key intermediate having the carbon skeleton of the desired product is 3-methyl-3-pentanol. This becomes apparent from the fact that alkanes may be prepared from alkenes, which in turn are available from alcohols. The desired alcohol may be prepared from reaction of an acetate ester with a Grignard reagent, ethylmagnesium bromide. 

The above type of addition has been used for the simple preparation, from alkenes and other unsaturated substances, of alcohols, ethers and amines— the additions of HgX2 are carried out in water, alcohols or acetonitrile, respectively. The mercury is removed from the intermediate by reduction with... 

Carbon-Oxygen Bond Formation. CAN is an efficient reagent for the conversion of epoxides into /3-nitrato alcohols. 1,2-cA-Diols can be prepared from alkenes by reaction with CAN/I2 followed by hydrolysis with KOH. Of particular interest is the high-yield synthesis of various a-hydroxy ketones and a-amino ketones from oxiranes and aziridines, respectively. The reactions are operated under mild conditions with the use of NBS and a catalytic amount of CAN as the reagents (eq 25). In another case, N-(silylmethyl)amides can be converted to A-(methoxymethyl)amides by CAN in methanol (eq 26). This chemistry has found application in the removal of electroauxiliaries from peptide substrates. Other CAN-mediated C-0 bondforming reactions include the oxidative rearrangement of aryl cyclobutanes and oxetanes, the conversion of allylic and tertiary benzylic alcohols into their corresponding ethers, and the alkoxylation of cephem sulfoxides at the position a to the ester moiety. 

Alcohols are important organic compounds not only for their own commercial applications but because they can be converted by chemical reactions into a wide variety of other useful products. Most alcohols do not occur naturally in commercial qnantities and therefore are prepared from alkenes. 

In a reaction resembling halohydrin formation (Section 6.17), vicinal haloethers are prepared from alkenes by reaction with an alcohol in the presence of halogens— usually bromine or iodine. This haleotherification proceeds through a cyclic halonium ion, which reacts with the alcohol. 1-Methylcyclohexene undergoes iodoetherification with ethanol in the presence of iodine to give ran -l-ethoxy-2-iodo-l-methylcyclohexane. 

Reactions that probe these disconnections are presented in the homework problems. The disconnection diagrams should be read as follows. Alkyl halides are prepared from alkenes, with regioselectivity. Similarly, alcohols are prepared from alkenes with regioselectivity. A chemical reaction involves treatment of an alkene with an acid HX to give the alkyl halide. Another chemical reaction is treatment of an alkene with aqueous acid to give the more substituted alcohol, or with borane and then basic hydrogen peroxide to give the less substituted alcohol. 

Recall from Section 9.5 that alcohols can be prepared from alkenes via a process called oxymercuration-demercuration. 

However, we know of no reactions that convert alcohols directly to epoxides. Because we do know that epoxides are prepared from alkenes, we expand our retrosynthesis to reflect that. 

Like alcohol dehydrations El reactions of alkyl halides can be accompanied by carbocation rearrangements Eliminations by the E2 mechanism on the other hand nor mally proceed without rearrangement Consequently if one wishes to prepare an alkene from an alkyl halide conditions favorable to E2 elimination should be chosen In prac tice this simply means carrying out the reaction m the presence of a strong base... 

We now have a new problem Where does the necessary alkene come from Alkenes are prepared from alcohols by acid catalyzed dehydration (Section 5 9) or from alkyl halides by dehydrohalogenation (Section 5 14) Because our designated starting material is tert butyl alcohol we can combine its dehydration with bromohydrm formation to give the correct sequence of steps... 

Mixtures of anhydrous hydrogen fluoride and tetrahydrofuran are successfully used as fluorinating agents to convert 1,1,2-trifluoro-l-allcen-3-ols, easily prepared from bromotrifluoroethene via lithiation followed by the reaction with aldehydes or ketones, to 1,1,1,2-tetrafluoro-2-alkenes The yields are optimal with a 5 1 ratio of hydrogen fluoride to tetrahydrofuran The fluorination reaction involves a fluonde lon-induced rearrangement (Sf,j2 mechanism) of allylic alcohols [65] (equation 40)... 

Just as the chemistry of alkenes is dominated by addition reactions, the preparation of alkenes is dominated by elimination reactions. Additions and eliminations are, in many respects, two sides of the same coin. That is, an addition reaction might involve the addition of HBr or H20 to an alkene to form an alkyl halide or alcohol, whereas an elimination reaction might involve the loss of HBr or H20 from an alkyl halide or alcohol to form an alkene. 

Problem 7.8 What alkenes might the following alcohols have been prepared from ... 

Alcohols occupy a central position in organic chemistry. They can be prepared from many other kinds of compounds (alkenes.. alkyl halides, ketones, esters, and aldehydes, among others), and they can be transformed into an equally wide assortment of compounds (Figure 17.3). 

P-Amino alcohols can be prepared by treatment of an alkene with a reagent prepared from HgO and HBF4 along with aniline to give an ami nomercurial... 

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