Alkenes preparative procedure

Figure 4. Preparative procedure of alkene thiol-stabilized gold nanoparticles. 50% of hexenethiol bounded to the particle surface is hydrogenated to hexanethiol [11b].

Preparation of Iodine Monofiuoride and Its Reactions with Alkenes General Procedure 4... 

The addition of aliphatic aldehydes and ketones to alkenes is less successful as a preparative procedure for oxetanes. An essential requirement for addition is that the triplet energy of the alkene must be considerably greater than that of the carbonyl. If this condition is not fulfilled, energy transfer to the alkene can occur,279 sensitizing, for example, dimerization of the alkene. This is clearly illustrated 280, 281 for norbornene (264) which on irradiation in the presence of benzophenone (ET 68.5 kcal/mole) forms the adduct 265 photolysis in acetone (ET 75 kcal/mole) affords only norbornene dimers (266 and 267), whereas acetophenone, which has intermediate triplet energy (Et 73.6 kcal/mole) forms both oxetanes and norbornene dimers. 

Electrophilic addition reactions of tetravalent tellurium compounds have been reviewed.64 2-Naph-thyltellurium trichloride (ArTeCb) adds to alkenes in an anti stereospecific manner (equation 11), whereas tellurium tetrachloride gives mixtures of 2 1 adducts with both syn and anti addition.72 A one-pot alkene inversion procedure has been developed, based upon TeCU addition to alkenes followed by treatment of the (3-chloroalkyltellurium trichloride adduct with aqueous Na2S (Scheme 37).73 Tellurium compounds such as tellurinyl acetates, ArTe(0)0Ac, prepared in situ through reaction of tellurinic acid anhydrides with acetic acid, can be employed in oxytelluration and aminotelluration procedures (Schemes 38 and 39).74 In the oxytelluration reaction intermediate triacetates of the type RCH2Te(OAc)2Ph are reduced with hydrazine to the corresponding tellurides. 

It is clear therefore that this preparative procedure may only be of value for the formation of a restricted range of alkene target molecules. Recent procedures include the use of the non-nucleophilic base, l,8-diazabicyclo[5.4,0]undec-7-ene (DBU),18 and the use of PTC methods.19 One useful example is provided by the ready conversion of 3-bromocyclohexene (Expt 5.68) into cyclohexa-1,3-diene (Expt 5.13) where the base employed is quinoline. 

The bis(trimethylsiloxy)alkene bromination procedure is a large-scale preparation that gives excellent yields of cyclic and acyclic 1,2-diones however, when enolizable 1,2-diketones are produced, some complications can be encountered.7,8... 

Nickel boride catalysts have been considered viable alternatives to Raney nickel since they are non-pyrophoric, are easily prepared and the preparation procedure is reproducible. In addition these catalysts appear to be somewhat more active than W2 Raney nickel for the hydrogenation of alkenes and nitriles but both types of catalyst have about the same activity for carbonyl group hydrogenation. While nickel boride and Raney nickel are both more resistant to... 

Thermolysis. Note The observation that an alkoxypyrazine can undergo thermolytic conversion into a pyrazinone plus an alkene has been studied kinetically - but does not appear to have been developed as a preparative procedure. For example, 2-ethoxypyrazine (24) gave 2(l//)-pyrazinone (25) plus ethylene (26). ... 

Several methods have been used for preparing the SAP catalysts. According to the preparation procedure, the methods can be classified into two groups (a) indirect methods, when the support is first impregnated with the hydrosoluble catalytic complex, then dried and rehydrated before use [6,14,15,17, 41, 42, 49] (b) direct methods, when the support, catalytic complex, and water are mixed at the same time in the reaction system [15, 21, 29]. In general, the best conversions in the hydroformylation of alkenes by SAPC have been obtained by indirect preparation. However, the direct methods being of much easier implementation than the indirect ones, they are most widely used. 

Although the reaction of ketones and other carbonyl compounds with electrophiles such as bromine is formally a substitution process, it is mechanistically closely related to electrophilic additions to alkenes. The enol or enolate derived from the carbonyl compound is the reactive species, and the initial attack is similar to the electrophilic attack on alkenes. The reaction is completed by restoration of the carbonyl bond, rather than by addition. The acid- and base-catalyzed halogenations of ketones, which were discussed briefly in Part A, Chapter 7, are the best-studied examples of the reaction. The most common preparative procedures for such... 

Another useful preparative procedure is transalkylation from the corresponding organoborane, which can be prepared from the alkene (see also below). 

The established routes to organo-selenoketones have been reviewed." New methods of preparation of these important synthetic intermediates include the oxidation of alkenes, e.g. using diphenyl diselenide, bromine, and hexabutyl-distannoxane (Scheme 2)." For terminal alkenes alternative procedures involve the intermediate formation of the corresponding bromoselenide or ethoxy-selenide the former species is converted into a phenyl selenomethyl ketone using silver hexafluorophosphate in DMSO followed by reaction with triethylamine." /3-Phenylselenoketones protected as the trimethylsilyl enol ether are available by reaction of an a/8-unsaturated ketone with phenyl trimethylsilyl selenide."... 

Experimental procedure for iodofnnctionalization/ ring contraction of alkenes. Preparation of 171 (Scheme 18.40) A... 

There are a wide variety of methods for introduction of substituents at C3. Since this is the preferred site for electrophilic substitution, direct alkylation and acylation procedures are often effective. Even mild electrophiles such as alkenes with EW substituents can react at the 3-position of the indole ring. Techniques for preparation of 3-lithioindoles, usually by halogen-metal exchange, have been developed and this provides access not only to the lithium reagents but also to other organometallic reagents derived from them. The 3-position is also reactive toward electrophilic mercuration. 

Among chiral dialkylboranes, diisopinocampheylborane (8) is the most important and best-studied asymmetric hydroborating agent. It is obtained in both enantiomeric forms from naturally occurring a-pinene. Several procedures for its synthesis have been developed (151—153). The most convenient one, providing product of essentially 100% ee, involves the hydroboration of a-pinene with borane—dimethyl sulfide in tetrahydrofuran (154). Other chiral dialkylboranes derived from terpenes, eg, 2- and 3-carene (155), limonene (156), and longifolene (157,158), can also be prepared by controlled hydroboration. A more tedious approach to chiral dialkylboranes is based on the resolution of racemates. /n j -2,5-Dimethylborolane, which shows excellent enantioselectivity in the hydroboration of all principal classes of prochiral alkenes except 1,1-disubstituted terminal double bonds, has been... 

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

Pyrazoles are formed when the diazo compounds react with alkynes or with functionalized alkenes, viz. the enols of /3-diketones. Pyrazolenines (353 Section 4.04.2.2.1) are isolated from disubstituted diazomethanes. Many pyrazoles, difficult to obtain by other methods, have been prepared by this procedure, for example 3-cyanopyrazole (616) is obtained from cyanoacetylene and diazomethane (7iJCS(C)2i47), 3,4,5-tris(trifiuoromethyl)pyrazole (617) from trifluorodiazoethane and hexafluoro-2-butyne (8lAHC(28)l), and 4-phenyl-3-triflylpyrazole (618 R =H) from phenyltriflylacetylene and diazomethane (82MI40402). An excess of diazomethane causes iV-methylation of the pyrazole (618 R = H) and the two isomers (618 R = Me) and (619) are formed in a ratio of 1 1. 

This procedure illustrates a general method for the preparation of alkenes from the pal 1 adium(Q)-cata1yzed reaction of vinyl halides with organo-lithium compounds, which can be prepared by various methods, including direct regioselective lithiation of hydrocarbons. The method is simple and has been used to prepare a variety of alkenes stereoselectively. Similar stoichiometric organocopper reactions sometimes proceed in a nonstereoselective... 

Seven procedures descnbe preparation of important synthesis intermediates A two-step procedure gives 2-(HYDROXYMETHYL)ALLYLTRIMETH-YLSILANE, a versatile bifunctional reagent As the acetate, it can be converted to a tnmethylenemethane-palladium complex (in situ) which undergoes [3 -(- 2] annulation reactions with electron-deficient alkenes A preparation of halide-free METHYLLITHIUM is included because the presence of lithium halide in the reagent sometimes complicates the analysis and use of methyllithium Commercial samples invariably contain a full molar equivalent of bromide or iodide AZLLENE IS a fundamental compound in organic chemistry, the preparation... 

Mercury(II) trifluoroacetate is a good electrophile that is highly reactive toward carbon-carbon double bonds [52, 53, 54] When reacting with olefins in nucleophilic solvents, it usually gives exclusively mercurated solvoadducts, but never products of skeletal rearrangement Solvomercuration-demercuratton of alkenes with mercury(II) trifluoroacetate is a remarkably effective procedure for the preparation of esters and alcohols with Markovnikov s regiochemistry [52, 5J] (equation 24)... 

Acid-catalyzed dehydration (Section 5.9) This is a frequently used procedure for the preparation of alkenes. The order of alcohol reactivity parallels the order of carbocation stability R3C" > R2CH " > RCH2 ". Benzylic alcohols react readily. Rearrangements are sometimes observed. 

Rearrangements and other side-reactions are rare. The ester pyrolysis is therefore of some synthetic value, and is used instead of the dehydration of the corresponding alcohol. The experimental procedure is simple, and yields are generally high. Numerous alkenes have been prepared by this route for the first time. For the preparation of higher alkenes (> Cio), the pyrolysis of the corresponding alcohol in the presence of acetic anhydride may be the preferable method." The pyrolysis of lactones 9 leads to unsaturated carboxylic acids 10 ... 

The preparation of an alkene 3 from an amine 1 by application of a /3-elimination reaction is an important method in organic chemistry. A common procedure is the Hofmann elimination where the amine is first converted into a quaternary ammonium salt by exhaustive methylation. Another route for the conversion of amines to alkenes is offered by the Cope elimination. 

The McMurry procedure is a valuable method for the synthesis of highly substituted, strained alkenes such compounds are difficult to prepare by other methods. Diisopropyl ketone 6 can be coupled to give tetraisopropylethene 7 in 87% yield attempts to prepare tetra-t-butylethene however were not successful. ... 

The (Horner-)Wadsworth-Emmons reaction generally is superior to the Wittig reaction, and has found application in many cases for the synthesis of a ,/3-unsaturated esters, a ,/3-unsaturated ketones and other conjugated systems. Yields are often better then with the original Wittig procedure. However the Wadsworth-Emmons method is not suitable for the preparation of alkenes with simple, non-stabilizing alkyl substituents. 

These alkene isomers are separately available (4) by treatment of threo-S-trimethylsilyloctan-4-ol, prepared by reduction of the corresponding ketone with DIBAL in pentane at —120°C, with base or acid. The preparation of 5-trimethylsilyloctan-4-one itself illustrates three general procedures the addition of alkyl lithium reagents to vinylsilanes to generate a-lithiosilanes, the preparation of complex /5-hydroxysilanes, as diastereoisomeric mixtures, and the oxidation of such compounds to /8-ketosilanes... 

Diphenylmethyl vinyl ether has also been prepared from benzhydrol and acetylene (ethyne) under high-pressure conditions.3 In the described method, which is an adaptation of the procedure ofWeinstock and Boekelheide,4 improved yields of the alkene are obtained by using more convenient experimental conditions. 

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