Reaction into alkenes

Typical nucleophiles known to react with coordinated alkenes are water, alcohols, carboxylic acids, ammonia, amines, enamines, and active methylene compounds 11.12]. The intramolecular version is particularly useful for syntheses of various heterocyclic compounds[l 3,14]. CO and aromatics also react with alkenes. The oxidation reactions of alkenes can be classified further based on these attacking species. Under certain conditions, especially in the presence of bases, the rr-alkene complex 4 is converted into the 7r-allylic complex 5. Various stoichiometric reactions of alkenes via 7r-allylic complex 5 are treated in Section 4. 

Thallation of aromatic compounds with thallium tris(trifluoroacetate) proceeds more easily than mercuration. Transmetallation of organothallium compounds with Pd(II) is used for synthetic purposes. The reaction of alkenes with arylthallium compounds in the presence of Pd(Il) salt gives styrene derivatives (433). The reaction can be made catalytic by use of CuCl7[393,394], The aryla-tion of methyl vinyl ketone was carried out with the arylthallium compound 434[395]. The /9-alkoxythallium compound 435, obtained by oxythallation of styrene, is converted into acetophenone by the treatment with PdCh[396]. 

The Wittig reaction, for which George Wittig received the 1979 Nobel Prize in Chemistry, is an important synthetic procedure for converting aldehydes and ketones into alkenes. The active reagent is a phosphorous ylide which undergoes nucleophilic addition to the carbonyl carbon, e.g., for addition of triphenylphosphinemethylidene to acetone. 

Amines are converted into alkenes by a two-step process called the Iiofnunn elimination. SN2 reaction of the amine with an excess of CH3I in the first stej yields an intermediate that undergoes E2 reaction when treated with silvei oxide as base. Pentylamine, for example, yields 1-pentene. Propose a structim for the intermediate, and explain why it undergoes ready elimination. 

Aldehydes and ketones are converted into alkenes by means of a nucleophilic addition called the Wittig reaction. The reaction has no direct biological counterpart but is important both because of its wide use in the laboratory and drug manufacture and because of its mechanistic similarity to reactions of the coenzyme thiamin diphosphate, which well see in Section 29.6. 

In 1970, it was disclosed that it is possible to achieve the conversion of dimethylformamide cyclic acetals, prepared in one step from vicinal diols, into alkenes through thermolysis in the presence of acetic anhydride." In the context of 31, this two-step process performs admirably and furnishes the desired trans alkene 33 in an overall yield of 40 % from 29. In the event, when diol 31 is heated in the presence of V, V-dimethylforrnamide dimethyl acetal, cyclic dimethylformamide acetal 32 forms. When this substance is heated further in the presence of acetic anhydride, an elimination reaction takes place to give trans olefin 33. Although the mechanism for the elimination step was not established, it was demonstrated in the original report that acetic acid, yV, V-dimethylacetamide, and carbon dioxide are produced in addition to the alkene product."... 

The metathesis reaction of alkenes constitutes a major development in the field of hydrocarbon chemistry in recent years. The first examples of the heterogeneously and the homogeneously catalyzed metathesis of linear alkenes have been published by Banks and Bailey (I) and Calderon et al. (2), respectively. By this reaction, linear alkenes are converted with high selectivity into equimolar amounts of two new alkenes, according to ... 

O Neill and Rooney 90) found that the Mo03-CoO-A1208 catalyst converts diazomethane into nitrogen and ethene under conditions where propene undergoes metathesis. However, because many catalysts are active for this conversion 91), their results cannot be considered as supporting the hypothesis that the metathesis reaction of alkenes proceeds via carbene complexes. 

By a similar process, stannylsilylthiomethanes have been prepared, and then converted by the Petersen reaction into stannylthiyl alkenes 56). 

Another interesting biooxygenation reaction with alkenes, recently identified, represents an enzymatic equivalent to an ozonolysis. While only studied on nonchiral molecules, so far, this cleavage of an alkene into two aldehydes under scores the diversity of functional group interconversions possible by enzymatic processes [121,122]. 

In 1980 and 1982, Callot and co-workers reported that Rh(Por)l catalyzed the reaction between alkenes and ethyl diazoacetate to give syn cyclopropoanes as the major products (Eq. 25). " This was unusual as most transition metal catalysts for this reaction give the anti isomers as the predominant products. Kodadek and co-workers followed up this early report and put considerable effort into trying to improve the syn/anti ratios and enantioselectivity using porphyrins with chiral substituents. 

Hence, the rate depends only on the ratio of the partial pressures of hydrogen and n-pentane. Support for the mechanism is provided by the fact that the rate of n-pentene isomerization on a platinum-free catalyst is very similar to that of the above reaction. The essence of the bifunctional mechanism is that the metal converts alkanes into alkenes and vice versa, enabling isomerization via the carbenium ion mechanism which allows a lower temperature than reactions involving a carbo-nium-ion formation step from an alkane. 

Silicon substituents can be introduced into alkenes and alkynes by hydrosilation.70 This reaction, in contrast to hydroboration, does not occur spontaneously, but it can be carried out in the presence of catalysts such as H2PtCl6, hexachloroplatinic acid. Other catalysts are also available.71 Halosilanes are more reactive than trialkylsilanes.72... 

SRN I reactions using related p-nitrophenyl or p-nitrocumyl systems41 as reductive alkylating agents have been studied by Komblum and co-workers these are well summarized in the reviews.39 At the same time, Russell discovered the S l reaction of geminal halonitroalkanes with stabilized carbanions (see Eq. 5.25).42 The products are readily converted into alkenes via elimination of nitro groups (see Section 7.3). 

Primary nitro compounds are good precursors for preparing nitriles and nitrile oxides (Eq. 6.31). The conversion of nitro compounds into nitrile oxides affords an important tool for the synthesis of complex natural products. Nitrile oxides are reactive 1,3-dipoles that form isoxazolines or isoxazoles by the reaction with alkenes or alky nes, respectively. The products are also important precursors for various substrates such as P-amino alcohols, P-hydroxy ketones, P-hydroxy nitriles, and P-hydroxy acids (Scheme 6.3). Many good reviews concerning nitrile oxides in organic synthesis exist some of them are listed here.50-56 Applications of organic synthesis using nitrile oxides are discussed in Section 8.2.2. 

Allyl acetates are more commonly used as electrophiles for the palladium-catalyzed allylic alkylation than allylic nitro compounds.20 However, the reaction of allylic nitro compounds has found wider applications. Allylic nitro compounds are readily available by nitration of alkenes. The regio- and stereoselective introduction of electrophiles and nucleophiles into alkenes is possible as outlined in Eq. 7.19. In fact, this strategy is applied to the synthesis of terpenoids.21... 

The /Tamino alcohol structural unit is a key motif in many biologically important molecules. It is difficult to imagine a more efficient means of creating this functionality than by the direct addition of the two heteroatom substituents to an olefin, especially if this transformation could also be in regioselective and/ or enantioselective fashion. Although the osmium-mediated75 or palladium-mediated76 aminohydroxylation of alkenes has been studied for 20 years, several problems still remain to be overcome in order to develop this reaction into a catalytic asymmetric process. 

Reactions between alkenes and 07 on MgO also lead to nonselect ive oxidation (21). One would hope to gain insight into the possible role of this ion in epoxidation catalysis, but rapid surface reactions, for example between ethylene oxide and MgO, make it difficult to obtain such information. The principal reaction products, CHi and CO2, are believed to be formed in a manner analogous to reactions 12-15. The initial hydrogen abstraction again is effected by the 07 ion. 

At one time considered as two distinct reactions occurring by different mechanisms [51], the fragmentations of Scheme 2 and the rearrangments of Scheme 5 are now seen as different facets of the same fundamental heterolysis of -substituted alkyl radicals into alkene radical cations, with the eventual outcome determined by the reaction conditions [52],... 

The reactions are related to the incorporation of CO and H20 into alkenes or alkynes leading to the corresponding saturated or unsaturated carboxylic acids. The general equation is shown below. Equation lb is related to the synthesis of acrylic acid discovered by Reppe [11]. 

A wide range of organic substrates can undergo an oxidative carbonylation reaction. Depending on reaction conditions, alkenes have been converted into -chloroalkanoyl chlorides (oxidative chloro-chlorocarbonylation) [1,2], succinic diesters (oxidative dialkoxycarbonylation) [3-20], a,/J-unsaturated esters [21,22] (oxidative monoalkoxycarbonylation), or /J-alkoxyalkanoic esters [11] (oxidative alkoxy-alkoxycarbonylation), according to Eqs. 10-13. 

Another method of making epoxides is the electrophilic reaction of alkenes with a peroxy acid such as peroxyacetic acid (sometimes simply peracetic acid). Thus, cyclohexene may be converted into the epoxide in a single reaction. 

Reactions of alkenes such as 120 with a-chloronitrosoalkanes of type 119 proceed under very mild conditions and result in the formation of nitrones 121 that can be easily hydrolyzed into hydroxylamines 122 (equation 86) . Chiral carbohydrate-derived a-chloronitrosoalkenes 123 possess enhanced reactivity and produce good stereoselectivity in reaction with prochiral alkenes such as 124 (equation 87) . ... 

One of the most important features of the ozonolysis reaction of alkenes is one in which ozone adds to the C=C bond to form a primary ozonide (1,2,3-trioxolane). The Criegee mechanism suggests that this unstable intermediate decomposes into a carbonyl compound and a carbonyl oxide that recombine to form a final isomeric ozonide (1,2,4-trioxolane). Direct spectroscopic evidence for a substituted carbonyl oxide has only recently been reported by Sander and coworkers for the NMR characterization of dimesityl carbonyl oxide. Kraka and coworkers have theoretically modeled dimesityl carbonyl oxide and confirmed the structural aspects reported by Sander and coworkers on the basis of NMR data. 

Thiiranes that are obtained from the reaction of diazo dipoles with C=S bonds can be transformed into alkenes by desulfurization. This reaction sometimes occurs spontaneously, but more often is achieved by treatment with phosphanes (225). This important methodology represents an alternative for the W ittig reaction and has high merit for the preparation of stericaUy hindered (226-229) and uncommonly functionalized alkenes (214,216,217,230,231). Some examples are given in... 

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