Olefin alkene electronics

An important virtue of the Heck reaction is that it can be applied with much success to essentially every type of olefin, although electron-deficient olefins are particularly well-suited. Moreover, the Heck reaction tolerates a variety of functional groups, and often does not require rigorous exclusion of oxygen and water.llb In fact, many alkene arylations proceed very efficiently in water.14... 

The electron-donating groups, if present in the alkene enhance the rate of the reaction. This is why the reaction is particularly rapid with tetraalkyl olefines. The electron-withdrawing groups in the peracids also increases the reaction rate. The following suggested mechanisms has been accepted for the conversions. 

Reaction of an alkene with a nitronium ion involves donation of the n -electron pair of the alkene into an orbital of the nitronium ion. Donation of a bonded electron pair necessarily means that the bond from which it comes is broken. Likewise population of an antibonding level by electron donation generally results in breaking of the bond to which the antibonding orbital corresponds. In this case electron donation of the olefinic r-electron pair results in the rupture of file olefinic n bond and acceptance into file N-O n orbital results in breakage... 

Simple examples of the reactions of olefins with diols are shown in Equations 16.104 and 16.105. Reactions of alkenes typically generate ketals, whereas reactions of olefins bearing electron-withdrawing groups, such as those in acrylates or acrylonitrile, tend to form acetals. This regioseJectivity is shown by the reactions of butene and... 

The enantioselective hydroformylation process confronts a series of challenges. First, the hydroformylation of alkenes tends to form linear over branched products. Thus, enantioselective hydroformylation of alkenes must be conducted with uns)rmmetrical "vinylidenic" olefins (2,2-disubstituted) that establish a stereocenter at the carbon p to the carbonyl group, or with mono-substituted olefins that form the branched product. As noted above, vinylarenes, vinyl acetates, and other olefins bearing electron-withdrawing groups, form branched products. Thus, reactions of these olefins have been the greatest focus of enantioselective hydroformylations. 

Metals in low oxidation states such as palladium(O) will enhance the backdonation component of the bond, and this is particularly important in the case of olefins with electron-withdrawing groups. The chemistry of stable palladium(O) alkene complexes is thus dominated by this type of olefin and a fairly large number of them have been synthesized in the last 12 years. In addition, the chelate effect favors coordination of chelating alkenes over similar monodentate systems, because of the stabilizing entropy contribution. 

Bromination of Olefins. Another example of TMS-Br as a source of bromine is its use in the bromination of alkenes (eq 42). This transformation is achieved via the use of tetradecyltrimethyl-ammonium permanganate [(Ci4H29(CH3)3N)(JCMn04)]. This reagent performs a trans relative addition of the bromine atoms and is selective for electron-rich olefins over electron-deficient ones. The yields for these reactions are generally high (60-91%). 

In the same way as oxidative addition of an olefin to a metallacyclopropane occurs only if the metal is very electron rich or the olefin very electron poor, oxidative coupling of two alkenes usually requires the same conditions. 

Inter- and intramolecular additions of alkenes and dienes to propargylic alcohols catalyzed by thiolate-bridged diruthenium complexes have been described. The processes, a kind of allenylidene-ene reaction, generate 1,5-enynes and dienynes by reaction of propargylic alcohols with 2-arylpropenes [196] and 1,3-conjugated dienes [197], respectively. The intramolecular version of this reac-ti(Mi has been developed to give diastereo- [196, 198] or enantioselective syn-substituted chromanes (Scheme 58) [199]. Recently, the results of DPT calculations indicated that nucleophilic attack of the olefinic Jt-electrons on a carbocationic... 

If alkyl groups are attached to the ylide carbon atom, cis-olefins are formed at low temperatures with stereoselectivity up to 98Vo. Sodium bis(trimethylsilyl)amide is a recommended base for this purpose. Electron withdrawing groups at the ylide carbon atom give rise to trans-stereoselectivity. If the carbon atom is connected with a polyene, mixtures of cis- and rrans-alkenes are formed. The trans-olefin is also stereoseiectively produced when phosphonate diester a-carbanions are used, because the elimination of a phosphate ester anion is slow (W.S. Wadsworth, 1977). 

A major difficulty with the Diels-Alder reaction is its sensitivity to sterical hindrance. Tri- and tetrasubstituted olefins or dienes with bulky substituents at the terminal carbons react only very slowly. Therefore bicyclic compounds with polar reactions are more suitable for such target molecules, e.g. steroids. There exist, however, several exceptions, e. g. a reaction of a tetrasubstituted alkene with a 1,1-disubstituted diene to produce a cyclohexene intermediate containing three contiguous quaternary carbon atoms (S. Danishefsky, 1979). This reaction was assisted by large polarity differences between the electron rich diene and the electron deficient ene component. 

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

Its reactions with olefins, governed by steric rather than electronic factors, are very sluggish. Even simple 1-alkenes require 8 h at 25°C for complete reaction. In contrast, alkynes are hydroborated with great ease to alkenylboranes, high steric requirements of the reagent preventing dihydroboration (117). 

The initial bond formation between the -> ir excited carbonyl compound and an alkene can occur by interaction of the half-filled n -orbital of the [I CO] with the ir-system of the alkene, in a sense transferring a tt-electron to the -orbital and making a bond between an alkene carbon and the carbonyl oxygen. In this process (common for electron rich olefins) the plane formed by the alkene carbons and their four substituents is perpendicular to the plane of the carbonyl groups and its two substituents (Figure 1). In the... 

Reaction of triethylsilyl hydrotrfoxide with electron-rich olefins to gh/e dioxetanes that react IntrarTMlecularly with a keto group in the presence of t-txrtyidimethyl silyl triflateto afford 1,2,4 Inoxanes also oxydatnre cleavage ol alkenes Also used in cleavage ol olefins... 

With very electrophilic olefins, an alternative hydrogen fluoride addition process is often preferred This process, involving reaction of the olefin with fluoride ion in the presence of a proton donor, is applicable to certain perhalogen ated alkenes [/] and substrates with other electron attracting groups attached to the double bond [i5, 36] (equations 4 and 5)... 

The reaction of perfluoroalkyl iodides with electron donor nucleophiles such as sodium arene and alkane sulfinates in aprotic solvents results in radical addition to alkenes initiated by an electron-transfer process The additions can be carried out at room temperature, with high yields obtained for strained olefins [4 (equations 3-5)... 

The pyrolysis of sodium chlorodinuoroacetate is still a widely used, classical method for generating difluorocarbene, especially with enol and allyl acetates [48, 49, 50, 51] (equation 21) A convenient alternative that avoids the hygroscopic salt uses methyl chlorodifluoroacetate with 2 equivalents of a lithium chlonde-hexa-methylphosphoric triamide complex at 75-80 °C in triglyme [52], Yields are excellent with electron-rich olefins but are less satisfactory with moderately nucleophilic alkenes (4-5% yields for 2-bulenes)... 

Olefins conjugated with electron-withdrawing groups other than a carbonyl group undergo reactions with enamines in a manner similar to the carbonyl-conjugated electrophilic alkenes described above. Namely, they condense with an enamine to form a zwitterion intermediate from which either 1,2 cycloaddition to form a cyclobutane ring or simple alkylation can take place. 

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