Activation of the Alkene

Ammonium salts have also been used as catalysts in the synthesis of t-BuNH2 from NH3 and isobutene in water (Eq. 4.7). The co-produced t-BuOH can be recycled [97]. 

Hegedus et al. have thoroughly studied the homogeneous hydroamination of olefins in the presence of transition metal complexes. However, most of these reactions are either promoted or assisted, i.e. are stoichiometric reactions of an amine with a coordinated alkene [98-101] or, if catalytic, give rise to the oxidative hydroamination products, as for example in the cyclization of o-allylanilines to 2-alkylindoles [102, 103], i.e. are relevant to Wacker-type chemistry [104]. 

The first transition metal-catalyzed hydroamination of an olefin was reported in 1971 by Coulson who used rhodium(I), rhodium(III) or iridium(III) catalysts (Eq. 4.8) [105,106]. 

This reaction is restricted to ethylene and to secondary amines of high basicity (nude-ophUicity) and low steric bulk (Me2NH, pyrrolidine, piperidine). No high molecular weight products are formed. However, the same catalysts [107,108] as well as PdQj [108] also exhibit some activity for the hydroamination of ethylene with PhNH2 (Eq. 4.9). 

The reaction can be made selective for the synthesis of N-ethylaniline (150°C, 10 bar) but at the expense of catalytic activity [107, 108]. 

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Hydrophosphination of ethyl acrylate using PH3 (R = C02Me, Equation (17)) is catalyzed by a mixture of the zero-valent platinum complexes (72a c), which are formed upon addition of P CH2CH2C02Et 3 to Pt(norbornene)3] (Scheme 44). Failure of these complexes to bring about P H addition to CH2 = CHCF3 indicates that Michael activation of the alkene through I and R effects of the substituents is crucial for catalytic activity in this class of metal complexes.190... 

The alkylation reaction is initiated by the activation of the alkene. With liquid acids, the alkene forms the corresponding ester. This reaction follows Markovnikov s rule, so that the acid is added to the most highly substituted carbon atom. With H2S04, mono- and di-alkyl sulfates are produced, and with HF alkyl fluorides are produced. Triflic acid (CF3S020H) behaves in the same way and forms alkyl triflates (24). These esters are stable at low temperatures and low acid/hydrocarbon ratios. With a large excess of acid, the esters may also be stabilized in the form of free carbenium ions and anions (Reaction (1)). 

The orthogonal reactivities exhibited by IBr (2.5equiv at —78°C) and Au(PPh3)BF4 (5mol% at 0°C) in the activation of the alkene and alkyne groups of trichloroacetimidate 440 could be utilized in regioselective cyclizations toward 5,6-dihydro-4//-l,3-oxazine derivatives 441 or 442, respectively (Scheme 84) <2006OL3537>. 

Activation of the alkene and electrophilic substitution, where the furan (electron-rich) would act as a nucleophile. 

Because of the centrality of the carbonyl group in synthesis, carbonyl-substituted radicals are especially useful. The above results indicate that, if planned addition or cyclization reaction of a carbonyl-substituted radical fails due to lack of reactivity of the acceptor, one should consider activation of the alkene not only with electron donors but also with electron acceptors. 

The reaction of isocyanates with alkenes toward azetidin-2-ones requires activation of the former by electro-with-drawing groups or activation of the alkenes by electron-donating groups <2004COR463>. 

Palladium-catalyzed oxidative cyclization of aryl homoallyl ethers affords 4-methyl-2//-chromenes in moderate yield. The reaction is proposed to proceed via activation of the alkene by coordination to Pd(ll) followed by intramolecular nucleophilic attack by the arene. Subsequent [1-hydride elimination and isomerization then affords 4-methyl-27/-chromenes (Scheme 13). Electron-rich aryl homoallyl ethers give the best yield and good regio-selectivity is observed for the reaction of unsymmetrical arenes <2005OL3355>. 

The cycloaddition of an alkene across the C=N bond of an isocyanate is a useful method of synthesis of p-lactams (equation 50). 4,iS2 j, g reaction requires the activation of the alkene by electron-releasing... 

A mechanism which involved the allylic carbon-hydrogen bond activation of the alkene moiety was proposed for the cycloisomerization of 1,6-diyne to alkylidenecy-cloheptene on the basis of stereochemical consideration and deuterium labeling experiment (Scheme 12.7). 

Using a cyano group for activation of the alkene may also lead to the cyclized product [208] as illustrated in Eq. (38). 

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(ii). in such palladium(ll) olefin complexes (1), which are square planar d complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled tt-orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin tt -orbital (back donation), leads to an electrophilic activation of the alkene. ... 

Wacker-type reactions are Pd(II)-catalyzed transformations involving heteroatom nucleophiles and alkenes or alkynes as electrophiles [108]. In most of these reactions, the Pd(ll) catalyst is converted to an inactive Pd(0) species in the final step of the process, and use of stoichiometric oxidants is required to effect catalytic turnover. For example, the synthesis of furan 113 from a-allyl-P-diketone 112 is achieved via treatment of the substrates with a catalytic amount of Pd(OAc)2 in the presence of a stoichiometric amount of C uC F [109]. This transformation proceeds via Pd(lt) activation of the alkene to afford 114,... 

Various heteroatoms have been used to facilitate carbenium ion formation at less substituted centres, including halogens,9"" nitrogen,12 selenium,13 and sulfur.14 These reactions generally proceed via activation of the alkene, through formation of a cyclic cationic intermediate 25.15... 

Most unusually, 8-endo cyclizations can be carried out with extraordinary efficiency when mediated by Sml2 [83, 87], Although the scope and limitations of the method have not been studied in detail, it is clear that some type of activation of the alkene is necessary to achieve reasonable yields of the desired products (Eq. 74). 

It was assumed that reactions of nickelocene with lithium - or magnesium alkyls possessing -hydrogen proceed with the formation of corresponding alkenes. The activation of the alkene by the active dinickel species (NiCp)2) formed in the reaction leads to the formation of (cyclopentadienyl)(alkylidyne)trinickel clusters. 

Alkenes are potential sources of electrophiles for alkylation of indoles, but require selectivity for activation of the alkene over attack on the indole ring. In one example, Zhao et al. found the indole could by alkylated by styrenes in the presence of M-phenylselenylphthalimide [109]. 

In contrast, Muniz reported an indole synthesis from 2-vinyl anilines 13, which relies on activation of the alkene moiety by the hypervalent iodine reagent [9]. The reaction employs a sterically congested Koser reagent generated in situ from iodosobenzene and the bulky sulfonic acid 15. The reaction provides synthetically unique access to 2,3-unsubstituted indoles 14 and can also be conducted under catalytic conditions. 

The determined dependence of the Gibbs free energy and the activation energy of recycHzation of the 3 (2-hydroxyethyl)-l-triflu-oromethyl-2,5-dioxolenium ions on their stmcture coincides with experimentally observed activity of the alkenes in trifluoroacetic acid in the 3-alkylsubstituted hydrogenated fiirans formation reaction. According to this dependence, the lecyclization of the dioxolenium ions formed from nonterminal alkenes is characterized by... 

Fe, Ru and Os - The synthesis and X-ray crystal structure of the n-bonded tetracyanoethene complex [Fe(NO)2(PPh3)(ti2-TCNE)] has been reported36. Whereas the complex [Fe(depe)2(N2>] [depe = 1,2 bis(diethylphosphinoethane)] was shown32 to react with ethene to form [Fe(depe)2(Tl2-C2H4)], reaction with 2-methylstyrene led to activation of the alkene C-H bond to afford a mixture of cis and trans [FeH(depe)2(CH=C6H4CH3)]. The selective formation of alkene rt-complexes or C-H activated products was explained in terms of steric considerations. 

These compounds are at present the most active of the alkene metathesis catalysts known. Catalysts developed by Grubbs, such as the second-generation Grubbs catalysts, tolerate a wide variety of functional groups being very stable. 

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