Alkene Activation

Alkanes, dipurin-8-yl-synthesis, 5, 574 Alkanes, poly-N-pyrazoIyl-synthesis, 5, 320 Alkanoic acids, tetrazolyl-anti-inflammatory activity, 5, 835 Alkanoic acids, 4-thienyi-cyclization, 4, 905-906 Alkene metathesis mechanism, 1, 668 Alkenes activated... 

VILSMEIER - HAACK - VIEHE Reagent Formylation of aremalics, alkenes, activated H compounds by MeaN -CHCI Cl (Vilsmeier-... 

Nitrones are a rather polarized 1,3-dipoles so that the transition structure of their cydoaddition reactions to alkenes activated by an electron-withdrawing substituent would involve some asynchronous nature with respect to the newly forming bonds, especially so in the Lewis acid-catalyzed reactions. Therefore, the transition structures for the catalyzed nitrone cydoaddition reactions were estimated on the basis of ab-initio calculations using the 3-21G basis set. A model reaction indudes the interaction between CH2=NH(0) and acrolein in the presence or absence of BH3 as an acid catalyst (Scheme 7.30). Both the catalyzed and uncatalyzed reactions have only one transition state in each case, indicating that the reactions are both concerted. However, the synchronous nature between the newly forming 01-C5 and C3-C4 bonds in the transition structure TS-J of the catalyzed reaction is rather different from that in the uncatalyzed reaction TS-K. For example, the bond lengths and bond orders in the uncatalyzed reaction are 1.93 A and 0.37 for the 01-C5 bond and 2.47 A and 0.19 for the C3-C4 bond, while those in... 

An alkene activated by an electron-withdrawing group—often an acrylic ester 2 is used—can react with an aldehyde or ketone 1 in the presence of catalytic amounts of a tertiary amine, to yield an a-hydroxyalkylated product. This reaction, known as the Baylis-Hillman reaction, leads to the formation of useful multifunctional products, e.g. o -methylene-/3-hydroxy carbonyl compounds 3 with a chiral carbon center and various options for consecutive reactions. 

Doyle et al. (1977 c) and Oae et al. (1980) reported modified Meerwein arylations with significant improvements in the yield by the use of aryl amines and alkyl nitrites in place of arenediazonium salts. However, good yields are only achieved if alkenes activated by electron-withdrawing groups are present. 

Alkenes activated by an electron-withdrawing group (Z may be C=C, halogen, C=0, Ar, CN, etc.) can be arylated by treatment with a diazonium salt and a cupric chloride catalyst. This is called the Meerwein arylation reaction Addition of... 

More generally, many combinations of EWG substituents can serve as the anion-stabilizing and alkene-activating groups. Conjugate addition has the potential to form a bond a to one group and (3 to the other to form a a,y-disubstituted system. 

Very recently, [2 + 2] cycloadditions with alkenes activated by electron-with drawing groups have teen performed on the double bond of methylenecyc-lopropanes exo-substituted with two electron-donating groups, e.g., dimethyleneketene acetals 546 (Scheme 75, Table 44) [145]. 

Finally, the catalytic enantioselective 1,3-dipolar cycloaddition reaction has recently been developed to be a highly selective reaction of nitrones with electron-deficient alkenes activated by chiral Lewis acids. High levels of regio-, diastereo-, and enantioselectivities can now be reached using catalysts 89 <2000JOC9080>, 90 <2002JA4968>, or 91 <2005JA13386> (Scheme 29). 

The cyclodimerisation of fluorinated alkenes and formation of cyclobutane from alkenes activated by electron withdrawing groups has formed great application in laboratory and industry... 

A -Protonated 2-azabuta-l,3-diene undergoes 4+- -2-cycloaddition with alkenes activated by electron-donating and electron-withdrawing substituents.The 2+ - - 4-cycloadditions of l,3-dithian-2-ylium ions with buta-1,3-dienes follow second-order kinetics which do not exclude a mechanism involving a concerted pathway. The... 

The presence of stable V-O-Si bonds also limits the possibility of insertion of O in the activated organic molecule and the stable coordination limits the possibility of alkene activation through an allylic mechanism. Both these factors are important in controlling the consecutive transformation of propylene. 

Hojo et al. [218,308,309] described the trifluoroacetylation of alkenes activated by heteroatom donors such as vinyl ethers and ketene acetals (Eq. 116) the reaction occurs under mild conditions and can be used to prepare bis(trifluoro-acetyl) products from highly activated derivatives [310]. The main use of the products appears to be in the synthesis of trifluoromethyl heterocycles both... 

Formation of 1,2-dioxetanes is usually characteristic of alkenes activated by heteroatom in the vinylic position. Most dioxetanes are unstable and decompose with concomitant chemiluminescence. The products are carbonyl compounds resulting from oxidative cleavage of the double bond. 

Co(III)-nitro,545 Rh(III)-nitro,546 and Pd(II)-nitro547 complexes were shown to transfer oxygen to alkenes in stoichiometric545 546 or catalytic545 547 processes. In some cases (Co and certain Rh complexes), when the alkene cannot coordinate to the metal, Pd(II) is required, which serves as an alkene activator through coordination.545 546... 

The rhodium(II) complex [Rh2(OAc)4] reduces terminal and cyclic alkenes, activated alkenes and alkynes.148 Various polar solvents could be used, but DMF was preferred. Following a kinetic investigation of the hydrogenation of 1-decene, the mechanism shown in equations (32)-(35) was proposed. 

IrCI(CO)(PCy3)2] Alkenes, activated alkenes, ketones... 

Preparatively useful yields are frequently obtained and a wide variety of alkene activators have been used. Authoritative reviews provide more details for this reaction, which is probably the most general method available for the addition of aryl radicals across carbon-carbon double bonds.187-188... 

The extensive data accumulated by Nakamura and Otsuka, although interpreted by them as being due to the intervention of metal carbene and metallocyclobutane intermediates, can also be rationalized by an alternative mechanism in which coordination of the chiral Co(II) catalyst with the alkene activates the alkene for electrophilic addition to the diazo compound (Scheme 5.4). Subsequent ring closure can be envisioned to occur via a diazonium ion intermediate, without involving at any stage a metal carbene intermediate. 

Fluoromethylbenzoic acids, metallation, 9, 26-27 Fluoro(phenyl) complexes, with platinum(II), 8, 482 Fluorosilanes, elimination in fluorinated alkene activation, 1, 732 in fluorinated aromatic activation, 1, 731 and hydrodefluorination, 1, 748 Fluorosilicate anions, hypercoordinated anions, 3, 484 Fluorotoluenes, metallation, 9, 21 Fluorous alkylstannanes, preparation, 3, 820 Fluorous biphasic system, as green solvent, 12, 844 Fluorous ligands, with supercritical carbon dioxide, 1, 82 Fluorous media... 

CO insertion, Ru and Os clusters, 6, 840 dynamic NMR studies, 1, 421 in fluorinated alkene activation, 1, 732 and fluorinated aromatic activation, 1, 731 to heteroatom ligands, Ru and Os trinuclear clusters,... 

Cyclometallation (also called oxidative coupling) is a rather special case of oxidative addition. In this reaction, two unsaturated molecules, X=Y and X =Y, add to the same metal atom M. One of the X—Y bonds and one of the X —Y bonds are broken, and new M X and M -Y bonds form. However, a new Y—Y bond also forms, and the overall result is a cyclometaUated compound (Figure 3.7a). As in oxidative addition, the oxidation state of the metal center increases by 2. Cyclometallation is common with alkynes (Figure 3.7b), as well as with alkenes activated by electron-withdrawing groups [21]. 

This unique reactivity was subsequently utilized in the functionalization of unsaturated hydrocarbon substrates with CO (Scheme 39) [285, 286]. In the course of these studies the stereospecific synthesis of a diolato unit was observed via a samarium-mediated CO and CH (alkene) activation. A bridging trans-5,10-dihydroindeno[2,l-a]-5,10-diolate ligand forms terminal Sm-O bonds in [Cp Sm]2(O2C16H10) and its THF adduct. Two molecules of CO are formally inserted into the carbon carbon double bond of l,2-di-2-pyridylethene to form a terminal bisenolate moiety [286] (Table 18). 

There has been a number of developments in the use of salicylaldehydes as precursors of both chromenes and chromans. Alkenes activated by acyl, formyl, nitrile and phenylsulfonyl groups react with 2-hydroxybenzaldehydes and 2-hydroxy-1-naphthaldehyde under Bayliss-Hillman conditions to yield 3-substituted chromenes via the in situ dehydration of the initially formed chroman-4-ol <02JCS(P1)1318>. In like manner, P-nitrostyrenes yield 2- and 2,2-substituted derivatives of 3-nitrochromenes <02H(57)1033>. A simple route to 2-phenyl-2H-chromenes starting from salicylaldehyde and utilising a Pd(0)-catalysed cyclisation of an allylic acetate has been described <02SC3667>. 

Alkynes can also be used as radical acceptors for the generation of cyclo-pentanols bearing an exocyclic alkene, but in nearly all cases the yields are modest compared with those obtained in the related cyclisations of alkenes. Activation of the alkynes by electron-withdrawing groups or silyl substituents leads to more efficient 5-exo-dig ring closure.55... 

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