Allylation reaction allylic substitution

Complete reactions are obtained by the combination of these unit exchanges into composite reactions. Figure 3-11 gives the example of an allylic substitution. 

Figure 3-11. Allylic substitution as a composite reaction in Hendrickson s scheme.

The allyl-substituted cyclopentadiene 122 was prepared by the reaction of cyclopentadiene anion with allylic acetates[83], Allyl chloride reacts with carbon nucleophiles without Pd catalyst, but sometimes Pd catalyst accelerates the reaction of allylic chlorides and gives higher selectivity. As an example, allylation of the anion of 6,6-dimethylfulvene 123 with allyl chloride proceeded regioselectively at the methyl group, yielding 124[84]. The uncatalyzed reaction was not selective. 

Another useful modification of this reagent is the reaction of CF3CCI3 with zinc and DMF in the presence of AICI3 [60, 63] (equation 53). The alcohol product can be treated subsequently with DAST, thionyl chloride, or phosphorus chlorides to afford the allyl substitution product regio- and stereoselectively [66] (equation 54). 

Copper-mediated Diastereoselective Conjugate Addition and Allylic Substitution Reactions... 

To explain tlie stereodieniistiy of tlie allylic substitution reaction, a simple stereoelectronic model based on frontier molecular orbital considerations bas been proposed fl55. Fig. G.2). Organocopper reagents, unlike C-nudeopbiles, possess filled d-orbitals fd - configuration), wbidi can interact botli witli tlie 7t -fC=C) orbital at tlie y-carbon and to a minor extent witli tlie cr -fC X) orbital, as depicted... 

Depending on the substrate and the other reaction parameters, very higli re-gioselectivilies towards either a or y suhstilution can he obtained. In cetLain cases, the regioselectivity can easily he switclied between the two modes by changing the reaction conditions [11]. Compared to, for example, palladiumiO)-catalyzed allylic substitution reactions, the possibility of switching between S j2 and S j2 selectivity... 

S ]2 -selective reactions between primary allylic substrates and otganocoppet reagents testiU in the creation of new Chirality in previously aChital molecules, and it is tempting to try to take advantage of this for the development of enantioselective allylic substitution reactions. 

Denniatk and co-wotkets teporied tlie brst example in 1990 [16], using substrates 1, s7ntliesized Grom adiital allylic alcohols and tead dy ava dable optically active amine auxdiaries. Substrates 1 were tlien employed in coppet-niediaied allylic substitution reactions, as shown in Sdienie 8.4. 

A result equivalent to an allylic substitution reaction with a chiral leaving group can also be achieved by a two-step procedure involving a conjugate addition reaction and a subsequent elimination reaction, as demonstrated by Tamura et al., wbo studied the reaction shown in Scheme 8.15 [27]. 

Hie use of tlie cliiral catalyst 19b for asymmetric allylic substitution of allylic substrates bas been studied in some deta d fSdieme 8.18) and, under ji-selective reaction conditions, asymmetric induction was indeed obtained [28, 34]. 

It may be concluded from die different examples sliown here tiiat die enantio-selective copper-catalyzed allylic substitution reaction needs ftirdier improvemetiL High enantioselectivities can be obtained if diirality is present in tiie leaving group of die substrate, but widi external diiral ligands, enantioselectivities in excess of 9096 ee have only been obtained in one system, limited to die introduction of die sterically hindered neopeatyl group. 

Nanaomycin A 103 and deoxyfrenolicin 108 are members of a group of naphthoquinone antibiotics based on the isochroman skeleton. The therapeutic potential of these natural products has attracted considerable attention, and different approaches towards their synthesis have been reported [65,66]. The key step in the total synthesis of racemic nanaomycin A 103 is the chemo-and regioselective benzannulation reaction of carbene complex 101 and allylacety-lene 100 to give allyl-substituted naphthoquinone 102 after oxidative workup in 52% yield [65] (Scheme 47). The allyl functionality is crucial for a subsequent intramolecular alkoxycarbonylation to build up the isochroman structure. However, modest yields and the long sequence required to introduce the... 

The other bromine atom comes from another bromine-containing molecule or ion. This is clearly not a problem in reactions with benzylic species since the benzene ring is not prone to such addition reactions. If the concentration is sufficiently low, there is a low probability that the proper species will be in the vicinity once the intermediate forms. The intermediate in either case reverts to the initial species and the allylic substitution competes successfully. If this is true, it should be possible to brominate an alkene in the allylic position without competition from addition, even in the absence of NBS or a similar compound, if a very low concentration of bromine is used and if the HBr is removed as it is formed so that it is not available to complete the addition step. This has indeed been demonstrated. ... 

Allylic silanes react with aldehydes, in the presence of Lewis acids, to give an allyl-substituted alcohol. In the case of benzylic silanes, this addition reaction has been induced with Mg(C104)2 under photochemical conditions. The addition of chiral additives leads to the alcohol with good asymmetric induction. In a related reaction, allylic silanes react with acyl halides to produce the corresponding carbonyl derivative. The reaction of phenyl chloroformate, trimethylallylsilane, and AICI3, for example, gave phenyl but-3-enoate. ... 

Example of Cyclopropanation, Diels-Alder Reaction, Nucleophilic Allylic Substitution... 

Keywords N,N-Containing ligands Asymmetric catalysis Cyclopropanation Diels-Alder reaction Nucleophilic allylic substitution... 

With the iron atom in its most negative oxidation state of —2 this complex possesses nucleophilic properties and thus can be used in nucleophilic substitution reactions. As the iron atom in this complex formally has ten valence electrons, it is isoelectronic with Pd(0), which is a well-known catalyst in allylic substitution reactions [49]. 

Recently, the scope of the allylic substitution has been extended to sulfinate salts 84 to obtain allylic sulfones 85. Due to solubility problems of both nucleophile 84 and carbonate leaving group, a polar solvent mixture of DMF and 2-methoxyethanol had to be employed, which limits the reaction to the use of a phosphine ligand. Thus, various aryl sulfinates 84 and functionalized carbonates 81 could be converted to the corresponding allylic sulfones 85 with good to excellent yields and regioselectivites and complete retention of stereochemistry (eq. 2 in Scheme 20) [65]. 

Besides allylic substitution reactions it was also shown that [Fe(CO)3(NO)] 76 is catalytically active in transesterification reactions under neutral conditions (Scheme 24) [70]. Various activated acyl donors 97 can be used to give rise to the corresponding carboxylic esters 100 in good to excellent yields. This reaction proceeds in the absence of additional ligands in nonpolar solvents, for example, hexane. Mechanistically, the reaction is assumed to proceed via a Fe-acyl-complex 98 (Scheme 24). 

Abstract The use of A-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation. 

Since Pd complexes are well-known catalysts for enantioselective allylic substitution reactions, here the catalytic behaviour of palladium NPs for this reaction is examined (Scheme 1). One example involving a chiral phosphite with a carbohydrate backbone, able to coordinate firmly at the surface of NPs together with oxygen atoms capable to interact weakly with this surface, is presented. In particular. 

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