Alkenes benzyl

Pd-C, cyclohexene or 1,4-cyclohexadiene, 25°, 1.5-6 h, good yields. Some alkenes, benzyl ethers, BOM groups, and benzylamines are compatible with these conditions. 

Acid chlorides also couple smoothly with lower-order zinc cyanocuprates (equation 59) to give ketones in high yields. Although most acid chloride substrates employed have been simple ones, alkenes, benzylic and alkyl chloride functional groups have been present in the acyl halide without incident25. 

EPR studies in acetonitrile at room temperature have shown that aliphatic carbonyl compounds promote the formation of LnCoOj superoxocobalt(III) complexes, which are responsible for the catalytic effect. According to the general procedure for catalytic oxidation of alkenes, benzylic compounds and alcohols, the mixture of 5 mmol substrate, 5 mol% of cobalt(II) complex, 10 mmol p-ketoester were stirred under 1 atm O2 for 20-35 hours at room temperature or 50-60°C. 

Catalytic hydrogenation is mostly used to convert C—C triple bonds into C C double bonds and alkenes into alkanes or to replace allylic or benzylic hetero atoms by hydrogen (H. Kropf, 1980). Simple theory postulates cis- or syn-addition of hydrogen to the C—C triple or double bond with heterogeneous (R. L. Augustine, 1965, 1968, 1976 P. N. Rylander, 1979) and homogeneous (A. J. Birch, 1976) catalysts. Sulfur functions can be removed with reducing metals, e. g. with Raney nickel (G. R. Pettit, 1962 A). Heteroaromatic systems may be reduced with the aid of ruthenium on carbon. 

Benzyl chloride reacts with alkenes bearing an EWG[8], The reaction with acrylate proceeds smoothly to give 7-phenylcrotonate (76) in the presence of BujN without a ligand. No reaction takes place with Pd(Ph,P)4[77]. 

A interesting and useful reaetion is the intramolecular polycyclization reaction of polyalkenes by tandem or domino insertions of alkenes to give polycyclic compounds[l 38]. In the tandem cyclization. an intermediate in many cases is a neopentylpalladium formed by the insertion of 1,1-disubstituted alkenes, which has no possibility of /3-elimination. The key step in the total synthesis of scopadulcic acid is the Pd-catalyzed construction of the tricyclic system 202 containing the bicyclo[3.2. Ijoctane substructure. The single tricyclic product 202 was obtained in 82% yield from 201 [20,164). The benzyl chloride 203 undergoes oxidative addition and alkene insertion. Formation of the spiro compound 204 by the intramolecular double insertion of alkenes is an exam-ple[165]. 

This IS a frequently used proce dure for the preparation of alkenes The order of alcohol reactivity paral lels the order of carbocation stability R3C > R2CH > RCH2 Benzylic al cohols react readily Rearrangements are sometimes observed... 

Dichlorodicyanoquinone (DDQ), CH2CI2, H2O, 40 min, it, 84-93% yield.This method does not cleave simple benzyl ethers. This method was found effective in the presence of a boronate. The following groups are stable to these conditions ketones, epoxides, alkenes, acetonides, to-sylates, MOM ethers, THP ethers, acetates, benzyloxymethyl (BOM) ethers, and TBDMS ethers. 

Carbocations can also be generated during the electrolysis, and they give rise to alcohols and alkenes. The carbocations are presumably formed by an oxidation of the radical at the electrode before it reacts or diffuses into solution. For example, an investigation of the electrolysis of phenylacetic acid in methanol has led to the identification of benzyl methyl ether (30%), toluene (1%), benzaldehyde dimethylacetal (1%), methyl phenylacetate (6%), and benzyl alcohol (5%), in addition to the coupling product bibenzyl (26%). ... 

The benzylic position in alkylbenzenes is analogous to the allylic position in alkenes. Thus a benzylic C—H bond, like an allylic one, is weaker than a C—H bond of an alkane, as the bond dissociation energies of toluene, propene, and 2-rnethylpropane attest ... 

Raney Nickel W2 or W4, EtOH, 85-100% yield. Mono- and dimethoxy-substituted benzyl ethers and benzaldehyde acetals are not cleaved under these conditions, and trisubstituted alkenes are not reduced. 

Reaction of 2-chloromethyl-4//-pyrido[l,2-u]pyrimidine-4-one 162 with various nitronate anions (4 equiv) under phase-transfer conditions with BU4NOH in H2O and CH2CI2 under photo-stimulation gave 2-ethylenic derivatives 164 (01H(55)535). These alkenes 164 were formed by single electron transfer C-alkylation and base-promoted HNO2 elimination from 163. When the ethylenic derivative 164 (R = R ) was unsymmetrical, only the E isomer was isolated. Compound 162 was treated with S-nucleophiles (sodium salt of benzyl mercaptan and benzenesulfinic acid) and the lithium salt of 4-hydroxycoumarin to give compounds 165-167, respectively. 

On the basis of this successful application of 23d, this catalyst was applied in a series of reactions (Scheme 6.22). For all eight reactions of nitrones 1 and alkenes 19 in which 23d was applied as the catalyst, diastereoselectivities >90% de were observed, and most remarkably >90% ee is obtained for all reactions involving a nitrone with an aromatic substituent whereas reactions with N-benzyl and N-alkyl nitrones led to lower enantioselectivities [65]. 

Cycloaddidon of the cychc nitrone derived ftom prolide benzyl ester v/ith alkenes proceeds readily to give isoxazohdines v/ith good regio-and stereoselecdvity fEq 8 47i The reaction favors exo-mode addidon However, certin cycloaddidons are reversible and therefore the product distribndon may reflect thermodynamic rather than kinedc control... 

The mechanism of benzylic bromination is similar to that discussed in Section 10.4 for allylic bromination of alkenes. Abstraction of a benzylic hydrogen atom generates an intermediate benzylic radical, which reacts with Br2 to yield product and a Br- radical that cycles back into the reaction to carry on the chain. The Br2 necessary for reaction with the benzylic radical is produced by a concurrent reaction of HBr with NBS. 

The benzylic position of an alkylbcnzene can be brominated by reaction with jV-bromosuccinimide, and the entire side chain can be degraded to a carboxyl group by oxidation with aqueous KMnCfy Although aromatic rings are less reactive than isolated alkene double bonds, they can be reduced to cyclohexanes by hydrogenation over a platinum or rhodium catalyst. In addition, aryl alkyl ketones are reduced to alkylbenzenes by hydrogenation over a platinum catalyst. 

Lanthanide triflates catalyze the Diels-Alder reaction of imines, generated from anilines and aldehydes, with both dienes and alkenes [26]. Thus N-benzyl-ideneaniline in the presence of Yb(OTf)3 (Scheme 6.16) reacts in organic solvent with open-chain dienes, such as Danishefsky s diene, to give tetrahy-dropyridine derivatives, while with cyclopentadiene and vinylethers and vinylthioethers it works like azadiene in both organic solvent and aqueous medium, affording tetrahydroquinoline derivatives. 

The method is quite useful for particularly active alkyl halides such as allylic, benzylic, and propargylic halides, and for a-halo ethers and esters, but is not very serviceable for ordinary primary and secondary halides. Tertiary halides do not give the reaction at all since, with respect to the halide, this is nucleophilic substitution and elimination predominates. The reaction can also be applied to activated aryl halides (such as 2,4-dinitrochlorobenzene see Chapter 13), to epoxides, " and to activated alkenes such as acrylonitrile. The latter is a Michael type reaction (p. 976) with respect to the alkene. 

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. ... 

---