Biphenyl, reductive alkylation

Table I. Reductive Alkylation of Anthracene, Naphthalene, and Biphenyl in Metal-Ammonia Solutions...

The reduction of three polynuclear aromatic compounds, namely, perylene, decacyclene, and dibenzothiophene, with potassium metal in tetrahydrofuran at 25 °C, followed by quenching with alkyl iodides, was investigated. With perylene, a dianion was obtained, which on reaction with methyl iodide added two methyl groups the main re-gioisomer had one methyl group on C-l and one methyl group on the adjacent nonprotonated carbon, C-14. The reductive alkylation of decacyclene also led to alkyl groups on quaternary carbon atoms. The reduction of dibenzothiophene led to a loss of sulfur and proton uptake from solvent to yield biphenyl as the product after 24 h of reduction. 

Dibenzothiophene Chemistry. Unlike with perylene and deca-cylene, for which reductive alkylation leads simply to the addition of alkyl groups to aromatic compounds, the reductive alkylation of dibenzothiophene can lead to biphenyl (8). This product occurs by the loss of sulfur from dibenzothiophene, followed by the uptake of two protons. Using reaction times of 1-4 h at room temperature, one can isolate a 2-biphenylyl, alkyl sulfide this finding indicates that the anion on sulfur can have some stability. We investigated this situation by performing 2H NMR on a product of the reaction of CD3I with a dibenzothiophene anion solution formed by adding 2 mol of K per mole of dibenzothiophene. 

Bis(aryl)cobalt(II) compounds have been prepared by reaction of R MgX (where R = C6H6 Cl n = 2-4) with Co(PR3)2Cl2.203 They undergo both thermally and oxidatively induced decomposition, with the corresponding biphenyl a product. The reactions of alkyl-cobalt complexes have been reviewed recently, and include thermolysis, photolysis, oxidation, and reduction reactions.25 Homolysis of the Co—C bond is a feature of reactions. 

Standard cyclisation methodology was used to access the cyclic monophosphinic acid derivative 78 by reaction of ammonium phosphonate and ethyldiisopropylamine, followed by the addition of chlorotrimethylsilane, with 2,2 -bis (bromomethyl)-l,l -biphenyl. Silane reduction of 78 gave the secondary phosphine. The secondary phosphine borane complex 79 could be used in alkylation or Michael addition reactions. For example the Michael adduct 80 was produced in high yield by treatment of 78 with a NaH suspension in THF followed by the addition of diethylvinylphosphonate . 

An important development followed in 1991,28 when Yus and Ramon showed that naphthalene, biphenyl and DBB could be used catalytically in the reduction of functionalised alkyl chlorides.29 9 Only 1% of the arene catalyst is required, and organolithiums are... 

Benzylic deprotonation is often an inefficient process. It may be more important than it would appear from the end products, however, since radical cation deprotonation followed by reduction of the radical and reprotonation may regenerate the starting material. This mechanism has been proposed to explain the inefficiency of some PET alkylations [68]. In suitable models such a process has been revealed, e.g. deuterium incorporation at the bis-benzylic position in 2-(4-methoxyphenyl)-2-phenylethyl methyl ether and cis-trans isomerization in 2-methoxy-l-(4-methoxyphenyl)indane (but not in the corresponding 3-methoxyphenyl derivatives) [204], as well as deconjugation of 1-phenylalkenes to 3-phenylalkenes in the presence of 1,4-dieyanobenzene, biphenyl (as a secondary donor) and a hindered pyridine as the base [205]. Deprotonation of N,N-dimethylaniline has likewise been observed (Scheme 38) [206-207],... 

Scheme IS depicts a high yield, general method for specific ortho alkylation of polycyclic aromatic hydrocarbons. In this example, biphenyl is subjected to reductive methylation followed by oxidative rearrangement with trityl tetrafluoroborate to give 2-methylbiphenyl. In unsymmetrical substrates the regioselectivity is poor phenanthrene gives a 3 2 mixture of 4-methyl- and 1-methyl-phenanthrene.

Fluorene has been reported to afford the 3,9a-dihydro product, but it is almost certain that this is the 2,4a-dihydro isomer (55 = 1) by analogy with biphenyl. 9,10-Dihydrophenanthrene (56) is reduced as expected to (55 n = 2), but spontaneously reverts to the starting material on standing. These systems do not require the presence of alcohol for reduction and it is consequently possible to alkylate the intermediate anions with alkyl halides, as (56) gives (57). These products are much more stable and structural analysis is simplified accordingly oxidation of the doubly allylic methylene occurs readily to afford the dienone (58 Scheme 7). Dienones of this type have potential as intermediates for the synthesis of natural products. Anthracene and phenanthrene are both readily reduced in the central ring to form the 9,10-di-hydro derivatives as might be expected, but to avoid further reduction it is necessary to have an iron salt present. Further examples are reviewed elsewhere. ... 

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