Aromatic ring isocyanides

The close compounds bearing three aromatic rings (two of them related by an ester bridge as part of an isocyanide promesogen, Figure 7.7) also show an SmA phase... 

Complexes with the simplest alkoxyphenylisocyanide and several halides are prepared by metathetical reactions of [AuCl(CNR)] with KX salts (Figure 7.19) [17]. The chloro-derivatives (n > 4) andthebromo-complexes (n > 6) display SmA phases. However, the ligands and the iodo-complexes are not liquid crystals. The transition temperatures decrease in the order Cl > Br > I, according to the decrease in polarity of the Au—X bond. It is important to note that the coordination of a very simple non-mesomorphic isocyanide (only one alkoxy chain and one aromatic ring) to Au—Cl allows the formation of a quite ordered and stable smectic mesophase. 

The a-amidoalkylation reaction, which involves the addition of carbon nucleophiles (primarily aromatic rings, alkenes, cyanides, isocyanides, alkynes, organometallic and active methylene-containing compounds) to substituted amides (89) where X is a leaving group (Scheme 17), has been extensively re-viewed. A variety of organometallic condensations have since been reported which extend the scope of this reaction. The reactive intermediates in these reactions are considered to be N-acylimines (88) or N-acyliminium salts (SKI). Direct displacement of the a-haloalkylamide precursors (89 X = halogen) cannot in certain cases be discounted. 

Some fungal species produce cyclopentenyl isocyanides, for example trichoviridin (166), which are known to be biosynthesised from tyrosine by cleavage of the aromatic ring. Despite much experimental study, the biosynthetic origin of the isocyanide carbon into the cyclopentenyl diene isocyanide acid (167) and related metabolites remains elusive [101-103], Notably the standard C j-tetrahydrofolate precursors such as methionine, formate, glycine or serine do not provide the source of this carbon, nor is cyanide incorporated into the isocyanide moiety [101-103], In summary, other than one terrestrial cyanophyte, the marine environment appears so far unique in accessing cyanide for isocyanide biosynthesis. 

It is well known that simple alkanes are ready to undergo hydrogen abstraction to form carbon-centered radicals in the presence of radical initiators (TBHP, DTBP, K2S2O8, etc), and the resulting nucleophilic alkyl radicals are able to attack unsaturated chemical bonds, such as alkenes, alkynes, aromatic rings, and isocyanides (Scheme 2.38). In 2008, Li et al. introduced a novel [Ru(p-cymene)Cl2]2-catalyzed,... 

It has been reportedthat a C-C bond in an aromatic ring is cleaved with a stoichiometric amount of a transition metal complex. When quinoxaline was treated with the tungsten complex 90, dihydrogen was released and the tungsten was inserted into the C-C bond to furnish the di(isocyanide) tungsten complex 92 (Scheme 7.37) [52]. The authors proposed the mechanism to be through sequential C-H... 

This study prompted us to examine stmcturally distinct azomethine imines. The reaction of a C,A-cyclic azomethine imine not fused to the aromatic ring, which was generated in situ from 37 in the presence of a base, was conducted with t-butyl isocyanide (35a) to afford a cyclized product 38 in 81% yield (Eq. 11.15). On the other hand, the AJV -cyclized azomethine imine 11a, which geometrically could not afford the iminoxadiazinone derivatives, did not react with the isocyanide 35a even under dichloromethane reflux conditions (Eq. 11.16). These results suggested that the direction of the amidocarbonyl oxygen was crucial to promote the nucleophilic addition of isocyanide. 

The fate of the isocyanide was suggested to depend on the nature of the substituent originally at C4 of the isothiazole ring. If the substituent is aromatic, the isocyanide is reprotonated at the isocyanide carbon to yield 32, which spontaneously cyclizes to the 4-subsituted thiazole 34, the observed N2-C3 interchange transposition product. If the substituent is not aromatic, 31 is reprotonated at sulfur to provide 33, which has a higher energy barrier to cyclization. In these cases, the isocyanosulfides can be observed spectroscopically and can be trapped as their N-formylaminobenzyl thioether derivatives. 

Liquid crystals based on aliphatic isocyanides and aromatic alkynyls (compounds 16) show enantiotropic nematic phases between 110 and 160 °C. Important reductions in the transition temperatures, mainly in clearing points (<100 °C), areobtained when a branched octyl isocyanide is used. The nematic phase stability is also reduced and the complexes are thermally more stable than derivatives of aliphatic alkynes. Other structural variations such as the introduction of a lateral chlorine atom on one ring of the phenyl benzoate moiety or the use of a branched terminal alkyl chain produce a decrease of the transition temperatures enhancing the formation of enantiotropic nematic phases without decomposition. 

The p-functionalized 2-hydroxyphenyl isocyanide not only contains the isocyanide and the nucleophile within the same molecule, but both functional groups are also arranged in one plane for an intramolecular nucleophilic attack [176]. This arrangement, in addition to the aromaticity of the five-membered ring obtained after cyclization to the carbene ligand, particularly favors the intramolecular nucleophilic attack. In contrast to 2-hydroxyethyl isocyanide, free 2-hydroxyphenyl isocyanide is not stable [177]. The stable 2-trimethylsiloxyphenyl isocyanide [178] can serve as a synthon for 2-hydroxyphenyl isocyanide. Carbene complexes 63 with an NH,0-stabilized NHC ligand can be obtained from the complexes 62 with the... 

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