Nitrile Ylide Dimerization:  Investigation of the Carbene Reactivity of Nitrile Ylides

Abstract

A series of novel hexaaryl diazatrienes 5 ("nitrile ylide dimers") were synthesized directly from the corresponding diaryl ketimines 12 and dichlorotoluenes 13 in a facile one-pot synthesis. The carbene character of the nitrile ylides was investigated by varying the substituents on the aromatic ring adjacent to the carbene center. The isolation of the corresponding carbene dimers as stable crystalline materials with absorption maxima (lambda(max)) from 363 to 422 nm was shown to be promoted by the absence of strongly electron-withdrawing substituents. The crystal structures indicate that the E-isomers were isolated when phenyl, 3-methylphenyl, and 3-chlorophenyl substituents are present at the carbene carbon; the Z-isomer was isolated when the more sterically hindered 2,4,6-trimethylphenyl substituent (Mes) is present. The (1)H NMR spectra of the E-isomers demonstrate the nonequivalence of the aromatic rings, in which two of the aromatic rings of the imine moiety are pseudoaxial and the remaining aromatic rings are pseudoequatorial. The reactions proceed via the intermediate nitrile ylides 1 generated by the base-promoted 1,1-elimination of HCl from the intermediate chloroimine 14. The nitrile ylide was also generated by 1,3-elimination of HCl from the imidoyl chloride 18, confirming common pathways via the nitrile ylide as the dimer products obtained from these different routes were identical. The strongly electron-withdrawing 4-nitrophenyl substituent promotes the linear carbanion character of the 1,3-dipole and no dimer is formed.

Dissociation of the P=C Ylidic Bond

“…[10] In general, there are only a few examples known of the behaviour of nitrile ylides as imine-substituted carbenes, all of which are devoted to their dimerisation. [11] For example, the imine-stabilized carbene 16, which results from the thermolysis or photolysis of 4,5-dihydro-1,2,3λ 5 -dioxaphosphole (15), has been postulated as one of the intermediates in the formation of compound 19 (Scheme 6). [12] Scheme 5.…”

Nitrogen–boron coordination versus OH⋯N hydrogen bonding in pyridoxaboroles – aza analogues of benzoxaboroles

“…The resulting yellow slurry was stirred for 15 min, and the reaction mixture was treated with a mesitaldehyde 38 (2.1 g, 14 mmol) diluted with Et 2 O (10 mL). The temperature was kept below -85°C.…”

Nitrile Ylides: Generation, Properties and Synthetic Applications

“…Several dimers were prepared and it was found that strong electron-withdrawing substituents suppressed the carbene-like reactivity of nitrile ylides [81]. Several dimers were prepared and it was found that strong electron-withdrawing substituents suppressed the carbene-like reactivity of nitrile ylides [81].…”