Glyoxal diimines

Primary 1,2-diamines can be obtained by hydrogenolysis of the benzylic N-auxiliary bond over Pd/C, e.g., the preparation of 249d. The cleavage is not selective in the presence of other benzylic bonds in the molecule, e.g., aryl substituents at Cl - C2. However, in the presence of methoxyphenyl or dimethoxyphenyl C substituents, the N substituents can be selectively removed, as exemplified by the preparation of the primary 1,2-diamine 

249e [122], which was then converted to the N,N -dibenzyl derivative 250 by routine N-benzoylation and borane reduction steps. 

Most importantly, the 4,5-diamino-1,7-octadienes 248a, 256 and 257, available by the double addition of allylic zinc reagents to the diimine 247, are 

---

Scheme 38 Addition of organometallic reagents to a chiral glyoxal diimine...

Figure 15 shows the pathway proposed for the formation of the isolated two-carbon product (glyoxal diimine derivative). 

When glyoxal diimine is reacted with lithium in THF and in the presence of a chlorosilane, formation of an SMA is not expected. It has been postulated, however, that a l,2-diaza-4-silacyclopentane derivative (SMA) is formed in low yield in addition to the expected product, l,3-diaza-2-sila-4-cyclopentene.178... 

The reactivity of the hetero ring, with electron deficient 2(3)-positions, has been compared to glyoxal diimine." Quinoxalines are deactivated towards electrophilic substitutions, however, substitution is most likely to occur at the equivalent positions 5 and 8, where electron-localization calculations show the highest electron density to be. Electron-donor substituents in the benzenoid ring facilitate electrophilic substitution and, when activating substituents are present in the hetero ring, the site of reaction depends on the reaction conditions. In deprotonated alkyl-substituted quinoxalines an increased number of resonance possibilities exists and mildly basic conditions arc usually required for condensation reactions. 

G) or cyclisation of an a-diimine or diazobutadiene (obtained by the condensation of the amine with glyoxal) with formaldehyde in the presence of a Brpnsted acid... 

Ir(cod)Cl]2 reacts with Q-diimines LL (derived from glyoxal and biacetyl) to yield cationic [Ir(cod)LL]+.523 If the reaction is carried out in the presence of SnCl2, then the pentacoordinate Ir(SnCl3)(cod)LL species results. The compounds are active catalysts in the homogeneous hydrogen transfer from isopropanol to cyclohexanone or to acetophenone followed by hydrogenation... 

The preparation of vicinal diallyldiamines is illustrated by the following sequence condensation of glyoxal with benzhydrylamine yields the diimine 189, which is converted into 190 by the action of allylmagnesium chloride. The diphenylmethyl groups are then removed reductively by treatment with triethylsilane196. 

Simple a-diimines are hydrolytically unstable, but can be stabilized as metal complexes by virtue of the formation of stable five-membered chelate rings.68 69 a-Diketones and glyoxal undergo metal template reactions with amines to yield complexes of multidentate ligands such as (34),70 (35)71 and (36).72>73 In the last case, the metal exerts its stabilizing influence on the a-diimine partner in an equilibrium process (Scheme 5). The same phenomenon occurs with amino alcohols74 75 in addition to amino thiols. The thiolate complexes (37) can be converted to macrocyclic complexes by alkylation in a kinetic template reaction (Scheme 5).76 77... 

Presumably, the oxidation of the iron(n) diimine complex derived from glyoxal and methylamine (encountered previously) by cerium(iv) occurs by a related mechanism (Fig. 9-37). This also provides an interesting example of the metal ion stabilising a particular tautomer of a ligand. The free ligand would be expected to exist as the amide tautomer 9.22. 

These findings confirmed that glyoxal dicyclohexylimine is one of the products from the reaction of glucose with cyclohexylamine in ethanol. Although the formation of similar imines by the reaction of glucose with other alkylamines was not directly established, detection of glyoxal by silica gel TLC, shown in Fig. 9, in all of these cases seems to justify the assumption that the two-carbon diimines are always among the products of this kind of reaction, under the conditions employed. 

After a careful theoretical analysis by ab initio MO methods, Schleyer and coworkers concluded that 1,2-dioxetenes and 1,2-diazetines are non-aromatic 67t-systems with completely localized C = C double bonds and normal C-X and X-X single bonds (X = O or NH). The unsubstituted 1,2-dioxetene and 1,2-diazetine undergo conrotatory ring opening to glyoxal and a-diimine. 

The preparation starts from glyoxal and /butylamine (Eq. 4). Lithiation of the resulting diimine 3 and coupling to silicon tetrachloride led to the five-membered ring 4. The final step was dehalogenation of 4. To our surprise this required vigorous reaction conditions contact with liquid potassium metal in THF... 

Blau reported in 1898 that the saturated a,a -dipiperidine (36a) forms a purple iron(II) complex. This surprising finding was later shown by Krumhok " to be due to air oxidation of the ligand in the presence of to give the tris chelate of (36b). He also demonstrated the template reaction of methylamine with glyoxal or biacetyl in the presence of Fe + that affords red tris complexes of the diimines (37a) and (37b). Although the free ligands are unstable, these complexes are very stable with respect to dissociation in acid but rather less so towards alkali. Their visible spectra are similar to those of [Fe(bipy)3] " and [Fe(phen)3] ". For [Fe(37b)3], = 568 (e = 1.07 x 1(f) and as... 

The new condensation reactions of amines with glyoxal to yield hexa-azaisowurtzitane derivatives 51 appear to be limited to benzylamine and certain phenyl-substituted benzylamines. Primary aliphatic amines and anilines usually form dicarbinolamines 52 or diimines 53 (Refs. 108-112). With certain arylamines, such as 2-chloroaniline or aniline itself, one may obtain 52 or l,r,2,2 -tetrakis(arylamino)ethanes (Ref. 109). The condensation of benzylamine with glyoxal was apparently not described in the literature prior to our report (Ref. 107). However, condensations of a-methylbenzylamine and a,o -dimethylbenzyiamine with glyoxal have been reported to produce diimines (53, R = QHjCHCHj and QH5C(CH3)2, respectively) (Refs. 111,112). 

Another approach, particularly valuable for the synthesis of imidazolium salts containing sterically hindered substituents, for example, multiple-substituted phenyl groups at the nitrogen atoms, consists of a two-step protocol (route D) [28]. Under mild conditions, lequiv of glyoxal is treated with 2equiv of arylamine in -propanol to form the corresponding diimine as an intermediate. Subsequent cyclization by condensation with chloromethylethyl ether leads to the desired diarylimidazolium salt in moderate yields (40 -47%) without the need of a purification step. 

---