2-Unsubstituted Imidazole N-Oxides as Novel Precursors of Chiral 3-Alkoxyimidazol-2-ylidenes Derived from trans-1,2-Diaminocyclohexane and Other Chiral Amino Compounds †

‘Desymmetrization’ of trans-1,2-diaminocyclohexane by treatment with α,ω-dihalogenated alkylation reagents leads to mono-NH2 derivatives (‘primary-tertiary diamines’). Upon reaction with formaldehyde, these products formed monomeric formaldimines. Subsequently, reactions of the formaldimines with α-hydroxyiminoketones led to the corresponding 2-unsubstituted imidazole N-oxide derivatives, which were used here as new substrates for the in situ generation of chiral imidazol-2-ylidenes. Upon O-selective benzylation, new chiral imidazolium salts were obtained, which were deprotonated by treatment with triethylamine in the presence of elemental sulfur. Under these conditions, the intermediate imidazol-2-ylidenes were trapped by elemental sulfur, yielding the corresponding chiral non-enolizable imidazole-2-thiones in good yields. Analogous reaction sequences, starting with imidazole N-oxides derived from enantiopure primary amines, amino alcohols, and amino acids, leading to the corresponding 3-alkoxyimidazole-2-thiones were also studied.

Along with trans-DACH, another primary amine widely explored in asymmetric synthesis is α-methylbenzylamine (α-MBA), but to date, reports on its application in the synthesis of chiral NHCs are limited [6]. Similarly, chiral β-amino alcohols and amino esters have scarcely been explored. Selected representatives of these two classes of compounds will also be involved in this study.
It is well established that the most efficient and preparatively useful method for the in situ generation of imidazol-2-ylidenes 3 comprises the deprotonation of the corresponding imidazolium salts of type 2 by treatment with a base (Scheme 2) [2,3]. by modifications of the amino groups. Derivatives of type 1 with a cyclic amine motif are promising building blocks for the synthesis of new bifunctional catalysts [11,12] and are important synthons for the preparation of some bioactive compounds [13][14][15][16][17]. An efficient method for the 'desymmetrization' of DACH, reported only in recent years, comprises two-fold N,N-alkylation of only one NH2 group using α,ω-dihaloalkylating reagents as shown in Scheme 1 [12].
Along with trans-DACH, another primary amine widely explored in asymmetric synthesis is αmethylbenzylamine (α-MBA), but to date, reports on its application in the synthesis of chiral NHCs are limited [6]. Similarly, chiral β-amino alcohols and amino esters have scarcely been explored. Selected representatives of these two classes of compounds will also be involved in this study.
It is well established that the most efficient and preparatively useful method for the in situ generation of imidazol-2-ylidenes 3 comprises the deprotonation of the corresponding imidazolium salts of type 2 by treatment with a base (Scheme 2) [2,3].
In our earlier publications, we described an efficient method for the synthesis of enantiomerically pure bis-and mono-imidazole N-oxides derived either from trans-1,2diaminocyclohexane [18,19], α-methylbenzylamine [20], β-amino alcohols [21], and amino acid esters [22]. In another study, we demonstrated that 2-unsubstituted imidazole N-oxides including some chiral derivatives can be O-alkylated by treatment with alkyl bromides [23]. However, these 1alkoxyimidazolium salts have never been used for the generation of N-alkoxy-substituted imidazol-2-ylidenes of type 3. In a very recent publication, we reported the synthesis of non-symmetric imidazolium salts prepared in a multistep synthesis starting with adamantyloxyamine [24]. In a test experiment, a symmetric 1,3-di(adamantyloxy)imidazolium salt was converted into the corresponding imidazole-2-thione derivative via an intermediate NHC. Prompted by these results, we decided to synthesize a series of new chiral 1-alkoxyimidazolium salts derived from 'desymmetrized' trans-1,2-diaminocylohexane derivatives of type 1, which can subsequently be used as precursors of chiral NHCs. The study was extended by the involvement of α-methylbenzylamine as well as selected amino alcohols and amino acid derivatives. Scheme 1. trans-1,2-diaminocyclohexane (DACH) and its 'desymmetrized' derivatives 1 (Reproduced with permission from [12]).

Scheme 2. Formation of imidazol-2-ylidenes by deprotonation of imidazolium salts.
In our earlier publications, we described an efficient method for the synthesis of enantiomerically pure bis-and mono-imidazole N-oxides derived either from trans-1,2-diaminocyclohexane [18,19], α-methylbenzylamine [20], β-amino alcohols [21], and amino acid esters [22]. In another study, we demonstrated that 2-unsubstituted imidazole N-oxides including some chiral derivatives can be O-alkylated by treatment with alkyl bromides [23]. However, these 1-alkoxyimidazolium salts have never been used for the generation of N-alkoxy-substituted imidazol-2-ylidenes of type 3. In a very recent publication, we reported the synthesis of non-symmetric imidazolium salts prepared in a multistep synthesis starting with adamantyloxyamine [24]. In a test experiment, a symmetric 1,3-di(adamantyloxy)imidazolium salt was converted into the corresponding imidazole-2-thione derivative via an intermediate NHC. Prompted by these results, we decided to synthesize a series of new chiral 1-alkoxyimidazolium salts derived from 'desymmetrized' trans-1,2-diaminocylohexane derivatives of type 1, which can subsequently be used as precursors of chiral NHCs. The study was extended by the involvement of α-methylbenzylamine as well as selected amino alcohols and amino acid derivatives.

Results and Discussion
In contrast to DACH, which reacts with two equivalents of formaldehyde forming a dimeric product identified as a tetraazaeicosane derivative [25], all 'primary-tertiary' diamines 1a-d reacted with formaldehyde yielded the expected formaldimines 4a-d as solid products (Scheme 3). In the solid state they exist as hexahydro-1,3,5-triazines (trimers), which in polar, aprotic solvents undergo almost total dissociation forming the monomeric imines. Similar observations were made for cyclohexylformaldimine [26]. For example, formaldimine 4b dissolved in CDCl 3 dissociates with t 1/2 of ca. 30 s at 298 K. On the other hand, the same imine observed in the C 6 D 6 solution exists as an equilibrium mixture of trimeric and monomeric forms in a ratio of ca. 2.5:1. The structures of formaldimines 4 were confirmed by 1 H and 13 C-NMR spectra in CDCl 3 . For example, in the 1 H-NMR spectrum of 4b, the characteristic AB-system of the monomeric form located at 7.13 and 7.28 ppm with J = 18 Hz was attributed to the N=CH 2 group. In the 13 C-NMR spectrum, the signal of this group appeared at 151.0 ppm, and similar data were found for all formaldimines 4. However, in all samples, the presence of trimeric forms interfered with the integration of signals. The presence of trimers was revealed by signals of the CH 2 groups located at 3.80-4.00 ppm. The products were unstable in solution and underwent gradual decomposition over a period of weeks during storage at room temperature. The molecular formulae of the stable, crystalline samples were confirmed by elemental analysis.

Results and Discussion
In contrast to DACH, which reacts with two equivalents of formaldehyde forming a dimeric product identified as a tetraazaeicosane derivative [25], all 'primary-tertiary' diamines 1a-d reacted with formaldehyde yielded the expected formaldimines 4a-d as solid products (Scheme 3). In the solid state they exist as hexahydro-1,3,5-triazines (trimers), which in polar, aprotic solvents undergo almost total dissociation forming the monomeric imines. Similar observations were made for cyclohexylformaldimine [26]. For example, formaldimine 4b dissolved in CDCl3 dissociates with t1/2 of ca. 30 s at 298 K. On the other hand, the same imine observed in the C6D6 solution exists as an equilibrium mixture of trimeric and monomeric forms in a ratio of ca. 2.5:1. The structures of formaldimines 4 were confirmed by 1 H and 13 C-NMR spectra in CDCl3. For example, in the 1 H-NMR spectrum of 4b, the characteristic AB-system of the monomeric form located at 7.13 and 7.28 ppm with J = 18 Hz was attributed to the N=CH2 group. In the 13 C-NMR spectrum, the signal of this group appeared at 151.0 ppm, and similar data were found for all formaldimines 4. However, in all samples, the presence of trimeric forms interfered with the integration of signals. The presence of trimers was revealed by signals of the CH2 groups located at 3.80-4.00 ppm. The products were unstable in solution and underwent gradual decomposition over a period of weeks during storage at room temperature. The molecular formulae of the stable, crystalline samples were confirmed by elemental analysis. In the case of 4e derived from (S)-α-methylbenzylamine ((S)-α-MBA), rapid trimerization led to hexahydro-1,3,5-triazine 4′e (Scheme 4) in the course of its synthesis, and after isolation, it could be used for further transformations only in this form [20]. An analogous tendency for trimerization was also observed for 4f-h used in the study. Scheme 4. Trimerization of formaldimine 4e derived from (S)-α-methylbenzylamine (α-MBA) [20] (Reproduced with permission from [20]) and structures of analogous formaldimines 4f-i.
In the case of 4e derived from (S)-α-methylbenzylamine ((S)-α-MBA), rapid trimerization led to hexahydro-1,3,5-triazine 4 e (Scheme 4) in the course of its synthesis, and after isolation, it could be used for further transformations only in this form [20]. An analogous tendency for trimerization was also observed for 4f-h used in the study.

Results and Discussion
In contrast to DACH, which reacts with two equivalents of formaldehyde forming a dimeric product identified as a tetraazaeicosane derivative [25], all 'primary-tertiary' diamines 1a-d reacted with formaldehyde yielded the expected formaldimines 4a-d as solid products (Scheme 3). In the solid state they exist as hexahydro-1,3,5-triazines (trimers), which in polar, aprotic solvents undergo almost total dissociation forming the monomeric imines. Similar observations were made for cyclohexylformaldimine [26]. For example, formaldimine 4b dissolved in CDCl3 dissociates with t1/2 of ca. 30 s at 298 K. On the other hand, the same imine observed in the C6D6 solution exists as an equilibrium mixture of trimeric and monomeric forms in a ratio of ca. 2.5:1. The structures of formaldimines 4 were confirmed by 1 H and 13 C-NMR spectra in CDCl3. For example, in the 1 H-NMR spectrum of 4b, the characteristic AB-system of the monomeric form located at 7.13 and 7.28 ppm with J = 18 Hz was attributed to the N=CH2 group. In the 13 C-NMR spectrum, the signal of this group appeared at 151.0 ppm, and similar data were found for all formaldimines 4. However, in all samples, the presence of trimeric forms interfered with the integration of signals. The presence of trimers was revealed by signals of the CH2 groups located at 3.80-4.00 ppm. The products were unstable in solution and underwent gradual decomposition over a period of weeks during storage at room temperature. The molecular formulae of the stable, crystalline samples were confirmed by elemental analysis. In the case of 4e derived from (S)-α-methylbenzylamine ((S)-α-MBA), rapid trimerization led to hexahydro-1,3,5-triazine 4′e (Scheme 4) in the course of its synthesis, and after isolation, it could be used for further transformations only in this form [20]. An analogous tendency for trimerization was also observed for 4f-h used in the study.  [20] (Reproduced with permission from [20]) and structures of analogous formaldimines 4f-i.
In contrast, the alkoxyformaldimine 4i derived from (S)-α-methylbenzyloxyamine (α-MBOA) [27] exists in CDCl3 solution in monomeric form, and in that case, no tendency to undergo Scheme 4. Trimerization of formaldimine 4e derived from (S)-α-methylbenzylamine (α-MBA) [20] (Reproduced with permission from [20]) and structures of analogous formaldimines 4f-i. In contrast, the alkoxyformaldimine 4i derived from (S)-α-methylbenzyloxyamine (α-MBOA) [27] exists in CDCl 3 solution in monomeric form, and in that case, no tendency to undergo trimerization was observed even after three days at room temperature. Moreover, a similar behavior was observed in the case of benzyloxyformaldimine (4j) prepared from benzyloxyamine (BOA) [28] and formaldehyde in the course of the present study. Apparently, the presence of an alkoxy moiety reduces the electrophilicity of the =CH 2 unit and thereby formation of the trimeric forms is substantially disfavored.
Enantiomerically pure, (R,R)-configured 2-unsubstituted imidazole N-oxides 6 used in this study as the key building blocks were prepared from enantiopure formaldimines 4, derived from the corresponding amines 1, and α-hydroxyiminoketones 5a,b in glacial acetic acid at room temperature [24]. In the case of 4b, along with the (R,R)-configured stereoisomer, the (S,S)-enantiomer was also involved in the study (Scheme 5 and Table 1).
Molecules 2019, 24, x 4 of 18 trimerization was observed even after three days at room temperature. Moreover, a similar behavior was observed in the case of benzyloxyformaldimine (4j) prepared from benzyloxyamine (BOA) [28] and formaldehyde in the course of the present study. Apparently, the presence of an alkoxy moiety reduces the electrophilicity of the =CH2 unit and thereby formation of the trimeric forms is substantially disfavored. Enantiomerically pure, (R,R)-configured 2-unsubstituted imidazole N-oxides 6 used in this study as the key building blocks were prepared from enantiopure formaldimines 4, derived from the corresponding amines 1, and α-hydroxyiminoketones 5a,b in glacial acetic acid at room temperature [24]. In the case of 4b, along with the (R,R)-configured stereoisomer, the (S,S)-enantiomer was also involved in the study (Scheme 5 and Table 1). Scheme 5. Synthesis of chiral (R,R)-configured imidazole N-oxides 6a-h (see also Table 1).
Reactions of 4 with α-hydroxyiminoketones 5 leading to imidazole N-oxides 6 were performed at room temperature in glacial acetic acid. After isolation, the products were identified by spectroscopic methods, and the most characteristic signal of HC(2) of the imidazole ring in the 1 H-NMR spectra appeared at ca. 8 ppm. Products 6 were alkylated with an equimolar amount of benzyl bromide in CH2Cl2 solution at room temperature. Due to the observed slow decomposition, the (Nbenzyloxy)imidazolium salts 7 were used for the next reaction step without further purification. In the case of the azepane derivative 7e, the structure of the crude product was confirmed by 1 H-NMR spectroscopy. The characteristic HC(2)-signal was significantly shifted toward the lower field and appeared at 11.35 ppm. In addition, chemoselective O-benzylation was confirmed by the appearance of only one AB-system (5.67 and 5.72 ppm, J = 12.0 Hz) of the OCH2Ph group. Thus, competitive Nbenzylation could be ruled out.  Table 1). Table 1. Synthesis of imidazole N-oxides 6 and imidazole-2-thiones 9 from chiral formaldimines 4a-d derived from 'desymetrized' trans-1,2-diaminocyclohexanes 1.

Config. 4
Molecules 2019, 24, x 5 of 18 The obtained crude imidazolium salts 7 were used as precursors of chiral imidazol-2-ylidenes 8 (Scheme 6). Their intermediacy was proven with known trapping reactions with elemental sulfur [24,29,30] leading to non-enolizable imidazole-2-thiones. The deprotonation of 7 was easily achieved by treatment with triethylamine in pyridine. In the presence of elemental sulfur, the in situ generated NHCs 8 exclusively reacted with the desired imidazole-2-thiones 9 (Scheme 6 and Table 1). Their structures were confirmed by spectroscopic data with the most characteristic signal in the 13 C-NMR spectra being the C(2)=S group resonating between 156 and 162 ppm. The obtained imidazole-2thiones 9a-h were shown to be optically active compounds. The optical rotation for all these products was determined in CHCl3 solutions and no racemization under the reaction conditions is expected.
Reactions of 4 with α-hydroxyiminoketones 5 leading to imidazole N-oxides 6 were performed at room temperature in glacial acetic acid. After isolation, the products were identified by spectroscopic methods, and the most characteristic signal of HC(2) of the imidazole ring in the 1 H-NMR spectra appeared at ca. 8 ppm. Products 6 were alkylated with an equimolar amount of benzyl bromide in CH 2 Cl 2 solution at room temperature. Due to the observed slow decomposition, the (N-benzyloxy)imidazolium salts 7 were used for the next reaction step without further purification. In the case of the azepane derivative 7e, the structure of the crude product was confirmed by 1 H-NMR spectroscopy. The characteristic HC(2)-signal was significantly shifted toward the lower field and appeared at 11.35 ppm. In addition, chemoselective O-benzylation was confirmed by the appearance of only one AB-system (5.67 and 5.72 ppm, J = 12.0 Hz) of the OCH 2 Ph group. Thus, competitive N-benzylation could be ruled out.
The obtained crude imidazolium salts 7 were used as precursors of chiral imidazol-2-ylidenes 8 (Scheme 6). Their intermediacy was proven with known trapping reactions with elemental sulfur [24,29,30] leading to non-enolizable imidazole-2-thiones. The deprotonation of 7 was easily achieved by treatment with triethylamine in pyridine. In the presence of elemental sulfur, the in situ generated NHCs 8 exclusively reacted with the desired imidazole-2-thiones 9 (Scheme 6 and Table 1). Their structures were confirmed by spectroscopic data with the most characteristic signal in the 13 C-NMR spectra being the C(2)=S group resonating between 156 and 162 ppm. The obtained imidazole-2-thiones 9a-h were shown to be optically active compounds. The optical rotation for all these products was determined in CHCl 3 solutions and no racemization under the reaction conditions is expected.
The obtained crude imidazolium salts 7 were used as precursors of chiral imidazol-2-ylidenes 8 (Scheme 6). Their intermediacy was proven with known trapping reactions with elemental sulfur [24,29,30] leading to non-enolizable imidazole-2-thiones. The deprotonation of 7 was easily achieved by treatment with triethylamine in pyridine. In the presence of elemental sulfur, the in situ generated NHCs 8 exclusively reacted with the desired imidazole-2-thiones 9 (Scheme 6 and Table 1). Their structures were confirmed by spectroscopic data with the most characteristic signal in the 13 C-NMR spectra being the C(2)=S group resonating between 156 and 162 ppm. The obtained imidazole-2thiones 9a-h were shown to be optically active compounds. The optical rotation for all these products was determined in CHCl3 solutions and no racemization under the reaction conditions is expected. Table 1).
The inspection of the 1 H-NMR spectra of 9 in CDCl3 solution at room temperature evidenced complex signal patterns. It is likely that there are equilibria of different rotamers and/or conformers existing in these solutions. To acquire more information, the 1 H-NMR spectra of analytically pure (R,R)-9b were recorded at different temperatures in a 1,1,2,2-tetrachloroethane solution. The spectrum at 294 K showed a set of broadened signals in the region of 4.2-4.8 ppm attributed to the PhCH2O fragment. In addition, two broad signals of H(C1) and HC(2) of the cyclohexane ring were found at 2.8 and 3.1 ppm. In the spectrum measured at 334 K, the benzylic region revealed one broad signal at 4.6 ppm and the signals of two distinct cyclohexane H-atoms were shifted high-field and overlap with the α-H-atoms of the pyrrolidine ring. This observation supports our assumption of the presence of a dynamic equilibrium of different conformers of (R,R)-9b. An analogous explanation is valid for all spectra of 9 bearing two Ph groups at C(4) and C(5) of the imidazole ring.
The inspection of the 1 H-NMR spectra of 9 in CDCl 3 solution at room temperature evidenced complex signal patterns. It is likely that there are equilibria of different rotamers and/or conformers existing in these solutions. To acquire more information, the 1 H-NMR spectra of analytically pure (R,R)-9b were recorded at different temperatures in a 1,1,2,2-tetrachloroethane solution. The spectrum at 294 K showed a set of broadened signals in the region of 4.2-4.8 ppm attributed to the PhCH 2 O fragment. In addition, two broad signals of H(C1) and HC(2) of the cyclohexane ring were found at 2.8 and 3.1 ppm. In the spectrum measured at 334 K, the benzylic region revealed one broad signal at 4.6 ppm and the signals of two distinct cyclohexane H-atoms were shifted high-field and overlap with the α-H-atoms of the pyrrolidine ring. This observation supports our assumption of the presence of a dynamic equilibrium of different conformers of (R,R)-9b. An analogous explanation is valid for all spectra of 9 bearing two Ph groups at C(4) and C(5) of the imidazole ring.
In extension of the studies performed with 'desymmetrized' trans-DACH derivatives, analogous experiments were performed starting with enantiomerically pure functionalized primary amines. Thus, the corresponding formaldimines 4f-h (Scheme 4) were converted into imidazole N-oxides 10a-c by reaction with the α-hydroxyiminoketone 5a under standard conditions (see Experimental Section). In addition, enantiopure 10d, readily available by aminolysis of the corresponding ester with (R)-α-MBA [22] was used. The obtained imidazole N-oxides 10 were used for the preparation of imidazolium salts 11a-d, which upon treatment with triethylamine in pyridine in the presence of elemental sulfur gave the optically active imidazole-2-thiones 13a-d in good yields (Scheme 7). In these reactions, optically active N-butoxyimidazol-2-ylidenes of type 12 were the in situ generated reactive intermediates. Section). In addition, enantiopure 10d, readily available by aminolysis of the corresponding ester with (R)-α-MBA [22] was used. The obtained imidazole N-oxides 10 were used for the preparation of imidazolium salts 11a-d, which upon treatment with triethylamine in pyridine in the presence of elemental sulfur gave the optically active imidazole-2-thiones 13a-d in good yields (Scheme 7). In these reactions, optically active N-butoxyimidazol-2-ylidenes of type 12 were the in situ generated reactive intermediates. In contrast to 9b,d,f, and g derived from (R,R)-trans-DACH, the 1 H-NMR spectra of 13 clearly indicated the presence of a single form in the solution. For example, the spectrum of compound 13b showed only one multiplet for the CH2O unit located at 4.34-4.41 ppm, and two characteristic singlets of the Me groups at C(4) and C(5) of the imidazole ring at 1.72 and 2.09 ppm, respectively. In the 13 C-NMR spectrum, the signal of the C=S group appeared at 157.4 ppm.
In order to check whether steric hindrance is the reason for the existence of different conformers of 9, four other compounds of that type (i.e., 15a-d derived from (S)-α-methylbenzylamine) were synthesized following the multistep procedure presented in Schemes 6 and 7. The starting 2unsubstituted imidazole N-oxides 14a,b are known compounds [20]. Upon treatment with alkylating reagents (benzyl bromide or n-pentyl bromide), they were converted into the corresponding imidazolium salts (Scheme 8). The latter products without purification were treated with triethylamine in the presence of elemental sulfur, yielding the desired 15a-d in good yields (66-81%).
Ph Me 1. R 2 Br/CHCl 3 /rt 2. NEt 3 /Py/S 8 /rt In that case, the less bulky (S)-α-methylbenzyl residue was placed at the N(1) atom. The 1 H-NMR spectrum of 15a reveals the presence of the expected AB system of the PhCH2O group at 5.45 and 5.58 (J = 10 Hz) ppm. This result emphasizes the importance of the steric demand in the series of compounds 9 derived from bulky trans-1,2-diaminocyclohexane.

Conclusions
Enantiomerically pure desymmetrized derivatives of trans-1,2-diaminocyclohexane containing one free NH2 group (i.e., so called 'primary-tertiary amines') can be efficiently converted into In contrast to 9b,d,f, and g derived from (R,R)-trans-DACH, the 1 H-NMR spectra of 13 clearly indicated the presence of a single form in the solution. For example, the spectrum of compound 13b showed only one multiplet for the CH 2 O unit located at 4.34-4.41 ppm, and two characteristic singlets of the Me groups at C(4) and C(5) of the imidazole ring at 1.72 and 2.09 ppm, respectively. In the 13 C-NMR spectrum, the signal of the C=S group appeared at 157.4 ppm.
In order to check whether steric hindrance is the reason for the existence of different conformers of 9, four other compounds of that type (i.e., 15a-d derived from (S)-α-methylbenzylamine) were synthesized following the multistep procedure presented in Schemes 6 and 7. The starting 2-unsubstituted imidazole N-oxides 14a,b are known compounds [20]. Upon treatment with alkylating reagents (benzyl bromide or n-pentyl bromide), they were converted into the corresponding imidazolium salts (Scheme 8). The latter products without purification were treated with triethylamine in the presence of elemental sulfur, yielding the desired 15a-d in good yields (66-81%). Section). In addition, enantiopure 10d, readily available by aminolysis of the corresponding ester with (R)-α-MBA [22] was used. The obtained imidazole N-oxides 10 were used for the preparation of imidazolium salts 11a-d, which upon treatment with triethylamine in pyridine in the presence of elemental sulfur gave the optically active imidazole-2-thiones 13a-d in good yields (Scheme 7). In these reactions, optically active N-butoxyimidazol-2-ylidenes of type 12 were the in situ generated reactive intermediates. In contrast to 9b,d,f, and g derived from (R,R)-trans-DACH, the 1 H-NMR spectra of 13 clearly indicated the presence of a single form in the solution. For example, the spectrum of compound 13b showed only one multiplet for the CH2O unit located at 4.34-4.41 ppm, and two characteristic singlets of the Me groups at C(4) and C(5) of the imidazole ring at 1.72 and 2.09 ppm, respectively. In the 13 C-NMR spectrum, the signal of the C=S group appeared at 157.4 ppm.
In order to check whether steric hindrance is the reason for the existence of different conformers of 9, four other compounds of that type (i.e., 15a-d derived from (S)-α-methylbenzylamine) were synthesized following the multistep procedure presented in Schemes 6 and 7. The starting 2unsubstituted imidazole N-oxides 14a,b are known compounds [20]. Upon treatment with alkylating reagents (benzyl bromide or n-pentyl bromide), they were converted into the corresponding imidazolium salts (Scheme 8). The latter products without purification were treated with triethylamine in the presence of elemental sulfur, yielding the desired 15a-d in good yields (66-81%). In that case, the less bulky (S)-α-methylbenzyl residue was placed at the N(1) atom. The 1 H-NMR spectrum of 15a reveals the presence of the expected AB system of the PhCH2O group at 5.45 and 5.58 (J = 10 Hz) ppm. This result emphasizes the importance of the steric demand in the series of compounds 9 derived from bulky trans-1,2-diaminocyclohexane.

Conclusions
Enantiomerically pure desymmetrized derivatives of trans-1,2-diaminocyclohexane containing one free NH2 group (i.e., so called 'primary-tertiary amines') can be efficiently converted into In that case, the less bulky (S)-α-methylbenzyl residue was placed at the N(1) atom. The 1 H-NMR spectrum of 15a reveals the presence of the expected AB system of the PhCH 2 O group at 5.45 and 5.58 (J = 10 Hz) ppm. This result emphasizes the importance of the steric demand in the series of compounds 9 derived from bulky trans-1,2-diaminocyclohexane.

Conclusions
Enantiomerically pure desymmetrized derivatives of trans-1,2-diaminocyclohexane containing one free NH 2 group (i.e., so called 'primary-tertiary amines') can be efficiently converted into corresponding formaldimines. The initially obtained formaldimines were transformed into 2-unsubstituted imidazole N-oxides, which upon treatment with benzyl bromide were selectively converted into benzyloxyimidazolium salts. After deprotonation with triethylamine, the latter compounds generated new types of chiral imidazol-2-ylidenes. Their appearance was evidenced by the trapping reaction with elemental sulfur leading to chiral non-enolizable imidazole-2-thiones. The same procedure applied to other optically active functionalized primary amines opens straightforward access for in situ generated nucleophilic carbenes derived from imidazole (imidazol-2-ylidenes) bearing an alkoxy group. This little-known class of alkoxy-substituted optically active NHCs is of interest for potential applications in asymmetric synthesis and in organometallic chemistry for the preparation of transition metal complexes. The multistep synthesis of the optically active imidazole-2-thiones occurs with preservation of the stereochemistry. In addition, many derivatives of chiral 1,2-diamines [13,15,31,32], imidazolium salts [33][34][35] as well as numerous imidazole-2-thiones [36,37] display diverse biological activities. Nevertheless, their alkoxy derivatives are practically unknown. For that reason, the new types of 'desymmetrized' derivatives of trans-1,2-diaminocyclohexane and other optically active primary amines reported in this work may also be of interest for medicinal chemistry.

General Information
Solvents and chemicals were purchased and used as received without further purification. Products were purified by standard column chromatography on silica gel (230-400 mesh, Merck, Kenilworth, NJ, USA). Unless stated otherwise, yields refer to analytically pure samples. NMR spectra were recorded with a Bruker Avance III 600 MHz instrument ( 1 H-NMR: 600 MHz; 13

Synthesis of Imidazole N-Oxides 6-General Procedure
A solution of 1.1 mmol of the respective formaldimine 4 and 1 mmol of α-hydroxyiminoketone 5 in 3 mL of glacial acetic acid was stirred magnetically overnight. The next day, 1 mL of aqueous hydrochloric acid was added and the solution was stirred for 15 min. Then, the resulting solution was evaporated to dryness, the solid residue was dissolved in 20 mL of dichloromethane, and the obtained solution was neutralized with 20 mL of diluted aqueous sodium hydroxide. The organic layer was separated, dried over mgSO 4 , filtrated, and evaporated. The resulting crude products were washed with two portions (each ca. 10 mL) of diethyl ether, and after separation, the obtained thick oils or amorphous solids were characterized spectroscopically and used for further transformations.  128.95, 128.89, 127.95, 127.91, 127.79, 127.4, 127.1, 68.0, 56.9

Synthesis of 3-Benzyloxyimidazolium Bromides 7-General Procedure
A solution of 0.5 mmol of crude imidazole N-oxide 6 and 86 mg (0.5 mmol) benzyl bromide in 1 mL of CHCl 3 was stirred magnetically overnight at room temperature. The next day, the solvent was evaporated and the obtained crude imidazolium salts were triturated with diethyl ether. The ethereal phase was separated and the non-soluble imidazolium salts 7 were used for the generation of carbenes 8 and their reaction with elemental sulfur without further purification. Whereas di-Me substituted imidazolium salts formed viscous, thick oils, the corresponding di-Ph derivatives were obtained as amorphous solids. The structures of selected imidazolium salts were confirmed by running the 1 H-NMR spectra. A representative example of the 1 H-NMR spectra registered for 7e is described below and the scanned spectrum is presented in the Supplementary Materials. The crude imidazolium salt 7 obtained from 0.5 mmol of the corresponding imidazole N-oxide 6 and 86 mg (0.5 mmol) of benzyl bromide according to the general procedure (see above) was dissolved in 1 mL of dry pyridine. Next, 38 mg (1.2 mmol) of elemental sulfur and 122 mg (1.2 mmol) of triethylamine were added to the magnetically stirred, homogenous solution. Stirring at room temperature was continued overnight. Next day, pyridine was removed under reduced pressure and the residual semi-solid material was purified by preparative thin layer chromatography with silica gel. A mixture of dichloromethane and methanol (99:1) was used as an eluent. Imidazole-2-thiones 9 formed a single fraction with R f ca. 0.3. Solid products were additionally purified by crystallization.    13   ppm. 13

Synthesis of Imidazole N-Oxides 10a-d
A solution of diacetyl monooxime (5a, 354 mg, 3.5 mmol) and the corresponding formaldimine 4 (3.0 mmol) in EtOH (10 mL) was refluxed for 3 h. The solvents were removed in vacuo, and the resulting oil was washed with Et 2 O (3 × 20 mL). The crude product 10 was either purified by column chromatography on silica gel using AcOEt/MeOH mixtures as the eluent or by recrystallization from the appropriate solvents to give spectroscopically pure imidazole N-oxides isolated as colorless materials. Compounds 10a [21] and 10c [22] were prepared following the analogous method, while imidazole N-oxide 10d was obtained in two steps by condensation of 5a with methyl glycinate-derived formaldimine of type 4 followed by aminolysis of the resulting product with α-MBA as described [22]; the NMR spectra of the obtained samples matches the data reported in the literature.

Synthesis of Imidazolium Bromides 11a-d
To a solution of imidazole N-oxide 10 (1.0 mmol) in dry CH 2 Cl 2 (1.0 mL) was added an excess of 1-bromobutane (548 mg, 4.0 mmol) and the resulting mixture was stirred until the starting material was fully consumed (TLC monitoring: SiO 2 , EtOAc/MeOH 7:1). The solvents were removed under reduced pressure to give the corresponding imidazolium bromide 11 quantitatively, which was used for the next step without further purification.