Stable N-heterocyclic carbenes: N-alkyl-N'-phosphanylbenzimidazol-2- ylidenes

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DOI: 10.1002/ejoc.201200282

Stable N-Heterocyclic Carbenes: N-Alkyl-NЈ-phosphanylbenzimidazol-2-

ylidenes

Anatoliy P. Marchenko,^[a]Heorgiy N. Koidan,^[a]Anastasiia N. Hurieva,^[a]Igor I. Pervak,^[a]

Svitlana V. Shishkina,^[b]Oleg V. Shishkin,^[b,c]and Aleksandr N. Kostyuk*^[a]

Keywords: Carbenes / Nitrogen heterocycles / Ligand design / Structure elucidation

Three original approaches to the synthesis of N-alkyl-NЈ-

phosphanylbenzimidazolium salts, which are precursors of

the corresponding carbenes, were developed. New stable N-

alkyl-NЈ-phosphanylbenzimidazol-2-ylidenes were prepared

in good yields. X-ray analysis of a carbene was made and

the chemical properties of the carbenes were studied. The

reactions with O-, N-, and C-nucleophiles were found to pro-

ceed with or without cleavage of the P–N bond. Thermal re-

arrangement of the carbenes afforded 2-phosphorylated

benzimidazoles.

backbone, was separated as an individual compound. An

attempt to synthesize the analogous carbene with less steri-

cally demanding substituents (methyl instead of isopropyl),

failed.^[4l]Stable six-membered ring carbene derivatives of

borazines D have been reported.^[4m]

Introduction

In the past 20 years N-heterocyclic carbenes (NHC) have

become one of the most promising and productive growth

areas of organic chemistry. Numerous articles, detailed re-

views,^[1]and books^[2]devoted to their synthesis, chemical

properties, and use as ligands for homogeneous catalysis as

well as organocatalysts have been published. The majority

of works in which carbenes have been characterized as indi-

vidual compounds concern NHCs with bulky wingtip sub-

stituents: aliphatic (tBu, Np), carbocyclic (adamantyl), or

aromatic (Mes).^[3]Electronic and steric properties of NHCs

can be fine-tuned by modifying the substituents on the do-

nor groups. NHCs with elementorganic wingtip substitu-

ents are much less common. Among them, one should in-

clude diaminocabenes featuring N–B bonds.^[4]Some of

these carbenes are quite stable and have been separated and

characterized by X-ray analysis (Figure 1). Scorpionate li-

gands based on NHCs of type C were introduced by

Fehlhammer and co-workers;^[4a–4f]these carbenes were used

extensively for the synthesis of various complexes, but the

carbenes themselves were not separated. Carbenes of type

B have been prepared and were used for the synthesis of

metal complexes.^[4f,4g]Carbenes of type A and B were sepa-

rated as individual compounds and characterized by X-ray

analysis. They proved to be, rather than free carbenes, lith-

ium carbene complexes.^[4h–k]Carbene E, with an inorganic

[a] Institute of Organic Chemistry, National Academy of Sciences

of Ukraine,

Murmanska Str. 5, Kyiv-94 02094, Ukraine

E-mail: a.kostyuk@yahoo.com

[b] SSI “Institute for Single Crystals”, National Academy of

Science of Ukraine,

60 Lenina ave., Kharkiv 61001, Ukraine

[c] V. N. Karazin Kharkiv National University,

4 Svobody sq., Kharkiv 61202, Ukraine

Supporting information for this article is available on the

WWW under http://dx.doi.org/10.1002/ejoc.201200282.

Figure 1. N-Heterocyclic carbenes.

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Complexes with NHCs bearing N–N bonds are also cult to make benzimidazolium salts compared to imid-

available. Complexes bearing ligands of type F, with chiral azolium salts, probably due to lower basicity of benzimid-

wingtip pyrrolidine substituents, have been synthesized.^[5a]azoles. (2) Triflate 3b was obtained by the reaction of 1

The pyrrolidine substituents seem to improve the σ-donor with phosphenium cation (iPr₂N)₂P⁺TfO^–. The method is

capacity of the ligands.^[5b]Bertrand et al. have synthesized general and independent of the nature of starting azoles.

persistent acyclic carbenes G, bearing a single N–N bond, The reaction proceeds readily and in high yield. (3) By alk-

that are rather weak; these compounds are stable in solu- ylation of N-1H-phosphorylated benzimidazole 2 with iso-

tion for a short period of time.^[5c]Carbenes of type F, propyl triflate. The scope of the method is limited by the

namely N-arylamino-substituted NHCs, have been used in low availability of alkyl esters of trifluoromethanesulfonic

the synthesis of 2-keto imidazoles, with the arylamino acid.

group being eliminated in the course of the reaction.^[5d]

Complexes F, with chiral ligands, are suitable catalysts in

allylic substitution reactions.^[5e]Rh^Icarbene complexes hav-

ing ligands of type H have been prepared. These ligands (R

= NMePh, NBn₂, or OBn) were shown to lose σ-donor

ability compared to ligands bearing aryl substituents.^[5f]

Catalysts bearing chiral ligands of type H have shown

promising results in the asymmetric hydrosilylation of

acetophenone.^[5g]

Concerning carbenes with an N–P bond, a literature se-

arch revealed carbenes of type I. Such carbenes were sepa-

rated as individual compounds and characterized by X-ray

analysis. Decreasing steric hindrance in carbene I by replac-

ing the iPr groups with Me groups results in facile dimeriza-

Scheme 1.

tion. Carbene I was used for the synthesis of the Rh com-

Compounds 3a–c are high-melting solids that are soluble

plex and was screened in a range of olefin metathesis reac- in THF, dichloromethane and are insoluble in diethyl ether

tions, showing low reactivity.^[6a,6b]

and petroleum ether.

In the previous work, we have experimentally shown the

High lability of the P–N bond is a characteristic feature

“carbene” mechanism of the reaction of 1-alkylimidazoles of the prepared N-phosphorylated benzimidazolium salts.

with phosphorus(III) halides. It was found that the reaction For example, benzimidazolium triflate 3a slowly reacted

occurred with initial formation of unstable N-phosphor- with water at 20 °C (24 h) and with methanol (4 days) de-

ylated imidazolium halides, but that their counterparts composing into starting benzimidazole 1 and phosphite 4a,

bearing a triflate anion can be separated as individual com- and with dimethylamine (4 days) to give phosphinous

pounds; upon treatment with sodium hexamethyldisilazide amide 4b (Scheme 2).^[7]Formation of compounds 4a and

these compounds are transformed into stable N- and N,NЈ- 4b was confirmed by ³¹P NMR spectroscopic analysis by

phosphorylated imidazol-2-ylidenes (carbenes J).^[6c]Re- adding an authentic sample to the reaction mixture. In con-

cently, we have shown that carbenes J can be prepared di- trast, the reaction with sodium hydroxide or sodium meth-

rectly by phosphorylation of bulky substituted lithium oxide proceeded fast while keeping the P–N bond intact. In

imidazolides and benzimidazolides.^[6d]

the case of sodium hydroxide, the reaction was ac-

Thus, we have shown that carbenes of type J are ther- companied by opening of the imidazole cycle to give di-

mally quite stable and that they can be isolated as individ- amide 5, however, with sodium methoxide the reaction led

ual compounds. They have two coordination sites (phos- to the formation of 2-methoxy-2H-benzimidazoline 6.

phorus and carbene) and might be used in the synthesis of

Strong bases such as methyllithium, lithium diisoprop-

polynuclear complexes.

ylamide (LDA), sodium dimethylamide, and sodium bis(tri-

In this work we report on the synthesis of N-phosphor- methylsilyl)amide deprotonated the 2nd position of salts

ylated benzimidazolium salts and the corresponding carb- 3a–c, affording carbenes 7a–c (Scheme 3). Benzimidazol-2-

enes, and we discuss their properties.

ylidenes 7a and 7c are yellowish crystalline solids, and 7c

could be distilled under reduced pressure, however, it was

highly susceptible to moisture and air oxygen. Carbene 7b

was analyzed by ³¹P NMR spectroscopy (δ_p= 68.4 ppm;

Results and Discussion

Et₂O; r.mixture). The structure of carbenes 7a and 7c was

NЈ-Phosphanylbenzimidazolium triflates 3a–c were ob- confirmed by H, ³¹P and ¹³C NMR spectroscopy, by X-

tained (Scheme 1) by three approaches: (1) By the reaction ray diffraction in the case of 7a (Figure 2), and by a set of

of 1-methylbenzimidazole (1) with di-tert-butylbromophos- chemical transformations.

1

phane in tetrahydrofuran (THF) in the presence of sodium

Analysis of the ¹³C NMR spectra of available benzimid-

triflate. The reaction equilibrium is shifted toward the for- azol-2-ylidenes bearing various alkyl substituents at the ni-

mation of triflate 3a due to the low solubility of the re- trogen atoms suggests that chemical shifts of the carbene

sulting sodium bromide. Using this method it is more diffi- atoms are not significantly affected by the size of the alkyl

2

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Stable N-Heterocyclic Carbenes

Scheme 2.

Scheme 3.

substituent. In our case, introduction of a phosphorus sub-

stituent instead of an alkyl group at the nitrogen atom does

not show any peculiarities. The ¹³C NMR chemical shifts

of the carbene atoms lie in the usual range δ = 233–235 ppm

typical for bulky substituted benzimidazol-2-ylidenes.^[8]

Compared to N-phosphorylated imidazol-2-ylidenes, the

chemical shifts of the carbene atom are shifted 10–12 ppm

downfield (Table 1).^[6c,6d]

Table 1. Selected ¹³C NMR shifts (ppm) for carbenes 7 and their

corresponding azolium salts 3.^[a]

Carbene δ C2

Azolium salt

δ C2 (d, J_C–P)

7a

7b

7c

235.4

231.8 (m)

233.08

3a

3b

3c

145.0 (11.9)

142.9

143.2 (14)

[a] NMR spectra are taken in CDCl₃, with the exception of 7b and

7c, which were recorded in C₆D₆.

The carbene structure of 7a was confirmed by an X-ray

diffraction study (Figure 2). The C1–N1 and C1–N2 bond

lengths [1.387(2) Å and 1.355(2) Å, respectively] are closer

Figure 2. X-ray crystal structure of 7a.

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to the mean value^[9]for the Csp²–N(3) ordinary bond

(1.355 Å) than to the Csp²=N double bond in the imidazole

ring (1.313 Å). Meanwhile, the heterocycle is planar within

0.001 Å and the hydrogen atoms at C1 are not observed in

electron-density difference maps. Analysis of intramolecular

interactions in 7a using NBO^[10]theory demonstrates the

presence of a lone pair at the C1 atom. The interactions of

this lone pair with σ-antibonding orbitals of the C3–N1

and C2–N2 bonds [the stabilization energy E(2) for these

interactions is 10.62 and 12.33 kcal/mol, respectively] allows

us to assume that one lies in the plane of the five-membered

heterocycle. The NBO theory describes the C1–N2 bond as

a double bond and postulates strong interactions between

its π-orbital and the lone pair of the N1 atom [the value of

E(2) is 67.56 kcal/mol]. However the bond orders for the

C1–N2 and C1–N1 bonds are very similar (1.276 and 1.197)

and indicate ordinary rather than double bond character.

Thus, 7a has a structure consistent with that of a carbene

(Scheme 3, molecules 7a–c).

The substituent at the N1 atom is turned in such way

that the lone pair (LP) of the P1 atom is almost antiper-

iplanar to the N1–C1 bond [the C1–N1–P1–LP(P1) torsion

angle is 166°]. The distances between the hydrogen atoms

of the methyl groups and the C1 carbene atom (see dis-

tances in Table 2, which are shorter than the van der Waals

radii sum^[11]of 2.87 Å) indicate that these interactions may

be considered as C–H···LP(C1) hydrogen bonds. However

detailed analysis of the electron density distribution within

Bader’s “Atoms in Molecules” (AIM) approach^[12]demon-

strated that the (3, –1) bond critical points are observed

only for two interactions, namely C11–H···C1 and C14–

H···C1, which should be considered as true hydrogen bonds.

Values of energy estimated from properties of bond critical

points indicate that both hydrogen bonds are rather weak

(Table 2, Figure 3).

Figure 3. Molecular graph of 7a obtained from analysis of electron

density distribution within Bader’s “Atoms in Molecules” theory.

Thus, handling these compounds requires rigorous ex-

clusion of moisture. The reaction of carbene 7a with water,

like the reaction of benzimidazolium salt 3a with NaOH,

was accompanied by opening of the benzimidazole cycle,

affording

a

mixed o-phenylenediamine derivative

5

(Scheme 4). Compound 5 is a crystalline solid that can be

distilled under reduced pressure. It easily added sulfur giv-

ing thiophosphinate 13. All isolated compounds are novel,

and their structures were established by analysis of their

NMR (¹H, ¹³C, ³¹P) spectra, and elemental analysis data.

Compound 13 (Figure 4) was analyzed by X-ray crystal-

lography; two molecules (A and B) are observed in the

asymmetric part of the unit cell in crystals of 13. The N-

methyl-formamide substituent in both molecules is turned

considerably relatively to the aromatic ring [the C7–N2–

C2–C1 torsion angle is –59.7(5)° in molecule A and 60.1(5)°

in B]. The thiophosphinamide fragment of the substituent

at the C1 atom is also turned relatively the benzene ring,

and the S1–P1 bond is slightly noncoplanar to the C1–N1

bond [the P1–N1–C1–C6 and S1–P1–N1–C1 torsion angles

are 44.4(5)° A and 43.9(5)° B, and 6.7(4)° A and 7.2(4)° B,

respectively]. Such orientation of the substituents is stabil-

ized by intramolecular hydrogen bonds: N1–H···O1 H···O

Table 2. The geometric parameters and characteristics of the (3,

–1) bond critical points for the intramolecular interactions with

participation of the C1 atom (ρ: value of electron density, ٌρ: La-

placian of electron density, ε: ellipticity, E_int: energy of interaction)

in 7a.

Interaction

H···A D-

H···A

[deg]

ρ

ν˜ρ

ε

E_int

[Å]

[e/au³] [e/au⁵]

[kcal/

mol]

C12-H12a···C1

C11-H11c···C1

C14-H14a···C1

2.66 126

2.67 130

2.59 122

–

0.0112 0.0326

0.0109 0.0369

0.062 1.85

0.743 2.03

Thus, results of these theoretical studies demonstrate the

absence of any interaction between the lone pair of the

carbene atom C1 and the phosphorus atom. Stabilization

of carbenes is mainly provided by steric shielding by the

bulky tert-butyl group, the orientation of which is addition-

ally stabilized by weak C–H···C hydrogen bonds.

In any description of the chemical properties of carbenes

7, one must start with a very important, although unwanted

property: its extremely high sensitivity towards moisture. Scheme 4.

4

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Stable N-Heterocyclic Carbenes

(2.07 Å A and 2.11 Å B), N–H···O (139° A and 141° B), known examples are the reaction of 1,3,4-triphenyl-4,5-di-

C6–H···S1 H···S (2.98 Å A and 2.96 Å B), and C–H···S hydro-1H-1,2,4-triazol-5-ylidene with alcohol and morph-

(111° A and B).

oline, and insertion into the C–H bond of acetonitrile for 1-

(1-adamantyl)-3,4-diphenyl-1,2,4-triazol-5-ylidene, 1,3-di(1-

adamantyl)benzimidazol-2-ylidene, and 1,3-dimesitylimid-

azolin-2-ylidene.^[13]

Carbene 7a was found to react at room temperature with

methanol (at the time of mixing), acetonitrile (2 h), dimeth-

ylamine (15 h), and morpholine (24 h) to give the 2H-

benzazolines 6, 14a, 15a, and 15b as an equilibrium mixture

of two isomers (Scheme 5), due to the different spatial and

electronic nature of substituents at the nitrogen atoms. With

the exception of the 2-methoxy derivative 6, azolines 14a,

15a, and 15b were formed as crystalline compounds, the

structures of which were established on the basis of their

NMR (¹H, ¹³C, ³¹P) spectroscopic data, and for 15a also

by an X-ray diffraction study.

Figure 4. Molecular structure of diamide 13.

The thermal stability of N-alkyl-NЈ-phosphanylbenz-

imidazol-2-ylidenes 7a–c strongly depends on steric and

electronic effects of substituents at the phosphorus atom

and correlates with the ease of formation of 2-phosphoryl-

ated imidazoles 8a–c. Thus, carbene 7b bearing a bis(diiso-

propylamino)phosphanyl group slowly isomerizes even at

18 °C to 2-phosphanyl-1H-benzimidazole 8b (Scheme 3).

Monitoring the reaction by ³¹P NMR spectroscopy re-

vealed that the reaction was 20% complete in 15 h, and in

three days at 25 °C the isomerization went to completion.

Thus, the ¹³C NMR spectrum of carbene 7b was taken in

C₆D₆as a mixture with phosphane 8b. Carbenes 7a and 7c

bearing a di-tert-butylphosphanyl group at the nitrogen

atom are much more stable and could even be distilled in-

tact under high vacuum. Nevertheless, at 150 °C in benzene

Scheme 5.

There is a very close literature analogy to compounds 6,

solution in a sealed vial over three hours they completely 14a, 15a, and 15b. Heinicke et al. confirmed the existence

transformed into 2-phosphorylated benzimidazoles 8a and of two rotamers of 1-dicyclohexylphosphanyl-2,5-dimethyl-

8c, thus proving the carbene mechanism of the reaction of 1H-1,3-benzazaphosphole at room temperature.^[14]Based

phosphorus halides with benzimidazoles. The process of on this analogy, we can draw a conclusion that, in solution,

isomerization probably takes place in an intermolecular the minor azolines correspond to the structure “B” with

fashion with participation of intermediate carbanions “A” more downfield shift. In the ¹H, ¹³C NMR spectra of

by the so-called “ylide” mechanism.

benzazolines 6, 14a, 15a, and 15b doubling of signals is ob-

Phosphanyl carbenes 7a–c were easily oxidized with served in the same proportion as observed in the ³¹P NMR

either sulfur or selenium (Scheme 3). The reaction pro- spectra (Scheme 5, Table 3). In addition, due to the pres-

ceeded in a stepwise manner with the formation of imid- ence of a chiral center in these compounds, groups (tBu₂P,

azol-2-thiones 9 or selones 10 at the first stage, leaving the -CH₂-, and iPr) are diastereotopic and exhibit doubled res-

phosphanyl group intact. Oxidation of the phosphorus onances.

atom occurred only when heated (100 °C, 3–4 h), affording

phosphorothioyl 11 or phosphoroselenoyl 12 derivatives in 15a (Figure 5) adopts an envelope conformation. Devia-

(Scheme 3). tion of the C1 atom from the mean plane of the remaining

According to X-ray diffraction data, the imidazoline ring

It was of interest to study insertion reactions of carbenes atoms of the ring is –0.29 Å. In contrast to carbene 7a, the

7a–c into HX (X = C, O, N) bonds, since such transforma- N1 atom has slightly pyramidal configuration (the sum of

tions are characteristic of singlet carbenes. In particular, bond angles centered on the N1 atom is 354.8°), which is

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Table 3. Selected spectroscopic data of compounds 6, 14a, 15a, and 15b.

X

OMe

b

6

CH₂CN

c

14a

NMe₂

d

15a

N(CH₂)₄O

3a

15b

δ

³¹P [ppm]

A

B

85

99

93

96

88

100

90

100

100.7

A/B

δ

J [Hz]

ca. 10:1

100 (14)

109.1 (55)

ca. 5:1

79.6 (15)

87.8 (46)

ca. 1.2:1

97 (14)

104.3 (48)

ca. 1.6:1

97 (14)

104.2 (46)

–

¹³C [ppm]

C²H/A

C²H/B

145.0 (12)

caused by strong steric repulsion between the heterocycle onitrile and formation of the corresponding benzimidazole

and the tert-butyl group [shortened intramolecular contact and triazole carbenes.^[8,13a,13b]This method is one of the

H1···H14C (2.03 Å), H1···H12A (2.01 Å), while van der applied synthetic routes to free NHCs that prompted us to

Waals radii sum^[11]is 2.32 Å]. Despite this, the length of the investigate the analogous properties of 2-H-benzazolines 6,

N1–P1 bond in 15a [1.727(1) Å] is considerably smaller 14a, 15a, and 15b. We found that their thermal stability

than in 7a [1.755(1) Å]. The substituent at the N1 atom is depends on the nature of the substituent X, and the ease of

turned in such a way that the lone pair of the phosphorus their decomposition is inversely proportional to the rate of

is located near the plane of the C1–N1 endocyclic bond (the their formation. Thus, the methoxy-2H-azoline 6, which is

C1–N1–P1–LP torsion angle is 154° where LP is an ideal- formed immediately upon mixing of the starting reagents,

ized position of the lone pair). The dimethylamino group even upon distillation (0.05 Torr), partially (38%) trans-

has axial orientation relative to the bicyclic fragment [the forms into the carbene 7a, while with preliminary heating

N3–C1–N2–C7 torsion angle is 102.4(1)°] and is turned in at 120 °C for 30 min, the formation of the carbene is quan-

such way that the lone pair of the N3 atom has sc-confor- titative. According to the ³¹P NMR spectroscopic data,

mation relative to the N1–C1 bond [the N1–C1–N3– vacuum distillation of the cyanomethyl-2H-azoline 14a

LP(N3) torsion angle is –46°]. The N3 atom has pyramidal (145 °C/0.05 Torr) afforded a mixture containing the start-

configuration (the sum of bond angles centered on the N1 ing azoline 14a (20%), carbene 7a (27%), and a new com-

atom is 343.3°).

pound 16 (53%), which became the sole product of distil-

lation when azoline 14a was preliminary heated (125 °C,

30 min). The ³¹P NMR chemical shift (δ = 55 ppm) of 16

coincides with the chemical shift of 5, which is the carbene

hydrolysis product. Under the same conditions, aminoazol-

ines 15a and 15b were distilled unchanged. Formation of

ene-1,2-diamine 16 may occur due to deprotonation of the

cyanomethyl group (Scheme 6) under the action of cyano-

methanide anion.

Figure 5. Molecular structure of azoline 15a.

Doubling of signals in the NMR spectra is due to the

presence of another stereoisomer, benzazoline B, in which

substituent X and the phosphanyl group are in an eclipsed

position. Relatively close values of C2 atoms in the ¹³C

Scheme 6.

This mechanism is based on the assumption of the exis-

NMR spectra of the azolines A and B that are markedly tence of an equilibrium between covalent and ionic forms

different from the value of the C2 atom in imidazolium of azolines 14 and 14Ј, which has been investigated in detail

triflate 3a testifies for their covalent structures (Table 3).

in the direct reaction of an imidazol-2-ylidene in aqueous

When heated under reduced pressure, non-phosphoryl- THF.^[15]

ated methoxy- and cyanomethyl-2H-azolines of type 6 are

One would expect that the thermal stability of azolines

known to decompose with elimination of methanol or acet- 6, 14a, 14b, 15a, and 15b will change in the presence of the

6

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Stable N-Heterocyclic Carbenes

Scheme 7.

pentavalent phosphorus atom. Indeed, the stability of the 88.0(5)°] and its isopropyl group is oriented in such a way

derivatives resulting from the interaction of these azolines that the H15–C15–N2–C6 torsion angle is 176.0(4)°. Such

with selenium and sulfur is not only lower but is also de- orientation of vicinal substituents is stabilized by the intra-

pendent on the nature of substituent X (Scheme 7). We molecular hydrogen bonds N1–H···N2 H···N (2.24 Å) and

found that the addition of sulfur to azoline 6 (X = OMe) N–H···N (119°).

at room temperature takes place ambiguously with the for-

mation of a complex mixture of products according to ³¹P

NMR spectroscopic data. However, when heated with an

excess of sulfur, only one compound was obtained, the ³¹P,

¹H, ¹³C NMR spectral characteristics of which corresponds

to disulfide 11a. Interaction of the cyanomethyl-2H-azol-

ines 14a and 14b with sulfur or selenium proceeded slowly

(3–4 days) at room temperature and led to the formation of

chalcogen-containing azolines 18a and 18b in the form of

one isomer plus a trace amount of acyclic compounds 17a

and 17b. Compound 19a was obtained as a sole product,

with no other compounds being observed. The structures

of these compounds were confirmed by NMR spectroscopic

methods and from an X-ray diffraction study of com-

pounds 17b and 18a (Figures 6 and 7). Both nitrogen atoms

of the substituents at the aromatic ring in the molecule 17b

have almost planar configuration (the sum of bond angles

centered at each is 358°). The thiophosphinamide fragment

of the substituent at the C1 atom is slightly noncoplanar to

the aromatic ring [the P1–N1–C1–C2 torsion angle is

14.4(7)°] and the S1–P1 bond is turned relative to the C1–

N1 bond [the S1–P1–N1–C1 torsion angle is 26.8(5)°]. The

substituent at the C6 atom has orthogonal orientation with

respect to the cycle [the C18–N2–C6–C1 torsion angle is Figure 6. Molecular structure of 17b without hydrogen atoms.

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Benzimidazole carbenes 7a–c, like N-heterocyclic non-

phosphorylated carbenes, are strong organic bases that re-

place weaker bases from their salts.^[16]Thus, the reaction of

7c with triethylamine hydrochloride formed benzimidazole

1c and released di(tert-butyl)chlorophosphane (Scheme 8).

Figure 7. Molecular structure of 18a.

Scheme 8.

Two molecules (A and B) are observed in the asymmetric

part of the unit cell for crystals of compound 18a. Similar

to molecule 15a, the five-membered heterocycle of the bicy-

clic fragment in 18a adopts an envelope conformation. De-

viation of the C1 atom from the mean plane of the remain-

ing atoms of the cycle is 0.36 Å in molecule A and –0.41 Å

in B. The phosphorus-containing substituent has an oppo-

site orientation compared to 15a. tert-Butyl groups are ori-

ented toward the benzene ring instead of the C1 atom as is

observed in 15a and 7a [the S1–P1–N1–C1 torsion angle is

–11.3(2)° for A and –13.9(1)° for B]. Such conformation of

the substituent also leads to significant steric strain due to

repulsion between the hydrogen atoms of the alkyl group

and the aromatic ring (shortened intramolecular contacts

H13···H6 1.93 Å in both molecules). However, despite this,

the N1 atom in 18a has an almost planar configuration (the

sum of bond angles centered at this atom is 358.7° in A and

358.6° in B) and the length of the N–P bond [1.704(2) Å in

molecule A and 1.705 Å in molecule B] is smaller than in

15a. The cyanomethyl group is almost orthogonal to the

bicyclic fragment and is turned relative to the C1–N2 bond

[the C8–C1–N2–C2 and C9–C8–C1–N2 torsion angles are

93.6(2)° for A, 90.1(2)° for B, and 73.0(2)° for A and

73.1(2)° for B, respectively].

Like its trivalent counterpart 14a, heating selenoic deriv-

ative 19a generates the open-form compound 21, but at a

lower temperature. It should be noted that these acyclic de-

rivatives (according to ³¹P NMR analysis) are formed (see

the Exp. Section for 17) in trace amounts even at room tem-

perature (25 °C) when preparing azolines 18. Reduction of

the selenophosphoryl group in compound 19a with hexa-

alkylphosphorus triamide proceeded readily, affording the

starting compound 14a, however, with the same isomers ra-

tio “A/B” (Scheme 7). Aminoazolines 15a and 15b readily

added sulfur (5 min, 25 °C) to form P^Vderivatives 20a and

20b, which were unstable when stored. It is important to

note that salts 3a–c, having an ionic structure, in contrast

to azolines, do not react with S or Se.

We also investigated the interaction of carbenes 7a–c

with phosphorus electrophiles. It was found that these carb-

enes at 18 °C (and even upon heating) do not react with

tBu₂PCl or tBuP(NMe₂)Cl. At the same time, compound

7c easily reacted with less sterically hindered diphenyl-

chlorophosphane and tert-butyldichlorophosphane, afford-

ing phosphanes 22 and 23, respectively (Scheme 8). The lat-

ter was reduced with lithium aluminum hydride to phos-

phane 24. It should be noted that compounds 23 and 24 are

the first representatives of chlorophosphanes and secondary

phosphanes bearing a benzimidazol-2-yl residue, respec-

tively.

The reactions, accompanied by cleavage of the P–N

bond, proceeded at the carbene atom affording diphosphor-

ylated benzimidazolium chloride. The latter, as noted above,

in view of the high nucleophilicity of the chloride anion,

are unstable and decomposed into the final products. A

competitive chloride anion attack at the carbene center is

unlikely because this would lead to analogues of unstable

α-chloroalkylamines.

In conclusion, we have developed three new approaches

to N-alkyl-NЈ-phosphanylbenzimidazolium salts that, by

treatment with strong bases, were transformed into stable

N-alkyl-NЈ-phosphanylbenzimidazol-2-ylidenes. The prop-

erties of both the salts and the carbenes have been studied.

These reagents could react either with cleavage or preserva-

tion of the P–N bond. X-ray studies on the carbenes and

on the products of their reaction have been made.

Experimental Section

General: All procedures with air- and moisture-sensitive com-

pounds were performed under an atmosphere of dry argon in

flame-dried glassware. Solvents were purified and dried by standard

methods. Melting points were determined with an electrothermal

capillary melting point apparatus and are uncorrected. ¹H NMR

spectra were recorded with

a Bruker Avance DRX 500

(500.13 MHz) or a Varian VXR-300 (299.94 MHz) spectrometer.

8

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Stable N-Heterocyclic Carbenes

¹³C NMR spectra were recorded with a Bruker Avance DRX 500

(125.75 MHz) spectrometer. ³¹P NMR spectra were recorded with

a Varian VXR-300 (121.42 MHz) spectrometer. Chemical shifts (δ)

are reported in ppm downfield relative to internal TMS (for ¹H,

¹³C) and external 85% H₃PO₄(for ³¹P). Chromatography was per-

formed on silica gel Gerudan SI60. Elemental analyses were per-

formed at the Microanalytical Laboratory of the Institute of the

Organic Chemistry, National Academy of Sciences of Ukraine.

4.25 (s, 3 H), 3.75 (m, 4 H), 1.21 (br. s, 24 H) ppm. ¹³C NMR

(125 MHz, CDCl₃): δ = 142.9, 133.7, 132.9, 126.5, 126.2, 120.8 (q,

J_C,F= 321 Hz), 115.5, 112.5, 48.1 (d, J_C,P= 13.8 Hz), 33.5, 23.8 (d,

J_C,P= 7.5 Hz) ppm. ³¹P (CDCl₃): δ = 107.6 ppm. C₂₃H₄₀F₃N₄O₃PS

(540.62): calcd. C 51.10, H 7.46, N 10.36, P 5.73, S 5.93; found C

51.18, H 7.54, N 10.17, P 5.64, S 6.15.

3-(Di-tert-butyl)phosphanyl-1-isopropyl-1H-benzimidazolium Tri-

flate (3c): To a cooled solution of the benzimidazole 2 (3.21 g,

22 mmol) in CH₂Cl₂(15 mL) at –20 °C, a solution of isopropyl

trifluoromethanesulfonate (2.35 g, 22 mmol) in CH₂Cl₂(10 mL)

was added. The reaction mixture was warmed to 20 °C and then

stirred for 12 h. The solvent was removed under reduced pressure

and Et₂O (30 mL) was added to the oily residue. The powdered

solid was filtered under argon to give 3c (4.63 g, 83%) as a white

1-(Di-tert-butylphosphanyl)-1H-benzimidazole (2): To a suspension

of oil-free sodium hydride (1.5 g, 62 mmol) in THF (15 mL), a

solution of benzimidazole (5.1 g, 43 mmol) in THF (40 mL) was

added dropwise over 15 min at 20 °C. The reaction mixture was

heated to reflux for 5 min, cooled to 0 °C, and then a solution of

di(tert-butyl)chlorophosphane (7.8 g, 43 mmol) in hexane (15 mL)

was added dropwise over 10 min. The reaction mixture was warmed

to room temperature, the precipitated solid was filtered off, the

filtrate was evaporated, and the residue was distilled to give the

target product; yield 10.7 g (95%); b.p. 135–138 °C/0.02 Torr; m.p.

84–86 °C. ¹H NMR (500 MHz, C₆D₆): δ = 8.22 (s, 1 H), 8.06 (d, J

= 8.1 Hz, 1 H), 7.90 (d, J = 7.6 Hz, 1 H), 7.25 (t, J = 7.6 Hz, 1 H),

7. 20 (t, J = 7.5 Hz, 1 H), 0.92 (s, 9 H), 0.89 (s, 9 H) ppm. ¹³C

NMR (125 MHz, C₆D₆): δ = 146.0 (d, J_C,P= 10.6 Hz), 144.6 (d,

J_C,P= 3.5 Hz), 140.8 (d, J_C,P= 20.8 Hz), 123.6, 122.7, 120.7, 112.7

(d, J_C,P= 16.4 Hz), 34.7 (d, J_C,P= 26.1 Hz), 29.0, 28.9 ppm. ³¹P

(C₆D₆): δ = 73.6 ppm. C₁₅H₂₃N₂P (262.33): calcd. C 68.68, H 8.84,

N 10.68, P 11.81; found C 68.79, H 8.91, N 10.81, P 11.69.

1

solid; m.p. 127–129 °C. H NMR (300 MHz, CDCl₃): δ = 9.59 (s,

1 H), 8.03–8.00 (m, 1 H), 7.94–7.91 (m, 1 H), 7.64–7.60 (m, 2 H),

5.44 (m, 1 H), 1.77 (d, J = 7.0 Hz, 6 H), 1.24 (d, J = 13.5 Hz, 18

H) ppm. ¹³C NMR (125 MHz, CDCl₃): δ = 143.2 (d, J = 14 Hz),

137.95 (d, J = 19 Hz), 129.8, 127.7, 127.45, 123.3 (q, J = 320 Hz),

115.8 (d, J = 17.6 Hz), 114.4, 53.6, 35.3 (d, J = 30 Hz), 28.6 (d, J

= 15 Hz), 21.7 ppm. ³¹P (CDCl₃): δ = 102.5 ppm. C₁₉H₃₀F₃N₂O₃PS

(454.49): calcd. C 50.21, H 6.65, N 6.16, P 6.81, S 7.05; found C

50.09, H 6.57, N 5.97, P 6.98, S 6.88.

P,P-Di(tert-butyl)-N-{2-[formyl(methyl)amino]phenyl}phosphinous

Amide (5): To a mixture of salt 3a (500 mg, 1.1 mmol) and sodium

hydroxide (190 mg, 4.7 mmol), THF (5 mL) was added. The sus-

pension was stirred at 25 °C for 10 min and then concentrated un-

der reduced pressure. The slurry residue was extracted with boiling

pentane (25 mL) in a stoppered flask, after cooling, insoluble solid

was filtered off. The filtrate was concentrated under reduced pres-

sure and the residue was distilled (b.p. 120–125 °C/0.05 Torr), to

give 5 (280 mg, 85 %) as a white solid. ¹H NMR (300 MHz,

CDCl₃): δ = 8.17 (s, 1 H), 7.53 (d, J = 6.6 Hz, 1 H), 7.22 (t, J =

7.5, 7.0 Hz, 1H), 7.00 (d, J = 7.5 Hz, 1 H), 6.73 (t, J = 7.0, 8.0 Hz,

1 H), 4.22 (d, J = 10.2 Hz, 1 H), 3.22 (s. 3 H), 1.12 (d, J = 10.8 Hz,

18 H) ppm; ¹³C NMR (125 MHz, CDCl₃): δ = 163.9, 146.2 (d, J =

16 Hz), 129.7, 128.7, 127.9 (d, J = 2.5 Hz), 118.0, 116.1 (d, J =

23 Hz), 34.2 (d, J = 20 Hz), 32.9, 28.0 (d, J = 15 Hz) ppm. ³¹P

NMR (81 MHz, CDCl₃): δ = 56.12 ppm. C₁₅H₂₅N₂OP (280.35):

calcd. C 64.26, H 8.99, N 9.99, P 11.05; found C 64.20, H 9.06, N

10.13, P 11.29.

3-(Di-tert-butyl)phosphanyl-1-methyl-1H-benzimidazolium Triflate

(3a): To a solution of the benzimidazole 2 (2.60 g, 20 mmol) and

sodium trifluoromethanesulfonate (dried at 200 °C under 0.05 Torr

for 2 h; 6.55 g, 38 mmol) in THF (30 mL), di-tert-butylbromophos-

phane (4.7 g, 21 mmol) was added by using a syringe. The reaction

mixture was stirred for 20 h at 20 °C. The solvent was removed

under reduced pressure (upon heating to 30–35 °C), the residue was

washed with pentane (110 mL), and filtered under argon. The solid

residue was dried at 30–35 °C under reduced pressure (0.05 Torr)

and then extracted with CH₂Cl₂(60 mL). Insoluble residue was

filtered off, the filtrate was concentrated under reduced pressure,

the residue was washed again with Et₂O (40 mL), and the solid

residue was filtered and dried under reduced pressure to give 3a

as a white solid; yield 6.75 g (85%); m.p. 134–136 °C. ¹H NMR

(500 MHz, CDCl₃): δ = 9.78 (s, 1 H), 8.02 (d, J = 7.1 Hz, 1 H),

7.87 (d, J = 7.4 Hz, 1 H), 7.67 (m, 2 H), 4.37 (s, 3 H), 1.28 (d, J =

14 Hz, 18 H) ppm. ¹³C NMR (125 MHz, CDCl₃): δ = 145.0 (d,

1-[Di(tert-butyl)phosphanyl]-2-methoxy-3-methyl-2,3-dihydro-1H-

benzimidazole (6). Path 1: To a mixture of imidazolium salt 3a

(3.2 g, 7.5 mmol) and sodium methylate (760 mg, 14 mmol), was

added THF (15 mL; previously degassed and cooled to –30 °C).

The reaction mixture was warmed to room temperature (25 °C) and

then stirred for 10 min. THF was removed under reduced pressure,

the residue was treated with pentane, and resulting solution was

filtered from undissolved precipitate and evaporated to give 6

(2.0 g, 90%) as a colorless oil.

J_C,P= 11.9 Hz), 137.2 (d, J_C,P= 17.7 Hz), 131.8, 127.6 (d, J_C,P

=

20.4 Hz), 125.1 (d, J_C,P= 27.9 Hz), 120.7 (q, J_C,F= 320 Hz), 115.3

(d, J_C,P= 17.3 Hz), 113.1, 35.4 (d, J_C,P= 29.6 Hz), 34.2, 28.6,

28.4 ppm. ³¹P (CDCl₃): δ = 100.7 ppm. C₁₇H₂₆F₃N₂O₃PS (426.43):

calcd. C 47.88, H 6.15, N 6.57, P 7.26, S 7.52; found C 48.13, H

6.27, N 6.55, P 7.03, S 7.44.

3-[Bis(diisopropylamino)]phosphinoyl-1-methyl-1H-benzimid-

azolium Triflate (3b): To a solution of bis(diisopropylamino)phos-

phenium triflate (2.35 g, 6.2 mmol) in dichloromethane (12 mL) co-

oled to –20 °C, a solution of 1-methylbenzimidazole (980 mg,

7.41 mmol) in dichloromethane (5 mL) was added. The reaction

Path 2: To a frozen solution of carbene 7a (380 mg, 1.4 mmol) in

benzene (3 mL), was added a 4.62% solution of methanol in ben-

zene (950 mg, 1.4 mmol). The mixture was warmed to room tem-

perature and, after 20 min, the solvents were evaporated to give 6.

mixture was warmed to 20 °C over 30 min and, after an additional ¹H NMR (500 MHz, C₆D₆): δ (major compound) = 7.25 (d, J =

30 min, the solvent was evaporated at 40 °C under reduced pres-

sure. Et₂O (22 mL) was added to the residual oil and the mixture

was cooled to –5 °C. Over the required time, the powdered precipi-

tate was collected by filtration under argon and washed with diethyl

ether (15 mL) to give 3b as a white solid; yield 2.53 g (80%); m.p.

7.5 Hz, 1 H), 6.73–6.82 (m, 2 H), 6.42 (d, J = 2.5 Hz, 1 H), 6.26

(d, J = 7.5 Hz, 1 H), 2.71 (s, 3 H), 2.66 (s, 3 H), 1.37 (d, J =

12.0 Hz, 9 H), 1.04 (d, J = 11.5 Hz, 9 H) ppm. ¹³C NMR

(125 MHz, C₆D₆): δ (major compound) = 142.0 (d, J = 22.6 Hz),

139.0 (d, J = 4 Hz), 120.2, 117.8, 110.1 (d, J = 18.9 Hz), 102.6,

100.2 (d, J = 13.8 Hz), 47.7, 37.2 (d, J = 35.2 Hz), 33.9 (d, J =

22.6 Hz), 30.05 (d, J = 16.3 Hz), 29.25 (d, J = 17.6 Hz), 29.0 ppm;

1

118–120 °C. H NMR (500 MHz, CDCl₃): δ = 9.22 (s, 1 H), 7.97

(d, J = 8.0 Hz, 1 H), 7.73 (d, J = 8.0 Hz, 1 H), 7.63–7.56 (m, 2 H),

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A. N. Kostyuk et al.

FULL PAPER

δ (minor compound) = 141.5, 136.6 (d, J = 13.8 Hz), 120.6, 116.5,

111.5, 103.3, 109.1 (d, J = 45.3 Hz), 49.2, 36.7 (d, J = 31.4 Hz),

34.9 (d, J = 27.7 Hz), 30.5 (d, J = 17.6 Hz), 29.8, 27.7 (d, J =

15.1 Hz) ppm. ³¹P NMR (81 MHz, C₆D₆): δ = 98.7, 84.6

(1:9.1) ppm. C₁₇H₂₉N₂OP (308.40): calcd. C 66.21, H 9.48, N 9.08,

P 10.04; found C 66.03, H 9.34, N 9.15, P 10.29.

δ = 5.03 ppm. C₁₆H₂₅N₂P (276.36): calcd. C 69.54, H 9.12, N 10.14,

P 11.21; found C 69.42, H 9.05, N 10.01, P 11.07.

1-Methyl-2-bis(diisopropylamino)phosphinoyl-1H-benzimidazole

(8b): To a suspension of triflate 3b (1.58 g, 3 mmol) in Et₂O

(15 mL) at –5 to 0 °C, a solution of sodium bis(trimethylsilyl)amide

(560 mg, 3 mmol) in Et₂O (15 mL) was added dropwise over

20 min. The reaction mixture was stirred at 0 °C for 20 min until

complete dissolution of triflate 3b and then the solution was con-

centrated to 1/2 volume. The formed precipitate was filtered under

argon and washed with Et₂O (2ϫ 10 mL). The filtrate was evapo-

rated under reduced pressure and the residue was dissolved in benz-

ene (10 mL). After 3 days, benzene was removed under reduced

pressure, the residue was distilled (b.p. 110–115 °C/0.05 Torr), to

give crude 8b, which was recrystallized from pentane (3 mL). The

precipitate formed at –18 °C was filtered and dried under reduced

1-(Di-tert-butylphosphanyl)-3-methyl-1H-benzimidazol-2-ylidene

(7a): To a suspension of triflate 3a (1.40 g, 3.29 mmol) in Et₂O

(10 mL) at 0 °C, a solution of sodium hexamethyldisilazide

(600 mg, 3.29 mmol) in Et₂O (12 mL) was added dropwise over

15 min. Within complete dissolution of triflate 3a the reaction mix-

ture became thick and cloudy from released sodium triflate. The

reaction mixture was stirred for 1 h at 0 °C, then the precipitate

was filtered under argon. The filtrate was cooled to –18 °C. After

12 h, the formed precipitate was filtered and dried under reduced

pressure to give 7a; yield 0.46 g (51%); m.p. 132–133 °C. ¹H NMR

(500 MHz, CDCl₃): δ = 7.96 (d, J = 7.8 Hz, 1 H), 7.10 (t, J =

7.5 Hz, 1 H), 7.02 (t, J = 7.5 Hz, 1 H), 6.90 (d, J = 7.8 Hz, 1 H),

3.58 (s, 3 H), 1.36 (s, 9 H), 1.33 (s, 9 H) ppm. ¹³C NMR (125 MHz,

CDCl₃): δ = 235.4, 142.9 (d, J_C,P= 24 Hz), 134.9 (d, J_C,P= 3.3 Hz),

121.8 (d, J_C,P= 24 Hz), 113.1 (d, J_C,P= 17.3 Hz), 109.6, 35.2 (d,

J_C,P= 23.5 Hz), 34.9, 29.5, 29.3 ppm. ³¹P (CDCl₃): δ = 78.1 ppm.

C₁₆H₂₅N₂P (276.36): calcd. C 69.54, H 9.12, N 10.14, P 11.21;

found C 69.37, H 9.01, N 10.12, P 11.11.

1

pressure to give pure 8b; yield 800 mg (72%); m.p. 110–111 °C. H

NMR (300 MHz, C₆D₆): δ = 8.00–7.97 (m, 1 H), 7.20–7.16 (m, 2

H), 7.03–7.00 (m, 1 H), 3.71 (m, 4 H), 3.45 (s, 3 H), 1.19 (dd, J =

1

6.6, 26.7 Hz, 24 H) ppm. H NMR (500 MHz, C₆D₆): δ = 7.97 (d,

J = 8.0 Hz, 1 H), 7.20–7.16 (m, 2 H), 7.03 (d, J = 7.0 Hz, 1 H),

3.73 (m, 4 H), 3.48 (s, 3 H), 1.24 (d, J = 7.0 Hz, 12 H), 1.14 (d, J

= 6.0 Hz, 12 H) ppm. ¹³C NMR (125 MHz, C₆D₆): δ = 158.5 (d, J

= 11.3 Hz), 144.7 (d, J = 2.5 Hz), 136.2 (d, J = 2.5 Hz), 121.9 (d,

J = 1.26 Hz), 121.5, 119.9, 108.9 (d, J = 1.26 Hz), 48.0 (d, J =

11.3 Hz), 30.2 (d, J = 17.6 Hz), 23.8 (d, J = 6.3 Hz) ppm. ³¹P NMR

(81 MHz, C₆D₆): δ = 28.6 ppm. C₂₀H₃₅N₄P (362.50): calcd. C

66.27, H 9.73, N 15.46, P 8.54; found C 66.13, H 9.63, N 15.32, P

8.52.

Synthesis of Carbene 7a from Azoline 6: Azoline 6 (1.7 g, 5 mmol)

was heated with stirring at 120 °C for 30 min and then distilled

(b.p. 120–130 °C/0.05 Torr) to give 7a (1.49 g). The crude product

was purified by crystallization from pentane (4 mL) to give pure 7a

(1.21 g, 76%).

1-Isopropyl-2-(di-tert-butylphosphanyl)-1H-benzimidazole (8c): Syn-

thesized according to the above procedure. Required time 2 h; yield

90 mg (90%); m.p. 102–103 °C (pentane). ¹H NMR (500 MHz,

C₆D₆): δ = 8.02 (d, J = 8.0 Hz, 1 H), 7.38 (d, J = 8.0 Hz, 1 H),

7.13–7.09 (m, 2 H), 5.73 (m, 1 H), 1.313 (s, 6 H), 1.309 (d, J =

12.5 Hz, 18 H) ppm. ¹³C NMR (125 MHz, C₆D₆): δ = 153.9 (d, J

= 20 Hz), 145.6 (d, J = 1.26 Hz), 133.3, 122.1, 121.6, 121.1, 112.1,

48.05 (d, J = 25 Hz), 33.1 (d, J = 17.6 Hz), 30.0 (d, J = 13 Hz),

20.7 ppm. ³¹P NMR (81 MHz, C₆D₆): δ = 4.90 ppm. C₁₈H₂₉N₂P

(304.41): calcd. C 71.02, H 9.60, N 9.20, P 10.17; found C 71.18,

H 9.71, N 9.03, P 10.18.

1-(Di-tert-butylphosphanyl)-3-isopropyl-2,3-dihydro-1H-benzimid-

azol-2-ylidene (7c): To a suspension of triflate 3c (4.5 g, 10 mmol) in

Et₂O (25 mL), a solution of sodium bis(trimethylsilyl)amide (1.8 g,

10 mmol) in Et₂O (20 mL) was added over 15 min. Stirring was

continued for a further 15 min, then the reaction mixture was

stirred at 15 °C for 15 h until complete dissolution of triflate 2c.

The Et₂O was evaporated under reduced pressure and the solid

residue was extracted with boiling pentane (30 mL) in a stoppered

flask. The insoluble part was filtered off and washed with pentane

(2 ϫ 20 mL), and the filtrate was evaporated. The residue was

recrystalized from pentane (10 mL) and the solid that precipitated

at –18 °C was collected to give 7c (1.53 g, 51%); m.p. 76–77 °C. ¹H

NMR (300 MHz, C₆D₆): δ = 7.96–8.0 (m, 1 H), 7.07–7.13 (m, 1

H), 7.04 (br. s, 2 H), 4.43 (m, 1 H), 1.49 (d, J = 6.6 Hz, 6 H), 1.34

(d, J = 12.6 Hz, 18 H) ppm. ¹³C NMR (125 MHz, C₆D₆): δ =

233.08, 142.75 (d, J = 24 Hz), 133.5 (d, J = 4 Hz), 121.5, 121.1,

112.95 (d, J = 17 Hz), 109.5, 49.3, 34.9 (d, J = 24 Hz), 29.1 (d, J

= 16 Hz), 23.0 ppm. ³¹P NMR (81 MHz, C₆D₆): δ = 78.1 ppm.

C₁₈H₂₉N₂P (304.41): calcd. C 71.02, H 9.60, N 9.20, P 10.17; found

C 71.20, H 9.67, N 9.11, P 10.22.

1-(Di-tert-butylphosphanyl)-3-methyl-1,3-dihydro-2H-benzimid-

azole-2-thione (9a): To a solution of carbene 7a (300 mg, 1.1 mmol)

in degassed benzene (5 mL), was added finely ground sulfur

(35 mg, 1.1 mmol). The reaction mixture was stirred for 1 h at

28 °C, the benzene was evaporated in vacuo, and the residue was

extracted with pentane (10 mL). The insoluble part was filtered off

under argon and washed with pentane (2ϫ 5 mL). The filtrate was

evaporated to 2/3 volume and allowed to stand at –15 °C. The

formed precipitate was collected by filtration to give 9a (240 mg,

72 %) as a light-yellow powder; m.p. 88–89 °C. ¹H NMR

(300 MHz, CDCl₃): δ = 7.79–7.83 (m, 2 H), 7.19 (m, 1 H), 7.04–

7.07 (m, 1 H), 3.77 (s, 3 H), 1.44 (d, J = 14 Hz, 18 H) ppm. ¹³C

NMR (125 MHz, CDCl₃): δ = 174.9 (d, J = 16 Hz), 138.9 (d, J =

24 Hz), 133.9 (d, J = 4 Hz), 122.9, 122.5 (d, J = 4 Hz), 114.0 (d, J

= 29 Hz), 108.3, 37.0 (d, J = 33 Hz), 31.9, 30.6 (d, J = 19 Hz) ppm.

³¹P NMR (81 MHz, CDCl₃): δ = 101.2 ppm. C₁₆H₂₅N₂PS (308.42):

calcd. C 62.31, H 8.17, N 9.08, P 10.04, S 10.40; found C 62.25, H

8.14, N 9.19, P 9.89, S 10.51.

1-Methyl-2-(di-tert-butylphosphanyl)-1H-benzimidazole (8a): A

solution of carbene 7a (100 mg, 0.36 mmol) in benzene (0.5 mL)

was heated in a sealed ampoule at 150 °C for 3 h. The benzene was

evaporated under reduced pressure, the residue was recrystallized

from pentane (3 mL), and the solution was cooled to –18 °C to give

colorless crystals of the target compound; yield 95 mg (95%); m.p.

1

89–90 °C. H NMR (500 MHz, C₆D₆): δ = 7.99–7.97 (m, 1 H, Ar-

H), 7.17–7.16 (m, 2 H, Ar-H), 7.05–7.04 (m, 1 H, Ar-H), 3.55 (s, 3

H, CH₃), 1.28 (d, J = 12.5 Hz, 18 H, tBu) ppm. ¹³C NMR

(125 MHz, C₆D₆): δ = 154.56 (d, J = 21 Hz), 144.6, 135.95 (d, J =

1.26 Hz), 122.5, 121.9, 120.5, 109.6, 33.2 (d, J = 17.6 Hz), 30.7 (d,

1-(Di-tert-butylphosphanyl)-3-methyl-1,3-dihydro-2H-benzimid-

azole-2-selone (10a): To a solution of carbene 7a (550 mg, 2 mmol)

in degassed benzene (6 mL), finely ground selenium (160 mg,

2 mmol) was added and the reaction mixture was stirred for 4 h at

J = 20 Hz), 29.96 (d, J = 15 Hz) ppm. ³¹P NMR (81 MHz, C₆D₆): 28 °C. The solvent was evaporated under reduced pressure and the

10

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Stable N-Heterocyclic Carbenes

residue was extracted with pentane (20 mL). After cooling, the pre-

cipitated impurities were filtered off; the crystals precipitated from

the filtrate at –6 °C were collected to give 10a (500 mg, 84%) as a

NMR (81 MHz, CDCl₃): δ = 124.1 ppm. MS (EI, 70 eV): m/z (%)

= 341 (100) [M + H]⁺. C₁₆H₂₅N₂PS₂(340.48): calcd. C 56.44, H

7.40, N 8.23, P 9.10, S 18.83; found C 56.37, H 7.32, N 8.29, P

9.04, S 18.99.

1

light-yellow solid; m.p. 114–116 °C. H NMR (300 MHz, CDCl₃):

δ = 7.91–7.96 (m, 1 H), 7.13–7.24 (m, 3 H), 3.90 (s, 3 H), 1.48 (d,

J = 14.4 Hz, 18 H) ppm. ¹³C NMR (125 MHz, CDCl₃): δ = 171.8

(d, J = 16.3 Hz), 140.6 (d, J = 25 Hz), 134.4 (d, J = 4 Hz), 123.3,

122.9 (d, J = 2.5 Hz), 114.7 (d, J = 31.4 Hz), 108.9, 37.2 (d, J =

24 Hz), 34.3, 30.8 (d, J = 19 Hz) ppm. ³¹P NMR (81 MHz, CDCl₃):

δ = 104.8 ppm. C₁₆H₂₅N₂PSe (355.32): calcd. C 54.09, H 7.09, N

7.88, P 8.72; found C 53.88, H 7.01, N 7.71, P 8.54.

Synthesis of 11a from Azoline 6: To a solution of the azoline 6

(1.38 g, 4.05 mmol) in benzene (5 mL), finely ground sulfur

(130 mg, 4.06 mmol) was added. Within 30 min, an additional

amount of sulfur (130 mg, 4.06 mmol) was added and the reaction

mixture was heated in a stoppered flask at 100 °C for 3 h. The

solvent was removed under reduced pressure, the oil residue was

purified by silica gel chromatography to give 11a (1.30 g, 94%) as a

yellow solid; m.p. 101–102 °C (pentane); R_f= 0.40–0.70 (CH₂Cl₂).

1-Methyl-3-[bis(diisopropylamino)]phosphinoyl-1,3-dihydrobenz-

imidazole-2-selone (10b): To a suspension of salt 3b (2.49 g,

5 mmol) in Et₂O (18 mL) at 0 °C, was added a solution of sodium

bis(trimethylsilyl)amide (890 mg, 5 mmol) in diethyl ether (15 mL)

over 20 min. The reaction mixture was stirred at 0 °C for 10 min

until complete dissolution of salt 3b. The reaction mixture was

evaporated under reduced pressure at 0 °C. To the residual oil,

pentane (45 mL) was added and stirring at 0 °C was continued until

the oil solidified. The precipitate was collected by cold filtration

under argon; to the filtrate (δ_P= 68.4–70%) selenium was added

(270 mg, 3 mmol). The reaction was stirred at 15 °C for 20 h, then

the mixture was concentrated under reduced pressure. The solid

residue was dissolved in benzene, unreacted selenium was filtered

off, the filtrate was evaporated under reduced pressure, and the

residue was recrystallized from pentane; yield of 10b: 1.0 g (47%);

m.p. 108–109 °C. ¹H NMR (500 MHz, C₆D₆): δ = 7.89 (d, J =

6.0 Hz, 1 H), 7.12–7.13 (m, 2 H), 6.94 (d, J = 6.0 Hz, 1 H), 3.50

(m, 7 H), 1.09 (dd, J = 6.5, 30.5 Hz, 24 H) ppm. ¹³C NMR

(125 MHz, C₆D₆): δ = 145.1, 144.8 (d, J = 10 Hz), 136.9, 128.0,

121.8, 121.4, 108.8, 48.8 (d, J = 12.6 Hz), 31.3 (d, J = 4.0 Hz), 23.8

(d, J = 6.3 Hz), 22.8 (d, J = 10.0 Hz) ppm. ³¹P NMR (81 MHz,

C₆D₆): δ = 108.9 ppm. C₂₀H₃₅N₄PSe (441.46): calcd. C 54.42, H

7.99, N 12.69, P 7.02; found C 54.41, H 8.04, N 12.50, P 7.18.

1-(Di-tert-butylthiophosphonyl)-3-isopropyl-1,3-dihydro-2H-benz-

imidazole-2-thione (11c): To a solution of carbene 7c (1.0 g,

3.3 mmol) in benzene (3 mL) was added finely ground sulfur

(100 mg, 3.1 mmol). The reaction mixture was stirred for 24 h at

25 °C. The solvent was evaporated under reduced pressure and the

residue was extracted with pentane (20 mL). The solution was al-

lowed to cool at –6 °C and then decanted from the settled oil. Ad-

ditional finely ground sulfur (100 mg, 3.1 mmol) was added and

the mixture was heated 100 °C for 3 h in a stoppered flask and

concentrated under reduced pressure. The residual solid was puri-

fied by silica gel chromatography. The crude product was recrys-

tallized from pentane (3 mL) to give 11c as a light-yellow powder;

yield 640 mg (53%); m.p. 147–148 °C; R_f= 0.45–0.55 (CH₂Cl₂). ¹H

NMR (300 MHz, CDCl₃): δ = 7.75–7.78 (m, 1 H, Ar-H), 7.59–7.62

(m, 1 H, Ar-H), 7.22–7.25 (m, 2 H, Ar-H), 5.26 (m, 1 H, CH of

iPr), 1.69 (d, J = 7.2 Hz, 6 H, 2CH₃of iPr), 1.56 (d, J = 17.4 Hz,

18 H, 3CH₃of tBu) ppm. ¹³C NMR (125 MHz, CDCl₃): δ = 144.2,

141.3 (d, J = 5 Hz), 134.3, 123.0, 122.0, 120.5, 112.1, 49.6, 44.9

(d, J = 35 Hz), 28.2, 21.6 ppm. ³¹P NMR (81 MHz, CDCl₃): δ =

122.5 ppm. C₁₈H₂₉N₂PS₂(368.53): calcd. C 58.66, H 7.93, N 7.60,

P 8.40, S 17.40; found C 58.62, H 7.97, N 7.51, P 8.27, S 17.58.

1-(Di-tert-butylselenophosphonyl)-3-methyl-1,3-dihydro-2H-benz-

imidazole-2-selone (12a): To a solution of selone 10a (500 mg,

1.4 mmol) in benzene (5 mL), selenium (110 mg, 1.4 mmol) was

added and the reaction mixture was heated at 100 °C for 4 h. The

solvent was removed under reduced pressure and the oil residue

was purified by chromatography on silica gel to give 12a as a light-

brown solid; yield 450 mg (75 %); m.p. 80–81 °C; R_f0.2–0.45

(CH₂Cl₂). ¹H NMR (500 MHz, CDCl₃): δ = 7.78 (d, J = 8.0 Hz, 1

H), 7.37 (d, J = 8.0 Hz, 1 H), 7.32 (t, J = 7.5 Hz, 1 H), 7.25–7.28

(m, 1 H), 3.95 (s, 3 H), 1.59 (d, J = 18.0 Hz, 18 H) ppm. ¹³C NMR

(125 MHz, CDCl₃): δ = 144.1, 140.1 (d, J = 7.5 Hz), 136.9, 123.5,

122.3, 119.9, 110.0, 44.7 (d, J = 21 Hz), 32.8, 28.6 (d, J =

45 Hz) ppm. ³¹P NMR (81 MHz, CDCl₃): δ = 123.0 ppm. MS (EI,

70 eV): m/z (%) = 434.0, 441.0 (100) [M + H]⁺. C₁₆H₂₅N₂PSe₂

(434.28): calcd. C 44.25, H 5.80, N 6.45, P 7.13; found C 44.08, H

5.69, N 6.40, P 7.25.

1-(Di-tert-butylphosphanyl)-3-isopropropyl-1,3-dihydro-2H-benz-

imidazole-2-selone (10c): To a solution of carbene 7c (120 mg,

4 mmol) in benzene (4 mL), selenium (350 mg, 4.4 mmol) was

added and the reaction mixture was stirred for 2 h. The unreacted

selenium was filtered off under argon and the filtrate was evapo-

rated. The residue was recrystallized from pentane (3 mL) to give

10c after cooling at –18 °C; yield 120 mg (80%); m.p. 131–132 °C.

¹H NMR (300 MHz, CDCl₃): δ = 7.98–7.81 (m, 1 H), 7.47–7.44

(m, 1 H), 7.19–7.16 (m, 2 H), 6.10 (m, 1 H), 1.59 (d, J = 7.0 Hz, 6

H), 1.48 (d, J = 14.0 Hz, 18 H) ppm. ¹³C NMR (125 MHz, CDCl₃):

δ = 171.1 (d, J_C,P= 17.6 Hz), 141.1 (d, J_C,P= 25.0 Hz), 131.9, 122.6

(d, J_C,P= 2.5 Hz), 122.3 (d, J_C,P= 4.0 Hz), 115.1 (d, J_C,P

32.7 Hz), 110.3, 52.1, 37.2 (d, J_C,P= 35.2 Hz), 30.9 (d, J_C,P

=

19.0 Hz), 19.6 ppm. ³¹P (CDCl₃): δ = 107.7 ppm. C₁₈H₂₉N₂PSe

(383.37): calcd. C 56.39, H 7.62, N 7.31, P 8.08; found C 56.57, H

7.74, N 7.21, P 7.95.

1-[Di(tert-butyl)selenophosphonyl]-3-isopropyl-1,3-dihydro-2H-

1-(Di-tert-butylthiophosphonyl)-3-methyl-1,3-dihydro-2H-benz- benzimidazole-2-selone (12c): To a solution of selone 10c (392 mg,

imidazole-2-thione (11a): A solution of thione 9a (220 mg,

0.7 mmol) and finely ground sulfur (50 mg, 1.6 mmol) in benzene

(3 mL) was heated at 100 °C for 15 h. The solvent was evaporated

under reduced pressure and the residue was purified by silica gel

chromatography [R_f= 0.30–0.50 (CH₂Cl₂)]. The isolated product

1.02 mmol) in benzene (5 mL), selenium (82 mg, 1.04 mmol) was

added and the reaction mixture was heated at 60 °C overnight. Un-

reacted selenium was filtered off, the filtrate was concentrated un-

der reduced pressure, and the solid residue was recrystallized from

pentane (8 mL) to give 12c after cooling at –18 °C; yield 410 mg

was recrystallized from pentane (18 mL) to give 11a (20 mg, 91%) (88 %); yellow powder; m.p. 146–147 °C. ¹H NMR (300 MHz,

as a deep-yellow solid; m.p. 101–102 °C. ¹H NMR (300 MHz,

CDCl₃): δ = 7.77–7.76 (m, 1 H), 7.64–7.61 (m, 1 H), 7.25–7.21 (m,

2 H), 5.20 (m, 1 H), 1.71 (d, J = 6.9 Hz, 6 H), 1.58 (d, J = 17.7 Hz,

CDCl₃): δ = 7.76 (d, J = 7.5 Hz, 1 H), 7.31–7.35 (m, 2 H), 7.25–

7.29 (m, 1 H), 3.94 (s, 3 H), 1.61 (d, J = 10.8 Hz, 18 H) ppm. ¹³C 18 H) ppm. ¹³C NMR (125 MHz, CDCl₃): δ = 145.1, 139.9 (d, J

NMR (125 MHz, CDCl₃): δ = 143.3, 142.0 (d, J = 6.3 Hz), 136.9,

= 6 Hz), 134.1, 122.9, 121.9, 120.5, 111.9, 50.6, 44.65 (d, J =

123.6, 122.4, 119.9, 110.0, 44.9 (d, J = 34 Hz), 32.2, 28.1 ppm. ³¹P

20 Hz), 28.7 (d, J = 1.2 Hz), 21.8 ppm. ³¹P NMR (81 MHz,

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A. N. Kostyuk et al.

FULL PAPER

CDCl₃): δ = 122.3 ppm. MS (EI, 70 eV): m/z (%) = 463.0 (100) [M

+ H]⁺. C₁₈H₂₉N₂PSe₂(462.33): calcd. C 46.76, H 6.32, N 6.06, P

6.70; found C 46.62, H 6.30, N 6.15, P 6.88.

1-[Di-tert-butylphosphanyl]-N,N,3-trimethyl-2,3-dihydro-1H-benz-

imidazole-2-amine (15a): A flask charged with solid carbene 7a

(890 mg, 3.2 mmol) was cooled in a bath with liquid ammonia. Dry

dimethylamine (2.7 g) was added. The flask was stoppered and left

at room temperature (25 °C) overnight, then dimethylamine was

removed under reduced pressure. The solid residue was recrys-

tallized from pentane (1 mL) to give (at –18 °C) 15a (440 mg, 43%)

as pale crystals; m.p. 73–75 °C. ¹H NMR (500 MHz, C₆D₆): δ (mix-

ture of isomeric compounds) = 7.30 (d, J = 7.0 Hz, 1 H), 7.03 (d,

J = 8.0 Hz, 1 H), 6.79–6.87 (m, 3 H), 6.65 (t, J = 8.0, 7.5 Hz, 1 H),

6.31 (d, J = 7.5 Hz, 1 H), 6.23 (d, J = 7.5 Hz, 1 H), 5.75 (d, J =

8.5 Hz, 1 H), 5.73 (d, J = 2.0 Hz, 1 H), 2.62 (s, 3 H), 2.59 (s, 3 H),

2.23 (br. s, 6 H), 2.08 (br. s, 6 H), 1.41 (d, J = 12.5 Hz, 18 H), 1.14

(t, J = 12.5, 11.5 Hz, 18 H) ppm. ¹³C NMR (125 MHz, C₆D₆): δ =

143.34 (d, J = 23.9 Hz), 143.33, 140.2 (d, J = 4 Hz), 137.5 (d, J =

14 Hz), 120.5, 119.6 (d, J = 2.5 Hz), 117.9, 116.2, 110.4, 109.96 (d,

J = 20 Hz), 104.3 (d, J = 48 Hz), 103.9, 102.9, 97.1 (d, J = 14 Hz),

37.9 (d, J = 33.9 Hz), 37.5, 37.1 (d, J = 35.2 Hz), 35.1 (d, J =

27.7 Hz), 34.7, 34.6 (d, J = 23.9 Hz), 31.3 (d, J = 17.6 Hz), 30.8,

30.4 (d, J = 16.3 Hz), 29.8 (d, J = 17.6 Hz), 29.5 (d, J =

17.6 Hz) ppm. ³¹P NMR (81 MHz, C₆D₆): δ = 98.6, 87.5

(1:1.2) ppm. {¹H}³¹P NMR (121 MHz, C₆D₆): δ = 99.2, 88.1

(22:25) ppm. C₁₈H₃₂N₃P (321.44): calcd. C 67.26, H 10.03, N 13.07,

P 9.64; found C 67.00, H 9.95, N 12.99, P 9.87.

P,P-Di(tert-butyl)-N-{2-[formyl(methyl)amino]phenyl}phosphino-

thioic Amide (13): A mixture of amide 5 (292 mg, 1.0 mmol) and

finely ground sulfur (100 mg, 3.1 mmol) in benzene (3 mL) was

stirred at 25 °C for 12 h. Unreacted sulfur was filtered off and the

filtrate was concentrated under reduced pressure. The residual solid

was purified by silica gel chromatography. The crude product was

recrystallized from pentane (30 mL) to give 13 (250 mg, 81%) as a

white powder; m.p. 114–115 °C; R_f= 0.20–0.50 (CH₂Cl₂). ¹H NMR

(300 MHz, CDCl₃): δ = 8.39 (d), 8.36 (m) (s, 1 H), 8.18 (s, 1 H),

8.14 (m) (d, J = 7.8 Hz, 1 H), 7.29–7.21 (m, 2 H), 7.135 (d, J =

7.5 Hz, 1 H), 7.08–7.05 (m, 1 H), 6.93 (d, J = 7.5 Hz, 1 H), 5.19

(d, J = 10.5 Hz, 1 H), 4.08 (d, J = 7.8 Hz, 1 H), 3.40, 3.21 (s, 3 H),

1.39 (d, J = 15.6 Hz, 18 H), 1.37 (d, J = 15.8 Hz, 18 H) ppm. ¹³C

NMR (125 MHz, CDCl₃): δ = 163.8, 162.5, 140.6 (d, J = 2.5 Hz),

137.9 (d, J = 2.5 Hz), 132.0 (d, J = 5.0 Hz), 129.6 (d, J = 5.0 Hz),

129.5, 128.5, 127.8, 125.2, 124.8, 123.1, 121.2, 119.0, 41.4 (d, J =

54 Hz), 40.75 (d, J = 55 Hz), 38.2, 33.3, 27.6, 27.5 ppm. ³¹P NMR

(81 MHz, CDCl₃): δ = 92.0 ppm. ³¹P-{¹H} NMR: δ = 93.31, 91.97

(1:2.3) ppm. MS (EI, 70 eV): m/z (%) = 327.2 (100) [M + H]⁺.

C₁₅H₂₅N₂OPS (312.41): calcd. C 57.67, H 8.07, N 8.97, P 9.91, S

10.26; found C 57.51, H 8.17, N 8.95, P 9.81, S 10.48.

1-[Di-tert-butylphosphanyl]-3-methyl-2-(morpholin-4-yl)-2,3-di-

hydro-1H-benzimidazole (15b): To solid carbene 7a (1.0 g,

3.6 mmol) at room temperature was added freshly distilled morph-

oline (5.0 g). After stirring at room temperature (25 °C) for 24 h,

excess morpholine was removed under reduced pressure. The resi-

due was dissolved in pentane (20 mL) and the resulting solution

was cooled to –18 °C overnight and then decanted from the oil.

After further cooling, the precipitated solid was collected to give

15b (960 mg, 70%) as a yellowish powder; m.p. 71–72 °C. ¹H NMR

(500 MHz, C₆D₆): δ (mixture of isomeric compounds) = 7.25 (d, J

= 7.5 Hz, 1 H), 7.01 (d, J = 7.5 Hz, 1 H), 6.77–6.85 (m, 3 H), 6.65

(t, J = 7.0, 7.5 Hz, 1 H), 6.33 (d, J = 7.0 Hz, 1 H), 6.23 (d, J =

7.0 Hz, 1 H), 5.575 (d, J = 9.0 Hz, 1 H), 5.541 (d, J = 2.5 Hz, 1

H), 2.66 (s, 3 H), 2.58 (s, 3 H), 2.53 (m, 4 H), 2.22 (m, 4 H), 1.40

(t, J = 12.5, 12.0 Hz, 18 H); 1.13 (d, J = 12.5 Hz, 9 H), 1.08 (d, J

= 11.5 Hz, 9 H) ppm. ¹³C NMR (125 MHz, C₆D₆): δ = 143.6 (d, J

= 22.6 Hz), 143.4, 140.1 (d, J = 4 Hz), 137.6 (d, J = 13.8 Hz), 120.6,

119.9 (d, J = 2.5 Hz), 118.1, 116.5, 111.5, 110.2 (d, J = 18.9 Hz),

104.6, 104.2 (d, J = 45.3 Hz), 103.2, 97.05 (d, J = 18.9 Hz), 66.8,

66.3, 38.05 (d, J = 33.9 Hz), 37.8, 37.4 (d, J = 37.7 Hz), 35.0 (d, J

= 27.7 Hz), 34.5 (d, J = 22.6 Hz), 31.4 (d, J = 16.3 Hz), 31.3, 30.4

(d, J = 17.6 Hz), 29.75 (d, J = 16.3 Hz), 29.5 (d, J = 16.3 Hz) ppm.

³¹P NMR (81 MHz, C₆D₆): δ = 99.8, 89.5 (1:1.35) ppm. {¹H}³¹P

NMR (121 MHz, C₆D₆): δ = 100.3, 90.1 (20:25) ppm. C₂₀H₃₄N₃OP

(363.48): calcd. C 66.09, H 9.43, N 11.56, P 8.52; found C 65.84,

H 9.30, N 12.05, P 8.65.

1-[Di-tert-butylphosphanyl]-2-cyanomethyl-3-methyl-2,3-dihydro-

1H-benzimidazole (14a): To a solution of carbene 7a (2.21 g,

8 mmol) in degassed benzene (10 mL) was added degassed acetoni-

trile (12 mL). The reaction mixture was stirred at 28 °C overnight,

the solvents were removed under reduced pressure, and the residue

was recrystallized from pentane (30 mL) to give (at –18 °C) the

corresponding solid product (1.80 g, 71%) as a mixture of isomeric

1

compounds; m.p. 95–96 °C. H NMR (500 MHz, C₆D₆): δ (major

compound) = 1.06 (d, J = 12.5 Hz, 9 H), 1.21 (d, J = 13.0 Hz, 9

H), 2.46 (s, 3 H), 2.19–2.29 (m, 2 H), 4.80 (m, 1 H), 6.24 (d, J =

7.5 Hz, 1 H), 6.62 (t, J = 7.5 Hz, 1 H), 6.73 (t, J = 7.5, 8.0 Hz, 1

H), 6.88 (d, J = 7.5 Hz, 1 H) ppm; δ (minor compound) = 0.96 (d,

J = 12.0 Hz, 9 H), 1.08 (d, J = 11.5 Hz, 9 H), 2.67 (s, 3 H), 2.02–

2.05 (m, 2 H), 5.27 (m, 1 H), 6.263 (d, J = 7.5 Hz, 1 H), 6.765–

6.81 (m, 2 H), 7.11 (d, J = 7.5 Hz, 1 H) ppm. ¹³C NMR (125 MHz,

C₆D₆): δ (major compound) = 143.4, 138.3 (d, J = 13.8 Hz), 120.8,

118.4, 116.6, 111.8, 107.2, 87.9 (d, J = 45.3 Hz), 37.8 (d, J =

32.7 Hz), 35.0, 34.6 (d, J = 25.1 Hz), 30.6 (d, J = 17.6 Hz), 29.96

(d, J = 16.3 Hz), 24.2 (d, J = 6.3 Hz) ppm; δ (minor compound) =

143.1 (d, J = 21.4 Hz), 140.15 (d, J = 4 Hz), 121.4 (d, J = 2.5 Hz),

119.4, 116.9, 113.0 (d, J = 17.6 Hz), 107.2, 79.5 (d, J = 15.1 Hz),

37.7 (d, J = 39.0 Hz), 33.6 (d, J = 22.6 Hz), 33.2, 29.7 (d, J =

16.3 Hz), 29.5 (d, J = 16.3 Hz), 22.96 ppm. ³¹P NMR (81 MHz,

C₆D₆): δ = 95.0, 92.3 (1:4.5) ppm. C₁₈H₂₈N₃P (317.41): calcd. C

68.11, H 8.89, N 13.24, P 9.76; found C 68.34, H 8.70, N 13.29, P

9.55.

1-[Di-tert-butylphosphanyl]-2-cyanomethyl-3-isopropyl-2,3-dihydro- P-Di(tert-butyl)-N-(2-{[(E)-2-cyanovinyl](methyl)amino}phenyl)phos-

1H-benzimidazole (14b): To a degassed solution of carbene 7c

(1.62 g, 6 mmol) in benzene (4 mL), was added degassed acetoni-

trile (5 mL). The reaction mixture was stirred at 25 °C for 5 d. The

solvent was removed under reduced pressure to give a mixture of

two isomeric compounds; yield 1.76 g (99 %). ¹H NMR (300 MHz,

phinous Amide (16): Solid azoline 14a (200 mg, 0.63 mmol) was

heated at 125 °C in a Claisen bulb on a magnetic stirrer for 30 min

and then distilled (140 °C/0.05 Torr) to give 16 as a colorless pow-

der. ¹H NMR (400 MHz, CDCl₃): δ = 7.50–7.47 (m, 1 H), 7.18–

7.11 (m, 2 H), 6.95 (d, J = 7.6 Hz, 1 H), 6.71 (t, J = 7.6 Hz, 1 H),

CHCl₃): δ = 1.24–1.36 (m, 15 H), 1.58 (d, J = 16 Hz, 9 H), 2.67 4.20 (d, J = 9.6 Hz, 1 H), 3.92 (br. s, 1 H), 3.16 (s, 3 H), 1.11 (d, J

(dd, J = 2.7, 16.8 Hz, 1 H), 2.96 (dd, J = 5.4, 16.8 Hz, 1 H), 3.75

= 12.0 Hz,18 H) ppm. ¹³C NMR (125 MHz, CDCl₃): δ = 153.8,

(m, 1 H), 6.60 (m, 1 H), 6.64 (t, J = 7.5 Hz, 1 H), 6.86 (dd, J = 144.6 (d, J = 16 Hz), 129.2, 126.8, 120.8, 118.3, 116.2 (d, J =

7.5, 8.1 Hz, 1 H), 7.16 (d, J = 8.1 Hz, 1 H) ppm. ³¹P NMR

(81 MHz, CDCl₃): δ = 96.5 (94%), 94.9 (6%). C₂₀H₃₂N₃P (345.47):

calcd. C 69.53, H 9.34, N 12.16, P 8.97; found C 69.80, H 9.21, N

12.33, P 8.78.

23.9 Hz), 65.5, 34.2 (d, J = 20 Hz), 28.0 (d, J = 15 Hz) ppm. ³¹P

NMR (81 MHz, CDCl₃): δ = 55.8 ppm. {¹H} ³¹P NMR (121 MHz,

C₆D₆): δ = 56.4 ppm. C₁₈H₂₈N₃P (317.41): calcd. C 68.11, H 8.89,

N 13.24, P 9.76; found C 68.18, H 8.75, N 13.38, P 9.59.

12

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Pages: 17

Stable N-Heterocyclic Carbenes

P,P-Di(tert-butyl)-N-(2-{[(E)-2-cyanovinyl](methyl)amino}phenyl)-

phosphinothioic Amide (17a)

13.5 Hz, 1 H, CH of vinyl), 1.40 (d, J = 5.5 Hz, 9 H, CH₃of tBu),

1.36–1.37 (m, 9 H + 3 H), 1.24 (d, J = 6.5 Hz, 3 H, CH₃of

iPr) ppm. ¹³C NMR (125 MHz, CDCl₃): δ = 150.7 (CH_vinyl), 140.3

(d, J = 2.5 Hz), 129.05, 128.1, 127.8 (d, J = 7.5 Hz), 121.2, 120.7

(CN), 118.7, 67.6 (CH_vinyl), 56.3, 41.7 (d, J = 50 Hz), 41.4 (d, J =

50 Hz), 21.5 (m), 22.9, 21.4 ppm. ³¹P NMR (81 MHz, CDCl₃): δ =

90.8 ppm. MS (EI, 70 eV): m/z (%) = 378 (93.4) [M + H]⁺.

C₂₀H₃₂N₃PS (377.53): calcd. C 63.63, H 8.54, N 11.13, P 8.20, S

8.49; found C 63.52, H 8.41, N 11.26, P 8.01, S 8.57.

1-(Di-tert-butyl-thiophosphonyl)-3-methyl-2,3-dihydro-1H-benzimid-

azole-2H-azoline (18a): To a solution of 14a (790 mg, 2.5 mmol) in

benzene (6 mL) was added finely ground sulfur (80 mg, 2.5 mmol).

The reaction mixture was stirred at 25 °C for 5 d. The solvent was

removed under reduced pressure and the residual solid was recrys-

tallized from pentane (18 mL) to give 18a (640 mg, 77%) as a white

1

powder; m.p. 91–92 °C. H NMR (300 MHz, CDCl₃): δ = 7.15 (d,

1-[Di(tert-butyl)phosphoroselenoyl]-2-cyanomethyl-3-methyl-2,3-

dihydro-1H-benzimidazole (19a): To a solution of the 2H-benz-

azoline 14a (720 mg, 2.3 mmol) in benzene (8 mL) was added sele-

nium (460 mg, 5.8 mmol) and the mixture was stirred at 25 °C for

72 h. Excess selenium was filtered, the filtrate was concentrated un-

der reduced pressure, and the residue was treated with pentane. The

clear solution was decanted from the oil and the solid formed at

–18 °C was filtered to give 19a (840 mg, 94%) as yellow crystals;

m.p. 95–97 °C. ¹H NMR (500 MHz, C₆D₆): δ = 7.04 (d, J = 8.0 Hz,

1 H), 6.81 (t, J = 8.0, 7.5 Hz, 1 H), 6.55 (t, J = 8.0 Hz, 1 H), 6.49–

6.53 (m, 1 H), 6.21 (d, J = 7.5 Hz, 1 H), 2.41 (s, 3 H), 2.33 (dd,

J = 8.1 Hz, 1 H), 6.92 (t, J = 7.5, 8.0 Hz, 1 H), 6.65 (t, J = 7.0,

7.5 Hz, 1 H), 6.52 (d, J = 7.8 Hz, 1 H), 6.40 (m, 1 H), 2.99 (s, 1

H), 2.73 (d, J = 4 Hz, 2 H), 1.59 (d, J = 16.5 Hz, 9 H), 1.30 (d, J

= 16.2 Hz, 9 H) ppm. ¹³C NMR (125 MHz, CDCl₃): δ = 144.4 (d,

J = 4 Hz), 134.3 (d, J = 1.25 Hz), 123.8, 118.1, 116.9, 114.5, 108.4,

81.4 (d, J = 6.3 Hz, C-2), 44.5 (d, J = 40 Hz), 41.2 (d, J = 53 Hz),

34.6, 29.1 (d, J = 1.25 Hz), 28.4 (d, J = 1.25 Hz), 24.5 (d, J =

1.25 Hz) ppm. ³¹P NMR (81 MHz, CDCl₃): δ = 108.4 ppm. MS

(EI, 70 eV): m/z (%) = 350 (98.7) [M + H]⁺. C₁₈H₂₈N₃PS (349.47):

calcd. C 61.86, H 8.08, N 12.02, P 8.86, S 9.17; found C 61.74, H

7.92, N 12.20, P 9.05, S 9.14.

3

4

3

⁴J_P,H= 16.5, J_H,H= 6.0 Hz, 1 H), 2.22 (dd, J_P,H= 5.0, J_H,H

=

Pentane was removed under reduced pressure from the mother

liquor left from recrystallization of 18a, the residue was purified by

silica gel chromatography [R_f= 0.10–0.30 (CH₂Cl₂)] to give 17a

(34 mg, 4 %) as a white solid; m.p. 114–115 °C. ¹H NMR

(500 MHz, CDCl₃): δ = 8.32 (d, J = 8.0 Hz, 1 H), 7.20 (t, J =

7.0 Hz, 1 H), 7.12 (d, J = 13.5 Hz, 1 H), 7.03 (d, J = 7.5 Hz, 1 H),

7.03 (d, J = 7.5 Hz, 1 H), 6.92 (t, J = 7.5 Hz, 1 H), 4.83 (d, J =

6.5 Hz, 1 H), 3.95 (br. d, J = 10.5 Hz, 1 H), 3.18 (s, 3 H), 1.39 (d,

J = 16.0 Hz, 18 H) ppm. ¹³C NMR (125 MHz, CDCl₃): δ = 153.5,

138.9 (d, J = 2.5 Hz), 128.8, 126.4, 121.7, 119.3, 119.0, 67.1, 41.3

(d, J = 54.0 Hz), 27.5 ppm. ³¹P NMR (81 MHz, CDCl₃): δ =

91.1 ppm. MS (EI, 70 eV): m/z (%) = 350 (89.4) [M + H]⁺.

C₁₈H₂₈N₃PS (349.47): calcd. C 61.86, H 8.08, N 12.02, P 8.86, S

9.17; found C 61.96, H 7.97, N 12.24, P 8.64, S 9.25.

2.5 Hz, 1 H), 1.49 (d, J = 17.0 Hz, 9 H), 1.15 (d, J = 16 Hz, 9

H) ppm. ¹³C NMR (125 MHz, C₆D₆): δ = 144.9 (d, J = 4.0 Hz),

134.2 (4.0), 123.9, 117.7, 116.1, 114.8, 108.1, 83.7 (d, J = 7.5 Hz),

44.35 (d, J = 30.0 Hz), 41.95 (d, J = 32.7 Hz), 33.1, 29.2 (d, J =

1.25 Hz), 28.5 (d, J = 1.25 Hz), 23.3 (d, J = 4.0 Hz) ppm. ³¹P NMR

(81 MHz, C₆D₆): δ = 113.6 ppm. {¹H} ³¹P NMR (121 MHz,

C₆D₆): δ = 114.2 ppm. MS (EI, 70 eV): m/z (%) = 398 (91) [M +

H]⁺. C₁₈H₂₈N₃PSe (396.37): calcd. C 54.54, H 7.12, N 10.60, P

7.81; found C 54.35, H 7.07, N 10.46, P 7.70.

1-[Di(tert-butyl)phosphorothioyl]-N,N,3-trimethyl-2,3-dihydro-1H-

benzimidazole-2-amine (20a): To a solution of the 2H-benzazoline

15a (760 mg, 2.4 mmol) in benzene (10 mL) was added powdered

sulfur (80 mg, 2.5 mmol) and the mixture was stirred at 25 °C over-

night. Benzene was evaporated under reduced pressure and the resi-

due was recrystallized from pentane (25 mL) to give (at –18 °C)

P,P-Di-tert-butyl-N-{2-[(2-cyanovinyl)(isopropyl)amino]phenyl}phos-

phinothioic Amide (17b)

1

20a (200 mg, 24%) as a white powder; m.p. 112–113 °C. H NMR

1-[Di(tert-butyl)phosphorothioyl]-2-cyanomethyl-3-isopropyl-2,3-

dihydro-1H-benzimidazole (18b): To a solution of 14b (1.58 g,

4.6 mmol) in benzene (15 mL) was added finely ground sulfur

(146 mg, 4.56 mmol). The reaction was exothermic at 25 °C. After

stirring for 30 min, the solvent was removed under reduced pres-

sure. The residue was purified by silica gel chromatography to yield

18b (970 mg, 52%) as the major product. White powder; m.p. 116–

(500 MHz, C₆D₆): δ = 7.15 (br. s, C₆D₆+ 1 H), 6.98 (br. s, 1 H),

6.88 (t, J = 7.5 Hz, 1 H), 6.55 (br. t, J = 7.5, 7.0 Hz, 1 H), 6.20 (d,

J = 7.5 Hz, 1 H), 2.52 (s, 3 H), 2.17 (s, 6 H), 1.47 (d, J = 16.0 Hz,

9 H), 1.22 (d, J = 15.0 Hz, 9 H) ppm. ¹³C NMR (125 MHz, C₆D₆):

δ = 145.0, 134.0, 123.4, 115.5, 114.5, 103.9, 97.7 (br. s), 43.55 (d, J

= 41.5 Hz), 41.8 (d, J = 59.1 Hz), 38.0, 29.0, 28.7 ppm. ³¹P NMR

(81 MHz, C₆D₆): δ = 108.0 ppm. {¹H} ³¹P NMR (121 MHz,

C₆D₆): δ = 108.2 ppm. C₁₈H₃₂N₃PS (353.50): calcd. C 61.16, H

9.12, N 11.89, P 8.76, S 9.07; found C 61.01, H 9.06, N 11.99, P

8.77, S 9.20.

1

117 °C; R_f= 0.40–0.80 (CH₂Cl₂). H NMR (300 MHz, CDCl₃): δ

= 7.60 (d, J = 7.5 Hz, 1 H), 6.87 (t, J = 7.5 Hz, 1 H), 6.59–6.49

(m, 2 H), 6.54–6.48 (m, 1 H), 3.75 (m, 2 H), 2.98, 2.925 (2ϫ d, J

= 4.8, 4.2 Hz, 1 H), 2.53, 2.59 (2ϫ d, J = 2.7 Hz, 1 H), 1.59 (d, J

= 16.8 Hz, 9 H), 1.30 (d, J = 16.0 Hz, 9 H), 1.29 (dd, J = 7.0,

22 Hz, 6 H) ppm. ¹³C NMR (125 MHz, CDCl₃): δ = 142.6 (d, J =

4 Hz, C-9), 134.9 (d, J = 1.3 Hz, C-8), 123.2, 117.9, 117.3 (CN),

114.4, 109.7, 76.8 (d, J = 4 Hz, C-2), 49.8, 44.0 (d, J = 38 Hz), 42.1

(d, J = 54 Hz), 29.4 (d, J = 2.5 Hz, CH₂), 29.0 (d, J = 1.3 Hz), 28.9

(d, J = 1.3 Hz), 21.4, 20.2 ppm. ³¹P NMR (81 MHz, CDCl₃): δ =

108.0 ppm. MS (EI, 70 eV): m/z (%) = 378 (97.4) [M + H]⁺.

C₂₀H₃₂N₃PS (377.53): calcd. C 63.63, H 8.54, N 11.13, P 8.20, S

8.49; found C 63.47, H 8.50, N 11.00, P 8.06, S 8.74.

1-[Di(tert-butyl)phosphorothioyl]-3-methyl-2-(morpholin-4-yl)-2,3-di-

hydro-1H-benzimidazole (20b): To a solution of the 2H-benzazoline

15b (760 mg, 2.4 mmol) in benzene (5 mL) was added finely ground

sulfur (80 mg, 2.5 mmol) and the mixture was stirred at 25 °C over-

night. The solvent was removed under reduced pressure and the

residue was recrystallized from pentane (25 mL) to give 20b as a

1

light-yellow solid; m.p. 113–114 °C. H NMR (500 MHz, C₆D₆): δ

= 7.15 (br. s, C₆D₆+ 1 H), 6.86–6.89 (t, J = 7.5, 7.0 Hz, 2 H), 6.55

(br. t, J = 7.0, 7.5 Hz, 1 H), 6.19 (d, J = 7.5 Hz, 1 H), 3.32–3.39

(m, 4 H), 2.48–2.56 (m, 7 H), 1.48 (d, J = 16.0 Hz, 9 H), 1.195 (d,

J = 15.0 Hz, 9 H) ppm. ¹³C NMR (125 MHz, C₆D₆): δ = 145.2,

Compound 17b: Obtained as the minor product; yield 100 mg (6%);

white powder; m.p. 159–160 °C; R_f= 0.20–0.30 (CH₂Cl₂). ¹H NMR 134.3, 123.6, 115.7, 114.6, 104.0, 97.2, 66.5, 46.7, 43.8 (d, J =

(500 MHz, CDCl₃): δ = 8.48 (d, J = 7.5 Hz, 1 H, Ar-H), 7.28 (d,

J = 14 Hz, 1 H, CH of vinyl), 7.22 (t, J = 7.0, 8.0 Hz, 1 H, Ar-H),

41.5 Hz), 41.65 (d, J = 55.3 Hz), 29.2 (d, J = 1.25 Hz), 28.5 ppm.

³¹P NMR (81 MHz, C₆D₆): δ = 107.9 ppm. C₂₀H₃₄N₃OPS (395.54):

6.99 (d, J = 7.5 Hz, 1 H, Ar-H), 6.89–6.92 (m, 1 H, Ar-H), 4.95 calcd. C 60.73, H 8.66, N 10.62, P 7.83, S 8.11; found C 60.95, H

(d, J = 8.0 Hz, 1 H, NH), 3.74 (m, 1 H, CH of iPr), 3.72 (d, J = 8.54, N 10.60, P 7.62, S 8.27.

Eur. J. Org. Chem. 0000, 0–0

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Pages: 17

A. N. Kostyuk et al.

FULL PAPER

P,P-Di(tert-butyl)-N-(2-{[(E)-2-cyanovinyl](methyl)amino}phenyl)-

phosphinoselenoic Amide (21): A solution of the 2H-benzazoline

19a (530 mg, 1.3 mmol) in benzene (5 mL) was heated at 100 °C

for 2 h. Benzene was evaporated under reduced pressure and the

residue was treated with hexane and heated on a water bath. The

solid formed after cooling was filtered to give 21 (460 mg, 87%);

(300 MHz, C₆D₆): δ = 8.02–8.00 (m, 1 H), 7.29–7.27 (m, 1 H),

7.22–7.15 (m, 2 H), 5.16 (m, 1 H), 1.55 (d, J = 15.3 Hz, 9 H), 1.14

(d, J = 12.0 Hz, 6 H) ppm. ¹³C NMR (125 MHz, C₆D₆): δ = 151.4

(d, J = 45 Hz), 145.25 (d, J = 2.5 Hz), 133.8, 123.3, 122.2, 121.6,

112.3, 49.2 (d, J = 18 Hz), 34.9 (d, J = 25 Hz), 25.7 (d, J = 18 Hz),

20.7 ppm. ³¹P NMR (81 MHz, C₆D₆): δ = 83.6 ppm. C₁₄H₂₀ClN₂P

m.p. 110–111 °C. ¹H NMR (500 MHz, C₆D₆): δ = 8.70 (d, J = (282.75): calcd. C 59.47, H 7.13, Cl 12.54, N 9.91, P 10.95; found

7.5 Hz, 1 H), 6.95 (t, J = 7.5, 8.0 Hz, 1 H), 6.62 (t, J = 7.5 Hz, 1 C 59.67, H 7.18, Cl 12.69, N 10.08, P 10.91.

H), 6.48 (d, J = 8.0 Hz, 1 H), 6.40 (d, J = 14 Hz, 1 H), 4.66 (d, J

2-(tert-Butylphosphanyl)-1-isopropyl-1H-benzimidazole (24): To a

= 4.5 Hz, 1 H), 3.53 (d, J = 13.5 Hz, 1 H), 2.28 (s, 3 H), 1.18 (d, J

solution of compound 22 (1.5 g, 5.3 mmol) in Et₂O (15 mL) cooled

= 16.0 Hz, 18 H) ppm. ¹³C NMR (125 MHz, C₆D₆): δ = 152.6,

to –20 °C, LAH (400 mg, 10 mmol) was added portionwise. The

reaction mixture was stirred at 0 °C for 15 min, then warmed to

20 °C. After an additional 30 min the mixture was frozen, H₂O

(1 mL) and 10% NaOH (3 mL) were added and the mixture was

stirred at 20 °C for 30 min. The precipitate was filtered off under

argon and washed with Et₂O three times. The filtrate was concen-

139.1 (d, J = 1.25 Hz), 132.7, 128.1, 126.2, 121.4, 119.6 (d, J =

1.25 Hz), 119.2, 67.5, 41.5 (d, J = 45.0 Hz), 40.0, 27.4 (d, J =

2.5 Hz) ppm. ³¹P NMR (81 MHz, C₆D₆): δ = 91.3 ppm. {¹H} ³¹P

NMR (121 MHz, C₆D₆): δ = 92.7 ppm. C₁₈H₂₈N₃PSe (396.37):

calcd. C 54.54, H 7.12, N 10.60, P 7.81; found C 54.47, H 7.15, N

10.56, P 7.64.

trated under reduced pressure and the residue was distilled (b.p.

100–110/0.05 Torr) to give 24 (1.18 g, 90%) as an oil. ¹H NMR

Reaction of Carbene 7c with Triethylamine Hydrochloride

(300 MHz, C₆D₆): δ = 8.04–8.00 (dm, J = 8.2 Hz, 1 H), 7.29–7.26

1-Isopropyl-1H-benzimidazole Hydrochloride: To a solution of carb-

(dm, J = 9.0 Hz, 1 H), 7.17–7.07 (m, 2 H), 4.76 (m, 1 H), 4.41 (d,

ene 7c (1.34 g, 4.4 mmol) in benzene (3 mL), triethylamine hydro-

J_P–H= 221.1 Hz, 1 H), 1.25 (d, J = 13.2 Hz, 9 H), 1.14 (dd, J =

chloride (580 mg, 4.2 mmol) was added and the mixture was stirred

6.9, 14.1 Hz, 6 H) ppm. ¹³C NMR (125 MHz, C₆D₆): δ = 151.55

(d, J = 15 Hz), 146.1 (d, J = 4 Hz), 134.0, 122.1, 121.6, 120.7, 111.7,

at 20 °C for 30 min. The solvent was removed under reduced pres-

sure and the slurry residue was extracted with hot hexane

(2ϫ12 mL). The organic extract was concentrated under reduced

49.4 (d, J = 12.8 Hz), 30.1 (d, J = 12.8 Hz), 30.0, 20.8 (d, J =

15 Hz) ppm. ³¹P NMR (81 MHz, C₆D₆): δ = 41.4 (d, J_P–H

=

pressure and the residue was dried and distilled (b.p. 100–102 °C/

0.05 Torr) to give the crude 1-isopropyl-1H-benzimidazole. ¹H

NMR (300 MHz, CDCl₃): δ = 7.99 (s, 1 H), 7.45–7.42 (m, 1 H),

7.30–7.27 (m, 2 H), 7.83–7.80 (m, 1 H), 4.64 (m, 1 H), 1.62 (d, J

= 6.6 Hz) ppm. The obtained product was dissolved in Et₂O

(15 mL) and 2% HCl solution in Et₂O (2 g) was added. The solid

formed after cooling at –18 °C was filtered to give the title hydro-

225 Hz) ppm. C₁₄H₂₁N₂P (248.31): calcd. C 67.72, H 8.52, N 11.28,

P 12.47; found C 67.77, H 8.45, N 11.15, P 12.59.

X-ray Diffraction Study: X-ray diffraction studies were performed

with an automatic “Bruker-Nonius” diffractometer (graphite-mo-

nochromated Mo-K_αradiation, CCD-detector, ω-scanning). The

structures were solved by direct methods using the SHELXTL

package.^[17]Positions of hydrogen atoms were located from electron

density difference maps and refined using a riding model with U_iso

= nU_eqof the carrier atom (n = 1.5 for methyl and hydroxy group

and for water molecule and n = 1.2 for other hydrogen atoms),

except the H-atoms of the NH groups for the 13 and 19a molecules,

which were refined within isotropic approximation. The crystallo-

graphic data and experimental parameters are listed in Table 4.

CCDC deposition numbers given in Table 4 contain the supple-

mentary crystallographic data for this paper. These data can be

obtained free of charge from The Cambridge Crystallographic

Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

1

chloride (750 mg, 87 %); m.p. 145–146 °C. H NMR (300 MHz,

CDCl₃): δ = 10.2 (s, 1 H), 8.09–8.05 (m, 1 H), 7.69–7.65 (m, 1 H),

7.59–7.56 (m, 2 H), 5.31 (s, 1 H), 5.00 (m. 1 H), 1.826 (d, J =

6.9 Hz) ppm.

2-(Diphenylphosphanyl)-1-isopropyl-1H-benzimidazole (22): At

20 °C, to a solution of carbene 7c (1.1 g, 3.62 mmol) in Et₂O

(10 mL), diphenylchlorophosphane (860 mg, 3.9 mmol) was added

by using a pipette. The reaction mixture was stirred at 20 °C for

1 h, then concentrated under reduced pressure. The residue was

distilled and a second crop was collected to give crude 22, which

was purified by crystallization from pentane (20 mL); yield 720 mg

(75%); pale-yellow solid; m.p. 118–119 °C. ¹H NMR (300 MHz,

CDCl₃): δ = 7.82–7.79 (m, 1 H), 7.53–7.46 (m, 5 H), 7.37–7.34 (m,

6 H), 7.24–7.19 (m, 2 H), 5.17 (m, 1 H), 1.475 (d, J = 6.9 Hz, 6

H) ppm. ¹³C NMR (125 MHz, CDCl₃): δ = 153.7 (d, J = 6 Hz),

145.4 (d, J = 4 Hz), 134.33 (d, J = 36 Hz), 134.31 (d, J = 20 Hz),

129.5, 128.7 (d, J = 7.5 Hz), 122.6, 121.7, 121.0, 111.9, 49.5 (d, J

= 2.5 Hz), 21.2 ppm. ³¹P NMR (81 MHz, CDCl₃): δ = –24.3 ppm.

C₂₂H₂₁N₂P (344.40): calcd. C 76.73, H 6.15, N 8.13, P 8.99; found

C 76.65, H 6.11, N 8.07, P 9.08.

Method of Calculations: The analysis of intramolecular interactions

in 7a using Natural Bonding Orbitals (NBO) theory^[10]and Bader’s

“Atoms in Molecules” (AIM) approach^[12]was performed on non-

optimized molecules. The geometry of the analyzed molecule was

obtained from X-ray diffraction data. Positions of hydrogen atoms

were normalized in idealized positions in compliance with those

obtained from neutron diffraction data.

The input data for NBO analysis were obtained from single point

calculations by the M06-2X/cc-pvtz method^[18,19]using the

Gaussian 09 program.^[20]The interactions with participation of the

lone pair of the C1 atom were investigated by using NBO 5.0 pro-

gram.^[21]For each donor NBO (i) and acceptor NBO (j), the stabili-

zation energy E(2) associated with delocalization (“2e stabiliza-

tert-Butyl(1-isopropyl-1H-benzimidazol-2-yl)phosphinous Chloride

(23): To a frozen solution of tert-butyldichlorophosphane (2.70 g,

17 mmol) in Et₂O (25 mL), whilst stirring, a solution of carbene 7c

(3.0 g, 10 mmol) in Et₂O was added dropwise over 30 min. The

reaction mixture was warmed to 20 °C over 4 h, then the solvent

was removed at 40–45 °C under reduced pressure. The oil residue

was extracted with boiling pentane (35 mL) in a stoppered flask.

The mixture was cooled to –18 °C overnight and the settled solid

was filtered off under argon. The filtrate was concentrated under

f(i,j)²

tion”) iǞj is estimated as: E(2) = ΔE_ij= q_i

where q_iis the

෈_j– ෈

i

donor orbital occupancy, ෈ and ෈ are the diagonal elements (or-

j

i

bital energies), and F(i,j) is the off-diagonal NBO Fock matrix ele-

ment.

reduced pressure and the residue was distilled (b.p. 115–120 °C/

An analysis of the electron density distribution was carried out

1

0.05 Torr) to give 23 (1.53 g, 54%) as a light-brown oil. H NMR within Bader’s “Atoms in Molecules” (AIM)^[12]approach using the

14

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Stable N-Heterocyclic Carbenes

Table 4. The crystallographic data and experimental parameters for compounds 7a, 13, 15a, 17b and 18a.

15a

7a

13

17b

18a

Unit cell

a [Å]

b [Å]

c [Å]

α [°]

β [°]

20.1527(3)

18.6509(8)

90

12.0458(3)

10.5179(3)

13.0347(3)

90

106.164(2)

90

8.5447(8)

13.859(1)

16.318(1)

106.924(4)

100.687(5)

90.090(5)

1813.4(3)

704

20.1758(7)

11.1498(3)

16.8028(5)

90

10.693(2)

11.989(2)

16.567(3)

90

γ [°]

V [Å³]

7574.7(4)

2816

1586.17(7)

600

3779.9(2)

1504

2123.9(7)

816

F(000)

Crystal system

Space group

Z

tetragonal

I4(1)/a

16

monoclinic

P2₁/c

4

triclinic

P1

4

orthorhombic

Pca2₁

8

orthorhombic

P2₁2₁2₁

4

¯

T [K]

173

296

0.164

1.157

53

296

0.259

1.228

60

296

μ [mm^–1]

0.147

1.127

60

0.268

1.196

53

0.236

1.181

53

D_calcd.[g/cm³]

2Θ_max[°]

Measured reflections

Independent reflec-

29038

5810

13472

3231

26401

7604

27483

9505

11665

4347

tions

R_int

0.049

4081

0.029

2673

0.091

4543

0.035

8357

0.087

2840

Reflections with F Ͼ

4σ(F)

Parameters

212

179

409

430

238

R₁

wR₂

S

0.047

0.125

1.023

853138

0.032

0.0859

1.029

853139

0.071

0.159

1.016

853135

0.039

0.094

1.020

853136

0.070

0.152

0.989

853137

CCDC number

shall, T. K. Prakasha, J. Am. Chem. Soc. 1997, 119, 12742–

12749.

M06-2X/cc-pvtz wave function. The characteristics of the (3,–1)

critical points for the intramolecular hydrogen bonds were calcu-

lated by using the AIM2000 program^[22]with all default options.

The energies of the nonbonded interactions can be estimated by

using the characteristics of the (3,–1) critical points in accordance

with Espinosa’s formula.^[22]

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Received: March 7, 2012

Published Online: ᭿

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Job/Unit: O20282

/KAP1

Date: 11-06-12 15:51:22

Pages: 17

Stable N-Heterocyclic Carbenes

Stable Carbenes

A number of approaches to N-alkyl-NЈ-

phosphanylbenzimidazolium salts were de-

veloped. These salts were used for the

preparation of stable N-alkyl-NЈ-phos-

phanylbenzimidazol-2-ylidenes. Chemical

properties of both the salts and the carb-

enes with O-, N-, and C-nucleophiles were

studied. Both salts and the carbenes are

potential ligands for polynuclear com-

plexes.

A. P. Marchenko, H. N. Koidan,

A. N. Hurieva, I. I. Pervak,

S. V. Shishkina, O. V. Shishkin,

A. N. Kostyuk* ............................... 1–17

Stable N-Heterocyclic Carbenes: N-Alkyl-

NЈ-phosphanylbenzimidazol-2-ylidenes

Keywords: Carbenes

/ Nitrogen hetero-

cycles / Ligand design / Structure eluci-

dation

Eur. J. Org. Chem. 0000, 0–0

www.eurjoc.org

17

Article Doi

Stable N-heterocyclic carbenes: N-alkyl-N'-phosphanylbenzimidazol-2- ylidenes

DOI: 10.1002/ejoc.201200282

Source and publish data:

Authors:

Article abstract of DOI:10.1002/ejoc.201200282

Full text of DOI:10.1002/ejoc.201200282

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