NbCl5-Mg reagent system in regio-and stereoselective synthesis of (2Z)-alkenylamines and (3Z)-alkenylols from substituted 2-alkynylamines and 3-alkynylols

Article abstract of DOI:10.3390/molecules26123722

The reduction of N,N-disubstituted 2-alkynylamines and substituted 3-alkynylols using the NbCl₅ –Mg reagent system affords the corresponding dideuterated (2Z)-alkenylamine and (3Z)-alkenylol derivatives in high yields in a regio-and stereoselective manner through the deuterolysis (or hydrolysis). The reaction of substituted propargylamines and homopropargylic alcohols with the in situ generated low-valent niobium complex (based on the reaction of NbCl₅ with magnesium metal) is an efficient tool for the synthesis of allylamines and homoallylic alcohols bearing a 1,2-disubstituted double bond. It was found that the well-known approach for the reduction of alkynes based on the use of the TaCl₅-Mg reagent system does not work for 2-alkynylamines and 3-alkynylols. Thus, this article reveals a difference in the behavior of two reagent systems—NbCl₅-Mg and TaCl₅-Mg, in relation to oxygen-and nitrogen-containing alkynes. A regio-and stereoselective method was developed for the synthesis of nitrogen-containing E-β-chlorovinyl sulfides based on the reaction of 2-alkynylamines with three equivalents of methanesulfonyl chloride in the presence of stoichiometric amounts of niobium(V) chloride and magnesium metal in toluene.

Full text of DOI:10.3390/molecules26123722

molecules
Article
NbCl5-Mg Reagent System in Regio- and Stereoselective
Synthesis of (2Z)-Alkenylamines and (3Z)-Alkenylols from
Substituted 2-Alkynylamines and 3-Alkynylols
Rita N. Kadikova *, Azat M. Gabdullin, Oleg S. Mozgovoj, Ilfir R. Ramazanov and Usein M. Dzhemilev
Institute of Petrochemistry and Catalysis of Russian Academy of Sciences, 141 Prospekt Oktyabrya, 450075 Ufa,
Russia; saogabdullinsao@gmail.com (A.M.G.); skill15@mail.ru (O.S.M.); ilfir.ramazanov@gmail.com (I.R.R.);
ink@anrb.ru (U.M.D.)
* Correspondence: kadikritan@gmail.com
Abstract: The reduction of N,N-disubstituted 2-alkynylamines and substituted 3-alkynylols using
the NbCl5–Mg reagent system affords the corresponding dideuterated (2Z)-alkenylamine and (3Z)-
alkenylol derivatives in high yields in a regio- and stereoselective manner through the deuterolysis (or
hydrolysis). The reaction of substituted propargylamines and homopropargylic alcohols with the in
situ generated low-valent niobium complex (based on the reaction of NbCl5 with magnesium metal)
is an efficient tool for the synthesis of allylamines and homoallylic alcohols bearing a 1,2-disubstituted
double bond. It was found that the well-known approach for the reduction of alkynes based on
the use of the TaCl5-Mg reagent system does not work for 2-alkynylamines and 3-alkynylols. Thus,
this article reveals a difference in the behavior of two reagent systems—NbCl5-Mg and TaCl5-Mg,
in relation to oxygen- and nitrogen-containing alkynes. A regio- and stereoselective method was
developed for the synthesis of nitrogen-containing E-β-chlorovinyl sulfides based on the reaction of
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
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Citation: Kadikova, R.N.; Gabdullin,
A.M.; Mozgovoj, O.S.; Ramazanov,
I.R.; Dzhemilev, U.M. NbCl5-Mg
Reagent System in Regio- and
Stereoselective Synthesis of
2-alkynylamines with three equivalents of methanesulfonyl chloride in the presence of stoichiometric
amounts of niobium(V) chloride and magnesium metal in toluene.
(2Z)-Alkenylamines and
Keywords: niobium(V) chloride; 2-alkenylamines; substituted 2-alkynylamines; magnesium metal;
homopropargylic alcohols; methanesulfonyl chloride; chlorothiolation
(3Z)-Alkenylols from Substituted
2-Alkynylamines and 3-Alkynylols.
Molecules 2021, 26, 3722. https://
doi.org/10.3390/molecules26123722
Academic Editor: Andrea Penoni
1. Introduction
The use of magnesium metal as a reducing agent for the generation of low-valent
complexes of transition metals is well-described for numerous reactions giving titanocene
Received: 18 May 2021
Accepted: 13 June 2021
Published: 18 June 2021
and zirconocene alkyne complexes. Rosenthal and Burlakov [
on the synthesis of titanocene and zirconocene alkyne complexes based on the pioneering
works done by Vol’pin and Shur [ ]. The synthesized transition metal alkyne complexes
are of value because these metallacyclopropenes are a source of reactive metallocenes
serving as precursors for the synthesis of various organic compounds [ 18]. As regards
1,2] have performed studies
3,4
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1,4–
the synthesis of niobium alkyne complexes, the chemistry of low-valent niobium com-
plexes with unsaturated substrates is poorly known. Thus, methods for the synthesis of
niobiacyclopropene complexes based on functionalized acetylenic compounds are lack-
ing. Investigations of the chemistry of low-valent niobium often involve related tantalum
complexes due to the similarity of their electronic and chemical properties. Niobium
and tantalum alkyne complexes, as representatives of Group V metal complexes, were
synthesized for the first time by Cotton and coworkers based on the reaction of sterically
bulky alkynes with M2Cl6(THT)3 (M = Ta, Nb; THT is tetrahydrothiophene) giving dimeric
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Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
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4.0/).
MV complexes [19,20]. The synthesis of mononuclear niobium alkyne trihalides by the
reactions of acetylenes with low-valent niobium complexes such as NbX3(DME) (X = Cl,
Br) was reported by Roskamp [21]. The preparation of related trihaloniobium alkyne
complexes by the reaction of alkynes with NbX3(DME), generated by the treatment of
Molecules 2021, 26, 3722. https://doi.org/10.3390/molecules26123722
https://www.mdpi.com/journal/molecules

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niobium(V) halides with tributyltin hydride, were described in the publications [21,22].
Five-membered niobacycles were prepared by the reaction of niobium alkyne complexes
with allyl Grignard [23]. Niobium- and tantalum-containing complexes are of interest for
organic synthesis as a simple and effective tool for obtaining amino alcohols [21], metal-
locyclic carbonyl derivatives [24], metallacyclic enamines [25] and vicinal diamines [26].
Low-valent niobium- and tantalum-containing complexes are known as effective catalysts
for oligo- and polymerization of disubstituted acetylenes [20,27,28], chemoselective cy-
cloaddition of alkynes to olefins [29 31] and nitriles [32 33]. One of the most commonly
–
,
used approaches to the generation of low-valent transition metal compounds involves the
reduction of high-valent metal salts with such metals as Zn, Mg, Al or Na/Hg amalgam.
The synthesis of niobium alkyne complexes by the reduction of high-valent niobium salts
with metals was performed using Al [34–36], Na/Hg amalgam [37–41] or Zn [42] as re-
ducing agents. Based on the approaches to the preparation of niobium-alkyne complexes
known in the literature, it can be assumed that the reduction reactions of alkynes using
low-valence niobium complexes may be of interest for the selective preparation of olefinic
compounds [43–47]. However, strangely enough, there are no published examples of the
reduction of functionalized acetylenic compounds and non-functionalized alkynes using
the reaction of alkynes with low-valent niobium complexes, generated by the treatment of
niobium(V) chloride with magnesium metal. Meanwhile, the approach to the synthesis
of tantalum alkyne complexes from tantalum(V) chloride and magnesium metal is well-
developed [48]. To develop a new effective method for stereoselective reduction of alkynes
using low-valent niobium complexes, we put forward the idea of the possibility of synthe-
sizing intermediate niobiumcyclopropenes by reacting NbCl5 with metallic magnesium in
the presence of 1,2-disubstituted acetylenes to obtain the target corresponding niobiumcy-
clopropenes. Hence, the goal of this work is to study the reaction of alkynes having different
structures with the Mg–NbCl5 reagent system. Previously, we have developed a selective
method for the EtMgBr–NbCl5-catalyzed reduction of substituted 2-alkynylamines with
Et2Zn [49]. A limitation of this method is the impossibility of obtaining reduction products
from acetylenic alcohols under organozinc synthesis conditions. Taking into account the
stability of the amine function in the discovered niobium–magnesium-catalyzed trans-
formation of tertiary 2-alkynylamines, it was of interest to study first and foremost the
transformation of propargylamines using low-valent niobium, generated by the reaction of
niobium(V) chloride with magnesium metal. Thus, this work is devoted to the study of the
transformations of acetylene substrates under the action of the reagent system-Mg-NbCl5.
2. Results
We found that the reaction of four equiv. NbC15 with three equiv. of Mg in a 1:1
mixture of benzene and 1,2-dimethoxyethane (DME) at room temperature for 40 min
followed by the addition of 2-alkynylamines
the formation of reduction products 2a and 3c,e in high yields (73–89%) in a regio- and
1
and by the heating at 40 ◦C for 7 h leads to
–h
stereoselective manner through the hydrolysis (Scheme 1). Therefore, the presence of a
nitrogen atom in the alkyne molecules does not hinder the formation of a niobium alkyne
complex. We have now studied behavior of N-aryl-substituted 2-alkynylamines derivatives
in the investigated reaction. We also found that the reduction of N,N-dibenzylnon-2-yn-1-
amine 1g under the action of the reagent system-Mg-NbCl5 affords reduction product 2g in
yields of 88% (Scheme 1). Thus, the presence of a benzyl substituent at the nitrogen atom in
the structure of tertiary propargylamine 1g does not prevent the reduction of the triple bond
using the NbCl5-Mg reagent system. It follows from the performed experiment that the
C-N-bond of the N-benzyl fragment is stable under the action of complexes of low-valent
niobium. N-(Hept-2-yn-1-yl)-N-phenylaniline was not converted by the NbCl5-Mg reagent
system. However, this NbCl5-Mg reagent system proved to be effective for the reduction
of secondary 2-alkynyl amine that has a hydrogen atom at a nitrogen atom. In this way the
reaction of N-(3-phenylprop-2-yn-1-yl)butan-1-amine 1h with reagent system-Mg-NbCl5
affords reduction product 2h in high yield (89%).

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Scheme 1. Reduction of substituted 2-alkynylamines to (Z)-2-alkenylamines with the Mg–NbCl5
reagent system. See the Supplementary Materials for details.
The structural identification of the synthesized compounds was performed by 1D
and 2D NMR spectroscopy. Based on the NOESY correlations, the Z configuration of the
double bond was established for all hydrolysis products 2a
spectra show nuclear Overhauser effects between the methylene group (
sp2-hybridized carbon atom of the double bond and phenyl protons (
is indicative of the formation of diastereomer 2f with the Z configuration of the double
bond. The absence of signals of carbon atoms at
123.39–137.80 in the 13C NMR spectra of
–
2h. In particular, the NOESY
3.29–3.31) at the
7.20–7.38), which
δ
δ
δ
deuterolysis products 3c and 3e attests to the presence of two deuterium atoms at the double
bond. The presence of two vicinal deuterium atoms at the double bond in the dideuterated
allylamines may be indicative of the formation of the niobiacyclopropene intermediate
A
(Scheme 2). The reduction of niobium(V) chloride to a niobium(III) compound with
mercury-activated magnesium metal was described in the study [50]. The formation
of a niobium(III) complex was also observed in the reaction of Nb( -C5Me5)Cl4 with
ŋ
magnesium metal [51]. On the other hand, the TaCl5-catalyzed carbomagnesiation of
alkenes with n-alkyl Grignard reagents is initiated by the generation of the tantalum(III)
chloride alkene complex [52–54]. Hence, we suggested that the reaction of niobium(V)
chloride with magnesium metal also leads to the generation of a low-valent niobium(III)
complex. The further coordination of the latter by a propargylamine molecule affords
the organoniobium intermediate
A
(Scheme 2). The deuterolysis of the complex
A gives
dideuterated (2Z)-alkenylamine
B. The reaction of 4-(non-2-yn-1-yl)morpholine 1a with
20 mol.% of NbCl5 and three equivalents of magnesium metal in a 1:1 mixture of benzene
◦
and 1,2-dimethoxyethane (DME) at room temperature, followed by heating to 40 C
for 7 h resulted in the formation of reduction product in minor amounts. Apparently
niobiumcyclopropenes can be considered reaction intermediates. The hydrolysis of the
latter gives allylamines.

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Scheme 2. Putative mechanism of the reduction of 2-alkynylamines 1 with the Mg–NbCl5 reagent
system.
Initially, we performed experiments in a two-component solvent system consisting
of benzene and 1,2-dimethoxyethane. It is well known that the generation of Ta and
Nb alkyne complexes via the reactions of TaCl5 with Mg and of NbCl5 with Zn in a
mixture of benzene and DME occurs with high selectivity and in high yields [48]. We
used this two-component solvent system taking into account good solubility of tantalum
and niobium halides and the corresponding organometallic intermediates in a mixture of
benzene and 1,2-dimethoxyethane. Besides, 1,2-dimethoxyethane is commonly employed
as the solvent in the generation of niobium alkyne complexes due to the stabilizing effect
of DME molecules on niobium complexes through the coordination of the oxygen atoms of
1,2-dimethoxyethane to niobium. This imparts high thermal stability to Nb and Ta alkyne
complexes and makes the transformation of Group V metal alkyne complexes into different
classes of organic compounds easy and simple, as opposed to related Ti and Zr complexes.
Titanium(II) and zirconium(II) alkyne complexes are the best-known low-valent early
transition metal complexes with alkynes. However, many alkyne complexes involving
TiII and ZrII are thermally unstable and cannot be used in further reactions [33]. It should
also be noted that NbCl3(DME) is a thermally stable and commercially available complex.
Such low-valent niobium species have been used both as reagents and as catalysts in
organic transformations. We studied the effect of the solvent nature on the possibility
of the reduction of 2-alkynylamines under the organoniobium synthesis conditions. The
possibility of the reduction of the triple bound under the conditions of organoniobium
synthesis in different solvents was investigated in relation to the reaction of 4-(non-2-yn-1-
yl)morpholine 1a with 4 equiv. of NbCl5 and 3 equiv. of Mg (Table 1).

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Table 1. Effect of the solvent nature on the reduction of 2-alkynylamines with the NbCl5–Mg reagent
system a.
Entry
Solvent
DME b: Benzene b
Et2O b: Benzene b
CH2Cl2 b: Benzene b
Toluene
Conversion of 2-Alkynylamine (%)
Yield of 2a (%)
1
2
3
4
5
6
>99
>80
>99
<5
89
81
72
4 (GC)
3 (GC)
<1 (GC)
Benzene
<5
DME
<5
a
reaction conditions: i. NbCl5 (4 equiv.), Mg (3 equiv.), solvent, r.t., 40 min. ii. 2-alkynylamine (1 equiv.), 40 ◦C,
b
7 h. iii. 25 wt% KOH solution. solvent 1: solvent 2 (1:1).
The reduction of 4-(non-2-yn-1-yl)morpholine was found to occur efficiently not only
in the DME–benzene solvent system but also in the diethyl ether–benzene system giving al-
lylamine 2a in 81% yield (entry 2). It is worth noting that the commonly employed methods
for the reduction of non-functionalized alkynes with TaCl5–Zn, TaCl5–Mg and NbCl5–Zn
reagent systems are generally accomplished in DME–benzene or DME–toluene solvent
systems [48,55]. Also, 2-Alkynylamine is efficiently reduced to 2a in a dichloromethane–
benzene mixture (Entry 3). Therefore, the reduction of 2-alkynylamines with the NbCl5–Mg
reagent system does not require the presence of an ethereal solvent. However, the use
a polar chlorine-containing solvent is accompanied by an insignificant decrease in the
yield of the reduction product due to side reactions giving an unidentified mixture of
high-molecular-weight compounds, which are apparently polymerization products of
2-alkynylamine involving low-valent niobium complexes (according to the gas chromato-
graphic analysis, the yield of 2a was 72%; entry 3). Meanwhile, in the reaction using pure
toluene, benzene or DME as the solvent, the conversion of the starting propargylamine was
less than 5%, and 2b was obtained in trace amounts (entries 4, 5 and 6). The observed low
conversion of propargylamine in the reaction under consideration performed in aromatic
solvents is apparently attributed to poor solubility of chlorine-containing niobium com-
plexes in toluene and benzene. It was found that 2-alkynylamines are not reduced with the
TaCl5–Mg reagent system. The reaction of 4-(non-2-yn-1-yl)morpholine 1a with four equiv.
of TaCl5 and three equiv. of Mg in a 1:1 benzene–DME solvent mixture by the heating
◦
to 40 C for 7 h gives the reduction product in trace amounts. Therefore, the presence
of the oxygen-containing morpholyl substituent in 2-alkynylamine does not hinder the
selective reduction of the triple bond (Scheme 1). In the next step of our study concerning
the reduction of functionalized acetylenic compounds under the organoniobium synthesis
conditions, we investigated the reaction of substituted acetylenic alcohols with the NbCl5–
Mg reagent system. We failed to reduce the triple bond in such propargylic alcohols as
non-2-yn-1-ol and 3-phenylprop-2-yn-1-ol under the organoniobium synthesis conditions.
The starting acetylenic substrates and their transformation products were not detected in
the reaction mixture by gas chromatography. These experimental data suggest that the
transformation of propargylic alcohols is accompanied by the oligo- or polymerization.
Apparently, in the case of propargylic alcohols, the formation of high-molecular-weight
compounds is attributed to the polymerization of niobium-containing allene, which is
generated via the β-elimination of the alkoxide group of the niobiacyclopropene interme-
diate. Previously, we have found that the Zr-catalyzed cycloalumination of substituted
propargylic alcohols was also accompanied by the polymerization as the side reaction
caused by the
tuted homopropargylic alcohols
mixture of benzene and DME performed at room temperature for 8 h affords reduction
products 5a in high yields (73–92%) in a regio- and stereoselective manner through the
β-elimination of the alkoxide group [56]. However, the reaction of substi-
4
with four equiv. of NbCl5 and three equiv. of Mg in a 1:1
–e
hydrolysis (Scheme 3). It should be emphasized that the reduction of homopropargylic
alcohols is inhibited when using TaCl5 instead of NbCl5. The conversion of dec-3-yn-1-ol 5a

Molecules 2021, 26, 3722
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in the reduction reaction under the conditions presented in Scheme 3 is less than 5%. As can
be seen in Scheme 3, the transformation of homopropargylic alc◦ohols
4 into unsaturated
alcohols
5 does not require warming of the reaction mixture to 40 C, as opposed to propar-
gylamines
1 (Scheme 1). Therefore, 3-alkynylols are more reactive than 2-alkynylamines in
the reduction of alkynes with the NbCl5–Mg reagent system. Apparently, the more distal
position of the hydroxyl substituent not only promotes the selective reduction of alkinols
under the action of NbCl5-Mg, preventing β-elemination of the hydroxyl group, but also
an increase in reactivity in comparison with 2- alkynyl amines. However, this system is
unsuitable for the reduction of dialkyl-substituted acetylenes. The reaction of 5-decyne (or
diphenylacetylene) with NbCl5(4 equiv.) and Mg (3 equiv.) in a mixture of benzene and
DME (1:1) by the heating to 40 ◦C for 7 h gives the reduction products in trace amounts.
Scheme 3. Reduction of substituted 3-alkynols with the NbCl5–Mg reagent system.
In order to further study transformations of acetylenic compounds under the organon-
iobium synthesis conditions, we made attempts to develop efficient reagents for transforma-
tions of niobiacyclopropene intermediates. Previously, we have demonstrated that sulfonic
acid derivatives, such as sulfonyl halides, sulfonic acid silyl esters, alkylthiosulfonates, or-
ganic disulfides and diselenides are efficient electrophilic reagents for the functionalization
of organoaluminum compounds, such as 1-alkenylalanes, which can be used to prepare
the corresponding alkenyl halides [57], alkenylsilanes [58], alkenyl sulphides [59,60] and
alkenyl selenides [61] in high yields under mild conditions. We expected that sulfonyl
halides would also be reactive toward niobiacyclopropene intermediate generated in this
reaction. However, all our attempts to perform the reaction of niobiacyclopropene, pre-
pared by the reduction of 1a (Scheme 1), with methanesulfonyl chloride, failed. Thus, the
reaction of the organoniobium intermediate (prepared by the reduction of 4-(non-2-yn-1-
yl)morpholine 1a with four equiv. of NbCl5 and three equiv. of Mg in a mixture of benzene
and DME) with three equiv. of methanesulfonyl chloride (MsCl) at room temperature
followed by alkaline hydrolysis afforded, within 7 h, exclusively reduction product 2a in
82% yield. Meanwhile, the reaction in a 1:1 mixture of diethyl ether and benzene also did
not lead to the desired functionalization of the organoniobium intermediate with MsCl.
As described above, 2-alkynylamines can be reduced with the NbCl5–Mg reagent system
in a mixture of dichloromethane and benzene (Table 1, entry 3). The reaction with three
equivalents of methanesulfonyl chloride (MsCl) in a 1:1 mixture of dichloromethane and
benzene, performed at room temperature for 8 h, gives (E)-4-(2-chloro-3-(methylthio)non-
2-en-1-yl)morpholine 6a as the chlorothiolation product in 18% isolated yield. In this case,
the formation of the chlorothiolation product with the E configuration of the double bond
and the absence of cross-coupling products with MsCl in two-component solvents, such as
DME–benzene and Et2O–benzene mixtures, cast doubt on the involvement of the putative
niobiacyclopropene intermediate in the chlorothiolation. We found that the reaction of
4-(non-2-yn-1-yl)morpholine 1a with four equiv. of NbCl5 and three equiv. of Mg in toluene

Molecules 2021, 26, 3722
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followed by the addition of three equiv. of MsCl at room temperature is accompanied by
the formation of (E)-4-(2-chloro-3-(methylthio)non-2-en-1-yl)morpholine 6a in 78% yield
within 8 h through the alkaline hydrolysis. The conversion of the starting propargylamine
1a was 88% (Scheme 4).
Scheme 4. Chlorothiolation of alkynes with the NbCl5–Mg reagent system.
An increase in the reaction time and the use of a higher concentration (6 equiv.) of
methanesulfonyl chloride did not lead to the complete conversion of the starting alkyne.
However, as described above, the reduction of 4-(non-2-yn-1-yl)morpholine 1a with the
NbCl5–Mg reagent system is inhibited in toluene (entry 4, Table 1). The observed inertness
of 2-alkynylamine in the reduction reaction under consideration in toluene is an addi-
tional argument in favor of the suggestion that the niobiacyclopropene intermediate is
not involved in the reaction with MsCl and that the observed transformation proceeds
through another nontrivial mechanism. It was found that stoichiometric amounts of NbCl5
and Mg are required for the transformation of the starting propargylamine using MsCl.
Meanwhile, the transformation of the starting alkyne requires the addition of MsCl (three
equiv.) to a mixture of 2-alkynylamine, NbCl5, and magnesium metal in toluene pre-
heated at 40 ◦C for 4 h. Apparently, the increase in the temperature to 40 ◦C is necessary
for the generation of the low-valent metal complex reactive in the chlorothiolation of
propargylamines. For example, it is known that the formation of NbCl4 in the reaction of
niobiu◦m(V) halides with aluminum metal under reduced pressure requires a temperature
of 250 C [62]. Here, we report the experimental data on the chlorothiolation of alkynes only
for propargylamines. Besides, we found that the reactions of 5-decyne and 4-octyne with
methanesulfonyl chloride in toluene afford p-tolylthiolation products, which is indicative
of the direct involvement of the solvent molecules in the reaction. Hence, the reaction of
sulfonyl chlorides having different structures with other heteroatom-containing alkynes
and non-functionalized alkynes requires a special study.
Scheme 5 shows the reaction pathway of the chlorothiolation of 2-alkynylamines.
Apparently, the sulfonic group is reduced to the sulfide moiety with the NbCl5–Mg reagent
system (Scheme 5, Equation (1)). According to the literature data [63–65], the addition of
the sulfonyl radical to the triple bond is the key step in the Cu-catalyzed chlorothiolation
of acetylenes and the FeCl2-catalyzed chlorosulfonation of 1-alkynyl chlorides [66]. Hence,
we suggested that the observed transformation can be initiated by the complex “NbCl4”
which is generated in the reduction of NbCl5 with magnesium metal in toluene (Scheme
5, Equation (2)). The paramagnetic complex “NbCl4” A can, in turn, initiate the S–Cl
bond homolysis of sulfenyl chloride giving the sulfenyl radical and NbCl5. Monomeric
A,
B
low-valent niobium coordination compounds, such as NbCl4L2, are known to be param-
agnetic [67]. On the other hand, as can be seen in Table 1, the reaction of 2-alkynylamine
1a with the NbCl5–Mg reagent system in toluene afforded reduction product 2a in minor
amounts (chromatography mass-spectrometry data), which is indicative of the inhibition

Molecules 2021, 26, 3722
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of the pathway toward the putative niobiacyclopropene intermediate. The addition of the
sulfenyl radical
B
to 2-alkynylamine is accompanied by the formation of the alkyl radical
-haloalkenyl sulfide accompanied by the regeneration of
C
. The latter is transformed into
β
“NbCl4”. A similar trans-addition of sulfonyl chlorides to the triple bond was described in
the literature [63,68,69].
NbCl5 − Mg
MeSO2Cl −−−−−−−→ MeSCl
(1)
(2)
NbCl5+Mg → “NbCl4” + MgCl2
Scheme 5. Chlorothiolation of alkynes with the NbCl5–Mg reagent system.
The new method for the synthesis of
β-haloalkenyl sulfides, along with the elec-
trophilic chlorothiolation of terminal alkynes by sulfenyl chloride [70], bromothiolation
with sulfenyl bromine [71], and the Pd- and Fe-catalyzed regio- and stereoselective addition
of sulfenyl chlorides to terminal acetylenes [72,73], is of high value because the halogen
atom can be easily transformed into different alkenyl sulfide derivatives.
3. Materials and Methods
3.1. General Information
The reagents were obtained from Sigma-Aldrich or Acros. Dichloromethane were dis-
tilled over P2O5. Diethyl ether, benzene, toluene and 1,2-dimethoxyethane were dried over
sodium. 2-Alkynylamines 1a
with aqueous formaldehyde and secondary amines under CuI catalysis [74]. 3-Alkynylols
were prepared by the reaction of alkynylmagnesium reagents with ethylene oxide [75].
–h were prepared by aminomethylation of terminal alkynes
4
IR spectra were recorded on Bruker VE Vertex 70v spectrometer as liquid films or in Nujol
and are reported in wavenumbers (cm−1). Nuclear magnetic resonance spectroscopy was

Molecules 2021, 26, 3722
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performed on a Brucker Avance 500. The 1H NMR spectra were recorded at 500 MHz and
13C-{1H} NMR spectra at 125 MHz in CDCl3. The chemical shifts are reported in ppm
relative to tetramethylsilane (TMS) as the internal standard. The numbering of atoms in
the 13C-{1H} and 1H NMR spectra of the compounds 2a
–h, 3c,e, 5a–e, 6a–d is shown in
Figures 1–3. Elemental analysis was performed using a Carlo-Erba CHN 1106 elemental
analyzer. Mass spectra were obtained on a Finnigan 4021 instrument. The yields were
calculated from the isolated amount of allylamines obtained from starting 2-alkynylamines.
Figure 1. The numbering of atoms in the 13C- and 1H-NMR spectra of the compounds 2a–h, 3c,e.

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Figure 2. The numbering of atoms in the 13C- and 1H-NMR spectra of the compounds 5a–e.
Figure 3. The numbering of atoms in the 13C- and 1H-NMR spectra of the compounds 6a–d.
3.2. Preparation of (Z)-2-Alkenylamines 2a–h, 3c,e via Reduction of Substituted 2-Alkynylamines
via Mg-NbCl5
(Z)-4-(non-2-en-1-yl)morpholine; Typical Procedure
In a 50-mL reaction flask was placed NbC15 (2160 mg, 8 mmol) under an argon
atmosphere. To the salt benzene was added at room temperature (12 mL) and DME
(12 mL), successively. Magnesium powder (144 mg, 6 mmol) was added to the stirring pale
yellow solution of NbC15 and the resulting mixture was stirred at room temperature for
40 min. To the mixture was added at room temperature of a 4-(non-2-yn-1-yl)morpholine
(418 mg, 2.0 mmol) and the whole mixture was stirred at 40 ◦C 7 h. After 7 h at 40 ◦C, the
reaction mixture was diluted with Et2O (20 mL), and 25 wt% KOH solution (15 mL) was
added dropwise while the reaction flask was cooled in an ice bath. The aqueous layer was

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extracted with diethyl ether (3
brine (20 mL), dried over anhydrous MgSO4. The reaction mixture was filtered through a
filter paper and concentrated in vacuo to give crude product as a yellow oil. The residue
×
20 mL). The combined organic layers were washed with
was distilled through a micro column at 2.4 mmHg to give 2a (321 mg, 76%) as a colorless
1
oil. b.p. 126–127 ◦C (2.4 mmHg). H NMR (500 MHz, CDCl3):
δ = 0.86 (t, J = 6 Hz, 3H,
C(13)H3), 1.22–1.28 (m, 6H, C(10–12)H2), 1.29–1.32 (m, 2H, C(4)H2), 2.04 (q, J = 7 Hz, 2H,
C(3)H2), 2.43 (s, 4H, C(6, 7)H2), 2.98 (d, J = 7 Hz, 2H, C(5)H2), 3.69 (t, J = 3 Hz 4H, C(8,
9)H2), 5.39–5.44 (m, 1H, C(1)H), 5.52–5.57 (m, 1H, C(2)H). 13C NMR (125MHz, CDCl3):
δ = 14.04 (C(13)), 22.59 (C(12)), 27.49 (C(3)), 28.90 (C(10)), 29.46 (C(4)), 31.68 (C(11)), 55.47
(C(5)), 53.61 (C(6, 7)), 66.99 (C(8, 9)), 125.32 (C(1)), 133.75 (C(2)). MS (EI): m/z, % = 211 (3)
[M+], 126 (5), 87 (100), 86 (40), 57 (30), 40 (15). Anal. calcd for C13H25NO, (%): C, 73.88; H,
11.92; N, 6.63; Found, %: C, 74.83; H, 11.88; N, 6.57.
(Z)-1-(hept-2-en-1-yl)piperidine (2b)
Using the procedure described above 358 mg of 1-(hept-2-yn-1-yl)piperidine (2 mmol)
gave crude product that was distilled through a micro column at 3.4 mmHg to afford 2b
1
(304 mg, 84%) as a colorless oil. b.p. 107–110 ◦C (3.4 mmHg). H NMR (500 MHz, CDCl3):
δ
= 0.90 (t, J = 7 Hz, 3H, C(11)H3), 1.32–1.35 (m, 4H, C(4, 10)H2), 1.44 (s, 2H, C(12)H2),
1.59–1.63 (m, 4H, C(8, 9)H2), 2.06 (q, J = 6 Hz, 2H, C(3)H2), 2.42 (s, 4H, C(6, 7)H2), 3.01
(d, J = 6 Hz, 2H, C(5)H2), 5.46–5.51 (m, 1H, C(1)H), 5.52–5.57 (m, 1H, C(2)H). 13C NMR
(125 MHz, CDCl3):
δ = 13.96 (C(11)), 22.31 (C(10)), 24.26 (C(12)), 25.81 (C(8, 9)), 27.19 (C(3)),
31.72 (C(4)), 54.34 (C(6, 7)), 55.68 (C(5)), 125.91 ((C(1)), 133.15 (C(2)). MS (EI): m/z, % = 181
(7) [M+], 138 (4), 124 (10), 98 (29), 84 (100), 55 (30), 41 (15). Anal. calcd for C12H23N, (%): C,
79.49; H, 12.79; N, 7.72; Found, %: C, 79.45; H, 12.83; N, 7.52.
(Z)-4-(hept-2-en-1-yl)morpholine (2c)
Using the procedure described above 362 mg of 4-(hept-2-yn-1-yl)morpholine (2 mmol)
gave crude product that was distilled through a micro column at 5 mmHg to afford 2c
1
(326 mg, 89%) as a colorless oil. b.p. 109–111 ◦C (5 mmHg). H NMR (500 MHz, CDCl3):
δ
= 0.82 (t, J = 6 Hz, 3H, C(11)H3), 1.25–1.29 (m, 4H, C(4, 10)H2), 1.99 (q, J = 7 Hz, 2H,
C(5)H2), 3.64 (s, 4H, C(8, 9)H2), 5.35–5.39 (m, 1H, C(1)H), 5.47–5.52 (m, 1H, C(2)H). 13
NMR (125 MHz, CDCl3): = 13.85 (C(11)), 22.21 (C(10)), 27.12 (C(3)), 31.61 (C(4)), 55.36
C
δ
(C(5)), 53.49 (C(6, 7)), 66.83 (C(8, 9)), 125.17 ((C(1)), 133.68 (C(2)). MS (EI): m/z, % = 183 (10)
[M+], 140 (4), 110 (28), 87 (100), 57 (70), 41 (21). Anal. calcd for C11H21NO, (%): C, 72.08; H,
11.55; N, 7.64; Found, %: C, 72.22; H, 11.56; N, 7.37.
(Z)-4-(3-cyclopropylallyl)morpholine (2d)
Using the procedure described above 330 mg of 4-(3-cyclopropylprop-2-yn-1-yl)morpholine
(2 mmol) gave crude product that was distilled through a micro column at 4 mmHg to
1
afford 2d (267 mg, 80%) as a colorless oil. b.p. 91–93 ◦C (4 mmHg). H NMR (500 MHz,
CDCl3):
δ = 0.29–0.32 (m, 2H (A), C(4, 10)H2), 0.69–0.74 (m, 2H (B), C(4, 10)H2), 1.55–1.57
(m, 1H, C(3)H), 2.46 (s, 4H, C(6, 7)H2), 3.09 (d, J = 7 Hz, 2H, C(5)H2), 3.69 (t, J = 4 Hz, 4H,
C(8, 9)H2), 5.32–5.37 (m, 1H, C(1)H), 4.89 (t, J = 10 Hz, 1H, C(2)H). 13C NMR (125 MHz,
CDCl3):
δ = 6.97 (C(4, 10)), 9.78 (C(3)), 53.62 (C(6, 7)), 55.86 (C(5)), 67.01 (C(8, 9)), 123.39
(C(1)), 137.80 (C(2)). MS (EI): m/z, % = 167 (10) [M+], 138 (33), 87 (70), 79 (87), 56 (69), 40
(100). Anal. calcd for C10H17NO, (%): C, 71.81; H, 10.25; N, 8.37; Found, %: C, 71.98; H,
10.35; N, 8.35.
(Z)-4-(3-phenylallyl)morpholine (2e)
Using the procedure described above 330 mg of 402 mg of 4-(3-phenylprop-2-yn-1-
yl)morpholine (2 mmol) gave crude product that was distilled through a micro column at
◦
1
1 mmHg to afford 2e (305 mg, 75%) as a colorless oil. b.p. 130–132 C (1 mmHg). H NMR
(500 MHz, CDCl3): = 2.49 (s, 4H, C(6, 7)H2), 3.29–3.31(dd, J = 2 Hz, J = 6 Hz, 2H, C(5)H2),
δ
3.75 (t, J = 4 Hz, 4H, C(8, 9)H2), 5.78–5.83 (m, 1H, C(1)H), 6.63 (d, J = 11 Hz, 1H, C(2)H),
7.26–7.29 (m, 3H, C(4, 10, 12)H), 7.37 (t, J = 8 Hz, 2H, C(11, 13)H). 13C NMR (125 MHz,
CDCl3):
δ = 53.71 (C(6, 7)), 59.05 (C(5)), 67.03 (C(8, 9)), 126.97 (C(12)), 128.17 (C(11, 13)),
128.89 (C(4, 10)), 129.00 (C(1)), 131.69 (C(2)), 137.00 (C(3)). MS (EI): m/z, % = 203 (20) [M+],

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172 (4), 144 (12), 117 (72), 112 (100), 91 (33), 56 (32). Anal. calcd for C13H17NO, (%): C, 76.81;
H, 8.43; N, 6.89; O, 7.87; Found, %: C, 76.90; H, 8.37; N, 7.01.
(Z)-1-(3-phenylallyl)piperidine (2f)
Using the procedure described above 398 mg of 1-(3-phenylprop-2-yn-1-yl)piperidine
(2 mmol) gave crude product that was distilled through a micro column at 1 mmHg to
1
afford 2f (293 mg, 73%) as a colorless oil. b.p. 122–124 ◦C (1 mmHg). H NMR (500 MHz,
CDCl3):
J = 6 Hz, 2H, C(5)H2), 5.82–5.87 (m, 1H, C(1)H), 6.57 (d, J = 12 Hz, 1H, C(2)H), 7.24–7.28 (m,
3H, C(4, 10, 12)H), 7.33–7.37 (m, 2H, C(11, 13)H). 13C NMR (125 MHz, CDCl3):
= 24.29
δ = 1.45 (C(14)), 1.59–1.64 (m, 4H, C(8, 9)H2), 2.42 (s, 4H, C(6, 7)H2), 3.28 (d,
δ
(C(14)), 26.01 (C(8, 9)), 54.70 (C(6, 7)), 57.12 (C(5)), 126.75 (C(12)), 128.09 (C(11, 13)), 128.91
(C(4, 10)), 130.31 (C(1)), 130.75 (C(2)), 137.29 (C(3)). MS (EI): m/z, % = 201 (12) [M+], 200
(15), 117 (44), 115 (38), 110 (100), 98 (30), 84 (12). Anal. calcd for C14H19N, (%): C, 83.53; H,
9.51; N, 6.96; Found, %: C, 83.61; H, 9.47; N, 7.12.
(Z)-4-(hept-2-en-1-yl-2,3-d2)morpholine (3c)
Using the procedure described above 362 mg of 4-(hept-2-yn-1-yl)morpholine (2 mmol)
and D2O gave crude product that was distilled through a micro column at 2.4 mmHg to
afford 3c (303 mg, 82%) as a colorless oil. b.p. 118–120 ◦C (2.4 mmHg). IR (liquid film):
2958, 2925, 2870, 2855, 2811, 1742, 1742, 1654, 1618, 1519, 1508, 1456, 1399, 1118, 1070, 1034,
1
1008, 943, 866, 801 cm−1. H NMR (500 MHz, CDCl3):
δ = 0.91 (t, J = 6 Hz, 3H, C(11)H3),
1.34–1.38 (m, 4H, C(4, 10)H2), 2.08 (t, J = 6 Hz, 2H, C(3)H2), 2.47 (s, 4H, C(6, 7)H2), 3.03 (s,
2H, C(5)H2), 3.74 (s, 4H, C(8, 9)H2). 13C NMR (125 MHz, CDCl3):
δ
= 13.96 (C(11)), 22.32
(C(10)), 27.09 (C(3)), 31.69 (C(4)), 53.61 (C(6, 7)), 55.36 (C(5)), 67.01 (C(8, 9)). MS (EI): m/z,
% = 185 (7) [M+], 156 (1), 128 (6), 112 (19), 87 (100), 57 (70), 57 (70), 42 (13). Anal. calcd for
C11H19D2NO, (%): C, 71.30; N, 7.56; Found, %: C, 71.46; N, 7.42.
(Z)-4-(3-phenylallyl-2,3-d2)morpholine (3e)
Using the procedure described above 402 mg of 4-(3-phenylprop-2-yn-1-yl)morpholine
(2 mmol) and D2O gave crude product that was distilled through a micro column at
◦
1 mmHg to afford 3e (316 mg, 77%) as a colorless oil. b.p. 130–132 C (1 mmHg). IR (liquid
film): 3021, 2959, 2926, 2855, 2807, 2759, 1519, 1493, 1454, 1316, 1294, 1216, 1117, 1007, 778,
1
700, 669, 598, 472 cm−1. H NMR (500 MHz, CDCl3):
δ = 2.49 (s, 4H, C(6, 7)H2), 3.29 (s, 2H,
C(5)H2), 3.74 (t, J = 4 Hz, 4H, C(8, 9)H2), 7.26–7.29 (m, 3H, C(4, 10, 12)H), 7.37 (t, J = 8 Hz,
2H, C(11, 13)H). 13C NMR (125 MHz, CDCl3):
δ
= 53.72 (C(6, 7)), 56.51 (C(5)), 67.02 (C(8,
9)), 126.97 (C(12)), 128.17 (C(11, 13)), 128.89 (C(4, 10)), 136.93 (C(3)). MS (EI): m/z, % = 205
(21) [M+], 204 (14), 146 (9), 119 (73), 113 (100), 86 (19), 56 (32). Anal. calcd for C13H15D2NO,
(%): C, 76.06; N, 6.82. Found, %: C, 76.13; N, 6.95.
(Z)-N,N-dibenzylnon-2-en-1-amine (2g)
Using the procedure described above 638 mg of N,N-dibenzylnon-2-yn-1-amine
(2 mmol) and H2O gave crude product that was distilled through a micro column at
◦
1
1 mmHg to afford 2g (567 mg, 88%) as a colorless oil. b.p. 193–195 C (1 mmHg). H NMR
(500 MHz, CDCl3): = 0.97 (t, J = 4 Hz, 3H, C(17)H3), 1.33–1.43 (m, 8H, C(4, 5, 14, 15)H2),
δ
2.04–2.08 (m, 2H, C(3)H2), 3.14–3.16 (m, 2H, C(5)H2), 3.64 (s, 4H, C(6, 7)H2), 5.62–5.65
(m, 2H, C(1, 2)H), 7.28–7.32 (m, 2H, C(11, 110)H), 7.37–7.39 (m, 4H, C(10, 12, 100, 120)H),
7.44–7.48 (m, 4H, C(9, 13, 90, 130)H). 13C NMR (125 MHz, CDCl3):
δ = 14.18 (C(17)), 22.71
(C(16)), 27.65 (C(3)), 29.04 (C(4)), 29.66 (C(14)), 31.80 (C(15)), 50.13 (C(5)), 58.03 (2C(6, 7)),
126.82 (3C (1, 11, 110)), 128.19 (4C (10, 12, 100, 120), 128.87 (4C (9, 13, 90, 130), 133.22 (C(2)),
0
139.88 (2C (8, 8 )). MS (EI): m/z, % = 322 (<1) [M+], 232 (3), 210 (59), 181 (4), 91 (100), 65 (4).
Anal. calcd for C23H31N, (%): C, 85.92; H, 9.72; N, 4.36. Found, %: C, 86.86; H, 9.68; N, 4.10.
(Z)-N-(3-phenylallyl)butan-1-amine (2h)
Using the procedure described above 374 mg of N-(3-phenylprop-2-yn-1-yl)butan-1-
amine (2 mmol) and H2O gave crude product that was distilled through a micro column at
◦
1
3 mmHg to afford 2h (336 mg, 89%) as a colorless oil. b.p. 130–132 C (3 mmHg). H NMR
(500 MHz, CDCl3): = 0.93 (t, J = 7 Hz, 3H, C(9)H3), 1.32–1.39 (m, 2H, C(8)H2), 1.46–1.52
δ

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(m, 2H, C(7)H2), 2.65 (t, J = 7 Hz, 2H, C(6)H2), 3.57 (d, J = 6 Hz, 2H, C(5)H2), 5.78–5.83 (m,
1H, C(1)H), 6.55 (d, J = 12 Hz, 1H, C(2)H), 7.25–7.28 (m, 3H, C(4, 10, 12)H), 7.34–7.37 (t,
J = 7 Hz, 2H, C(11, 13)H). 13C NMR (125 MHz, CDCl3):
δ = 13.99 (C(9)), 20.48 (C(8)), 32.14
(C(7)), 47.71(C(5)), 49.34 (C(6)), 126.89 (C(12)), 128.17 (2C(11, 13)), 128.78 (2C(4, 10)), 130.41
(C(2)), 131.15 (C(1), 137.12 (C(3)). MS (EI): m/z, % = 189 (7) [M+], 146 (10), 117 (100), 91 (14),
84 (10). Anal. calcd for C13H19N, (%): C, 82.48; H, 10.12; N, 7.40. Found, %: C, 82.35; H,
10.01; N, 7.33.
3.3. Preparation of (3Z)-Alkenylols 5a–e via Reduction of Substituted Alkynylols via Mg-NbCl5
(Z)-dec-3-en-1-ol; Typical Procedure
In a 50-mL reaction flask was placed NbC15 (2160 mg, 8 mmol) under an argon
atmosphere. To the salt was added at room temperature benzene (12 mL) and DME (12 mL),
successively. Magnesium powder (144 mg, 6 mmol) was added to the stirring pale yellow
solution of NbC15 and the resulting mixture was stirred at room temperature for 40 min.
To the mixture was added at room temperature of a dec-3-yn-1-ol (616 mg, 3.0 mmol) and
the whole mixture was stirred at room temperature 8 h. After 8 h at room temperature, the
reaction mixture was diluted with Et2O (20 mL), and 25 wt% KOH solution (15 mL) was
added dropwise while the reaction flask was cooled in an ice bath. The aqueous layer was
extracted with diethyl ether (3
brine (20 mL), dried over anhydrous MgSO4. The reaction mixture was filtered through a
filter paper and concentrated in vacuo to give crude product as a yellow oil. The residue
×
20 mL). The combined organic layers were washed with
was distilled through a micro column at 4 mmHg to give 5a (275 mg, 88%) as a colorless oil.
1
b.p. 99–101 ◦C (4 mmHg). H NMR (500 MHz, CDCl3):
δ = 0.90 (t, J = 6 Hz, 3H, C(10)H3),
1.27–1.29 (m, 8H, C(4, 7, 8, 9)H2), 2.04–2.06 (m, 2H, C(3)H2), 2.35 (q, J = 7 Hz, 2H, C(5)H2),
3.66 (t, J = 6 Hz, 2H, C(6)H2), 5.36–5.41 (m, 1H, C(1)H), 5.56–5.61 (m, 1H, C(2)H). 13C NMR
(125 MHz, CDCl3):
δ = 14.09 (C(10)), 22.63 (C(9)), 27.38 (C(3)), 29.67 (C(4)), 29.70 (C(7)),
30.90 (C(5)), 31.75 (C(8)), 62.37 (C(6)), 124.93 (C(1)), 133.58 (C(2)). MS (EI): m/z, % = 170 (3)
[M+], 111 (11), 95 (25), 83 (47), 69 (74), 55 (100), 41 (46). Anal. calcd for C10H20O, (%): C,
76.86; H, 12.90; Found, %: C, 77.04; H, 12.83.
(Z)-non-3-en-1-ol (5b)
Using the procedure described above 280 mg of non-3-yn-1-ol (2 mmol) gave crude
product that was distilled through a micro column at 10 mmHg to afford 5b (256 mg, 90%)
as a colorless oil. b.p. 93–95 ◦C (10 mmHg). The spectral properties (1H NMR, 13C NMR,
MS) were in good agreement with those that were reported in the literature [76].
(Z)-oct-3-en-1-ol (5c)
Using the procedure described above 252 mg of oct-3-yn-1-ol (2 mmol) gave crude
product that was distilled through a micro column at 14 mmHg to afford 5c (210 mg, 82%)
◦
as a colorless oil. b.p. 88–90 C(14 mmHg). 1H NMR (500 MHz, CDCl3):
δ = 0.93 (t,
J = 7 Hz, 3H, C(8)H3), 1.39–1.44 (m, 2H, C(7)H2), 1.47–1.53 (m, 2H, C(4)H2), 2.19 (t, J = 7 Hz,
2H, C(3)H2), 2.44–2.47 (m, 2H, C(5)H2), 3.69 (br. s, 2H, C(6)H2), 5.36–5.40 (m, 2H, C(1)H),
5.53–5.61 (m, 1H, C(2)H). 13C NMR (125 MHz, CDCl3):
δ = 13.61 (C(8)), 18.92 (C(3)), 21.96
(C(7)), 23.19 (C(5)), 31.07 (C(4)), 61.39 (C(6)), 124.93 (C(1)), 133.56 (C(2)). Anal. calcd for
C8H16O, (%): C, 74.94; H, 12.58; Found, %: C, 75.12; H, 12.67.
(Z)-dodec-3-en-1-ol (5d)
Using the procedure described above 362 mg of dodec-3-yn-1-ol (2 mmol) gave crude
product that was distilled through a micro column at 2 mmHg to afford 5d (339 mg, 92%)
as a colorless oil. b.p. 115–117 ◦C (2 mmHg). The spectral properties (1H NMR, 13C NMR,
MS) were in good agreement with those that were reported in the literature [77].
(Z)-4-phenylbut-3-en-1-ol (5e)
Using the procedure described above 292 mg of 4-phenylbut-3-yn-1-ol (2 mmol) gave
crude product that was distilled through a micro column at 1.8 mmHg to afford 5e (216 mg,
1
73%) as a colorless oil. b.p. 117–121 ◦C (1.8 mmHg). H NMR (500 MHz, CDCl3):
δ = 2.64

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(q, J = 7 Hz, 2H, C(5)H2), 3.74 (t, J = 6 Hz, 2H, C(6)H2), 5.71–5.76 (m, 1H, C(1)H), 6.61 (d,
J = 12 Hz, 1H, C(2)H), 7.23–7.29 (m, 1H, C(9)H), 7.33–7.39 (m, 4H, C(4, 7, 8, 10)H). 13C NMR
(125 MHz, CDCl3):
δ = 32.02 (C(5)), 62.37 (C(6)), 126.83 (C(8)), 128.46 (C(1)), 131.36 (C(2)),
128.26 (C(8, 10)), 128.78 (C(4, 7)), 137.32 (C(3)). MS (EI): m/z, % = 148 (40) [M+], 117 (100),
115 (95), 104 (89), 91 (66), 77 (9), 65 (15). Anal. calcd for C10H12O (%): C, 81.04; H, 8.16.
Found, %: C, 80.73; H, 8.19.
3.4. Preparation of (3Z)-Alkenylols 6a–d via Chlorothiolation of Alkynes with the NbCl5–Mg
Reagent System
(E)-4-(2-chloro-3-(methylthio)non-2-en-1-yl)morpholine; Typical Procedure
In a 50-mL reaction flask was placed NbC15 (2160 mg, 8 mmol) under an argon
atmosphere. To the salt was added at room temperature benzene (12 mL) and DME (12 mL)
successively. Magnesium powder (144 mg, 6 mmol) and a 4-(non-2-yn-1-yl)morpholine
(418 mg, 2.0 mmol) were added to the stirring pale yellow solution of NbC15 the resulting
mixture was stirred at 40 ◦C for 4 h. After 4 h at 40 ◦C to the mixture was added at room
temperature of a methanesulfonyl chloride (690 mg, 6.0 mmol) and the whole mixture
was stirred at room temperature 8 h. After 8 h at room temperature, the reaction mixture
was diluted with Et2O (20 mL), and 25 wt% KOH solution (15 mL) was added dropwise
while the reaction flask was cooled in an ice bath. The aqueous layer was extracted with
diethyl ether (3
×
20 mL). The combined organic layers were washed with brine (20 mL),
dried over anhydrous MgSO4. The reaction mixture was filtered through a filter paper
and concentrated in vacuo to give crude product as a yellow oil. The residue was distilled
through a micro column at 1.6 mmHg to give 6a (456 mg, 78%) as a colorless oil. b.p. 97–100
1
◦C (1.6 mmHg). H NMR (500 MHz, CDCl3):
δ = 0.91 (t, J = 7 Hz, 3H, C(13)H3), 1.33 (s,
6H, C(12, 11, 10)H2), 1.56–1.60 (m, 2H, C(4)), 2.36 (s, 3H, C(14)H3), 2.52 (t, J = 7 Hz, 4H,
C(6, 7)H2), 2.74 (t, J = 7 Hz, 2H, C(3)H2), 3.42 (s, 2H, C(5)H2), 3.73 (t, J = 4 Hz, C(8, 9)). 13C
NMR (125 MHz, CDCl3):
δ = 14.05 (C(13)), 16.35 (C(14)), 22.57 (C(12)), 27.49 (C(4)), 28.50
(C(10)), 31.58 (C(11)), 37.23(C(3)), 53.19 (C(6, 7)), 59.09 (C(5)), 67.08 (C(8, 9)), 127.42 (C(2)),
136.95(C(1))MS (EI): m/z, % = 292 (<1) [M+], 291 (2), 189 (<1), 100 (100), 86 (7). Anal. calcd
for C14H26ClNOS, (%): C, 57.41; H, 8.98; N, 4.80. Found, %: C, 57.74; H, 8.83; N, 4.89.
(E)-1-(2-chloro-3-(methylthio)hept-2-en-1-yl)piperidine (6b)
Using the procedure described above 358 mg of 1-(hept-2-yn-1-yl)piperidine (2 mmol)
gave crude product that was distilled through a micro column at 2 mmHg to afford 6b (419
1
mg, 80%) as a colorless oil. b.p. 97–99 ◦C (2 mmHg). H NMR (500 MHz, CDCl3):
δ = 0.96
(t, J = 7 Hz, 3H, C(11)H3), 1.37 (q, J = 7 Hz, 2H, C(10)H2), 1.44–1.47 (m, 2H, C(12)), 1.55–1.61
(m, 6H, C(4, 8, 9)H2), 2.36 (s, 3H, C(13)H3), 2.43 (s, 4H, C(6, 7)H2), 2.74 (t, J = 7 Hz, 2H,
C(3)H2), 3.36 (s, 2H, C(5)H2). 13C NMR (125 MHz, CDCl3):
δ = 13.95 (C(11)), 16.35 (C(13)),
22.02 (C(10)), 24.45 (C(12)), 26.06 (C(8, 9)), 29.72 (C(4)), 36.98 (C(3)), 54.13 (C(6, 7)), 59.63
(C(5)), 128.52 (C(2)), 135.89 (C(1)). MS (EI): m/z, % = 262 (1) [M+], 261 (4), 178 (1), 98 (100),
84 (12). Anal. calcd for C13H24ClNS, (%): C, 59.63; H, 9.24; N, 5.35. Found, %: C, 59.78; H,
9.41; N, 5.59.
(E)-4-(2-chloro-3-(methylthio)hept-2-en-1-yl)morpholine (6c)
Using the procedure described above 362 mg of 4-(hept-2-yn-1-yl)morpholine (2 mmol)
gave crude product that was distilled through a micro column at 1.9 mmHg to afford 6c
1
(375 mg, 71%) as a colorless oil. b.p. 128–130 ◦C (1.9 mmHg). H NMR (500 MHz, CDCl3):
δ = 0.96 (t, J = 7 Hz, 3H, C(11)H3), 1.34–1.39 (m, 2H, C(10)H2), 1.55–1.61 (m, 2H, C(4)H2),
2.36 (s, 3H, C(12)H3), 2.52 (s, 4H, C(6, 7)H2), 2.75 (t, J = 7 Hz, 2H, C(3)H2), 3.42 (s, 2H,
C(5)H2), 3.74 (s, 4H, C(8, 9)H2). 13C NMR (125 MHz, CDCl3):
δ = 13.92 (C(11)), 16.35
(C(12)), 22.00 (C(10)), 29.69 (C(4)), 36.99 (C(3)), 53.17 (C(6, 7)), 59.07 (C(5)), 67.06 (C(8, 9)),
127.41 (C(2)), 136.89 (C(1)). MS (EI): m/z, % = 264 (13) [M+], 263 (65), 230 (18), 228 (19), 176
(10), 101 (98), 100 (100), 86 (47), 56 (29). Anal. calcd for C12H22ClNOS, (%): C, 54.63; H, 8.41;
N, 5.31. Found, %: C, 54.78; H, 8.21; N, 5.29.

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4. Conclusions
To conclude, we have demonstrated for the first time that the NbCl5–Mg reagent
system is an efficient tool for the regio- and stereoselective reduction of substituted
2-alkynylamines and 3-alkynylols to the corresponding (2Z)-alkenylamines and (3Z)-
alkenylols. However, substituted homopropargylic alcohols and 2-alkynylamines do
not undergo a transformation when treated with stoichiometric amounts of tantalum(V)
chloride and magnesium metal. Thus, we have demonstrated for the first time the efficiency
of the NbCl5-Mg reagent system for the reduction of 2-alkynylamines and 3-alkynylols. It
was found that a similar method known in the literature for the reduction of alkynes using
TaCl5-Mg does not work for such functionally substituted alkynes as 2-alkynylamines
and 3-alkynylols. We also reported the first results of the study of the reaction between
2-alkynylamines and methanesulfonyl chloride in the presence of stoichiometric amounts
of NbCl5 and Mg. A regio- and stereoselective method was developed for the synthesis of
nitrogen-containing E-β-chlorovinyl sulfides based on the reaction of 2-alkynylamines with
methanesulfonyl chloride in the presence of stoichiometric amounts of niobium(V) chloride
and magnesium metal in toluene. In connection with the discovered selective reduction
of functionally substituted alkynes using the NbCl5-Mg reagent system, we plan to study
the reaction of phosphorus- and sulfur-containing alkynes with NbCl5 in the presence
of metallic magnesium. One of the interesting findings of this article is the reaction of
chlorothiolation of 2-alkynyl amines with methanesulfonyl chloride in the presence of stoi-
chiometric amounts of NbCl5-Mg. The observed transformation indicates the generation of
paramagnetic niobium complexes. This finding can serve as a basis for the development of
a new method for the synthesis of multisubstituted olefins using the reaction of reductive
coupling of carbonyl compounds under the action of paramagnetic niobium complexes.
The observed tolerance of nitrogen- and oxygen-containing substituents under the con-
ditions of organoniobium synthesis can serve as a basis for the development of effective
methods for the synthesis of functionalized polysubstituted olefins.
Supplementary Materials: The following are available online, analytical data and NMR spectra for
all compounds.
Author Contributions: Conceptualization and methodology, A.M.G. and O.S.M.; investigation and
writing—original draft preparation, R.N.K.; review and editing, I.R.R. and U.M.D. All authors have
read and agreed to the published version of the manuscript.
Funding: This work was financially supported by the Russian Science Foundation (grant No. 19-73-
10113).
Data Availability Statement: The data used to support the findings of this study are included within
the article and supplementary materials.
Acknowledgments: We acknowledge the Center of collective use of the unique equipment «Agidel»
at the Institute of Petrochemistry and Catalysis of the Russian Academy of Sciences.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are available from the authors.
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