Substrate structural effects in yttrium(III)-catalyzed hydroamination/ cyclizations of 1,2-disubstituted and 1,1,2-trisubstituted aminoalkenes terminated by 2-(phenyl) and 2-(2-heteroarenyl) groups

Article abstract of DOI:10.1055/s-0032-1317843

A series of 2-phenyl- and 2-(2-heteroarenyl)-bearing amines possessing 1,2-disubstituted and 1,1,2-trisubststuted alkenes have been evaluated in intramolecular hydroaminations catalyzed by Y[N(TMS)(1. Aminoalkenes possessing a terminal 2-(5-trimethylsilyl)thienyl group exhibited substantially enhanced reactivity compared to their 2-(phenyl)-containing counterparts. Cyclization efficiencies imparted by the 2-[(5-trimethylsilyl)furanyl] substituent were comparable or only slightly better than those obtained with the simple the 2-(phenyl) group. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0032-1317843

LETTER
▌193
lSetuterbstrate Structural Effects in Yttrium(III)-Catalyzed Hydroamination/
Cyclizations of 1,2-Disubstituted and 1,1,2-Trisubstituted Aminoalkenes
Terminated by 2-(Phenyl) and 2-(2-Heteroarenyl) Groups
Yttrium(III)-Catalyzed Hydroamination/Cyclization
Tao Jiang, Khoi Huynh, Tom Livinghouse*
Department of Chemistry and Biochemistry, 103 CBB, Montana State University, Bozeman, MT 59717, USA
Fax +1(406)9945407; E-mail: livinghouse@chemistry.montana.edu
Received: 03.10.2012; Accepted after revision: 06.11.2012
(E)-1-chloro-3-phenyl-2-propene followed by hydrolysis
Abstract: A series of 2-phenyl- and 2-(2-heteroarenyl)-bearing
amines possessing 1,2-disubstituted and 1,1,2-trisubststuted
alkenes have been evaluated in intramolecular hydroaminations cat-
alyzed by Y[N(TMS)2]3 (1Y). Aminoalkenes possessing a terminal
2-(5-trimethylsilyl)thienyl group exhibited substantially enhanced
reactivity compared to their 2-(phenyl)-containing counterparts.
Cyclization efficiencies imparted by the 2-[(5-trimethylsilyl)fura-
nyl] substituent were comparable or only slightly better than those
obtained with the simple the 2-(phenyl) group.
(H3O+) and reductive amination (NH4OAc, NaBH3CN).
Catalytic Hydroamination and Mechanistic Studies
Involving Aminoalkenes Terminated by the 2-(Phe-
nyl)ethenyl Group
We commenced this investigation by examining the hy-
droamination/cyclization of 3aE catalyzed by simple
group 3 tris(amide) compounds Y[N(TMS)2]3 (1Y) and
Sc[N(TMS)2]3 (1Sc). Exposure of 3aE to 1Y (10 mol%) in
C6D6 at 60 °C resulted in efficient cyclization over 2 h
to provide the corresponding pyrrolidine 4a (ca. 90%,
NMR; Scheme 1),3 which was isolated as the correspond-
ing 4-methylbenzenesulfonamide 4aTs in 83% yield. As
expected, the cyclization of 3aE catalyzed by 1Sc was con-
siderably more lethargic, giving 4a (90%, NMR) after ful-
ly 480 h at 60 °C.2c It is of interest that the rates of
hydroamination/cyclization were typically not linear
throughout the course of the reaction, but instead de-
creased as the reaction progressed. Accordingly, cycliza-
tion of 3aE in the presence of 1Y (10 mol%) required only
20 min at 60 °C to reach 50% conversion, whereas 1Sc (10
mol%) catalyzed 50% cyclization in 180 h under these
conditions. In light of the above results, all subsequent
studies focused on intramolecular hydroaminations cata-
lyzed by 1Y. The effect of alkene geometry on the rate of
cyclization was probed by subjecting 3aZ to 1Y (10 mol%,
C6D6, 60 °C). In this instance, a pronounced rate enhance-
ment was observed because conversion into 4a required
only 30 min (90%, NMR) furnishing 4aTs in 82% isolated
yield. For this substrate, 50% cyclization was achieved in
only 8 min under identical conditions (Scheme 1).
Key words: hydroamination, cyclization, yttrium, heteroarenes
The catalytic hydroamination/cyclization of alkenes con-
stitutes one of the most efficient synthetic routes to nitro-
gen-containing heterocycles. Whereas main-group metal
complexes have recently been used as catalysts for alkene
hydroamination,1a,b the most general cases for the synthe-
sis of amines and their derivatives by this important reac-
tion involve transition-metal catalysis using complexes of
rhodium,1c–e ruthenium,1f–g nickel,1h–j palladium,1k–q
gold1r–t as well as the group 32a–o and group 42p–z metals.
In contrast to catalysis by many complexes of the late
transition metals, hydroaminations involving complexes
of the group 3 metals are believed to proceed via insertion
of the C=C double bond into the metal-amido (M–N)
bond with overall syn-addition.2a,b It is also noteworthy
that the relative rates associated with intramolecular
hydroaminations by group 3 metals are correlated with
both the covalent radius of the metal (i.e., Y > Sc) and its
ligand environment.2a,c
In this communication, we document hydroamination/
cyclizations involving 1,1,2-trisubstituted alkenes and de-
scribe substantial rate enhancements that are obtained
when 2-[(5-trimethylsilyl)thienyl] groups are employed
as addends. In addition, we provide direct experimental
evidence for syn-metalloamination4 as well as the geomet-
rical dependence of cyclization efficiency. The amino-
alkenes 3a–g employed in this study were prepared by the
alkylation of the requisite allylic chlorides with lithioiso-
butyronitrile followed by reduction (LiAlH4). Amino-
alkene 3h was prepared by alkylation of the lithio
derivative of 2-propanone-N,N-dimethylhydrazone with
It is generally accepted that alkene hydroamination cata-
lyzed by complexes of the group 3 metals proceeds via
syn-addition of the metal-amido (M–N) bond to the C=C
double bond.2a,b,4 To probe this experimentally, amino-
alkenes 3aE(D2) and 3aZ(D2) were prepared by H–D ex-
change and subjected to cyclization catalyzed by 1Y
(C6D6, 10 °C). Under these conditions, 3aE(D2) provided
rac-(R,R)-4a(D2) in 3.5 days (83% NMR) and 3aZ(D2)
gave rac-(R,S)-4a(D2) in 2 days (>95% NMR). Subse-
quent conversion of rac-(R,R)-4a(D2) and rac-(R,S)-
4a(D2) into the corresponding 4-methylbenzenesulfon-
amides afforded rac-(R,R)-4a(D)Ts and rac-(R,S)-4a(D)Ts
in 76 and 85% yield, respectively. Experimental valida-
tion for the structural assignments of these diastereomers
was based on 1H NMR H–H coupling constants conferred
SYNLETT 2013, 24, 0193–0196
Advanced online publication: 12.12.2012
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DOI: 10.1055/s-0032-1317843; Art ID: ST-2012-R0847-L
© Georg Thieme Verlag Stuttgart · New York

194
T. Jiang et al.
LETTER
by hindered rotation arising from the N-tosyl substituent.
For rac-(R,R)-4a(D)Ts, the vicinal J value observed for the
benzylic proton was 9.5 Hz, which is consistent with that
expected for a pseudo-anti orientation. In contrast, for
rac-(R,S)-4a(D)Ts, the corresponding J value was 2.5 Hz,
as would be expected for a pseudo-syn disposition. In ad-
dition, the 2H NMR chemical shifts for rac-(R,R)-4a(D)Ts
and rac-(R,S)-4a(D)Ts were unique, appearing as singlets
at δ = 3.47 and 2.65 ppm, respectively (Scheme 1).
Y[N(SiMe3)2]3
(10 mol%)
NH2
NH
R
R
°C, t
3b: R = n-C5H11
3c: R = Ph
4b: 120 °C, 156 h, 90% (24 h, 50%))
4c: 60 °C, 57 h, 90% (7 h, 50%)
Y[N(SiMe3)2]3
NH2
(10 mol%)
NH
NH
+
Ph
Ph
Ph
60 °C
4htrans
(6:1)
4hcis
(9 d, 90%)
(3 d, 50%)
3h
M[N(SiMe3)2]3
NH2
(10 mol%)
NH
Ph
Ph
C6D6, 60 °C
Scheme 2 Hydroamination/cyclization of 1,1,2-trisubstituted ami-
noalkenes and diastereoselectivity studies
3aE
4a
(90%)
M = Y: 2 h
Y[N(SiMe3)2]3
(10 mol%)
M = Sc: 480 h
M = Y: 20 min
M = Sc: 180 h
NH2
(50%)
Catalytic Hydroamination Involving 1,2-Disubstituted
and 1,1,2-Trisubststuted Aminoalkenes Terminated
by 2-(2-Heteroarenyl) Groups
C6D6, 60 °C, 30 min (90%)
8 min (50%)
Ph
3aZ
We have previously shown that the incorporation of a 2-
(2-thienyl) moiety leads to a marked acceleration of
Sc(III)-catalyzed bicyclization of an aminodiene en-route
to the pheromone xenovine.2b The synthetic utility of 2-
heteroarenyl substituents is significant in that the thienyl
nucleus is readily convertible into the corresponding al-
kane by reductive desulfurization,2b and because the fura-
nyl group can serve as a synthon for a 1,4-dione through
acid-catalyzed hydrolysis.6
Y[N(SiMe3)2]3
ND2
Ts
N
(10 mol%)
ND
H
TsCl
Ph
Ph
Ph
C6D6, 10 °C
3.5 d
Py
3aE (D2)
H
D
D
rac-(R,R)-4a (D2) (83%)
rac-(R,R)-4a (D)Ts (76%)
Y[N(SiMe3)2]3
(10 mol%)
ND2
Ts
N
ND
TsCl
Py
Ph
Ph
C6D6, 10 °C
2 d
H
H
D
Ph
3aZ (D2)
D
rac-(R,S)-4a (D2) (>95%) rac-(R,S)-4a (D)Ts (85%)
In a set of studies to determine the efficacy of alternatives
to the simple 2-phenyl group as conjugative adjuvants in
simple hydroaminative ring-closures, we investigated the
utility of 2-[(5-trimethylsilyl)thienyl] and 2-[(5-trimethyl-
silyl)furanyl] substituents as potential aromatic facilita-
tors for Y(III)-catalyzed hydroamination/cyclization. To
this end, aminothiophenes 3d and 3f, and aminofurans 3e
and 3g were subjected to the standard protocol (1Y, 10
mol%, C6D6, 60 °C). In all cases the 2-[(5-trimethylsi-
lyl)thienyl] subunit proved to be superior to the corre-
sponding 2-[(5-trimethylsilyl)furanyl] group as assistants
in efficient ring closures to furnish pyrrolidines 4d,f and
4e,g, respectively.7a,b As with previous cases, rates of cy-
clization diminished over the course of the reaction, with
50% conversion being attained in 0.16, 1.0, and 8.0 h for
3e, 3f, and 3g, respectively (Table 1).
Scheme 1 Hydroamination/cyclization of aminostyrenes
We have previously demonstrated that simple, nonconju-
gated 1,2-disubstituted alkenes are among the least reac-
tive carbon–carbon double bonds toward intramolecular
hydroamination catalyzed by group 3 amide complex-
es.2b,d It is therefore of interest that nonconjugated 1,1,2-
trisubstituted alkenes have experienced limited evaluation
in the context of hydroamination/cyclization using these
catalysts.5 The latter objective was approached by subject-
ing 3b in toluene-d8 to 1Y (10 mol%) at 120 °C. Under
these admittedly forcing conditions, 3b was indeed con-
verted into 4b (90%, NMR), albeit after 156 hours, with
50% conversion being achieved in 24 hours. In sharp con-
trast, the conjugated 1,1,2-trisubstituted aminoalkene 3c
underwent relatively facile ring closure in the presence of
1Y (10 mol%, C6D6) at 60 °C in 57 hours to provide 4c
(90%, NMR). In this case, 50% conversion occurred in
only 7 hours. As would be expected, lower catalyst load-
ings resulted in diminished rates of product formation. For
3c, hydroamination/cyclization using 1Y (5 mol%) re-
quired 5 days (50% conversion, NMR). In an experiment
designed to reveal the diastereoselectivity associated with
hydroamination/cyclization involving a simple 2-(phe-
nyl)ethenyl substituent, 3h was treated with 1Y (10 mol%,
C6D6) at 60 °C to provide 4htrans and 4hcis (t/c = 6:1, 90%,
NMR). Under identical conditions, 50% conversion was
observed in 3 days. The results of this study are summa-
rized in Scheme 2.
As expected, cyclization of 3f in the presence of 1Y (5
mol%) required fully 2 hours (50% conversion) and 12
hours (90% conversion, NMR). It is of considerable inter-
est that although the 2-[(5-trimethylsilyl)thienyl] group
imparts a markedly enhanced predisposition toward hy-
droamination/cyclization than its simple 2-(phenyl) coun-
terpart, the 2-[(5-trimethylsilyl)furanyl] substituent is
roughly comparable in reactivity to the latter.
We have shown that structurally diverse 1,1,2-trisubstitut-
ed alkenes can serve as effective participants in intramo-
lecular hydroaminations catalyzed by Y[N(TMS)2]3 (1Y),
particularly when used in conjugation with a 2-(trimethyl-
silyl)thienyl substituent. Cyclization efficiency for
a representative aminoalkene possessing a Z-configured
Synlett 2013, 24, 193–196
© Georg Thieme Verlag Stuttgart · New York

LETTER
Yttrium(III)-Catalyzed Hydroamination/Cyclization
195
Table 1 Hydroamination/Cyclization of Amines Tethered to 2-(2-Heteroarenyl)ethenyl Groups
Aminoalkene
Pyrrolidine
Time
(h)a,b
Time
(h)a
Yield by NMR Isolated yield
(%)c
(%)d
NH2
NH
SiMe3
SiMe3
SiMe3
S
–
0.5
1.5
4
95
87
S
4d
3d
NH2
NH
SiMe3
O
0.16
95
95
92
76
O
4e
3e
NH2
NH
SiMe3
S
SiMe3
SiMe3
1
S
3f
4f
NH2
NH
SiMe3
O
8
57
83
68
O
3g
4g
a All reactions were conducted at 60 °C.
b Time required for 50% conversion.
c Integration relative to p-xylene as an internal standard.
d Isolated as the p-toluenesulfonamide.
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Acknowledgment
Generous financial support from the National Institute of General
Medical Science and the National Science Foundation is gratefully
acknowledged.
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Supporting Information for this article is available online at
http://www.theme-connect.com/ejournals/toc/synlett.SunpfgIopi
o
nr
itSartnuIpofoirpmntgirat
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 193–196

196
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LETTER
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added and the reactant mixture was subsequently held at
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ylmethyl]tetrahydropyrrole (4fTs): Tetrahydropyrrole 4f
was prepared from (E)-2,2,4-trimethyl-5-[5-(trimethylsilyl)-
2-thienyl]pent-4-enylamine (3f; 28 mg, 0.10 mmol) by the
general hydroamination procedure. The Teflon screw cap
was then removed and the crude product was diluted with
anhydrous CH2Cl2 (3 mL). TsCl (22 mg, 0.12 mmol) and
pyridine (9.70 μL, 0.12 mmol) were added in succession.
The reactant mixture was stirred at room temperature for 12
h, then the reactant mixture was diluted with Et2O (10 mL),
washed with saturated NaHCO3 (3 mL) and brine (3 mL),
and the organic phase was subsequently dried with Na2SO4.
Concentration in vacuo followed by flash chromatography
on silica gel (hexane–EtOAc, 15:1) afforded 2,4,4-
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(3) All 1H NMR yields are based on integration relative to p-
xylene as the internal standard.
trimethyl-1-(toluene-4-sulfonyl)-2-[5-
(trimethylsilyl)thiophene-2-ylmethyl]tetrahydropyrrole
(4fTs; 33.2 mg, 76%). 1H NMR (500 MHz, CDCl3): δ = 7.75
(d, J = 8.0 Hz, 2 H, 2Ar-H), 7.26 (d, J = 8.0 Hz, 2 H, 2Ar-H),
7.07 (d, J = 3.0 Hz, 1 H, Ar-H), 6.98 (d, J = 3.0 Hz, 1 H, Ar-
H), 3.57 (d, J = 14.0 Hz, 1 H, CH), 3.29 (d, J = 14.0 Hz, 1 H,
CH), 3.06 (d, J =9.5 Hz, 1 H, CH), 2.98 (d, J = 10.0 Hz, 1 H,
CH), 2.40 (s, 3 H, ArCH3), 2.15 (d, J = 13.0 Hz, 1 H, CH),
1.51 (s, 3 H, CH3), 1.42 (d, J = 13.5 Hz, 1 H, CH), 0.97 (s,
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3 H, CH3), 0.90 (s, 3 H, CH3), 0.27 (s, 9 H, 3 SiCH3); 13
C
NMR (500 MHz, CDCl3): δ = 145.2, 142.8, 138.4, 133.9,
129.4, 129.3, 127.3, 68.7, 61.8, 52.6, 42.3, 36.2, 27.6, 27.2,
27.1, 21.5, −0.02; IR (film): 2957, 1439, 1342, 1251, 1209,
1154, 1092, 1055, 984, 840, 814, 759, 714, 659, 592, 548
cm−1; HRMS (ESI): m/z [M + H]+ calcd for C22H33NO2S2Si:
436.1816; found: 436.1897.
(7) (a) General Cyclization/Hydroamination Procedure: In
an argon-filled glove box, Y[N(TMS)2]3 (5.70 mg, 0.010
mmol) and benzene-d6 (0.5 mL) were added into a J. Young
NMR tube equipped with a Teflon screw cap. (E)-2,2,4-
Synlett 2013, 24, 193–196
© Georg Thieme Verlag Stuttgart · New York