Chelating bis(thiophosphinic amidate)s as versatile supporting ligands for the group 3 metals. An application to the synthesis of highly active catalysts for intramolecular alkene hydroamination

Article abstract of DOI:10.1021/ja021445l

Bis(thiophosphinic amidate) complexes (i.e., 1) of representative group 3 and lanthanide metals have been quantiatively prepared in situ from the corresponding thiophosphinic amides and Ln[N(TMS)₂]₃. These unusual pentacoordinate complexes exhibit very high activity as catalysts for intramolecular alkene hydroamination. Copyright

Full text of DOI:10.1021/ja021445l

Published on Web 07/16/2003
Chelating Bis(thiophosphinic amidate)s as Versatile Supporting Ligands for
the Group 3 Metals. An Application to the Synthesis of Highly Active
Catalysts for Intramolecular Alkene Hydroamination
Young Kwan Kim, Tom Livinghouse,* and Yoshikazu Horino
Department of Chemistry, Montana State UniVersity, Bozeman, Montana 59717
Received December 13, 2002; E-mail: livinghouse@chemistry.montana.edu
Permethylmetallocene and related derivatives of the group 3
metals have been shown to be effective catalysts for alkene polym-
erization,1 hydrosilylation,2 hydroboration,3 hydrogenation,4 as well
as intramolecular hydroamination.5 By virtue of the importance of
these and related reactions that are catalyzed by group 4 metal-
locenes, we were interested in evaluating chelating thiophosphinic
amidates as an alternative ligand class for the complexation of early
metals. The attractive features of this ligand motif include pseudot-
etrahedral bonding geometry at phosphorus for increased steric
shielding of the complexed metal relative to the planar central car-
bon present in the more familiar amidinato6 group and diminished
predisposition toward aggregate formation with oxophilic metals
by virtue of terminal sulfur (vs oxygen) substitution. Surprisingly,
thiophosphinic amides have thus far only been employed as simple
additives to influence the outcome of selected reactions involving
the addition of organometallic reagents to aldehydes.7 In this
Communication, we report the structure of the first low-coordinate
yttrium-bis(thiophosphinic amidate) complex. The utilization of this
complex as well as related chelates of representative group 3 and
lanthanide metals (e.g., Y, Nd, and Dy) for the efficient catalysis
of intramolecular alkene hydroaminations is then described.
The thiophosphinic amides 1a,c,d and f-i employed in this study
were prepared by the reaction of the diamine with the appropriate
dialkyl- or diarylchlorophosphine (2.1 equiv) followed by the
addition of sulfur (2.2 equiv) or selenium in the case of 1b (Figure
1).8 Attachment of the proligands 1a-i to group 3 metals could be
quantitatively achieved by direct metalation with 1 equiv of the
appropriate amide Ln[N(TMS)2]3 3a-c (Ln ) Y, Nd, or Dy) in
C6D6 or toluene to deliver complexes 4a-i (Scheme 1).
Figure 1. Proligands used in amine elimination reactions with 3a-c.
Figure 2. Chem 3D perspective view of 4h(Y) from the ORTEP diagram
program.
Scheme 1
Crystals of complex 4h(Y) suitable for X-ray diffraction were
obtained directly from the reaction mixture (toluene as solvent).
The molecular structure is shown in Figure 2. The complex has a
distorted square-pyramidal geometry, with the bis(trimethylsilyl)-
amido ligand at the apex. The structure confirms the bidentate nature
of the thiophosphinic amidate (NPS) ligand, with S-Y-N angles
of 67.94(7)° and 69.88(7)° and the apical amido possessing
N-Y-N and S-Y-N deflections of 120.86(11)°, 115.71(11)° and
105.53(8)°, 107.88(8)°, respectively. The bond length of Y-N(3)
[2.234(3) Å] is considerably shorter than Y-N(1) [2.314(3) Å] or
Y-N(2) [2.309(3) Å] in consonance with the greater ability of N(3)
to engage in π-dative bonding to Y. It is also of interest that the
bond length of Y-N(3) [2.234(3) Å] in 4h(Y) is shorter than those
of the 16 e- complex Cp*2Y-N(TMS)2 [Y-N: 2.274(5) Å and
2.253(5) Å] (which has two independent molecules in the asym-
metric unit)9 or that of the 14 e- complex [Cp*Si(Me)2N-(t-
C4H9)Y-N(TMS)2 [Y-N(TMS)2: 2.255(8) Å].10 In light of these
structural features, complexes belonging to the NPS class differ
considerably from the majority of those (i.e., metallocenes and
amido complexes) that possess activity as catalysts for small
molecule synthesis.11
The catalyzed intramolecular hydroamination of alkenes,5a-f
allenes,5g and alkynes12 has attracted considerable attention as a
valuable method for the synthesis of nitrogenous heterocycles.13
Whereas various transition metal complexes are capable of mediat-
ing addition reactions involving alkynes, those involving the less
reactive alkene linkage are less common. A limited number of group
3 metal complexes appear very well suited for effecting this process
chemoselectively and under mild reaction conditions.5a-d Our recent
interest in this area led us to its exploration as an initial venue for
catalysis by group 3 NPZ complexes.
The cyclization of 2-aminohex-5-ene (5) was selected for initial
examination.5a,d Successful implementation of the desired reaction
proved remarkably straightforward as the requisite catalytic species
could, in all cases, be generated quantitatively and in situ. Accord-
9
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J. AM. CHEM. SOC. 2003, 125, 9560-9561
10.1021/ja021445l CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Catalyzed Cyclization of 2-Aminohex-5-ene (5)
Scheme 2
reaction
time
diastereomeric
ratio (6a:6b)b
c
complex
% conversion
a
b
c
d
e
f
g
h
i
4a(Y)
4b(Y)
4c(Y)
4d(Y)
4e(Y)
4f(Y)
4g(Y)
4h(Y)
4i(Y)
15 min
1 h
1 h
4.5 h
6 daysa
3 h
22 h
15 h
2 h
13:1
13:1
7:1
8:1
- -
9:1
7:1
8:1
16:1
6:1
>95
>95
>95
>95
<5a
>95
>95
>95
>95
>95
41d
complex 4a(Nd) proved less reactive than 4a(Y). As would be
expected, however, cyclization diastereoselectivity using the Nd
chelate was also diminished.5a Interestingly, the dysprosium
complex 4a(Dy) was found to have considerably lower activity than
the corresponding yttrium chelate. To the best of our knowledge,
this is the first instance where the catalytic activity of complexes
derived from Y and a lanthanide of similar covalent radius (Dy)
has been directly compared in alkene hydroamination.
j
k
4a(Nd)
4a(Dy)
45 min
15 minc
10:1
a Reaction was conducted at 120 °C. b Ratio based on 1H NMR
1
integration: 6a/6b ) 3.14 (septet)/2.93 (m) (NCH peaks). c Based on H
NMR integration relative to p-xylene as the internal standard. d Reaction
not run to completion.
Complexes 4a(Y), 4a(Nd), and 4a(Dy) were then evaluated in terms
of relative catalytic activity in intramolecular hydroaminations
involving aminoalkenes 7 and 9 (Scheme 2). Although these
reactions were slower and in the case of 7 less stereoselective than
those involving aminoalkene 5, similar trends in catalytic activity
were observed, with 4a(Y) showing the highest activity. It is of
interest in a preparative context that all of the complexes 4a(Ln)
show high activity toward the cyclization of substrates containing
1,2-disubstituted alkenes (i.e., 9), as these are known to be reluctant
participants in the hydroamination reaction.5c
ingly, incubation of the appropriate bis(thiophosphinic amide) [or
bis(selenophosphinic amide)] 1 (5 mol %) with Ln[N(TMS)2]3 3
(Ln ) Y, Nd, and Dy) (5 mol %) in C6D6 at 120 °C led to
smooth “amine elimination”10 with concomitant formation of the
active NPZ catalyst 4. Subsequent addition of 5 followed by heating
at 60 °C then provided the pyrrolidines 6a,b, typically in >95%
yield. Alternatively, cyclization of 5 on a 3 mmol scale followed
by separation of the products from the catalyst by vacuum trans-
fer and protonation (HCl-MeOH) furnished 6a,b as the hydro-
chloride salts in quantitative yield. A compilation of reaction times
and diastereoselectivities observed for the cyclization of aminoal-
kene 5 in the presence of the NPS and related chelates 4 appears
in Table 1.
Acknowledgment. Generous financial support for this research
was provided by the National Institutes of Health and the National
Science Foundation.
Several of the trends that emerge from the foregoing examples
are worthy of comment. Yttrium bis(thiophosphinic amidate)s 4a(Y)
and 4c(Y) that possess comparatively hindered and electron-rich alkyl
substituents have high catalytic activity, with 4a(Y) being comparable
to the best metallocene or nonmetallocene group 3 hydroamination
reported to date.5e As a means of perspective, the cyclization of 5
to the pyrrolidines 6a,b in the presence of 5 mol % of the simple
amide 3a at 60 °C serves as a benchmark and was extremely
lethargic, requiring 31 days to reach 95% conversion.5e Excessive
shielding of the active metal center results in decreased activity
(e.g., 4d(Y) vs 4c(Y)), consistent with the observed trend in
lanthanocenes.5a Replacement of sulfur with selenium as the
chalcogen substituent of phosphorus (cf., 4b(Y)) leads to a reduction
of catalytic activity, albeit not dramatic. Significantly, replacement
of the chalcogen with oxygen (complex 4e(Y)) results in complete
suppression of catalysis. In consonance with this result, it is
significant that the yttrium chelate that is derived from the
corresponding tethered bis(enamide) 2a is of low catalytic activity.
In this case (5 mol % cat.), cyclization of 5 to 6a,b (6a:b ) 4:1)
proceeded only to 92% conversion at 120 °C oVer 6 days. By way
of contrast, the yttrium complex analogously generated from the
bis(thioenamide) 2b is of considerably higher activity. For this
complex, the cyclization of 5 to 6a,b required only 5 h at 30 °C to
reach >95% conVersion and proceeded with remarkably high
diastereoselectiVity (6a:b ) 33:1). The relative activity of com-
plexes derived from 1a and alternative group 3 metals was
subsequently explored using an early lanthanide (e.g., Nd) and one
comparable in size to yttrium (e.g., Dy). In variance with the activity
dependence expected for group 3 metallocenes, the neodymium
Supporting Information Available: Experimental procedures and
spectroscopic data for all new compounds and crystallographic data
for complex 4h(Y) (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
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