The coordination chemistry and reactivity of amino-dithiaphospholanes with rhodium, iridium, and ruthenium

Article abstract of DOI:10.1016/j.tetlet.2008.12.034

Novel amino-dithiaphospholane complexes of ruthenium, iridium, and rhodium were synthesized, and their properties were studied. Reaction of the new amino-dithiaphospholane (RS)₂PNR₂^′ (R = binaphthyl, R′ = CH₂Ph,

Full text of DOI:10.1016/j.tetlet.2008.12.034

Tetrahedron Letters 50 (2009) 922–925
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
The coordination chemistry and reactivity of amino-dithiaphospholanes
with rhodium, iridium, and ruthenium
*
Stephen Costin, Sergey L. Sedinkin, Eike B. Bauer
University of Missouri—St. Louis, Department of Chemistry and Biochemistry, One University Boulevard, St. Louis, MO 63121, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel amino-dithiaphospholane complexes of ruthenium, iridium, and rhodium were synthesized, and
their properties were studied. Reaction of the new amino-dithiaphospholane (RS)2PNR0 (R = binaphthyl,
Received 27 October 2008
Revised 2 December 2008
Accepted 5 December 2008
Available online 13 December 2008
2
R0 = CH2Ph, (rac)-4) with [RuCl2(p-cymene)]2 afforded [RuCl2(p-cymene)((rac)-4)] in 67% isolated yield.
Similarly, the new amino-dithiaphospholanes (RS)2PNR0 (R = cyclohexyl, (rac)-7) and (RS)2PNR20 (R = phe-
2
nyl, 9) gave upon reaction with [RhCl(CO)2]2 and [IrCp*Cl2]2 the novel complexes [RhCl(CO)(L)2] and
[IrCp*Cl2(L)] (L = (rac)-7, 9) in 61–96% yields. The ruthenium complex is catalytically active for the ether-
ification of propargylic alcohols with methanol and ethanol (8–48 h, 90 °C, 40–85% isolated yields).
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Ruthenium
Iridium
Rhodium
Amino-dithiaphospholanes
Nicholas reaction
Phosphoramidites
Phosphoramidites (1, Fig. 1) have recently attracted consider-
able interest as ligands in transition metal catalyzed organic trans-
formations.1 Phosphoramidites are a versatile ligand class, which
can serve as two-, four-, six-, or eight-electron donors.2 We have
recently reported that phosphoramidite complexes of ruthenium
are catalytically active for b-oxo ester formation from propargylic
alcohols and carboxylic acids3a and in the Mukaiyama aldol reac-
tion.3b During our investigations we found that steric properties
of the phosphoramidites can influence the catalytic activity of
the corresponding ruthenium complexes. However, thus far we
have not observed the impact of electronic factors on the catalytic
performance of our phosphoramidite complexes. Placing bromo
substituents on the aromatic backbone of the phosphoramidites
(1c in Fig. 1) as well as employment of ligands with partially
hydrogenated naphthyl units (1d in Fig. 1) did not show significant
impact on the catalytic activity of the corresponding ruthenium
complexes in b-oxo ester formation.3a
R'
R'
O
O
O
O
P NR2
P NR2
1d
R=CH2Ph,
R
1a
R=CH3, '=H,
1b
R=CH2Ph, R'=H,
R
1c
R=CH3, '=Br,
S
N
P
S
We were interested in determining whether electronic tuning
closer to the phosphorous atom (which coordinates directly to
the metal) can influence the catalytic performance of the corre-
sponding metal complexes. Replacement of the two oxygen atoms
directly bonded to the phosphorous center by sulfur seemed to
hold the greatest promise. Sulfur analogs of phosphoramidites
(such as 2 in Fig. 1) are known in the literature and they have been
referred to as amino-dithiaphospholanes.4 However, to the best of
2
Figure 1. Phosphoramidite ligands and an amino-dithiaphospholane derivative (2).
our knowledge, amino-dithiaphospholanes have never been
employed as ligands in transition metal complexes. Herein, we
describe the first amino-dithiaphospholane complexes of ruthe-
nium, iridium, and rhodium as well as a catalytic application of
the ruthenium complex.
First, we synthesized the BINOL-based amino-dithiaphospho-
lane (rac)-4 and a ruthenium complex thereof. According to
* Corresponding author.
E-mail address: bauere@umsl.edu (E.B. Bauer).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.12.034

S. Costin et al. / Tetrahedron Letters 50 (2009) 922–925
923
standard procedures previously applied in our laboratory,3a known
racemic 1,10-binaphthyl-2,20-dithiol (rac)-35 was reacted with N,N-
dibenzyl-1,1-dichlorophosphinamine (Cl2PN(CH2Ph)2), which was
generated in situ from PCl3 and dibenzylamine (Scheme 1). The
amino-dithiaphospholane (rac)-4 was isolated in 58% yield as a
white powder after flash chromatography. It is known that the
commercial ruthenium precursor [RuCl2(p-cymene)]2 forms ruthe-
nium complexes with the general formula [RuCl2(p-cymene)(L)]
when treated with phosphoramidite ligands.3a,6 Accordingly, the
amino-dithiaphospholane ligand (rac)-4 was reacted with
[RuCl2(p-cymene)]2 under standard conditions (CH2Cl2, room tem-
perature, 6 h). After recrystallization, the corresponding amino-
dithiaphospholane ruthenium complex (rac)-5 was obtained as a
tan solid in 67% yield.
Due to the lengthy, three-step synthesis of dithiol (rac)-3, we
sought simpler dithiols for amino-dithiaphospholane syntheses.
Accordingly, the known7 racemic trans-cyclohexane-1,2-dithiol
(rac)-6 was treated with Cl2PN(CH2Ph)2 to obtain the correspond-
ing amino-dithiaphospholane (rac)-7 as a white powder in 85%
yield (Scheme 2).
Similarly, commercial benzene-1,2-dithiol 8 was converted to
the aromatic amino-dithiaphospholane 9, which was isolated as a
pale pink solid in 83% yield (Scheme 3).
It is known that the commercial rhodium precursor
[RhCl(CO)2]2 forms complexes with the general formula
[RhCl(CO)(L)2] when treated with monodendate phosphines L.8
Similarly, iridium complexes of the general formula [IrCp*Cl2(L)]
(Cp* = pentamethylcyclopentadienyl) can be obtained9 from the
known precursor [IrCp*Cl2]2.10 Accordingly, when the amino-
dithiaphospholanes (rac)-7 and 9 were reacted with [RhCl(CO)2]2,
the corresponding rhodium complexes (rac)-10 and 12 were ob-
tained as tan powders in 88% and 61% yields after washing with
pentane (Schemes 2 and 3). Employing similar protocols with
[IrCp*Cl2]2, the corresponding iridium complexes (rac)-11 and 13
were synthesized in 96% and 92% yield, respectively (Schemes 2
and 3).
The novel amino-dithiaphospholanes (rac)-4, (rac)-7, and 9 and
their metal complexes (rac)-5 and 10–13 were analyzed by NMR
(1H, 13C, 31P), IR, and mass spectrometry (including HRMS). The
FAB mass spectra showed a molecular ion peak for all ligands
and metal complexes. For the complexes, a diagnostic fragmenta-
tion pattern due to loss of Cl and/or CO ligands was observed.11
S
SH
SH
Cl2PNR2
NEt3
P
NR2
Cl2PNR2
SH
SH
S
S
P NR2
58%
S
toluene,
rt, NEt3
8
9, 83%
R=CH2Ph
R=CH2Ph
4
(rac)-
0.5 [IrCp*Cl2]2
CH2Cl2, rt
30 min
3
(rac)-
0.25 [RhCl(CO)2]2
toluene, rt
0.5 [RuCl2(p-cymene)]2
CH Cl ,
10 min
rt,6h
2
2
S
Ru
S
P
NR2
Ir
Cl
CO
Cl
P
S
S
P
S
Cl
R2N
Cl
Rh
12
Cl
NR2
S
S
P
S
R2N
13
5
(rac)-
92%
61%
67%
Scheme 1. Synthesis of an amino-dithiaphospholane (rac)-4 and its ruthenium
complex (rac)-5.
Scheme 3. Synthesis of an aromatic amino-dithiaphospholane and rhodium and
iridium complexes thereof.
S
SH
SH
Cl2PNR2
NEt3
P
NR2
rac-
rac-
S
6
(rac)-
7
(rac)- ,85%
R=CH2Ph
0.5 [IrCp*Cl2]2
0.25 [RhCl(CO)2]2
toluene, rt
CH Cl ,
rt
2
2
30 min
10 min
S
S
P
NR2
Ir
Cl
CO
P
P
NR2
S
Cl
Rh
R2N
S
Cl
S
S
88%
10
(rac)-
11
(rac)-
96%
Scheme 2. Synthesis of an aliphatic amino-dithiaphospholane and rhodium and iridium complexes thereof.

924
S. Costin et al. / Tetrahedron Letters 50 (2009) 922–925
The coordination of the amino-dithiaphospholanes was best
Attempts to obtain X-ray quality crystals of the iridium complex 13
resulted in needles, which turned out to be dibenzylammonium
chloride. Further NMR tube experiments revealed that the ligands
as well as the metal complexes appeared to be sensitive to hydro-
lysis, and some representative decomposition pathways are com-
piled in Scheme 5.
When NMR samples of the amino-dithiaphospholane (rac)-7
and the metal complexes 12 and 13 were treated with one drop
of water, 1H and 31P spectra showed the absence of starting mate-
rial. Instead, for all samples 1H NMR resonances around 3.9 ppm
(NCH2 group of dibenzylamine) and in some cases broad reso-
nances around 5 ppm (tentatively assigned to –SH) were observed.
The spectra of the hydrolyzed metal complexes exhibited broad
signals around 11 ppm in the 1H NMR. Their IR spectra showed a
broad band around 950 cmꢀ1, and the NMR of the hydrolyzed
(rac)-7 exhibited a 31P NMR signal around 0 ppm suggesting the
presence of phosphorous acid. Species 16 could be identified by
HRMS upon hydrolysis of 13. The hydrolysis of ligand (rac)-7 was
much slower than that of the metal complexes and has literature
precedent. The structurally related amino-dithiaphospholane 2
seen by a shift of their 31P NMR signals in the spectra of their cor-
responding metal complexes. For example, the free amino-dithia-
phospholane (rac)-7 showed a 31P NMR signal at 102.0 ppm,
whereas its corresponding metal complexes (rac)-10 and (rac)-11
resonated around 121.5 and 90.5 ppm. In addition, the rhodium
complexes (rac)-10 and 12 exhibited rhodium-phosphorus cou-
plings (JPRh = 160.6–163.8 Hz), which are diagnostic of a square
planar complex of composition [RhCl(CO)(L)2].8b For complexes
(rac)-5, (rac)-11, and 13, diagnostic signals for coordinated p-cym-
ene and Cp* were observed in the 13C NMR. Six signals between
110.6 and 80.0 ppm indicated coordinated p-cymene for (rac)-5,
and resonances between 95.0–93.3 and 9.5–9.1 ppm indicated
coordinated Cp* for (rac)-11 and 13.
Finally, the IR spectra of the rhodium carbonyl complexes (rac)-
10 and 12 also supported the proposed structures. They exhibited a
single, strong mC„O signal at 1983 and 1984 cmꢀ1, respectively.
These values are also typical for structurally related rhodium
monocarbonyl complexes.12 No IR signals for the dimeric
[RhCl(CO)2]2 starting material were observed, which shows multi-
ple mC„O absorptions.
Next, we were interested in determining if the novel amino-
dithiaphospholane complexes could be employed as catalysts. It
is known that a number of ruthenium complexes activate propar-
gylic alcohols catalytically.13 The catalytic exchange of the OH of
propargylic alcohols (14 in Scheme 4) by nucleophiles appeared
to be especially appealing. A multistep sequence to perform this
transformation, referred to as the ‘Nicholas reaction’, is known,14
and only a few metal complexes have been reported so far to cat-
alyze the Nicholas reaction.13b,d Accordingly, the ruthenium com-
plex (rac)-5 was tested as a catalyst for the transformation of
propargylic alcohols 14 to the corresponding propargylic methyl
and ethyl ethers (15) employing methanol and ethanol as
nucleophiles.
Indeed, it was observed that methanolic and ethanolic solutions
of a number of tertiary propargylic alcohols with ruthenium com-
plex (rac)-5 as catalyst resulted in the formation of the correspond-
ing methyl and ethyl ethers upon heating. The results of the
catalytic experiments are compiled in Scheme 4, and the isolated
yields ranged from 40% to 85%. Mechanistically, an allenylidene
intermediate [Ru@C@C@CR2] might be involved in the catalytic cy-
cle,13b,d but that the ruthenium complex simply acts a Lewis acid
cannot be excluded. Identification of the catalytically active species
and mechanistic investigations for the reaction shown in Scheme 4
are currently underway.
All NMR spectra of the ligands and metal complexes described
above showed baseline purity, and spectra for the rhodium and
iridium complexes are given in the Supplementary data. When
the rhodium and iridium complex syntheses were performed in
deuterated solvents, 1H and 31P NMR spectra showed complete
conversion within minutes. However, it was not possible to obtain
correct elemental analyses for most of the ligands and complexes.
(Fig. 1) is known to phosphorylate alcohols with concomitant P-
4a
N bond cleavage.
Accordingly, extensive workup attempts by
chromatography and recrystallization did not result in purer com-
pounds. The ruthenium complex (rac)-5 might follow the same
decomposition pathways in alcohols as shown in Scheme 5 for
aqueous hydrolysis, when utilized as a catalyst for the reaction
shown in Scheme 4. Detailed investigations of these processes
are currently underway.
In conclusion, this work shows for the first time the coordina-
tion chemistry of amino-dithiaphospholanes with transition
metals. Ruthenium, rhodium, and iridium complexes of amino-
dithiaphospholanes were synthesized, which turned out to be sen-
sitive to hydrolysis. The ruthenium amino-dithiaphospholane
complex (rac)-5 was shown to be catalytically active in the conver-
sion of propargylic alcohols to their corresponding alkyl ethers.
Further investigations of the coordination chemistry of amino-
dithiaphospholanes and their potential application as catalysts in
organic transformations are currently underway.
S
NR2
R =
P
NR2
S
Ir
Cl
P
S
Cl
S
7
(rac)-
13
CH Ph
2
H2O
SH
SH
H2O
(rac)-5 (1.5 mol%)
R3OH, 90 °C, 8-48 h
OR3
R2
R1
15
R2 OH
R1
+( H2NR2)+Cl-
(detected by X-
ray)
Ir
S
S
+ HNR2
14
(detected
NMR)
by
+H3PO3
1
2
3
a
+H3PO3
R = Ph, R = CH3, R = CH2CH3, , 40%
16
1
2
3
(detected by
NMR and IR)
b
R = R = Ph, R = CH3, , 58%
detected by MS
R1 = Ph, R2 = R3 = CH3, c, 85%
Scheme 5. Representative hydrolysis pathways of amino-dithiaphospholanes and
Scheme 4. Application of (rac)-5 in the etherification of propargylic alcohols 14.
their metal complexes.

S. Costin et al. / Tetrahedron Letters 50 (2009) 922–925
925
Prietob, O.; Woodward, S. Chem. Commun. 2005, 2843; (k) van den Berg, M.;
Minnaard, A. J.; Haak, R. M.; Leeman, M.; Schudde, E. P.; Meetsma, A.; Feringa,
B. L.; de Vries, A. H. M.; Maljaars, C. E. P.; Willans, C. E.; Hyett, D.; Boogers, J. A.
F.; Henderickx, H. J. W.; de Vries, J. G. Adv. Synth. Catal. 2003, 345, 308; (l) de
Vries, A. H. M.; Meetsma, A.; Feringa, B. L. Angew. Chem., Int. Ed. 1996, 35, 2375.
2. Mikhel, I. S.; Rüegger, H.; Butti, P.; Camponovo, F.; Huber, D.; Mezzetti, A.
Organometallics 2008, 27, 2937.
3. (a) Costin, S.; Rath, N. P.; Bauer, E. B. Adv. Synth. Catal. 2008, 350, 2414; (b)
Costin, S.; Rath, N. P.; Bauer, E. B. Inorg. Chim. Acta 2008. doi:10.1016/
j.ica.2008.09.003.
4. (a) Miller, G. P.; Silverman, A. P.; Kool, E. T. Bioorg. Med. Chem. 2008, 16, 56; (b)
Okruszek, A.; Sierzchała, A.; Fearon, K. L.; Stec, W. J. J. Org. Chem. 1995, 60, 6998;
(c) Okruszeck, A.; Sierzchała, A.; Sochacki, M.; Stec, W. J. Tetrahedron Lett. 1992,
33, 7585; (d) Farschtschi, N.; Gorenstein, D. G. Tetrahedron Lett. 1988, 29,
6843.
Acknowledgments
We thank Tihana Tomas for technical assistance in performing
the catalytic studies. We thank the University of Missouri—St.
Louis (Research Award) for support. Funding from the National
Science Foundation for the purchase of the NMR spectrometer
(CHE-9974801) and the purchase of the mass spectrometer
(CHE-9708640) is acknowledged.
Supplementary data
Full experimental details and spectroscopic data of all new
compounds, 1H and 13C NMR spectra of the catalysis products
shown in Scheme 4 and of the rhodium and iridium complexes
10–13.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.12.034.
5. Cram, D. J.; Helgeson, R. C.; Koga, K.; Kyba, E. P.; Madan, K.; Sousa, L. R.; Siegel,
M. G.; Moreau, P.; Gokel, G. W.; Timio, J. M.; Sogah, G. D. Y. J. Org. Chem. 1978,
43, 2758.
6. Huber, D.; Anil Kumar, P. G.; Pregosin, P. S.; Mikhel, I. S.; Mezzetti, A. Helv. Chim.
Acta 2006, 89, 1696.
7. Iqbal, S. M.; Owen, L. N. J. Chem. Soc. 1960, 1030.
8. (a) McCleverty, J. A.; Wilkinson, G. Inorg. Synth. 1966, 8, 214; (b) Mann, B. E.;
Masters, C.; Shaw, B. L. J. Chem. Soc. A 1971, 1104.
9. (a) Durran, S. E.; Smith, M. B.; Dale, S. H.; Coles, S. J.; Hursthouse, M. B.; Light, M.
E. Inorg. Chim. Acta 2006, 359, 2980; (b) Yamamoto, Y.; Sugawara, K.; Kakeya,
M. Inorg. Chim. Acta 2002, 340, 21.
References and notes
10. White, C.; Yates, A.; Maitlis, P. M. Inorg. Synth. 1992, 228.
11. Branan, D. M.; Hoffman, N. W.; McElroy, E. A.; Ramage, D. L.; Robbins, M. J.;
Eyler, J. R.; Watson, C. H.; DeFur, P.; Leary, J. A. Inorg. Chem. 1990, 29, 1915.
12. Deeming, A. J.; Shaw, B. L. J. Chem. Soc. A 1969, 597.
13. (a) Bustelo, E.; Dixneuf, P. H. Adv. Synth. Catal. 2007, 349, 933; (b) Cadierno, V.;
Diez, J.; Garcia-Garrido, S. E.; Gimeno, J. Chem. Commun. 2004, 2716; (c)
Cadierno, V.; Gimeno, J.; Nebra, N. Adv. Synth. Catal. 2007, 349, 382; (d) Inada,
Y.; Yoshikawa, M.; Milton, M. D.; Nishibayashi, Y.; Uemura, S. Eur. J. Org. Chem.
2006, 881. and literature cited therein.
1. Key literature, recent examples and reviews: (a) Eberhardt, L.; Armspach, D.;
Harrowfield, J.; Matt, D. Chem. Soc. Rev. 2008, 37, 839; (b) Toselli, N.; Martin, D.;
Achard, M.; Tenaglia, A.; Bürgi, T.; Buono, G. Adv. Synth. Catal. 2008, 350, 280;
(c) Kostas, I. D.; Vallianatou, K. A.; Holz, J.; Börner, A. Tetrahedron Lett. 2008, 49,
´
331; (d) Mršic, N.; Lefort, L.; Boogers, J. A. F.; Minnaard, A. J.; Feringa, B. L.; de
Vries, J. G. Adv. Synth. Catal. 2008, 350, 1081; (e) Minnaard, A. J.; Feringa, B. L.;
Lefort, L.; de Vries, J. G. Acc. Chem. Res. 2007, 40, 1267; (f) Zhang, W.; Zhang, X. J.
Org. Chem. 2007, 72, 1020; (g) Shekar, S.; Trantow, B.; Leitner, A.; Hartwig, J. F. J.
Am. Chem. Soc. 2007, 128, 11770; (h) Zhao, B.; Wang, Z.; Ding, K. Adv. Synth.
Catal. 2006, 348, 1049; (i) Alexakis, A.; Albrow, V.; Biswas, K.; d’Augustin, M.;
14. Nicholas, K. M. Acc. Chem. Res. 1987, 20, 207.