Synthesis, characterization, and reactivity of 1-ethoxysilatrane

Article abstract of DOI:10.1080/15533174.2012.756023

The synthesis of four new silatranes with all six-membered rings N[CH ₂(Me₂C₆H₂O)]₃SiR [3-6, R = OEt (3); OCH₂CH₂N(Et)₂ (4); SPh (5); OOC(C₅H₄N)

Full text of DOI:10.1080/15533174.2012.756023

This article was downloaded by: [UZH Hauptbibliothek / Zentralbibliothek Zürich]
On: 22 July 2013, At: 03:39
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,
37-41 Mortimer Street, London W1T 3JH, UK
Synthesis and Reactivity in Inorganic, Metal-Organic,
and Nano-Metal Chemistry
Publication details, including instructions for authors and subscription information:
http://www.tandfonline.com/loi/lsrt20
Synthesis, Characterization, and Reactivity of 1-
Ethoxysilatrane
Gurjaspreet Singh a , Shally Girdhar a , Mridula Garg a & Promila a
a Department of Chemistry and Centre of Advanced Studies in Chemistry , Panjab
University , Chandigarh , India
Accepted author version posted online: 04 Apr 2013.
To cite this article: Gurjaspreet Singh , Shally Girdhar , Mridula Garg & Promila (2013) Synthesis, Characterization, and
Reactivity of 1-Ethoxysilatrane, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 43:9,
1107-1111, DOI: 10.1080/15533174.2012.756023
To link to this article: http://dx.doi.org/10.1080/15533174.2012.756023
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained
in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the
Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and
should be independently verified with primary sources of information. Taylor and Francis shall not be liable for
any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever
or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of
the Content.
This article may be used for research, teaching, and private study purposes. Any substantial or systematic
reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any
form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://
www.tandfonline.com/page/terms-and-conditions

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 43:1107–1111, 2013
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2012.756023
Synthesis, Characterization, and Reactivity
of 1-Ethoxysilatrane
Gurjaspreet Singh, Shally Girdhar, Mridula Garg, and Promila
Department of Chemistry and Centre of Advanced Studies in Chemistry, Panjab University,
Chandigarh, India
thesis, crystal structures, and dynamic NMR behavior of six-
membered ring silatranes derived from symmetric tetrapodal
ligands tris(2-hydroxy-3,5-dimethylbenzyl)amine and tris(2-
hydroxy-3-tert-butyl-5-methylbenzyl)amine. Besides these, 1-
isothiocyanato six-membered silatrane with Si-NCS linkage
and its adducts with Lewis acids and bases have also been re-
ported.[16]
Six-membered ring silatranes being structurally flexible are
of broad theoretical and experimental interest. This flexibility
permits changes in the N→Si bond lengths as well as 29Si NMR
chemical shifts, which are indeed correlated to the electronic
influences of the axial or exocyclic substituents. There are ample
reports of systematic theoretical studies also on the silatrane
systems.[17–19] These studies have provided vital information
regarding the structure and reactivity of such species.
Owing to our interest in such larger rings, in the present
context we report the synthesis and structural analysis of novel
silatranes 3–6 (Figure 1) with all six-membered rings by em-
ploying transesterifications of ethoxy silatrane 3.
The ethoxy silatrane 3 itself has been prepared by the reac-
tion of trichloroethoxysilane 1 with the tetrapodal ligand tris(2-
hydroxy-3,5-dimethylbenzyl)amine 2, unlike the conventional
method of preparation of alkoxy silatranes by use of tetraalkyl
orthosilicates.[13] Theoretical calculations have also been per-
formed, using density functional theory (DFT) with B3LYP
basis set and an account for the parameter of N→Si bond length
has been given with regard to the electronegativity of the ax-
ial/exocyclic substituent.
The synthesis of four new silatranes with all six-membered rings
N[CH2(Me2C6H2O)]3SiR [3–6, R = OEt (3); OCH2CH2N(Et)2
(4); SPh (5); OOC(C5H4N) (6)] using tris(2-hydroxy-3,5-
dimethylbenzyl)amine as encapsulating agent is described. Sila-
tranes 4, 5, and 6 have been prepared by transesterifications of
ethoxy silatrane 3. Structures of these silatranes have been estab-
1
lished by elemental analyses; infrared spectroscopy; H, 13C, and
29Si NMR spectroscopy; and mass spectrometry. The results have
been correlated with DFT studies using B3LYP basis set at 3–21G
level.
Keywords ethoxy silatrane, six-membered ring silatrane, trichloroe
thoxysilane, tris(2-hydroxy-3,5-dimethylbenzyl)amine
INTRODUCTION
The amine-tris(phenolate) ligands[1–4] are typical tetrapodal
ligands and there are numerous reports of the wide applica-
bility of their complexes. Among them, tris(2-hydroxy-3,5-
dialkylbenzyl)amines are reported to be stable scaffolds for
the synthesis of flexible six-membered ring metallatrane sys-
tems. For instance, metallatranes[5] of Al(III), Ga(III), Sn(IV),
Sb(III), Zr(IV), and Nb(V) have been reported previously.
Fe(III), Ta(IV), W(IV), and Bi(III) complexes[6–9] have also
been documented. Recently, Ti(IV) and V(V) complexes[10,11]
of tris(2-hydroxy-3,5-dialkylbenzyl)amine have been prepared,
both finding applications in catalysis of organic reactions.
More recently, boratranes with all six-membered rings and
with two different ring sizes derived from tris(2-hydroxy-3,5-
dimethylbenzyl)amine have been synthesized and characterized
by Kim et al.[12]
EXPERIMENTAL
With regard to complexes of silicon with amine-
Materials and Physical Measurements
tris(phenolate) ligands, Holmes et al.[13–15] reported the syn-
Solvents were freshly dried according to standard proce-
dures. All the reactions involving silanes and silatranes were
carried out in dry nitrogen atmosphere. Silicon(IV) chloride
(Aldrich, St. Louis, MO, USA), absolute alcohol (CYC China),
2,4-dimethylphenol (Merck, Darmstadt, Germany), hexam-
ethylenetetramine (Aldrich), p-toluenesulfonic acid (SDFCL,
Chandigarh, India), 2-diethylaminoethanol (CDH, Chandigarh,
India), thiophenol (Merck), and 2-picolinic acid (SDFCL) were
Received 31 May 2012; accepted 3 December 2012.
The authors gratefully acknowledge University Grants Commission
(UGC), New Delhi for providing financial support.
Address correspondence to Gurjaspreet Singh, Department of
Chemistry and Centre of Advanced Studies in Chemistry, Panjab Uni-
versity, Chandigarh 160-014, India. E-mail: gjpsingh@pu.ac.in.
1107

1108
G. SINGH ET AL.
(1-(N,N-diethyl)aminoethoxy)sila-2,10,11-trioxa-6-aza-3,4;
Compound
Exocyclic Group (R)
8,9;12,13-tris(4ꢁ,6ꢁ-dimethylbenzo) [4.4.4.0]tricyclo
N
3
4
5
6
OEt
tetradecane, N[CH2(Me2C6H2O)]3SiOCH2CH2N(Et)2 (4)
To a stirred solution of 3 (1.10 g, 2 mmol) in acetoni-
trile (30 mL), 2-diethylaminoethanol (0.30 mL, 2 mmol) was
added and the contents were heated to reflux for 6 h. Solvent
was removed from the reaction mixture by vacuum evapo-
ration and the pale yellow residue was washed with diethyl
ether (2 mL), followed by vacuum drying to give 4. Yield:
0.85 g (68%); mp 208–210◦C (decomposed). Anal. Calcd.
(%) for C33H44N2O4Si (560): C, 70.71; H, 7.86; N, 5.00;
Si, 5.00. Found (%): C, 70.05; H, 7.60; N, 4.90; Si, 4.80.
IR (KBr, cm–1): υ = 568(N→Si), 1110(Si-O), 1262(C-O).
1H NMR (300 MHz, DMSO-d6/CDCl3): δ = 2.16(s, 9H, Ar-
CH3), 2.22(s, 9H, Ar-CH3), 4.00(s, 6H, NCH2), 6.76(s, 3H,
Ar-H), 6.87(s, 3H, Ar-H), 3.88(t, 2H, OCH2CH2N), 3.29(t, 2H,
OCH2CH2N), 3.14(q, 2H, NCH2CH3), 1.34(t, 3H, NCH2CH3).
13C NMR (75.5 MHz, DMSO-d6/CDCl3): δ = 16.00(Ar-
CH3), 19.82(Ar-CH3), 55.45(NCH2), 128.86, 150.84(Ar-C),
54.54(OCH2CH2N), 53.53(OCH2CH2N), 47.12(NCH2CH3),
8.28(NCH2CH3). MS (ESI) m/z [assignment]: 583[M+Na+].
Si
OCH2CH2N(Et)2
O
R
SPh
3
OOC(C5H4N)
FIG. 1. Structures of silatranes synthesized.
used as supplied. Pyridine (Merck) was refluxed over KOH
pellets and vacuum distilled before use. Tris(2-hydroxy-3,5-
dimethylbenzyl)amine[13] and trichloroethoxysilane[20] were
prepared according to reported literature methods. Elemen-
tal analyses were performed using a Flash Organic Elemental
(Model 2000) CHNS-O Analyzer (Panjab University, Chandi-
garh, India). The % mass compositions of sulfur, silicon and
chlorine were determined by standard gravimetric methods. IR
spectra were recorded in the range 4000–400 cm–1 in Nujol/KBr
plates on a Perkin-Elmer RX1 FTIR Spectrophotometer (Panjab
University, Chandigarh, India). 1H (300 MHz), 13C (75.5 MHz),
and 29Si (59.6 MHz) NMR spectra were obtained in DMSO-
d6/CDCl3 on a Jeol AL 300 spectrophotometer (Panjab Uni-
versity, Chandigarh, India). Chemical shifts were reported as
positive downfield shifts in ppm, as relative to tetramethylsi-
lane. Mass spectra (ESI, 3500V) were recorded on a Micromass
Q-TOF Spectrometer (Panjab University, Chandigarh, India).
Computational studies (DFT) were performed on a Gaussian 03
program (Gaussian, Inc., Wallingford, CT, USA) system using
B3LYP basis set at 3–21G level.
1-Thiophenoxysila-2,10,11-trioxa-6-aza-3,4;8,9;12,13-
tris(4ꢁ,6ꢁ-dimethylbenzo)[4.4.4.0]tri-cyclotetradecane,
N[CH2(Me2C6H2O)]3SiSPh (5)
A solution of 3 (1.00 g, 2 mmol) and thiophenol (0.20 mL,
2 mmol) in dry acetonitrile (30 mL) was heated to reflux for
4 h. Solvent was removed by vacuum evaporation; the yellow
residue got was washed with hexane (2 mL) and vacuum dried
to give 5. Yield: 0.92 g (81%); mp 218–220◦C (decomposed).
Anal. Calcd. (%) for C33H35NO3SiS (553): C, 71.61; H, 6.33;
N, 2.53; Si, 5.06; S, 5.79. Found (%): C, 71.00; H, 6.30; N, 2.55;
Si, 5.00; S, 5.70. IR (KBr, cm–1): υ = 658(N→Si), 1072(Si-O),
1296(C-O), 859(Si-S). 1H NMR (300 MHz, DMSO-d6/CDCl3):
δ = 2.22(s, 9H, Ar-CH3), 2.36(s, 9H, Ar-CH3), 4.10(s, 6H,
NCH2), 6.80(s, 3H, Ar-H), 6.98(s, 3H, Ar-H), 8.28–8.77(m,
5H, SC6H5). 13C NMR (75.5 MHz, DMSO-d6/CDCl3): δ =
15.62(Ar-CH3), 19.36(Ar-CH3), 54.20(NCH2), 116.21, 124.83,
126.16, 128.55, 128.67, 132.51, 141.10, 150.22(Ar-C, SC6H5).
MS (ESI) m/z [assignment]: 554[M+H+].
Synthesis of Silatranes
1-Ethoxysila-2,10,11-trioxa-6-aza-3,4;8,9;12,13-tris(4ꢁ,6ꢁ-
dimethylbenzo)[4.4.4.0]tricyclotetradecane, N[CH2
(Me2C6H2O)]3SiOEt (3)
A solution of silane 1 (2.60 mL, 14 mmol), tetrapodal lig-
and 2 (6.10 g, 14 mmol), and pyridine (3.50 mL, 43 mmol) in
dry acetonitrile (50 mL) was stirred for 2 h at ambient tem-
perature. Pyridinium chloride formed as white solid was fil-
tered off from the reaction mixture. Removal of solvent from
the filtrate in vacuo gave the ethoxy silatrane 3 as a yellow
residue which was washed with hexane (5 mL) and dried. Yield:
6.05 g (85%); mp 198–200◦C (decomposed). Anal. Calcd. (%)
for C29H35NO4Si (489): C, 71.16; H, 7.16; N, 2.86; Si, 5.72.
Found (%): C, 71.00; H, 7.06; N, 2.90; Si, 5.45. IR (KBr, cm–1):
544(N→Si), 1108(Si-O), 1244(C-O). 1H NMR (300 MHz,
DMSO-d6/CDCl3): δ = 2.20(s, 9H, Ar-CH3), 2.15(s, 9H, Ar-
CH3), 4.14(s, 6H, NCH2), 6.78(s, 3H, Ar-H), 6.87(s, 3H, Ar-
H), 3.34(q, 2H, OCH2CH3), 1.10(t, 3H, OCH2CH3). 13C NMR
(75.5 MHz, DMSO-d6/CDCl3): δ = 16.41(Ar-CH3), 20.18(Ar-
CH3), 55.68(NCH2), 116.45, 126.16, 129.03, 129.47, 133.64,
150.98(Ar-C), 57.73(OCH2CH3), 18.20(OCH2CH3). 29Si NMR
(59.6 MHz, DMSO-d6/CDCl3): δ = –94.75. MS (ESI) m/z [as-
signment]: 512[M+Na+].
1-Picolinatosila-2,10,11-trioxa-6-aza-3,4;8,9;12,13-tris
(4ꢁ,6ꢁ-dimethylbenzo)[4.4.4.0]tri-cyclotetradecane,
N[CH2(Me2C6H2O)]3SiOOC(C5H4N) (6)
To a solution of 3 (1.00 g, 2 mmol) in dry acetonitrile
(30 mL), 2-picolinic acid (0.25 g, 2 mmol) was added. The
contents were stirred and refluxed for 6 h. After removal of
solvent in vacuo, the yellow residue got was washed with
hexane (2 mL) and vacuum dried to yield 6. Yield: 0.76 g
(66%); mp 238–240◦C (decomposed). Anal. Calcd. (%) for
C33H34N2O5Si (566): C, 69.96; H, 6.01; N, 4.95; Si, 4.95.
Found (%): C, 69.90; H, 5.98; N, 4.90; Si, 4.50. IR (KBr,
cm–1): υ = 675(N→Si), 1156(Si-O), 1301(C-O), 1602(C O).
1H NMR (300 MHz, DMSO-d6/CDCl3): δ = 2.16(s, 9H,

1-ETHOXYSILATRANE
1109
N
3 equiv
N
N
Si
3
+
+
.HCl
N
O
EtOSiCl3
OEt
OH
CH3CN/r.t.
3
3
3
1
2
SCH. 1.
Ar-CH3), 2.21(s, 9H, Ar-CH3), 4.08(s, 6H, NCH2), 6.76(s, IR Spectra
3H, Ar-H), 6.86(s, 3H, Ar-H), 7.46–8.66(m, 4H, NC5H4).
The infrared spectra of the silatranes showed υ N→Si ab-
sorptions ranging from 544 to 675 cm–1. It was observed
that the stretching frequency for N→Si increased as the ni-
trogen donor interaction increased, which is indeed attributed
to the increase in electron withdrawing effect of the exo-
cyclic groups while going from 3–6. Bands at 661, 910,
and 945 cm–1 were attributed to typical silatranyl skeletal
vibrations.[21]
13C NMR (75.5 MHz, DMSO-d6/CDCl3): δ = 15.52(Ar-
CH3), 19.28(Ar-CH3), 54.01(NCH2), 116.18, 116.21, 124.77,
126.39, 128.47, 128.58, 132.40, 140.27, 150.11, 151.71(Ar-C,
NC5H4), 194.26(C O). MS (ESI) m/z [assignment]: 566[M+],
567[M+H+].
RESULTS AND DISCUSSION
Synthesis and Spectroscopic Studies
NMR Spectra
Multinuclei (1H, 13C, and 29Si) NMR spectra were consistent
with the structure of the synthesized compounds. On compar-
Silatranes 3–6 were synthesized using two different methods.
Silatrane 3 was synthesized by the reaction of the tetrapodal
ligand 2 with trichloroethoxysilane 1 in the presence of three
equivalents of pyridine (Scheme 1). Silatranes 4–6 were ob-
tained by transesterifications of the preformed silatrane 3 with
appropriate ligands like 2-diethylaminoethanol, thiophenol, and
2-picolinic acid respectively, through the replacement of the
ethoxy group, as illustrated in Scheme 2.
1
ing the H and 13C NMR spectra of the silatranes with that of
the tetrapodal ligand 2, a downfield shift for the protons and
carbon atoms of the -CH2N- moiety in all compounds was ob-
1
served. Also, the H NMR spectra of the silatranes 3–6 were
devoid of the signals due to phenolic –OH of 2, thus confirming
complexation. 29Si NMR spectrum of 3 displayed a singlet at
–94.75 ppm, the value being in the range of that of a typical
pentacoordinate silicon environment.[22]
Et
Et
N
N
Et
N
OH
Si
O
O
O
Mass Spectral Analysis
O
Et
3
The mass spectra (ESI, 3500V) of the compounds clearly
revealed molecular ion peaks which were consistent with the
molecular masses of the individual compounds.
SH
4
N
N
Si
O
Si
S
OEt
Computational Study
3
3
3
5
6
The structures of the compounds were further correlated
with theoretical results as obtained from geometry optimiza-
tions by performing DFT calculations (unrestricted, B3LYP) on
a Gaussian 03 program system, at 3–21G level. The computed
bond lengths for the optimized structures of 3–6 rightly give an
COOH
N
N
O
C
Si
O
N
3
SCH. 2.
TABLE 1
Computational data for silatranes 3–6
All of the six-membered ring silatranes 3–6 are novel
and the ethoxy silatrane 3 was prepared by the reaction of
trichloroethoxysilane 1 with tetrapodal ligand 2, rather than by
employing the use of tetraalkyl orthosilicate in the presence of a
strong fluoride catalyst.[13] The yields of the prepared silatranes
are quite appreciable, ranging from 66% to 85%. The structures
of the silatranes 3–6 were analyzed on the basis of spectral data
(IR, multinuclei NMR, and mass spectrometry).
Parameter
3
4
5
6
Energya
−1761.92 −1973.35 −2234.91 −2042.18
Dipole momentb
Point group
N→Sic
5.60
C1
5.43
C1
8.20
C1
6.21
C1
2.076
2.114
2.110
2.085
aCalculated in a.u. bCalculated in debyes. cComputed in Å.

1110
G. SINGH ET AL.
FIG. 2. Optimized structures of silatranes 3–6 showing TBP arrangement around silicon atom (color figure available online).
insight into the influence of the varying exocyclic substituents REFERENCES
1. Chmura, A. J.; Davidson, M. G.; Frankis, C. J.; Jones, M. D.; Lunn, M. D.
Chem. Commun. 2008, 1293–1295.
on the N→Si bond; the variation range being 2.076–2.114 Å, as
shown in Table 1. The shortest and hence the most stable bond
among 3–6 was observed for 6 due to maximum –I effect of the
exocyclic group in this case which not only shortens the N→Si
bond length but also creates a perfect TBP arrangement around
the silicon atom (Figure 2).
2. Axe, P.; Bull, S. D.; Davidson, M. G.; Gilfillan, C. J.; Jones, M. D.; Robin-
son, D. E. J. E.; Turner, L. E.; Mitchell, W. L. Org. Lett. 2007, 9,223–226.
3. Gendler, S.; Segal, S.; Goldberg, I.; Goldschmidt, Z.; Kol, M. Inorg. Chem.
2006, 45, 4783–4790.
4. Bull, S. D.; Davidson, M. G.; Johnson, A. L.; Robinson, D. E. J. E.; Mahon,
M. F. Chem. Commun. 2003, 1750–1751.
5. Ghadwal, R. S.; Singh, A. Ind. J. Chem. 2007, 46A, 768–771.
6. Hwang, J. W.; Govindaswamy, K.; Koch, S. A. Chem. Commun. 1998,
1667–1668.
7. Groysman, S.; Goldberg, I.; Kol, M. Organometallics 2003, 22,
3793–3795.
8. Lehtonen, A.; Sillanpaa, R. Organometallics 2005, 24, 2795–2800.
9. Turner, L. E.; Davidson, M. G.; Jones, M. D.; Ott, H.; Schulz, V. S.; Wilson,
P. J. Inorg. Chem. 2006, 45, 6123–6125.
10. Bull, S. D.; Davidson, M. G.; Johnson, A. L.; Mahon, M. F.; Robinson, D.
E. J. E. Chem. Asian J. 2010, 5, 612–620.
11. Kikushima, K.; Moriuchi, T.; Hirao, T. Tetrahedron 2010, 66, 6906–6911.
12. Kim, D. J.; Hong, Y.; Kim, S. H.; Lee, K. M.; Mun, S.-D.; Yoon, S.; Lee,
J.; Do, Y.; Kim, Y. Inorg. Chim. Acta 2011, 378, 311–314.
13. Chandrasekaran, A.; Day, R. O.; Holmes, R. R. J. Am. Chem. Soc. 2000,
122, 1066–1072.
14. Timosheva, N. V.; Chandrasekaran, A.; Day, R. O.; Holmes, R. R.
Organometallics 2000, 19, 5614–5622.
15. Timosheva, N. V.; Chandrasekaran, A.; Day, R. O.; Holmes, R. R.
Organometallics 2001, 20, 2331–2337.
CONCLUSION
The formation and reactivity of ethoxy silatrane 3 has been
studied. The ethoxy exocyclic group unlike the alkyl counter-
parts is potentially reactive and opens up the scope for syn-
thesis of further new six-membered ring silatranes without af-
fecting the silatranyl cage. Six-membered ring silatranes ex-
hibit structural flexibility, which allows for the study of effect
of exocyclic substituents on the N→Si bond lengths as well
as infrared stretching frequencies of the bond. This feature of
flexible rings is absent in the case of five-membered ring sila-
tranes, which are quite rigid. In the present case, the donor
interaction has been found to depend primarily on the elec-
tronegativity and electron withdrawing effect of the exocyclic
ligands.

1-ETHOXYSILATRANE
1111
16. Puri, J. K.; Singh, G.; Duggal, P. Phosphorus Sulfur Silicon Relat. Elem.
2008, 183, 1853–1861.
19. Sidorkin, V. F.; Belogolova, E. F.; Gordon, M. S.; Lazarevich, M. I.;
Lazareva, N. F. Organometallics 2007, 26, 4568–4574.
17. Sakaki, S.; Kawai, D.; Tsukamoto, S. Collect. Czech. Chem. Commun.
2011, 76, 619–629.
18. Ishmaeva, E. A.; Timosheva, A. P.; Gazizova, A. A.; Chachkov, D. V.;
20. Anderson, H. H. J. Am. Chem. Soc. 1949, 71, 1801–1803.
21. Narula, S. P.; Shankar, R.; Kumar, M.; Chadha, R. K.; Janaik, C. Inorg.
Chem. 1997, 36, 1268–1273.
Vereshchagina, Y. A.; Kataev, V. E.; Timosheva, N. V. Russ. J. Gen. Chem. 22. Iwamiya, J. H.; Maciel, G. E. J. Am. Chem. Soc. 1993, 115,
2008, 78, 1350–1353. 6835–6842.