trans-Disubstituted diamido/diamine cyclam zirconium complexes

Article abstract of DOI:10.1016/j.inoche.2008.07.002

The reaction of H₂[1,8-dibenzyl-1,4,8,11-tetraazacyclotetradecane] (H₂Bn₂Cyclam) with 2 equiv of LiBu gives Li₂Bn₂Cyclam(L)₂ (L = Et₂O or THF) that reacts with ZrCl₄(THF)₂ to afford (Bn₂Cyclam)ZrCl₂ in 40% yield. Alternatively (Bn₂Cyclam)ZrCl₂ may be obtained via ZrCl₂(CH₂SiMe₃)₂(Et₂O)₂ in a significantly higher yield (87%), avoiding the lithiation of H₂Bn₂Cyclam. The syntheses of other H₂^*Bn₂Cyclam ligand precursors (^*Bn = 3,5-dimethylbenzyl and 3,5-di(tert-butyl)benzyl) and the corresponding Zr derivatives ({3,5-Me₂Bn}₂-Cyclam)ZrCl₂ and ({3,5-^tBu₂Bn}₂-Cyclam)ZrCl₂ are also reported. Results on preliminary tests of ε-caprolactone and ethylene polymerisation reactions are presented.

Full text of DOI:10.1016/j.inoche.2008.07.002

Inorganic Chemistry Communications 11 (2008) 1174–1176
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
trans-Disubstituted diamido/diamine cyclam zirconium complexes
Rui F. Munhá a, Luís G. Alves a, Nuno Maulide b, M. Teresa Duarte a, István E. Markó b, Michael D. Fryzuk c,
Ana M. Martins a,
*
a Centro de Química Estrutural, Instituto Superior Técnico, Av. Rovisco Pais n° 1, 1049-001 Lisboa, Portugal
b Université Catholique de Louvain, Département de Chimie, Bâtiment Lavoisier Place Louis Pasteur 1, 1348 Louvain-la-Neuve, Belgium
c Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada V6T 1Z1
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of H2[1,8-dibenzyl-1,4,8,11-tetraazacyclotetradecane] (H2Bn2Cyclam) with 2 equiv of LiBu
gives Li2Bn2Cyclam(L)2 (L = Et2O or THF) that reacts with ZrCl4(THF)2 to afford (Bn2Cyclam)ZrCl2 in 40%
yield. Alternatively (Bn2Cyclam)ZrCl2 may be obtained via ZrCl2(CH2SiMe3)2(Et2O)2 in a significantly
higher yield (87%), avoiding the lithiation of H2Bn2Cyclam. The syntheses of other H2*Bn2Cyclam ligand
precursors (*Bn = 3,5-dimethylbenzyl and 3,5-di(tert-butyl)benzyl) and the corresponding Zr derivatives
({3,5-Me2Bn}2-Cyclam)ZrCl2 and ({3,5-tBu2Bn}2-Cyclam)ZrCl2 are also reported. Results on preliminary
Received 15 May 2008
Accepted 5 July 2008
Available online 12 July 2008
Keywords:
Zirconium
Cyclam
Tetraazamacrocycles
Amido ligands
Ethylene polymerisation
tests of e-caprolactone and ethylene polymerisation reactions are presented.
Ó 2008 Elsevier B.V. All rights reserved.
e-Caprolactone polymerisation
The search for alternative systems to Group 4 cyclopendienyl-
H2(3,5-di(tert-butyl)benzyl)Cyclam, 2, in 70% and 55% global yield,
respectively [6]. The benzyl bromide required for the synthesis of 2
was prepared by bromination of commercially available 3,5-di(-
tert-butyl)toluene with an aqueous H2O2/HBr system in ca. 85%
yield [7]. Under these convenient and environmentally friendly
conditions, multigram amounts of 3,5-di(tert-butyl)benzyl bro-
mide (contaminated with residual 3,5-di(tert-butyl)toluene) could
be directly obtained without further purification (Eq. (1)). In our
hands, the known NBS-AIBN procedure (or its variants) reported
in the literature [8] always gave mixtures of mono- and di-bromi-
nated material, unless conversion was halted at around 60%.
based complexes is well documented and has been intensively
explored over the last years [1]. Diamido hybrid ligands – the
combination of two amido units with other neutral donors –
proved adequate in stabilizing early transition metal complexes
that found application in numerous catalytic processes [2]. Despite
the level of sophistication ligand design has reached, and though
various combinations of hard and soft donors have been employed,
early transition metal chemistry of saturated azamacrocycles still
remains relatively unexplored. This type of ligand set is attractive
to transition metal chemistry due to (i) the enhanced stability that
a cyclic array presents, (ii) the flexibility to adopt different confor-
mations and (iii) a macrocycle cavity size that forces the metal cen-
tres to sit above the plane of the donors leaving two adjacent
coordination positions available at the metal coordination sphere
[3]. In addition, and contrasting with azamacrocycles such as por-
phyrins or tetraazannulenes, saturated macrocycles are less prone
to undesired secondary reactions [4]. The scarce number of diam-
ido/diamine transition metal complexes derived from cyclam lies
on the difficulty of its selective functionalisation. This difficulty
was overcome by Guilard and co-workers who reported a facile
and high yield method for the selective trans-functionalisation of
cyclam with benzyl groups [5]. Using Guilard’s procedure we have
prepared ligand precursors H2(3,5-dimethylbenzyl)Cyclam, 1, and
Br
H2O2, HBr, H2O
ð1Þ
85% conv.
t Bu
But
t Bu
t Bu
Addition of 2 equiv of LiBu to a Et2O or THF solution of
(H2Bn2Cyclam), 3, results in the formation of Li2Bn2Cyclam(Et2O)2,
4, or Li2Bn2Cyclam(THF)2, 5, that have been isolated in 65% and 70%
yield, respectively [9,10]. The crystal structure of 5 is depicted in
Fig. 1. The geometry around the lithium atoms is best described
as tetrahedral, with each lithium coordinated by two amido and
one amine macrocyclic moieties and one THF molecule. The lith-
ium atoms are located at opposite faces of the macrocycle at an
average distance of 0.94(32) Å from the plane containing the four
macrocyclic nitrogen atoms [11].
* Corresponding author. Tel.: +351 218419172; fax: +351 218464455.
E-mail address: ana.martins@ist.utl.pt (A.M. Martins).
1387-7003/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2008.07.002

R.F. Munhá et al. / Inorganic Chemistry Communications 11 (2008) 1174–1176
1175
ZrCl2(CH2SiMe3)2(Et2O)2 in 61% and 56% yield, respectively
(Scheme 1) and characterised by 1H, 13C{1H}, COSY, HSQC and
NOESY NMR experiments [14]. The 1H NMR spectra of the
compounds are similar as ten chemically non-equivalent hydrogen
resonances are defined in the range 1.53–4.33 ppm for 7 and 1.46–
4.29 ppm for 8, corresponding to the twenty macrocyclic protons.
The diastereotopic benzylic protons appear as AB systems for both
7 (4.66 and 4.27 ppm) and 8 (4.64 and 4.27 ppm). The average C2
symmetry in solution is confirmed by the 13C{1H} NMR spectra of
7 and 8, where five ligand core resonances and one benzylic carbon
environment can be distinguished. Crystals of 7 suitable for X-ray
diffraction were obtained from a concentrated dichloromethane
solution at ꢀ20 °C [15]. An ORTEP depiction of the molecular struc-
ture is shown in Fig. 2, along with selected bond lengths and an-
gles. The zirconium is coordinated to the four nitrogen atoms of
the macrocycle and to two chloride ligands in a distorted trigonal
prismatic geometry. The size of the cavity forces the metal to sit
1.05(16) Å above the plane defined by the four nitrogen atoms,
forcing the chloride ligands to be cis to each other. Comparison
of the molecular structures of 6 and 7 shows that the different ben-
zylic groups have no influence on the Zr–N bonds.
Fig. 1. ORTEP diagram of 5 showing thermal ellipsoids at 50% probability level. Half
molecule is generated by symmetry operation –x + 3/2, y, ꢀz + 3/2. Hydrogens are
0
omitted for clarity. Selected bond lengths and angles: Li(1)–N(1)0 2.053(3) AÅ, Li(1)–
0
0
N(2) 1.981(4) ÅA, Li(1)–O(1) 1.955(4) ÅA, Li(1)–[N4Plane] 0.94(32) ÅA, N(1)–Li(1)–O(1)
109.88(16)°, N(2)–Li(1)–N(1) 91.17(14)°, N(2)–Li(1)–N(2_$1) 105.11(16)°.
Complexes 6, 7 and 8 have been tested as catalysts for e-capro-
As previously reported by some of us, the reaction of
ZrCl4(THF)2 with 5 led to (Bn2Cyclam)ZrCl2, 6, in 40% yield [10].
The relatively low yield obtained by the chloride metathesis reac-
tion impelled us to investigate acid–base reactions as alternative
synthetic approaches to 6, avoiding the lithiation of H2Bn2Cyclam,
3. Treatment of ZrCl2(CH2SiMe3)2(Et2O)2 [12] with 3 proved to be
the most suitable route for the preparation of (Bn2Cyclam)ZrCl2,
6 which is reproducibly formed in 87% yield (Scheme 1) [13].
Although ZrCl2(CH2SiMe3)2(Et2O)2 presents a low thermal stability,
the reaction was carried under mild THF reflux, evidencing the ra-
pid deprotonation reaction of the macrocycle 3 and its coordina-
tion to the metal centre.
lactone (CL) ring opening polymerisation [16]. At 80 °C, 6 is the
most active catalyst showing an activity of 1.1 kg/molZr[CL]h,
while 7 and 8 display much lower activities. The discrepancy be-
tween the reactivity of the three compounds is surprising and de-
serves further studies. Complex 6 was also tested as a catalyst
precursor for ethylene (E) polymerisation in the presence of MAO
[17]. The system is fairly active. The highest activity (1.7 kg/mol-
The low yield obtained in the preparation of 6 through 4 or 5
could be due to the stability of the lithium linkages in Li2Bn2Cy-
clam(L)2 and to an inadequate pre-conformation of the ligand pre-
cursor to metal coordination. In fact, the square defined by the
bridging lithium cations (Li–Namido–Li–Namido, see Fig. 1) is a stable
motif currently observed in lithium polyamides. Furthermore, be-
fore coordinate to the zirconium, the macrocyclic ring has to un-
dergo
a
structural reorganisation (equivalent to nitrogen
Fig. 2. ORTEP diagram of 7 showing thermal ellipsoids at 50% probability level. Half
inversion) to attain a bowl conformation and place the four nitro-
gen atoms in adequate position to bind to one metal centre.
Complexes ({3,5-Me2Bn}2-Cyclam)ZrCl2, 7, and ({3,5-tBu2Bn}2-
Cyclam)ZrCl2, 8, were obtained from the reaction of 1 or 2 with
molecule is generated by symmetry operation –x + 1/2, y, ꢀz. Hydrogens are
0
omitted for clarity. Selected bond lengths and angles: Zr(1)–N(1) 2.441(2) AÅ, Zr(1)–
0
0
0
N(2) 2.044(2) ÅA, Zr(1)–Cl(1) 2.5039(8) ÅA, Zr(1)–[N4Plane] 1.05(16) ÅA, N(2)–Zr(1)–
N(1) 72.32(8)°, N(1)–Zr(1)–Cl(1) 80.00(5)°, N(2)–Zr(1)–Cl(1) 133.92(6)°.
R
H
N
N
N
N
R = 3,5 MePh
R = 3,5 tBuPh
R = Ph
1
2
3
N
2 LiBu
N
S
Bn
S = Et2
S =THF
O
4
5
Li
Li
Et2O orTHF
Bn
N
S
H
N
R
ZrCl4(THF)2
THF
ZrCl2(CH2SiMe3)2(Et2O)2
THF
Cl
Cl
Zr
R
R
N
N
N
N
6
7
8
R = Ph
R =3,5 MePh
R =3,5 tBuPh
Scheme 1.

1176
R.F. Munhá et al. / Inorganic Chemistry Communications 11 (2008) 1174–1176
temperature. The white solution turned yellow and a white precipitate was
instantly formed. The solution was left stirring for 1 h, concentrated to a
minimum volume of ether and filtered. The white powder was washed with n-
hexane and isolated in 65% yield. NMR (C6D6): 1H d (ppm) 7.10–7.02 (10H, Ph),
Zr[E]h) was observed at 18 °C with Al/Zr = 2000. At higher temper-
atures the system activity decreases, probably due to thermal
decomposition of the active species. The Al/Zr ratio also affects
the activity of 6, increasing in the range Al/Zr 500–2000.
To the best of our knowledge, these continue to represent rare
examples of Group 4 metals supported by saturated macrocyclic
arrays. The reactivity and assessment of catalytic applications for
these compounds are in progress.
3.80–3.58 (8H, NCH2, NCH2Ph), 3.40 (d, 2H, NCH2Ph), 3.27 (m, 10H, NCH2
e
OCH2CH3), 3.02 (m, 2H, NCH2), 2.89 (m, 2H, NCH2), 2.71 (m, 2H, NCH2), 2.42 (m,
4H, NCH2), 2.04 (m, 2H, CCH2C), 1.78 (m, 2H, CCH2C), 1.12 (t, 12H, OCH2CH3);
13C{1H} d (ppm) 136.8, 130.2, 127.4 (Ph), 65.9 (OCH2CH3), 59.3, 58.9, 56.8, 55.9,
54.2 (CH2N), 30.1 (CCH2C), 15.6 (OCH2CH3). Anal. calcd for C32H54Li2N4O2: C,
71.09; H, 10.07; N, 10.36. Found: C, 71.20; H, 9.82; N, 13.78.
[10] R.F. Munhá, S. Namorado, S. Barroso, M.T. Duarte, J.R. Ascenso, A.R. Dias, A.M.
Martins, J. Organomet. Chem. 691 (2006) 3853.
Acknowledgement
[11] Crystallographic data for 5: C32H50N4Li2O2, Fw = 536.64, Triclinic, P-1,
0
0
0
a = 8.740(2) ÅA,
b = 82.339(13)°,
b = 9.928(3) ÅA,
c = 10.0012(3) ÅA,
a = 64.522(13)°,
c
= 84.472(15)°, V = 776.5(4) AÅ3, Z = 1, F(000) = 292, Bruker
We are grateful to Fundação para a Ciência e a Tecnologia, Por-
tugal that funded this work (POCI/QUI/55744/2004 and SFRH/BD/
29986/2006).
AXS KAPPA APEX II difractometer, T = 150(2) K, Colourless, Plate,
0.1 ꢁ 0.1 ꢁ 0.2,
= 0.070 mmꢀ1, Dx = 1.148 g/cm3, hmax = 29.24, 4178 unique
reflections, 2221 with I > 2 (I), 181 parameters refined. Final indices
[I > 2 (I)]: R1 = 0.0629, R2 = 0.1263, all data: R1 = 0.1375, R2 = 0.1476.
l
r
R
r
x
x
[12] H. Brand, J.A. Capriotti, J. Arnold, Organometallics 13 (1994) 4469.
[13] (1,8-Dibenzyl-1,4,8,11–tetraazacyclotetradecane)ZrCl2 (6): Compound 3 (0.86 g,
Appendix A. Supplementary material
2.3 mmol) was dissolved in 30 mL of THF and was rapidly added to
a
concentrated THF solution of ZrCl2(CH2SiMe3)2(Et2O)2 (1.10 g, 2.3 mmol). A
reflux condenser was adapted and the reaction mixture was left under mild
reflux. After 3 h, 90% of the THF was evaporated. The remaining THF was
filtered and the bright yellow solid was rinsed with hexanes and taken to
dryness (1.06 g, 87%).
CCDC 687687 and 687688 contain the supplementary crystallo-
graphic data for 5 and 7. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
[14] (1,8–(3,5-Dimethylbenzyl)–1,4,8,11–tetraazacyclotetradecane)ZrCl2
Compound 1 (0.50 g, 1.1 mmol) was dissolved in 15 mL of THF and was
rapidly added to concentrated THF solution of ZrCl2(CH2SiMe3)2(Et2O)2
(7):
a
References
(0.55 g, 1.1 mmol). A reflux condenser was adapted and the reaction mixture
was left under mild reflux. After 2 h a light yellow precipitate was formed. The
mixture was refluxed for further 3 h and 90% of the THF was evaporated. The
remaining THF was filtered and the bright yellow solid was rinsed with
hexanes and taken to dryness (0.40 g, 61%). Crystals suitable for X-ray
diffraction were obtained from a dichloromethane solution stored at ꢀ20 °C.
NMR (CDCl3): 1H d (ppm) 6.99 (b, 2 H, p-Ph), 6.94 (b, 4H, o-Ph), 4.66 (d, 2H,
PhCH2N), 4.33 (m, 2H, NCH2CH2CH2N(CH2Ph)), 4.27 (d, 2H, PhCH2N), 3.78 (m,
2H, NCH2CH2N(CH2Ph)), 3.55 (m, 2H, NCH2CH2CH2N(CH2Ph)), 3.35 (m, 2H,
NCH2CH2N(CH2Ph)), 3.15 (m, 2H, NCH2CH2N(CH2Ph)), 2.78 (m, 2H,
NCH2CH2CH2N(CH2Ph)), 2.63 (m, 2H, NCH2CH2CH2N(CH2Ph)), 2.51 (m, 2H,
NCH2CH2N(CH2Ph)), 2.33 (b, 12H, Ph-CH3), 1.91 (m, 2H, CH2CH2CH2), 1.53 (m,
2H, CH2CH2CH2); 13C{1H} d (ppm) 137.7 (ipso-PhCH3), 131.0 (ipso-PhCH2N),
130.5 (o-Ph), 129.9 (p-Ph), 56.4 (NCH2CH2CH2N(CH2Ph)), 56.0 (PhCH2N), 55.0
(NCH2CH2CH2N(CH2Ph), 53.6 (NCH2CH2N(CH2Ph), 49.1 (NCH2CH2N(CH2Ph)),
24.8 (CH2CH2CH2), 21.3 (CH3PhCH2N). Anal. calcd for C28H42Cl2N4Zr(CH2Cl2)2:
C, 47.00; H, 6.05; N, 7.31. Found: C, 46.97; H, 6.80; N, 7.38.(1,8-(3,5-Di-tert-
butylbenzyl)–1,4,8,11–tetraazacyclotetradecane)ZrCl2 (8): Compound 2 (0.70 g,
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1.2 mmol) was dissolved in 15 mL of THF and was rapidly added to
a
concentrated THF solution of ZrCl2(CH2SiMe3)2(Et2O)2 (0.56 g, 1.2 mmol). A
reflux condenser was adapted and the reaction mixture was left under mild
reflux. No precipitate was formed after 6 h of reflux. The solution was taken to
dryness. The resulting yellow solid was washed with toluene, resulting in a
bright yellow solid that was taken to dryness (0.50 g, 56%). Crystalline material
was obtained from a cold dichloromethane solution. NMR (CDCl3): 1H d (ppm)
7.32 (b, 2H, p-Ph), 7.06 (b, 4H, o-Ph), 4.64 (d, 2H, PhCH2N), 4.29 (m, 2H,
[3] S.J. Archibald, Annu. Rep. Prog. Chem. Sect. A 103 (2007) 264;
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Lecomte, Eur. J. Org. Chem. (1998) 1971.
NCH2CH2CH2N(CH2Ph)),
4.27
(d,
2H,
PhCH2N),
3.75
(m,
2H,
NCH2CH2N(CH2Ph)), 3.50 (m, 2H, NCH2CH2CH2N(CH2Ph)), 3.27 (m, 2H,
NCH2CH2N(CH2Ph)), 3.12 (m, 2H, NCH2CH2N(CH2Ph)), 2.76 (m, 2H,
NCH2CH2CH2N(CH2Ph)), 2.58 (m, 2H, NCH2CH2CH2N(CH2Ph)), 2.39 (m, 2H,
NCH2CH2N(CH2Ph)), 1.85 (m, 2H, CH2CH2CH2),1.46 (m, 2H, CH2CH2CH2), 1.25
(b, 36H, Ph-C(CH3)3); 13C{1H} d (ppm) 150.5 (ipso-PhC(CH3)3), 129.9 (o-Ph),
126.9
(ipso-PhCH2N),
122.0
55.1
(p-Ph),
(NCH2CH2CH2N(CH2Ph)),
56.9
(PhCH2N),
56.3
53.5
[6] Compound 1: NMR (CDCl3): 1H d (ppm) 6.88 (b, 4H, p-Ph), 6.83 (b, 2H, o-Ph),
3.58 (s, 4H, PhCH2N), 2.98 (b, 2H, NH), 2.71 (m, 8H, NCH2), 2.59 (m, 4H, NCH2),
2.49 (m, 4H, NCH2), 2.24 (s, 36H, Ph-(CH3)), 1.82 (m, 4H, CH2CH2CH2). 13C{1H} d
(ppm) 137.4 (ipso-PhCH2N), 128.6 (p-Ph), 127.3 (o-Ph), 57.6 (PhCH2N), 53.9,
52.1, 49.7, 47.5 (NCH2), 26.0 (CH2CH2CH2), 21.3 (Ph(CH3)). Anal. calcd for
C28H44N4(CH2Cl2)0.4: C, 72.48; H, 9.59; N, 11.90. Found: C, 72.42; H, 10.04; N,
12.38.Compound 2: NMR (CDCl3): 1H d (ppm) 7.30 (b, 2H, p-Ph), 7.17 (b, 4H, o-
Ph), 3.61 (s, 4H, PhCH2N), 2.76 (m, 8H, NCH2), 2.57 (m, 8H, NCH2), 2.31 (b, 2H,
NH), 1.86 (m, 4H, CH2CH2CH2), 1.33 (s, 36H, Ph-C(CH3)3. 13C{1H} d (ppm) 150.4
(ipso-PhC(CH3)3), 137.2 (ipso-PhCH2N), 123.3 (o-Ph), 120.8 (p-Ph), 57.9
(PhCH2N), 53.60, 49.1, 47.4, 47.3 (NCH2), 34.7 (PhC(CH3)3), 31.5 (PhC(CH3)3),
26.1 (CH2CH2CH2). Anal. calcd for C40H68N4(CH2Cl2)0.2: C, 77.63; H, 11.08; N,
9.01. Found: C, 77.80; H, 11.62; N, 8.66.
(NCH2CH2CH2N(CH2Ph)),
(NCH2CH2N(CH2Ph)), 49.0 (NCH2CH2N(CH2Ph)), 34.7 (PhC(CH3)3), 31.5
(PhC(CH3)3), 24.9 (CH2CH2CH2). Anal. calcd for C40H66Cl2N4Zr(CH2Cl2): C,
57.93; H, 8.06; N, 6.59. Found: C, 58.73; H, 8.99; N, 6.56.
[15] Crystallographic data for 7:0 C28H42N4ZrCl2, Fw = 596.8, Monoclinic, P2/a,
0
0
a = 13.411(4) ÅA, b = 7.380(2) ÅA, c = 14.324(4) ÅA,
a
= 90.00°, b = 97.722(19)°,
0
c
= 90.00°, V = 1404.8(7) ÅA3, Z = 2, F(000) = 624, Bruker AXS KAPPA APEX II,
T = 150(2) K, Colourless, Plate, 0.05 ꢁ 0.1 ꢁ 0.1,
l
= 0.605 mmꢀ1, Dx = 1.411 g/
(I), 159 parameters
r(I)]: R1 = 0.0549, xR2 = 0.0922, all data:
cm3, hmax = 32.64, 5132 unique reflections, 3065 with I > 2
r
refined. Final
R1 = 0.1217,
R
indices [I > 2
xR2 = 0.1055.
[16] Polymerization studies were carried out using standard Schlenk techniques
and the mixture was quenched with acidic methanol (1% CH3COOH). The
solution was evaporated, the polymer was precipitated by addition of water
and dried in a vacuum desiccator during two days.
[17] J.C.W. Chien, B.-P. Wang, J. Polym. Sci., Part A: Polym. Chem. 26 (1988) 3089.
The polymerization mixture was quenched with acidic methanol (2% HCl) and
the polymer was filtered, washed with methanol and dried in a vacuum oven
at 60 °C during two days..
[7] A. Podgorsek, S. Stavber, M. Zupan, J. Iskra, Tetrahedron Lett. 47 (2006) 7245.
[8] I. Karame, M. Jahjah, A. Messaoudi, M.L. Tommasino, M. Lemaire, Tetrahedron:
Asymmetry 15 (2004) 1569;
M.F. Mayer, S. Nakashima, S.C. Zimmerman, Org. Lett. 7 (2005) 3005.
[9] (1,8-Dienzyl-1,4,8,11–tetraazacyclotetradecane)Li2(Et2O)2 (4): Compound
3
(1.00 g, 2.6 mmol) was dissolved in 40 mL of Et2O and 2 equiv of a 1.6 M
LiBu solution in n-hexane (3.28 mL, 5.3 mmol) were added drop wise at room