Solvolysis reactions at the 13th carbon of 1-aryl organoiron complexes

Article abstract of DOI:10.1016/j.tetlet.2009.11.052

Acid-catalysed solvolysis procedures exchange protecting groups on benzyl alcohol derivatives in the synthesis of η⁵ 1-arylcyclohexadienyliron building blocks for alkaloid synthesis. The reaction proceeds via a carbocation intermediate which can also be intercepted by intramolecular electrophilic addition to the tricarbonyl(η⁴-diene)iron(0) moiety to provide a novel and high-yielding cyclisation reaction forming a 5aα-cyanomethyl-5a,8a-dihydrofluorene tricarbonyliron complex in 96% yield.

Full text of DOI:10.1016/j.tetlet.2009.11.052

Tetrahedron Letters 51 (2010) 591–595
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Solvolysis reactions at the 13th carbon of 1-aryl organoiron complexes
*
Caroline Roe, Elizabeth J. Sandoe, G. Richard Stephenson
School of Chemistry, University of East Anglia, Norwich, NR4 7TJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Acid-catalysed solvolysis procedures exchange protecting groups on benzyl alcohol derivatives in the
synthesis of g5 1-arylcyclohexadienyliron building blocks for alkaloid synthesis. The reaction proceeds
via a carbocation intermediate which can also be intercepted by intramolecular electrophilic addition
Received 6 October 2009
Revised 4 November 2009
Accepted 13 November 2009
Available online 18 November 2009
to the tricarbonyl(
forming a 5a -cyanomethyl-5a,8a-dihydrofluorene tricarbonyliron complex in 96% yield.
Ó 2009 Elsevier Ltd. All rights reserved.
g
4-diene)iron(0) moiety to provide a novel and high-yielding cyclisation reaction
a
Keywords:
Organoiron
Solvolysis
Deprotection
Dihydrofluorene
Electrophilic cyclisation
Our synthetic strategy1–5 introduces the ‘13th’ benzylic carbon
atom (Fig. 1) early in our organoiron-mediated6,7 routes to Amaryl-
lidaceae alkaloids.8,9 In many alkaloid syntheses this carbon is
added much later10 by a Pictet–Spengler reaction. In comparison,
our routes are more convergent, and hence potentially more effi-
cient. However, the advantage has been somewhat offset by the
need to employ relatively bulky protecting groups on the benzylic
alcohol.3 The choice of a 1,1 iterative strategy2,11 to elaborate 1 by
repeated use of the metal lies at the heart of this application of
arylcyclohexadienyliron complexes (e.g., 2a) as key building
blocks, and we have shown2,12–14 that when the 1-aryl group bears
an ortho substituent, it is important that the arene can adopt a rel-
atively flat conformation aligned with the dienyl ligand to allow
nucleophiles to add ipso. Large protecting groups on the CH2O at
this ortho position are more likely to hinder the approach of the
nucleophile next to the aromatic ring, and so complicate the opti-
misation of synthetic routes because of the need to balance perfor-
mance and selectivity in two separate areas: the fundamental issue
hexafluorophosphoric acid method was used with the methyl ether
4 (Scheme 1), the product, isolated in 83% yield, was the acetate 5,
which was clearly identified by the replacement of the OMe 1H and
13C resonances at 4.34 and 73.0 ppm with signals at 4.94 and
64.5 ppm for the CH3CO2 group, and the presence of an additional
stretching vibration at 1728 cmÀ1 in the IR spectrum of the prod-
uct. This structure was confirmed by FAB high resolution mass
spectrometry which gave the expected ion for C21H21FeO8PF6.
The two additional OMe groups on the arene are clearly sufficiently
electron-donating to activate the CH2OMe group for an efficient
SN1 solvolysis reaction19 with acetic acid generated by mixing
aqueous hexafluorophosphoric acid and acetic anhydride. This
observation led us to examine the possibility that SN1 conditions
might provide a way to use the simple (and small) CH2OMe substi-
tuent in synthetic routes that later required a free OH group at this
position, by employing an SN1 process to replace OMe by OH dur-
ing other acid-catalysed reactions in the closing stages of the syn-
thesis. In the course of this work, we have discovered an
unexpected intramolecular C–C bond formation that gives rise to
a dihydrofluorene-derived cyclohexadieneiron complex.
of i versus
x
nucleophile addition,12 and practical issues concern-
ing the introduction, stability and removal of protecting groups.
Our preferred method1,15–18 to prepare 1-aryl-cyclohexadienyl-
iron electrophiles uses the removal of an allylic OMe group next to
Addition of the 2-methoxymethyl-4,5-dimethoxyarene to tri-
carbonyl(
g
5-2-methoxycylohexadienyl)iron(1+) hexafluorophos-
the
g
4-diene complex. TFA followed by the addition of ammonium
phate (7)20 (Scheme 1) was performed21,22 by generating
a
hexafluorophosphate,15 aqueous hexafluorophosphoric acid in ace-
tic anhydride,16 triphenylcarbenium ion reagents17 and Meerw-
ein’s reagent18 are all procedures that we have employed
successfully in a wide variety of reactions. However, when the
diarylcuprate 6 from our normal5 aryllithium reagent 3 by reaction
with copper(I) iodide in THF at À78 °C over a period of 90 min. The
electrophile 7 was then added, and after a further 2 h at À78 °C, the
reaction mixture was warmed to room temperature overnight and
worked up to give the methoxybenzyl ether complex 8 in 56%
yield. A simple SN1 hydrolysis procedure with dilute aqueous
HCl was then employed to evaluate the practicality of replacing
* Corresponding author.
E-mail address: g.r.stephenson@uea.ac.uk (G.R. Stephenson)
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.11.052

592
C. Roe et al. / Tetrahedron Letters 51 (2010) 591–595
OMe
+
MeO
MeO
Li
Fe(CO)3
MeO
t
OSi BuPh2
1
benzylic carbon introduced during initial nucleophile addition
OMe
OMe
+
Fe(CO)3
Fe(CO)3
MeO
MeO
MeO
MeO
MeO
t
OSi BuPh2
OR
2a,b
t
a: R = Si BuPh2; b: R = Me
OH
NC
OMe
Fe(CO)3
steps
MeO
MeO
MeO
N
OH
ref. 3, ref. 5
MeO
maritidine
Figure 1. Use3 of an aryllithium reagent with an ortho CH2OTBDPS substituent to extend our ‘C12 building block’1–5 approach in a formal total synthesis of the Amaryllidaceae
alkaloid maritidine (the ‘13th carbon’ is ringed).
the OMe group by OH. Despite quite vigorous reaction conditions
(overnight at reflux), the desired benzyl alcohol product 9 was ob-
tained in 48% yield. Using methyl 3,4-dimethoxybenzyl ether itself
as a simple model compound (Scheme 2), a range of milder reac-
tion conditions were tried (Table 1). The conditions used success-
fully with 4 to form the acetate, followed by saponification with
aqueous sodium hydroxide gave only traces of the expected 3,4-
dimethoxybenzyl alcohol23 (11). The use of aqueous HCl worked
better, and with a longer reaction time would probably give the
most efficient procedure, as was the case (Scheme 1) with the tri-
OMe
MeO
MeO
Li
+
ref. 5
+
Fe(CO)3
OMe
MeO
3
1
OMe
OMe
+
MeO
Fe(CO)3
Fe(CO)3
HPF6, Ac2O
0 ºC, 30 min
MeO
MeO
MeO
MeO
OMe
OAc
5
4
83 %
OMe
MeO
(
CuLi
THF, -78 ºC, 2 h
then to rt overnight
)
2
+
+
Fe(CO)3
OMe
MeO
6
7
OMe
OMe
Fe(CO)3
MeO
Fe(CO)3
2N aq HCl
MeO
MeO
OMe
56%
THF, reflux, 12 h
MeO
48%
OH
9
8
Scheme 1. Solvolysis reactions in the synthesis of cationic g5 and neutral g4 aryl-substituted organoiron complexes.

C. Roe et al. / Tetrahedron Letters 51 (2010) 591–595
593
MeO
OMe
MeO
MeO
Me3SiCl, H2SO4
Ac2O, Et2O, 2 h, rt
OMe
MeO
OMe
HCl, H2O / THF,
70 ºC, 1.5 h
10
MeO
MeO
11
44%
OH
MeO
OMe
19%
Scheme 2. Cyclisation and solvolysis of methyl 3,4-dimethoxybenzyl ether (see Table 1).
eratrylene 1025 in 44% yield. This is a known26 product from the
benzyl alcohol, formed by reaction with glacial acetic acid contain-
ing a few drops of concd sulfuric acid. This trimerisation reaction
would give an interesting product in the case with the tricarbonyl-
iron complex attached, but when 7 was treated with trimethylsilyl
chloride and concd sulfuric acid, decomposition of the more sensi-
tive organometallic substrate was observed.
Table 1
Solvolysis reactions of the model compound methyl 3,4-dimethoxybenzyl ether
Entry
Conditions
1
2
3
4
5
HPF6 (aq), Ac2O, 0 °C, 90 min, then NaOH (aq) Trace of product 11
p-TSA,a THF, H2O (3:2)
No reaction
p-TSA,a AcOH, Ac2O (1:1)
Trace of product 11
44% yield of trimer 10
19% yield of product 11
TMSCl,b H2SO4, Ac2O, Et2O, 2 h
HCl, H2O, THF (10:1), 70 °C, 1.5 h
Reaction of [Me3SiCH2CH2OC(O)CHCN]Na27 with 2b was fol-
lowed by desilylative dealkylation/carboxylation promoted by
adding TBAF to the reaction mixture and heating at reflux for
90 min, to give an efficient one-pot formation of the nitrile 12
which is the methyl ether analogue of the silyl ethers used3
(Fig. 1) in our maritidine work. Reaction of 12 with dilute aqueous
HCl overnight (Scheme 3) afforded the benzyl alcohol 13 in 22%
yield. Addition of the SEM-protected aryllithium reagent 14 to
a
para-Toluene sulfonic acid.
Trimethylsilyl chloride.
b
carbonyliron complex 5. Sarma has described24 rather unusual SN1
conditions (trimethylsilyl chloride and concd sulfuric acid), and
with methyl 3,4-dimethoxybenzyl ether this formed cyclotriv-
NC
OMe
NC
OMe
Fe(CO)3
Fe(CO)3
2N aq HCl
MeO
MeO
MeO
MeO
THF, reflux, 15 h
OH
OMe
12
22%
13
Scheme 3. Access to the benzyl alcohol 13.
OMe
OMe
+
Ph3CPF6,
CH2Cl2, 0 ºC,
1 h
Fe(CO)3
OMe
MeO
Fe(CO)3
MeO
MeO
MeO
MeO
Li
+
MeO
MeO
+
Fe(CO)3
OSEM
CH2Cl2,
OSEM
MeO
OSEM
-100o C,
1 h
1
14
66%
15
16
46%
NC
NC
OMe
[Me3SiCH2CH2OC(O)CHCN]Na,
THF, 0 ºC, 1 h
(66%)
OMe
N
NC
OMe
Fe(CO)3
Fe(CO)2PPh3
Me3NO.2H2O,
PPh3
MeO
MeO
Fe(CO)3
HPF6, CH2Cl2,
0 ºC, 10 min
MeO
MeO
MeO
H
44%
acetone, reflux,
18 h
H
then TBAF, THF,
reflux, 1 h (64%)
OSEM
19
MeO
18
96%
17
H2N
OMe
Raney Ni, H2,
EtOH, NH3,
rt, 1 h
Fe(CO)3
MeO
MeO
H
66%
20
Scheme 4. Synthetic route to the novel cyclisation product 18 (box) and preparation of the dicarbonyltriphenylphosphine and alkylamine derivatives.

594
C. Roe et al. / Tetrahedron Letters 51 (2010) 591–595
the 1,4-dimethoxy salt 1 gave 15 in 46% yield (Scheme 4). The
product was converted in 66% yield into the 1-arylcyclohexadie-
nyliron salt 16 using triphenylcarbenium hexafluorophosphate in
dichloromethane. Addition of [Me3SiCH2CH2OC(O)CHCN]Na fol-
lowed by reaction with TBAF gave the nitrile 17 in 64% yield. The
intention had originally been to perform both the deprotection of
the benzyl alcohol and the desilylative dealkylation/decarboxyl-
ation in a single step, but unexpectedly, the SEM-protecting group
proved resistant to TBAF in this example. Lipshutz had reported28
an improved desilylation method using dry TBAF. When 17 was
heated at 40 °C in THF with TBAF and 4 Å molecular sieves, traces
of an unexpected product were observed by TLC. This was clearly
not the desired benzyl alcohol because it had a higher Rf value than
the starting material. Consequently, we examined this problem
further (Table 2) in our survey of SN1 procedures. In an earlier
model study29 for lycoramine, decomposition had been observed30
when deprotection of a phenolic MOM ether had been attempted
using 2 N HCl, so we chose alternative reaction conditions for use
with compound 17. Interestingly, we discovered that the less polar
compound observed with dry TBAF now became the major product
under SN1 conditions. This reaction was performed at 40 °C with
aqueous hexafluorophosphoric acid in THF, and gave a 67% yield
of 18. This was improved31 to 96% yield by dropping the reaction
temperature to 0 °C and using dichloromethane instead of THF.
The product from these reactions had only two OMe groups, and
since these had signals at the position in the 1H NMR spectrum ex-
pected for methyl aryl ethers, it was clear that the reaction had
been partially successful in the sense that the benzylic OMe group
had been removed. The typical 4.72 ppm 7 Hz doublet for the res-
onance of the benzylic CH2O methylene group was also replaced by
two doublet of doublet resonances at 3.29 and 2.69 ppm (Table 3),
each integrating for a single hydrogen, which, in view of the shift to
higher field, ruled out the possibility that the product contained a
CH2O group adjacent to the arene.32 Although 18 was crystalline,
X-ray quality crystals could not be obtained, and when the product
was converted into the Fe(CO)2PPh3 derivative 19 the product
failed to crystallise at all. The nitrile in 18 was reduced with Raney
nickel in ethanol saturated with ammonia to afford the primary
amine 20 which again had the pair of doublet of doublet 1H sig-
nals, this time at 3.14 and 2.62 ppm. All three products had very
similar features in their 1H NMR spectra for the CH2 group originat-
ing from the benzyl ether. On this basis, the identity of the product
was finally established. The larger coupling in each doublet of dou-
blets is clearly the geminal coupling between the two hydrogens,
showing that the CH2 group remains intact. The smaller couplings
of about 10 Hz and 4 Hz indicate the presence of a CH adjacent to
the CH2, indicating that a C–C bond-formation reaction had been
caused by the hexafluorphosphoric acid. The initial product was
thus assigned as the dihydrofluorene 18, produced by intramolec-
ular electrophilic C–C bond formation between the carbocation
formed in the SN1 process and the
g
4-dieneiron complex. The cis
ring junction is structurally the most reasonable and is consistent
with the normal33 endo addition of electrophiles to tricar-
bonyl(g
4-cyclohexadiene)iron complexes.
In conclusion, we have shown that SN1 conditions can cause
efficient replacement of benzylic ethers during the preparation of
cationic cyclohexadienyliron(1+) complexes, that this same proce-
dure works for the deprotection of neutral
g
4-cyclohexadiene
complexes, though in lower yield, and, when performed in less po-
lar non-nucleophilic solvents, provides an efficient method to ef-
fect a novel stereoselective intramolecular cyclisation reaction.
The mild conditions (0 °C) and high yield (96%) of this reaction
indicate that it will be suitable to gain access to a new class of tri-
cyclic
systems.
g
4-dieneiron complexes with fused cyclopentane ring
Acknowledgements
We thank the EPSRC and GlaxoSmithKline for financial support
and the EPSRC National Mass Spectrometry Service, Swansea Uni-
versity, for high resolution mass spectrometric data.
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Solvolysis reaction conditions to form 18
Entry
Conditions
Yield (%)
1
2
3
TBAF,a THF, 4 Å sieves, 40 °C
HPF6, THF, H2O, 40 °C
HPF6, CH2Cl2, 0 °C
Traceb
67
96
a
Tetrabutylammonium fluoride.
Observed by TLC.
b
Table 3
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between hydrogens in the rigid 8a,9 section of a dihydrofluorene
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18
19
20
3.29
3.28
3.14
16.8,a 9.6b
16.5,a 9.9b
16.5,a 9.9b
2.69
3.02
2.62
16.8,a 3.8c
16.5,a 4.3c
16.5,a 3.3c
a
b
c
J9Hexo,9Hendo,.
J8aHexo,9Hexo,.
J8aHexo,9Hendo
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5a -cyanomethyl-2,3,7-trimethoxy-5a,8a-dihydrofluorene]iron(0) 18 as a pale
a
yellow powder (15 mg, 0.034 mmol) in 96% yield. 1H NMR (270 MHz, CDCl3): d
2.54 (m, 3H, CH2CN and H-8a); 2.69 (dd, J 16.8, 3.8, 1H, H-8); 2.78 (d, J 6.3, 1H,
H-5); 3.29 (dd, J 16.8, 9.6, 1H, CH2); 3.30 (m, 1H, H-8); 3.85 (s, 3H, Ar-OMe);
3.90 (s, 3H, Ar-OMe); 5.17 (dd, J 6,3, 2.3, 1H, H-6); 6.66 (s, 1H, H-1); 6.69 (s, 1H,
H-4); 13C NMR (67.5 MHz, CDCl3): d 32.8, 38.5, 46.2, 54.6, 56.0, 56.1, 56.5, 58.0,
59.0, 65.9, 104.9, 109.5, 118.3, 134.0, 137.3, 138.8, 149.0, 150.0, 209.7; mmax
(CH2Cl2) 2307, 2049, 1974, 1609, 1507, 1423, 1266, 1112, 896 cmÀ1; m/z (EI)
409 (M+ÀCO, 4), 381 (8), 353 (24), 234 (23), 206 (47), 178 (61), 150 (100%);
Elemental Anal. Calcd for C21H19FeNO6: C, 57.7; H, 4.4, N, 3.2. Found: C, 57.7; H,
4.2; N, 3.0.
15. Astley, S. T.; Stephenson, G. R. Synlett 1992, 507–509; Stephenson, G. R.; Finch,
H.; Owen, D. A.; Swanson, S. Tetrahedron 1993, 49, 5649–5662.
16. McKillop, A.; Stephenson, G. R.; Tinlk, M. J. Chem. Soc., Perkin Trans. 1 1993,
1827–1828; Howell, J. A. S.; Bell, A. G.; O’Leary, P. J.; Stephenson, G. R.;
Hastings, M.; Howard, P. W.; Owen, D. A.; Whitehead, A. J.; McArdle, P.;
Cunningham, D. Organometallics 1996, 15, 4247–4257.
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Rose-Munch, F.; Rose, E.; Stephenson, G. R. Tetrahedron Lett. 2001, 42, 3311–
3313.
32. The MOM ether can be removed with MgBr2 to give the alcohol 13 which was
shown to be different (TLC) from the product 18.
33. Franck-Neumann, M.; Sedrati, M.; Mokhi, M. New J. Chem. 1990, 14, 471–480;
Rivera, A. V.; Sheldrick, G. M. Acta Crystallogr., Sect. B 1978, B34, 1716–1718.
18. Simpkins, S. M. E. Ph.D. Thesis, University of East Anglia, 2001.
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