Direct reaction of aryl iodides with activated aluminium powder and reactions of the derived aryl sesquiiodides

Article abstract of DOI:10.1055/s-0029-1219179

High temperature (110-120 °C) direct reaction of ArI (Ar = Ph, 1-C ₁₀H₇, 3-Tol) with aluminium powder in the presence of HgCl₂ (1 mol%) or liquid gallium metal (10mol%) leads to quantitative conversion to the aryl sesquiiodides Al₂Ar₃I₃. The latter, highly Lewis acidic, compounds are effective in opening of cyclic ethers and direct ester to amide conversions. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0029-1219179

636
LETTER
Direct Reaction of Aryl Iodides with Activated Aluminium Powder and
Reactions of the Derived Aryl Sesquiiodides
D
irec
t
Reaction of
i
A
ryl Io
a
dides
w
ith
o
Activated A
p
luminium Po
i
wder ng Tang, Daniel Rawson, Simon Woodward*
School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, UK
Fax +44(115)9513541; E-mail: simon.woodward@nottingham.ac.uk
Received 10 November 2009
Dedicated to Prof. G. Pattenden on the occasion of his 70th birthday
tion even after prolonged heating (entry 1). However, the
presence of catalytic amounts of HgCl2 led to high yields
of naphthalene in the presence or absence of solvent (en-
tries 2–4). Under these conditions reduction of HgCl2 and
Abstract: High temperature (110–120 °C) direct reaction of ArI
(Ar = Ph, 1-C10H7, 3-Tol) with aluminium powder in the presence
of HgCl2 (1 mol%) or liquid gallium metal (10 mol%) leads to quan-
titative conversion to the aryl sesquiiodides Al2Ar3I3. The latter,
highly Lewis acidic, compounds are effective in opening of cyclic subsequent Al-amalgam formation is known to take
place.9 As the use of even catalytic amounts of mercury is
ethers and direct ester to amide conversions.
not always acceptable in organic syntheses we sought a
complimentary activation method. It is known that galli-
um forms a 97:3 alloyed phase with aluminium that shows
enhanced redox properties under certain conditions.15 Use
of liquid Ga allowed sesquiiodide formation (entry 5), al-
beit at a slower rate than that observed with HgCl2 activa-
tion.16 Neither Hg or Ga activation worked for 1-BrC10H7,
nor did other promoters tried (PbI2, TiCl4).
Key words: organometallic reagents, Lewis acids, amides, halides,
electron transfer
Elemental aluminium is the most abundant metal in the
Earth’s crust and has been widely available since the in-
troduction of the Hall–Héroult process in the 1890s. How-
ever, despite its potential as a reducing agent (Al3+/Al0
–1.66 volts) its use in synthetic organic chemistry has
been limited by its ready passivation through the forma-
tion of inert surface oxide coatings (Al-O ca. 101 kcal
mol–1).1 Amalgam formation has been used as a method to
activate aluminium; Al-Hg acts as an exemplary reducing
agent for sensitive nitro2,3 and sulfoximine4 compounds.
Aluminium metal also serves as a terminal reductant for
Barbier-type reactions of aldehydes,5 carboxylic esters,6
Baylis–Hillman-derived acetates7 and imines.8 In many
cases the presence of redox catalysts is required to pro-
mote the reaction including, SbIII, FeII, NiII, CrIII and PbII.
Formation of isolable sp3 organoaluminiums is dominated
by the industrial (Hüls) process for the sesquichlorides
Al2R3Cl3 (R = Me, Et).9 Through the use of reactive allylic
halides the synthetic utility of such compounds has been
extended to 1,2-additions of allylic sesquihalides to
carbonyls10 and imines.11,12
Table 1 Optimization of C–I Insertion Process
I
Nap
I
Nap
Nap
H2O
Al
Al
Al (1.1 equiv)
C10H8
I
activating
conditions
I
Nap = 1-C10H7
Entry Conditions
Yield of
C10H8 (%)a
1
2
3
Al, neat C10H7I, 120 °C, 2–350 h
0
Al, neat C10H7I, 120 °C, 3 h, HgCl2 (1 mol%) >95
Al, C10H7I (1 M in toluene), 110 °C, 3.5 h,
HgCl2 (1 mol%)
89
4
Al, C10H7I (1 M in p-xylene), 138 °C, 3.5 h,
91
HgCl2 (1 mol%)
In contrast, direct preparations of sp2 sesquihalides are
limited to two papers. An early report suggests PhI reacts
with Al at very high temperatures providing a very poor
conversion,13 while Ti–Al alloy is claimed to allow the
formation of Al2Ph3Br3.14 As such species are expected to
be potentially useful Lewis acids we sought to identify a
quick, technically simple approach to their preparation. In
5
Al, neat C10H7I, 120 °C, 20 h, Ga (10 mol%)
>95
a Determined by GC against internal standard (tridecane).
To confirm that the sesquiiodides were being formed un-
der these reaction conditions, a number of them were in-
tercepted with representative acyl chlorides (Table 2).17
preliminary reactions with 1-naphthyliodide, aluminium In all cases (entries 1–8) near quantitative yields of the ex-
powder of mesh size ca. 200 (Acros) was found to be the pected carbonyl compounds were attained. Attempts to
most convenient reagent (Table 1). In the absence of any extend this chemistry to more functionalised sesquiio-
promoters, unlike Grosse,13 we could not attain any inser- dides (containing OMe, Ac or CN groups) resulted in low
to negligible yields. The sesquiiodides prepared by our
procedure are intensely Lewis acidic. For example, cyclic
ethers underwent spontaneous ring-opening reactions in
the presence the PhI-derived reagent leading to near quan-
SYNLETT 2010, No. 4, pp 0636–0638
Advanced online publication: 07.01.2010
DOI: 10.1055/s-0029-1219179; Art ID: D32709ST
0
1.
0
3.
2
0
1
0
© Georg Thieme Verlag Stuttgart · New York

LETTER
Direct Reaction of Aryl Iodides with Activated Aluminium Powder
637
Ph
Ph
Table 2 Reaction of Sesquiiodides with Acid Chlorides
I
Al
N
N
Al
Ph
O
AlXn
XnAl
I
I
Al (1.1 equiv)
O
R1
R1
R2
toluene
Cl
Ga (10 mol%)
I
I
I
R1
I
Al
Al
R1
[(DABCO)Al2Ph3I3]n
R2
R1
120 °C, 20 h
r.t., 3 h
Figure 1
Entry
R1
Ph
R2
Ph
Ph
Ph
Bn
Yield (%)a
1
2
3
4
5
6
7
8
97
96
91
83
98
79
80
96
While the 1H NMR spectrum is in accord with this formu-
lation the material’s low solubility limits its utility com-
pared to its commercial AlMe3 analogue.18–20 However,
like the latter species (DABCO)Al2Ph2I3 shows a similar
degree of air stability and may be handled on the bench for
short periods.
1-naphthyl
3-Tol
Ph
Ph
3-BrC6H4
4-BrC6H4
Bn
Due to its ease of preparation and very high Lewis acidity
we speculated that Al2Ph3I3 would promote the direct for-
mation of amides from methyl esters. An equivalent reac-
tion using AlMe3 is known,21 but an ability to tune the
Lewis acidity at aluminium is not available in this case.
The phenyl sesquiiodide proves to be an effective reagent
(Table 3).22
Ph
1-naphthyl
1-naphthyl
3-BrC6H4
a Isolated yields. Equivalent yields were realised in reagents generated
from HgCl2 catalysis.
Table 3 Al2Ph3I3-Promoted Formation of Amides
titative yields of iodide-transfer products (Scheme 1). In
no case was phenyl transfer observed.
I
I
I
Ph
Ph
Al
Al
O
Ph
O
R3
In line with this behaviour Al2Ph3I3 acted as a demethylat-
ing reagent for anisole, but its performance was not supe-
rior to BBr3; additionally, attempts to prepare these
sesquiiodides in ethereal solvents failed. In an attempt to
mitigate this extreme Lewis acidity Al2Ph3I3 was reacted
with DABCO at low temperature leading to an instanta-
neous white precipitate of (DABCO)Al2Ph3I3 (Figure 1).
H2N R3
R1
N
+
R1
OR2
toluene, 80 °C, 3 h
H
Entry R1
R2
R3
Yield (%)a
1
2
3
4
Ph
Me
Et
Bn
Bn
Bn
Bn
97
98
98
83
Bn
t-Bu
Et
I
I
I
Ph
Ph
Al
Al
O
O
Ph
I
OH
OH
r.t., 1 h
O
84%
I
5
6
Ph
Ph
Me
Me
t-Bu
96
56
I
I
Ph
Ph
Al
Al
O
H2N
Ph
I
7
8
Ph
Ph
Me
Me
Ph
98
98
toluene, –78 °C, 1 h
93%
79%
Me
I
I
I
Ph
Ph
Al
Al
H2N
Ph
Ph
O
OH
I
a Isolated yield.
toluene, –78 °C, 1 h
OH
In all but one case (entry 6) highly effective direct trans-
formations were attained. We could not detect any car-
boalumination products in reactions using propargylic
amine; polymerisation events seem to account for the
missing mass.
I
I
I
Ph
Ph
Al
Al
Ph
O
toluene, 0 °C, 2 h
96%
AlPhxIx
O
In conclusion we have developed a convenient route to the
preparation of rare, highly Lewis acidic, sp2 aryl sesquiio-
dides. Such compounds, which are not easily accessed in
a ‘salt-free’ form via transmetallation reactions, constitute
an interesting, tunable addition to the organic Lewis acid
family.
H
I
Scheme 1
Synlett 2010, No. 4, 636–638 © Thieme Stuttgart · New York

638
X. Tang et al.
LETTER
for 30 min for the excess aluminium metal to settle. The light
brown supernatant solution of organoaluminium sesqui-
halide was ready to use. Method 2: Aluminium powder
(59 mg, 2.2 mmol) and Ga (14 mg, 0.2 mmol) were added to
a dry argon-flushed Schlenk tube. Organohalide (2 mmol)
was added and the reaction was stirred at 120 °C for 20 h.
Anhydrous toluene (1 mL) was added and the mixture was
stirred vigorously for 5 min. The reaction mixture was
allowed to cool to r.t. and left for 30 min for the excess
aluminium metal to settle. The light brown supernatant
solution of organoaluminium sesquihalide was ready to use.
(17) General Procedure for Reactions of Sesquihalide and
Acetyl Chloride: Acetyl chloride (1 mmol) was dissolved in
anhydrous toluene (2 mL), and sesquihalide (1 mmol)
prepared by method 2 was added. The reaction mixture was
stirred at r.t. for 3 h. The reaction was quenched with HCl (2
M) and extracted with Et2O. The organic phase was dried
over MgSO4 and concentrated to give the crude compound.
The crude material was purified by column chromatography
(light petroleum–Et2O) over silica gel to give the pure
product.
Acknowledgment
We thank the Engineering and Physical Sciences Research Council
for partial support of this programme through Grant EP/G026882/
1. X.T. thanks the Marie Curie Office of the European Commission
for support through contract MEST-CT-2005-019780 (INDAC-
CHEM).
References and Notes
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(16) General Procedure for the Synthesis of Sesquihalide;
Method 1: Aluminium powder (81 mg, 3 mmol) and HgCl2
(5 mg, 0.02 mmol) were added to a dry argon-flushed
Schlenk tube. Organohalide (2 mmol) was added and the
reaction was stirred at 120 °C for 3 h. Anhydrous toluene (1
mL) was added and the mixture was stirred vigorously for 5
min. The reaction mixture was allowed to cool to r.t. and left
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Chem. Int. Ed. 2005, 44, 2232.
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(22) General Procedure for Amide Formation: Amine
(1 mmol) was dissolved in anhyd toluene (2 mL), and sesqui-
halide (0.67 mmol) prepared by method 2 was added. The
reaction mixture was stirred at r.t. for 10 min. The ester (0.5
mmol) was added to the reaction mixture and the mixture
was heated to 80 °C for 3 h. The reaction was quenched with
HCl (2 M) and extracted with Et2O. The organic phase was
dried over MgSO4 and concentrated to give the crude
compound. The crude material was purified by column
chromatography (light petroleum–Et2O) over silica gel to
give the pure product.
Synlett 2010, No. 4, 636–638 © Thieme Stuttgart · New York