InCl3-catalyzed synthesis of 1,2-dimetallic compounds by direct insertion of aluminum or zinc powder

Article abstract of DOI:10.1002/anie.201205169

In-sertion of metal: Catalytic amounts of InCl₃ allow the insertion of aluminum and zinc into aromatic 1,2-dibromides or 1,2-bromotriflates (see scheme). These 1,2-dimetallic species can undergo Cu or Pd-catalyzed acylations, allylations, or cross-couplings. Copyright

Full text of DOI:10.1002/anie.201205169

Angewandte
Chemie
DOI: 10.1002/anie.201205169
1,2-Dimetallic Compounds
InCl3-Catalyzed Synthesis of 1,2-Dimetallic Compounds by Direct
Insertion of Aluminum or Zinc Powder**
Tobias D. Blꢀmke, Thomas Klatt, Konrad Koszinowski, and Paul Knochel*
Organometallic species containing two carbon–metal bonds
in the same molecule (dimetallic compounds) often show
special reactivity patterns and offer unique synthetic applica-
tions.[1] The chemical properties of such organometallic
species depend on the nature of the two metals and their
topological proximity.[2] Whereas 1,n-dimetallic species (n =
1, 3–6)[3] have been prepared by several methods and
efficiently employed as multi-coupling reagents,[4] the prep-
aration of organometallic compounds bearing two adjacent
carbon–metal bonds (1,2-dimetallic species) in general is
difficult. The reduction of acetylenes with lithium metal at
low temperatures (À788C) has furnished 1,2-dilithio-
alkenes.[5] Transmetalations, or transition-metal-catalyzed
trapping of arynes, have led to 1,2-dimetallic species derived
from boron,[6] aluminum,[7] tin,[8] and silicon.[9] However, the
direct insertion of a metal into a 1,2-dihalide, such as 1,2-
dibromobenzene, would be the most straightforward method
to generate a 1,2-dimetallic compound, but all attempts so far
have proved to be highly problematic.[10] Owing to the
thermal instability of the intermediately formed b-halo
organometallic compound, side reactions occur, leading to
low yield of the 1,2-dimetallic species.[11] The best results to
date have been obtained using a zinc insertion into aromatic
iodides in a polar solvent,[12] under ultrasound irradiation,[13]
or with electrochemical assistance.[14] Because of the similar
electronegativity of Al (1.66) and Zn (1.65),[15] we envisioned
the synthesis of 1,2-dimetallic reagents by aluminum inser-
tion. As Takai[16] and others[17] have shown recently, aluminum
inserts readily into different unsaturated halides after activa-
tion of the metal surface by various additives, of which InCl3
was especially effective.
65% yield.[19,20] The reagent reacted almost quantitatively in
a second step with iodine, furnishing 1,2-diiodocyclohexane
(3) in 92% yield (Scheme 1). No reaction occurred between
the dibromide 1 and Al powder in the absence of either LiCl
or InCl3, indicating that both salts are required for the
insertion of Al powder.
Scheme 1. Preparation of the dialuminum reagent 2.[21] THF=tetra-
hydrofuran, TMS=trimethylsilyl.
To prove the formation of a 1,2-dimetallic reagent and
gain information about the structure of the dimetallic
intermediate, we conducted ESI mass spectrometric experi-
ments.[22] Interestingly, these experiments showed that several
aluminates are formed during the insertion reaction (Figure 1,
A–C).[23] The main charged component (Figure 1, A) pro-
duced by the insertion reaction, corresponded to the sum
formula [C10H16Al2BrCl4O]À (m/z = 427).[24] The simulated
isotope pattern is in good agreement with the measured
pattern.
The proposed structure of the aluminate is derived from
the measured m/z values and analysis of the fragmentation
Thus, in preliminary experiments we treated 1,2-dibro-
mocyclohexene (1) with Al powder (3 equiv), LiCl (3 equiv),
5% TMS-Cl,[18] and 7.5% InCl3 in THF. Within 24 h at 508C
we obtained the cyclohexenyl 1,2-dimetallic compound 2 in
[*] T. D. Blꢀmke, T. Klatt, Prof. Dr. P. Knochel
Department Chemie, Ludwig Maximilians-Universitꢁt Mꢀnchen
Butenandtstrasse 5-13, Haus F, 81377 Mꢀnchen (Germany)
E-mail: paul.knochel@cup.uni-muenchen.de
Prof. Dr. K. Koszinowski
Institut fꢀr Organische und Biomolekulare Chemie
Georg-August Universitꢁt Gçttingen
Tammannstrasse 2, 37077 Gçttingen (Germany)
[**] We thank the Fonds der Chemischen Industrie, the European
Research Council (ERC), and the Deutsche Forschungsgemein-
schaft (SFB 749) for financial support. We also thank Evonik
Industries AG (Hanau), and BASF AG (Ludwigshafen) for generous
donations of chemicals.
Figure 1. Negative-ion-mode ESI mass spectrum of a circa 10 mm
solution of the products formed by reaction of 1 with Al powder in the
presence of LiCl and InCl3 in THF (m/z ratios of the most abundant
isotopologues: A: 427 [C10H16Al2BrCl4O]À; B: 473 [C10H16Al2Br2Cl3O]À;
C: 687 [C20H32Al3Br2Cl4O2]À). Inset: measured (line, black) and simu-
lated isotope pattern (bars, red) of A.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205169.
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Table 1: InCl3-catalyzed insertion of Al powder into substrates 4 and
subsequent functionalization affording products 7.
patterns obtained from collision-induced dissociation (CID)
experiments. Remarkably, one aluminum atom in the dime-
tallic species remains uncharged and is complexed by THF,
which could be verified by using deuterated THFas solvent.[25]
The exchange of a chlorine by a bromine atom leads to the
related species B.[26] A dimeric structure was also detected
(Figure 1, C).
Entry Substrate
Time[a] Electrophile[c]
Yield[b]
Product[d]
12 h
54%
1
The synthesis of 2 could be extended to various aromatic
1,2-dibromides of type 4 (Table 1). Thus, the addition of 1,2-
dibromobenzene (4a) to Al (3 equiv), LiCl (3 equiv), and
7.5% InCl3 in THF furnished the expected dialuminum
reagent 5a in about 58% yield[20] (508C, 4 h). Again, ESI mass
spectrometric analysis indicated that several aluminates are
produced.[25] This 1,2-dimetallic reagent 5a reacted well with
ethyl 3-bromobenzoate (6a) in a Pd-catalyzed cross-coupling
reaction in the presence of Zn(OAc)2 and 1.4% PEPPSI-
iPr,[27] affording the diester 7a in 63% yield (Scheme 2).[28]
Similarly, 3,4-dibromotoluene (4b) and 3,4-dibromo-o-xylene
(4c) were converted into the corresponding 1,2-dimetallic
reagents in 54–61% yield.[20] After transmetalation with
Zn(OAc)2, these reagents reacted in an allylation reaction
(10% CuCN·2LiCl)[29] with 2-(bromomethyl) ethyl acry-
late[30] (6b), and in a Liebeskind–Srogl acylation[31] (1.4%
PEPPSI-iPr) with the thioester (6c) providing the bis-
(acrylate) 7b and the heterocyclic diketone 7c in 70–93%
yield (Table 1, entries 1 and 2).
4b
6b
7b: 93%[f]
12 h
61%
2
4c
6c
7c: 70%[e]
12 h
51%
3
4e
6e: R=p-Cl-Ph 7e: 66%[e]
12 h
58%
4
4 f
6 f
7 f: 87%[e]
7g: 64%[f]
7h: 65%[f,g]
12 h
55%
5
4g
6b
6b
12 h
66%
6
4h
Scheme 2. Preparation of 1,2-dimetallic reagents 5a and 5d and
subsequent cross-coupling reactions.
8 h
40%
7
4i
6b
6g
7i: 84%[f,g]
Remarkably, using the same insertion conditions, readily
available aromatic ortho-bromotriflates[32] also afforded
a variety of 1,2-dimetallic reagents. Thus, the naphthalene
derivative 4d led to the dimetallic species 5d in 53% yield[20]
(4 h, 508C), which was converted into the dinitro-substituted
terphenyl 7d by a Pd-catalyzed cross-coupling reaction with
4-nitrophenyl triflate (6d) in the presence of Zn(OAc)2 (80%;
Scheme 2). ESI mass spectrometric analysis proved again the
formation of a mixture of aluminates.[25]
6 h
38%
8
4j
7j: 61%[f,g]
6 h
45%
9
Several other electron-rich aromatic triflates (4e–g)
underwent a smooth insertion, furnishing 1,2-dialuminum
reagents in 50–60% yield.[20] After transmetalation with
Zn(OAc)2, these reagents could be readily functionalized in
Cu- or Pd-catalyzed reactions, leading to the bis-functional-
ized aromatic products (7e–g) in 64–87% yield (Table 1,
entries 3–5). In the case of electron-deficient aromatic
triflates (4h–k), the typical reaction conditions led to aryne
4k
6h
7k: 44%[e,g]
[a] Reaction time for completion at 508C. [b] Yield determined by GC-
analysis of iodolyzed reaction aliquots. [c] Up to 200% of electrophile was
used. [d] Yield of isolated analytically pure compounds. [e] 1.4% PEPPSI-iPr
was used. [f] 10% CuCN·2LiCl was added. [g] No LiCl was used.
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formation, and no dimetallic reagent was obtained. However,
performing the aluminum insertion in the absence of LiCl
smoothly produced (508C, 6–12 h) the 1,2-dimetallic species,
although in somewhat lower yields (38–66%).[20] Again, Cu-
mediated or Pd-catalyzed reactions led to ortho-functional-
ized aromatics (7h–k) in 44–84% yield (Table 1, entries 6–9).
Moreover, cyclization reactions could be achieved using
the 1,2-dimetallic species. Thus, the dialuminum reagent 5a
reacted with 2-iodobenzoyl chloride (6i) in the presence of
Zn(OAc)2 and 5–10% [Pd(PPh3)4] in an acylation followed by
a cross-coupling reaction, furnishing 9H-fluorenone (8a;
208C, 1 h) in 60% yield (Scheme 3). In an analogous way,
the dimetallic species 5c also underwent the ring-closure,
leading to the fluorenone derivative 8b in 73% yield
(Scheme 3).
tallic compound underwent a Negishi cross-coupling[34] with
ethyl 3-bromobenzoate (6a), furnishing the triester 7n in
61% yield (Table 2, entry 1). Ketones and aldehydes were not
tolerated under the insertion conditions. However, the
protected aldehyde 4o reacted well with Zn powder in the
presence of 7.5% InCl3 and led to the 1,2-dizinc reagent in
60% yield.[20] Negishi cross-coupling[34] followed by acetal
cleavage afforded the aldehyde 7o in 60% yield (Table 2,
entry 2). Even dimetallic species bearing a cyano or two ester
groups could be readily prepared, and afforded highly
functionalized terphenyls (7p–q) in 55–69% yield after
cross-coupling reactions (Table 2, entries 3 and 4). Moreover,
aromatic dibromides also underwent a smooth Zn insertion in
the presence of 7.5% InCl3. Thus, 1,2-dibromobenzene (4a)
or an electron-rich 3,4-dibromoanisole (4r) furnished dime-
tallic reagents that could be converted into the 1,2-disubsti-
Table 2: InCl3-catalyzed Zn insertion into substrates 4 and subsequent
functionalization leading to products 7.
Entry Substrate
Time[a] Electrophile[c]
Yield[b]
Product[d]
Scheme 3. Preparation of 1,2-dimetallic species 5a, 5c and subsequent
acylation/cross-coupling sequence affording 9H-fluorenone derivatives
8a, 8b.
2 h
70%
1
4n
6a
7n: 61%[e]
As InCl3 proved to be effective in generating 1,2-
dialuminum reagents, we also investigated whether it could
mediate the reaction of Zn powder with the readily available
2-bromophenyl triflate (4l) in N,N’-dimethylpropyleneurea
(DMPU) at 908C to give the corresponding 1,2-dizinc reagent
(Scheme 4). The addition of 7.5% InCl3 furnished the 1,2-
dimetallic zinc reagent 5l (508C, 1 h) in 90% yield,[20] whereas
the zinc insertion into the bromotriflate 4l does not proceed
in the absence of InCl3.[33]
Remarkably, using this InCl3 catalysis, even highly func-
tionalized 1,2-dizinc dimetallic species could be prepared
under relatively mild conditions. Thus, the reaction of the
functionalized triflate 4m with Zn powder (3 equiv) and 7.5%
InCl3 furnished an ester-substituted 1,2-dizinc 5m reagent
(508C, 2 h) in 75% yield[20] in DMPU. Cross-coupling with 4-
bromobenzaldehyde (6j, 1.4%, PEPPSI-iPr)[27] led to the
highly functionalized terphenyl 7m in 69% yield (Scheme 4).
A sensitive methyl ester (4n) was also compatible with the
reaction conditions (508C, 2 h) and the functionalized dime-
2 h
60%
2
4o
6k
7o: 60%[e]
2 h
57%
3
4p
6 f
7p: 55%[e]
2 h
45%
4
4q
6l
7q: 69%[e]
2 h
59%
5
4r
6b
7r: 61%[f]
7s: 72%[g]
2 h
59%
6
4s
6m
[a] Reaction time for completion at 508C. [b] Yield determined by GC-analysis
of iodolyzed reaction aliquots [c] Up to 200% of electrophile was used. [d] Yield
of isolated analytically pure compounds. [e] 1.4% PEPPSI-iPr. [f] 10%
CuCN·2 LiCl was added. [g] 5% [Pd(PPh3)4] was used.
Scheme 4. Reaction of the bromotriflates 4l and 4m with Zn powder
in the absence and in the presence of catalytic amounts of InCl3.
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and 6).
We speculate that the formation of the dimetallic reagents
proceeds over an intermediately generated aryne, as both
aluminum and zinc are known not to insert into carbon–
triflate bonds. This hypothesis is also supported by the fact
that 1,2-dibromocyclopentene furnished exclusively a mono-
metallic reagent (elimination to the strained cyclopentyne
does not occur). We propose that the triflate plays, as usual,
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trapped by low-valent zinc or aluminum at the metal surface,
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cases, aryne-derived side-products were observed.
In summary, we have demonstrated that a catalytic
amount of InCl3 dramatically enhances the insertion of
aluminum and zinc powder, allowing the preparation of 1,2-
dimetallic species of aluminum or zinc by direct insertion
using commercially available metal powders under mild
conditions. These 1,2-dimetallic species react in Cu- or Pd-
catalyzed reactions, affording various interesting ortho-bis-
functionalized aromatics for potential pharmaceutical or
materials science applications. The reactivity of such organ-
ometallic species can be used for the synthesis of fluorenone
derivatives. Further synthetic extensions of this method as
well as mechanistic studies are currently being performed in
our laboratories.
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Experimental Section
Typical procedure: Preparation of 8b: LiCl (254 mg, 6 mmol), InCl3
(33 mg, 0.15 mmol), and Al powder (162 mg, 6 mmol) were placed in
an argon-flushed Schlenk flask, followed by THF (4 mL) along with
the internal standard. 3,4-Dibromo-o-xylene (4c, 417 mg, 2 mmol)
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Received: July 2, 2012
Published online: && &&, &&&&
Keywords: 1,2-dimetallic compounds · aluminum ·
.
cross-coupling · indium · zinc
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used instead of InCl3.
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[31] a) J. Srogl, G. D. Allred, L. S. Liebeskind, J. Am. Chem. Soc.
1997, 119, 12376; b) H. Prokopcovꢃ, C. O. Kappe, Angew. Chem.
2009, 121, 2312; Angew. Chem. Int. Ed. 2009, 48, 2276.
[32] D. E. Frantz, D. G. Weaver, J. P. Carey, M. H. Kress, U. H.
Dolling, Org. Lett. 2002, 4, 4717.
[33] Without the presence of InCl3, zinc inserts readily only into
aromatic iodotriflates; see Ref. [11].
[34] E. Negishi, Acc. Chem. Res. 1982, 15, 340.
[20] The yield was determined by GC analysis of iodolyzed reaction
aliquots in THF.
[21] The insertion of aluminum usually leads to a mixture of
organometallic species, namely R2AlX and RAlX2. These
combined give R3Al2Cl3, which is shortened to RAl2/3X.
[22] a) J. E. Fleckenstein, K. Koszinowski, Organometallics 2011, 30,
5018; b) A. Putau, K. Koszinowski, Organometallics 2011, 30,
4771; c) K. Koszinowski, J. Am. Chem. Soc. 2010, 132, 6032;
d) K. Koszinowski, P. Bçhrer, Organometallics 2009, 28, 771.
[23] ESI mass spectrometry detects only charged species in solution.
[24] The m/z ratio is given for the most abundant isotopologue.
[25] See the Supporting Information.
[35] a) T. Matsumoto, T. Sohma, H. Yamaguchi, S. Kurata, K. Suzuki,
Synlett 1995, 263; b) T. Hamura, Y. Ibusuki, K. Sato, T.
Matsumoto, Y. Osamura, K. Suzuki, Org. Lett. 2003, 5, 3551.
[26] The experiments cannot determine to which of the two Al
centers the bromine atom is bound; most likely, a mixture is
present.
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!

.
Angewandte
Communications
Communications
1,2-Dimetallic Compounds
T. D. Blꢁmke, T. Klatt, K. Koszinowski,
P. Knochel*
&&&& — &&&&
InCl3-Catalyzed Synthesis of 1,2-
Dimetallic Compounds by Direct
Insertion of Aluminum or Zinc Powder
In-sertion of metal: Catalytic amounts of
InCl3 allow the insertion of aluminum and
zinc powder into aromatic 1,2-dibrom-
ides or 1,2-bromotriflates (see scheme).
These 1,2-dimetallic species can undergo
Cu or Pd-catalyzed acylations, allylations,
or cross-couplings.
6
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!