Tandem insertion of halocarbenoids and lithium acetylides into zirconacycles: A novel rearrangement to zirconium alkenylidenates by β-addition to an alkynyl zirconocene

Article abstract of DOI:10.1002/chem.201002962

Tandem insertion of 1,1-dihalo-1-lithio species (halocarbenoids) and lithium alkynides into zirconacyclopentenes and zirconcyclopentanes affords carbocyclic products in high yields via an unusual rearrangement that probably involves addition of an organol

Full text of DOI:10.1002/chem.201002962

DOI: 10.1002/chem.201002962
Tandem Insertion of Halocarbenoids and Lithium Acetylides into
Zirconacycles: A Novel Rearrangement to Zirconium Alkenylidenates by
b-Addition to an Alkynyl Zirconocene
Jozef Stec, Emma Thomas, Sally Dixon, and Richard J. Whitby*[a]
´
Dedicated to Prof. Philip J. Kocienski
Abstract: Tandem insertion of 1,1-
dihalo-1-lithio species (halocarbenoids)
and lithium alkynides into zirconacy-
clopentenes and zirconcyclopentanes
affords carbocyclic products in high
yields via an unusual rearrangement
that probably involves addition of an
organolithium species to the b-position
of a zirconium–alkyne complex to give
an alkenylidene–zirconate species. A
wide variety of cyclopentanoid organic
structures are rapidly assembled in
good yield using this multicomponent
coupling. The main side reaction,
which becomes exclusive in some cases,
is b-hydride elimination of an inter-
mediate cyclopentyl- or cyclopentenyl
zirconocene.
Keywords: carbenoids
· carbocy-
cles · metallacycles · rearrangement ·
zirconium
Introduction
conocene h2-imine complexes with a variety of additional p
components to provide a useful multicomponent coupling.[7]
Elaboration of carbon–zirconium bonds by insertion of 1-
halo-1-lithio species (ꢀcarbenoidsꢁ)[8] was first reported for
acyclic systems in 1989[9] and later applied to the ring expan-
sion of zirconacyclopentanes and zirconacyclopentenes 1 to
Zirconacyclopentanes, -enes and -dienes are readily formed
by reaction between zirconocene h2-alkene or -alkyne com-
plexes and alkenes or alkynes.[1] The most widely used
method is the intramolecular reductive coupling of unsatu-
rated moieties induced by a zirconocene (ꢀCp2Zrꢁ) equiva-
lent, most often zirconocene (1-butene) generated in situ
from dibutylzirconocene (the Negishi reagent).[2] A variety
of methods have been developed for elaboration of zircona-
cycles with carbon–carbon bond formation. Direct addition
to simple electrophiles (aldehydes and aroyl cyanides) is
known,[3] but generally transmetallation from zirconium to
another element such as AlCl3 or BiCl3 is needed first.[4]
Transmetallation from zirconium to copper or nickel, usually
under catalytic conditions, has allowed a wide range of
carbon–carbon bond-forming reactions such as addition to
acid chlorides, enones, aryl, allyl, and alkynyl halides, 1,n-di-
halides, b-haloenones or propynoates; and 1,2-addition to al-
kynes, particularly in the formation of aromatic rings.[5]
Insertion of carbon monoxide and isonitriles, reagents
with carbenoid properties, into zirconacyclopentanes and
-enes is facile. Carbonylation yields cyclic ketones or alco-
hols depending on conditions and work-up.[6] Insertion of
isonitriles may be followed by trapping of the so-formed zir-
afford zirconacyclohexanes and zirconacyclohexenes
3
(Scheme 1).[10] Since then a wide variety of carbenoids have
Scheme 1. Ring expansion of a zirconacyclopentene by insertion of carbe-
noids.
been used,[11] the regiochemistry of insertion into unsymmet-
ric zirconacycles studied,[12] and the chemistry applied to
natural product synthesis.[13] A reasonable mechanism for
the carbenoid insertion is by initial donation of an electron
pair to the 16-electron zirconium center to form an 18-elec-
tron zirconate complex 2 followed by a 1,2-metallate rear-
rangement[14] to give ring-expanded zirconacycle 3.
A special case of carbenoid insertion into zirconacycles is
that of 1-lithio-1,1-dihalo species, in which the zirconacycle
ring expansion is followed by a ring closure with loss of a
second halide (Scheme 2).[15] In this example, the 1,2-metal-
late rearrangement to give 4 is followed by dyotropic rear-
rangement[16] to give the cyclopentenylzirconocene 5 and
carbocycles 6 or 7 on work-up. Poor yields were obtained
[a] Dr. J. Stec, Dr. E. Thomas, Dr. S. Dixon, Prof. R. J. Whitby
School of Chemistry, University of Southampton
Highfield, Southampton, Hampshire SO17 1BJ (UK)
Fax : (+44)23-80593781
E-mail: rjw1@soton.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002962.
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FULL PAPER
lide substantially increased the yields of 12 and 14 to 72%
and 86%, respectively.
A reasonable mechanism for the formation of 12 is given
in Scheme 4. The first proposed step is the insertion of a
1,1-dihalo-1-lithio species into 10 to form the ring-expanded
Scheme 2. Insertion of dihalogenocarbenoids into zirconacycles followed
by dyotropic rearrangement.
for the insertion of dichloromethane, but were reasonable in
other cases.
In our previous communication[17] we presented prelimi-
nary results showing that the rearrangement of 4 to 5 was
substantially accelerated by the addition of a lithium alky-
nide, and this was followed by an unusual rearrangement to
incorporate the alkyne in the organic product 8 (Scheme 2).
We now report full details of this unusual reaction sequence
and related chemistry.
Scheme 4. Proposed mechanism for the three-component coupling on zir-
conocene.
Results and Discussion
Addition of lithium 2,2,6,6-tetramethylpiperidide (LiTMP)
to a mixture of zirconacycle 10, formed by co-cyclisation of
enyne 9,[6d] and dichloromethane (1.5 equivalents) at À788C
followed by addition of lithium phenylacetylide (1.1 equiva-
lents) and quenching with MeOH and saturated aqueous
NaHCO3 gave the bicyclic compound 12 in 52% yield
(Scheme 3).[17] Work-up with MeOD/D2O gave predomi-
six-membered zirconacycle 16 via 1,2-metallate rearrange-
ment of 15.[9,14] After addition of lithiated phenylacetylide to
afford the ꢀateꢁ complex 17 ring closure occurs by 1,2-bond
migration to give intermediate 18. From previous work[15]
we know that the 1,2-rearrangement of the neutral inter-
mediate 16 is slow at À788C, hence the requirement for the
formation of 17 before rearrangement (Scheme 4).
We believe the next step to be addition of a second equiv-
alent of lithiated alkyne to give the intermediate 19 which
fragments into the allyl anion 20 and bis
ACHTUNGTRENNUNG
cene 21. Beta addition of the anion 20 to the bisACHTUNGTRENNUNG
zirconium 21 could give the zirconocene alkenylidenate
complex 22. The known[18] rapid loss/re-addition of anions
from zirconate complexes provides precedent for the frag-
mentation of 19. Although b-addition to a metal alkyne as
suggested for the recombination of 20 and 21 is novel, for
mid-late transition metals the isomerisation of alkyne to al-
kenylidene ligands is known.[19]
Alternative mechanisms for the conversion of 18 to 22
were considered. Initially a [3,3] sigmatropic rearrangement
of 18 to the alkenylidene 24 was suspected (Scheme 5), but
theoretical calculations[20] indicated a high activation energy,
inconsistent with the low temperature at which the reaction
occurs. An alternative mechanism is 1,3-migration of the zir-
conium to the ring junction to give 23 followed by a 1,3-
alkyl shift from the metal to the b-position of the alkyne to
give 24. There is a driving force for the initial 1,3-zirconium
rearrangement in release of strain energy, as we have previ-
Scheme 3. Tandem insertion of lithium halocarbenoids and lithium phe-
nylacetylide.
nantly the bis-deuterated compound 13 (77% bis-deuterat-
ed, 23% of E- and Z-mono-deuterated) suggesting a zirconi-
um–alkenylidene or 1-metalloACTHNUTRGNEUGN(Zr or Li)-1-zircono–alkene
intermediate. When BuCHBr2 was used as the carbenoid
precursor compound 14 was obtained in 42% yield. An im-
portant observation, with implications for the mechanism,
was that the use of 2.2 equivalents of lithium phenylacety-
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R. J. Whitby et al.
Scheme 5. Alternative mechanisms explaining formation of compounds
12 and 14.
Scheme 7. Crossover experiment.
ously observed for a 1,3-amine rearrangement,[7b] indeed it
may be 23, which is attacked by acetylide to give the ion
pair 20 + 21. There is also precedent for the second migra-
tion converting 23 into 24 in the ring contraction of zircona-
cycle 25 to afford the zirconium alkenylidene complex 26[21]
(Scheme 6), as well as similar transformations known for
addition of the first equivalent of 28, strongly implying that
28 was all reacting with bis(phenylethynyl)zirconocene 21 to
give an ate complex [Cp2ZrACTHNUTRGNEUNG
(CCPh)2(CCC6H4-p-tBu)]À Li+
(31). Since 21 is much less sterically hindered at the metal
centre than 27 the selectivity is reasonable. Addition of a
second equivalent of acetylide 28 gave the adducts 29 and
14 (ca. 50% conversion) in a 1.9:1 ratio on protic quench.
Addition of the third equivalent of acetylide 28 caused the
remaining intermediate 27 to quickly convert into 29 with a
final ratio of 6.4:1. Alternatively, addition of bis(p-tert-butyl-
phenylethynyl)zirconocene (30) to 27, followed by portion-
wise addition of PhCCLi (11), provided the final compounds
29 and 14 in the ratio of 1:9.7, respectively.
Scheme 6. Zirconocene–alkyne to ziconocene–alkylidene rearrangement.
neutral or cationic mid/late transition metal complexes.[19]
For either route to 24 rapid trapping by the lithium acetylide
11 to give 22, which calculations indicate to be a very fa-
vourable process, could explain the need for more than two
equivalents of this reagent.[20] Our failure to trap the sup-
posed alkenylidene intermediate 24 with alkynes or carbonyl
compounds indicates that even if formed, it must be rapidly
trapped as 22. We would not expect 22 to show normal
metal carbene reactivity as it lacks the empty orbital on the
metal required for concerted [2+2] additions.[22] Surprising-
ly, we also failed to trap the intermediate 22 with electro-
philes such as methyl iodide, N-bromosuccinimide and ben-
zaldehyde.[23] It can be noted that rearrangement of the ꢀateꢁ
complex 19 to 22 by successive 1,3-rearrangements is not
likely as the metal lacks the empty orbital needed for these
to be concerted processes.[22]
The normal test for a fragmentation/recombination mech-
anism would be a crossover experiment. Although aware
that rapid exchange between organolithium species and or-
ganozirconocenes via ꢀateꢁ complexes[18] was likely to inva-
lidate the result, the experiment was tried (Scheme 7).
The zirconacyclohexane intermediate 27 (Scheme 7) was
formed by addition of BuCBr2Li to zirconacycle 10 at
À788C, then previously prepared bis(phenylethynyl)zircono-
cene 21 was added followed by portionwise addition of p-
tBuPhCCLi (28). The progress of the reaction was moni-
tored by GC, which showed no formation of 29 or 14 after
Ultimately the result of the crossover experiment was in-
conclusive. The fast trapping of bisACTHNUTRGNEUNG(alkynyl) zirconocene
with lithium alkynide to give tris-alkynyl zirconates such as
31, which are not susceptible to nucleophilic attack, negates
the basis for the experiment and we would expect to see
only non-crossover products from either inter- or intramo-
lecular mechanisms. The observation of small, but significant
amounts of crossover products is most easily explained by
exchange between the added alkynide and bisACTHNURTGNEU(GN alkynyl) zir-
conocene. Since this mixes both components the crossover
product could arise from either the exchanged alkynide or
bisACHTNUTRGNEUNG(alkynyl) zirconocene, and this experiment does not allow
us to distinguish between intra- and intermolecular process-
es.
An indication that the proposed mechanism is reasonable
is that addition of tBuLi to bis(phenylethynyl)zirconocene
(21) (Scheme 8) at À788C with immediate protic quench af-
forded the alkene 33 (88% isolated yield), probably via the
alkenylidenate complex 32. Addition of nBuLi to 21 gave
(Z)-1-phenylhexene (36; 73%) only on warming to room
temperature before protonolysis, presumably by initial addi-
tion to the metal to afford 34 followed by the type of 1,2-mi-
gration previously reported by Negishi and co-workers for
Cp2ZrÀ(CCR)3Li+ and Cp2ZrÀAr (CCR)Li+ systems,[24] to
ACHTNUTRGENNUG 2ACHTUNGTRENNUGN
afford 35. The result indicates that a bulky nucleophile is
important for successful b-addition to the alkyne by inhibit-
ing direct attack on the metal.
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Insertions into Zirconacycles
FULL PAPER
taining a cis-bicycloACTHNUTRGNE[NUG 3.3.0]oct-2-ene skeleton, which have
found use as ligands for the orphan nuclear receptors LRH-
1 and SF-1.[7e] Yields are generally excellent for a three-com-
À
ponent coupling, in which four new C C bonds and two
rings are formed. The low yields in Table 1 entries 1 and 2
are due to the use of only 1.1 equivalents of the lithium ace-
tylide component. Variation in the substituent R1 had little
effect on yields. The choice of dihalo component R2CX2H
was more limited. Alkyl dihalides work well, but introduc-
tion of an alpha branch (Table 1, entry 14) gave a very low
yield due to instability of the halocarbenoid. Rapid conver-
sion of (dibromo(cyclohexyl)methyl)lithium to bromomethy-
lenecyclohexane occurs by a 1,2-H shift and loss of LiBr, the
process being accelerated by metal-assisted ionisation.[26]
Silyl-substituted carbenoids also worked well (Table 1,
entry 15), but benzyl carbenoids (R2 =Ar) did not insert. In
the choice of R3 alkyl acetylides gave slightly lower yields
than aryl acetylides (e.g. compare entry 7 with 5 in Table 1).
The tandem reaction sequence also worked well for for-
mation of the pyrrolidine-fused system 41 derived from
enyne 40 (Scheme 9). The cis-fused ring junction stereo-
Scheme 8. a versus b addition to zirconocene–alkyne complexes.
We were able to extend the multicomponent reaction to
the use of a wide range of starting enynes, dihalocarbenoids
and lithiated alkynes (Table 1). The cis-ring junction stereo-
chemistry of the so-formed bicycloACHTUNTRGNEUNG[3.3.0]oct-2-enes 29 and
Table 1. Zirconium-mediated multicomponent synthesis.
Entry R1
R2
R3
Product Yield [%][a]
1
2
3
4
5
6
7
8
Ph
Pr
Pr
Pr
Pr
Pr
Ph
Ph
Ph
Ph
H
H
p-PhC6H4
Bu
39a
39b
39c
29
39d
39e
39 f
43[c]
47[c]
72[b], 54[c]
66
82
86
65
78
72
60
ACHTUNGTRENNUNG(CH2)2Ph Ph
Bu
p-tBuC6H4
Ph
p-tBuC6H4
nC6H13
m-MeOC6H4 39g
p-MeOC6H4 39h
p-PhC6H4
Ph
Ph
Bu
Ph
Ph
n-C6H13
n-C6H13
n-C6H13
n-C6H13
n-C6H13
n-C6H13
9
10
11
12
13
14
15
39i
m-MeOC6H4 n-C6H13
39j
39k
39l
39m
39n
75
80
68
10
c-C6H11
nC6H13
nC6H13
Bu
n-C6H13
n-C6H13
c-C6H11
SiMe2Ph
69
Scheme 9. Formation of pyrrolidine- and methyl-substituted systems. Re-
agents and conditions: a) [Cp2ZrBu2], THF, À788C to RT, 2 h; b)
nC6H13CHBr2, LDA or LiTMP, À788C, 15 min; c) 3.0 equiv PhCCLi,
À788C to À608C over 0.5 h; d) MeOH, aq NaHCO3, RT, 12–16 h; e)
À608C to RT over 2 h.
[a] Isolated yield of 29 or 39a–n based on 37 using 3.0 equiv alkynyl lithi-
um. [b] With 2.2 equiv alkynyl lithium. [c] With 1.1 equiv alkynyl lithium.
39a–n, as well as 12–14 followed from the coupling con-
stants between the protons Ha and Hb, and the bridgehead
proton Hc of approximately 2 Hz and 8-9 Hz, respectively,
which were observable in the 1H NMR spectra in most
cases. Molecular modelling predicted the dihedral angles
HaCCHc and HbCCHc to be about 958 and 258, respectively,
for the cis-fused bicycles, but about 408 and 1658 for the
trans-fused, only the former being consistent with the ob-
served coupling constants when considering the Karplus re-
lationship[25] between dihedral angles and coupling con-
chemistry followed from the 1.8 Hz coupling constant ob-
served between Ha and the bridgehead proton. When a
methyl group was incorporated at the ring junction in the
carbocyclic series only a poor yield of the expected product
44 was obtained from enyne 42. The cis-fused ring junction
stereochemistry of 44 was established by observation of a
6% nuclear Overhauser effect (NOE) enhancement of the
alkene proton Ha on irradiation of the bridgehead methyl
group. The halocarbenoid insertion and acetylide induced
ring closure to produce intermediates such as 43 occurred
quickly, but formation of the product 44 (after quenching)
from re-addition to zirconocene acetylide required warming
to room temperature. The main by-products were 45 and 46
stants. The trans-fused bicycloACTHNUGRTNEUNG[3.3.0]oct-2-enes are around
70 kJmolÀ1 less stable than the cis-fused isomers according
to DFT calculations.[20]
The synthetic route introduced above provides an effi-
cient method for construction of rigid core structures con-
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R. J. Whitby et al.
derived from protonation and b-hydride elimination, respec-
tively, of intermediate 43. The cis-ring junction stereochem-
istry of 45 has not been proven.
Variation in the ring size fused to the zirconacycles was
sought in respect of the multicomponent coupling sequence.
Co-cyclisation with Negishi reagent of the homologous
enyne, 1-(oct-7-en-1-ynyl)benzene (47) resulted in the cyclo-
hexane-fused zirconacycle 48 (Scheme 10). Subsequent in-
served coupling pattern is consistent with an interconverting
mixture of these conformers.[24] The structure of the minor
product 52 was also determined by 2D NMR experiments;
3
for example, there was a long-range (2J or J) correlation be-
tween the PhC=CH2 carbon and the cyclopentyl CH2 pro-
tons, but not between the quaternary alkyl carbon and the
allylic cyclohexyl CH2 protons. The stereochemistry of 52
was not determined.
Monocyclic zirconacyclopentenes 53 (Scheme 11) are
readily formed by addition of alkynes to in situ generated
zirconocene (ethylene).[27] Insertion of lithiated 1,1-dibromo-
heptane into 53 followed by addition of lithium phenylace-
Scheme 10. Extension of the method to a cyclohexyl-fused zirconacyclo-
pentene. Reagents and conditions: a) [Cp2ZrBu2], THF À788C, 0.5 h,
then 2 h at RT; b) nC6H13CHBr2, LiTMP, À908C to À788C, 25 min; c)
3.0 equiv PhCCLi, À788C to À558C over 45 min; d) MeOH, aq
NaHCO3, RT, 16 h.
Scheme 11. Insertion of halocarbenoids into monocyclic zirconacyclopen-
tenes.
sertion of lithiated dibromoheptane and lithium phenylace-
tylide provided a mixture of compounds 51 and 52 in 52%
combined yield and initial GC ratio of 4.7:1 respectively.
Compounds 51 and 52 were partly separated by careful
column chromatography to obtain the final products as
single compounds in 26% and 4% yield, respectively. The
formation of 51 and 52 required warming to À558C rather
than À788C needed for formation of 14, 29 and 39a–n.
GCMS analysis of the crude reaction mixture showed the
main side reactions to be protonation and b-hydride elimi-
nation from 49. The gross structure of 51 was established by
tylide did not give the acetylide incorporated products ob-
tained above. Instead, in the case of R=nC6H13 the sup-
posed intermediate 54 (M=ZrCp2ACTHUNTGRNEUNG(CCPh)) undergoes exo-
or endocyclic b-hydride elimination to give a 64:36 mixture
of 55a:56a. In the case of R=Ph the main product was 57b
resulting from protonation of 54, probably reflecting the
greater stability of the lithiated species (54, M=Li). The
two endocyclic (56b and 56’b), and exocyclic (55) b-hydride
elimination products were also formed.
3
2D NMR experiments, particularly for long-range (2J and J)
Elaboration of saturated zirconacycles: Extension of the
one-pot three-component coupling, described in the previ-
ous section, to saturated zirconacycles resulted in the syn-
thesis of a series of novel compounds containing a saturated
bicyclic core as described below. The general mechanism is
similar to that described above but the intermediate cyclo-
pentylzirconium species were more prone to b-hydride elim-
ination than were the cyclopentenylzirconocene intermedi-
ates 18 above.
Thus [Cp2ZrBu2]-induced co-cyclisation of diene 58 to
give trans-fused zirconacyclopentane 59[11b] followed by se-
quential addition of 1,1-dibromoheptane, LDA and lithium
phenylacetylide and aqueous work-up gave the bicyclo-
carbon–proton couplings. For example, the CH2pentyl pro-
tons did not correlate to the quaternary alkyl carbon centre.
The cis-stereochemistry of the ring fusion followed from ob-
servation of the bridgehead proton as a quintet (J=6 Hz).
Molecular modelling, including DFT studies showed that
the trans-fused diastereoisomer had only one reasonable
conformation of the bicyclic system, in which the HCCH di-
hedral angles made by the four adjacent protons to the
bridgehead proton were around 2ꢃ1808 and 2ꢃ608, not
compatible with the observed coupling pattern.[24] The cis-
fused isomer has two reasonable conformations, 6 kJmolÀ1
apart (comparable with error in DFT calculations) with
HCCH dihedral angles (less stable conformer in brackets)
of 1558 (908), 368 (288), 458 (438), and 708 (1588). The ob-
AHCTUNGTRENNUNG
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Insertions into Zirconacycles
FULL PAPER
Scheme 13. Attempted insertions into a-substituted zirconacyclopentanes.
Reagents and conditions: a) [Cp2ZrBu2], THF À788C, 0.5 h, then 2 h at
RT; b) nC6H13CHBr2, LiTMP, À788C, 25 min; c) 3.0 equiv PhCCLi,
À788C to À158C over 4.5 h (71a) or À358C over 3 h (71b) or 208C over
2 h (73); d) MeOH, aq. NaHCO3, RT, 16 h.
Scheme 12. Cyclisations to form bicycloACTHNUGTRNEUNG[3.3.0]octanes. Reagents and con-
ditions: a) [Cp2ZrBu2], THF À788C, 0.5 h, then 2 h at RT; b)
nC6H13CHBr2, LDA, À788C, 15 min; c) PhMe2SiCH2Cl, LDA, À78 to
À608C, 0.5 h; d) 3.0 equiv PhCCLi, À788C to À558C over 0.5 h (61) or
to À258C over 2.5 h (62) or to À158C over 3 h (63) or to À308C over
1.5 h (67); e) MeOH, aq. NaHCO3, RT, 16 h.
intermediate 70 (Scheme 13). Product 71a was obtained as a
9:1 mixture of (E)- and (Z)-isomers as established from the
13C NMR shifts of the carbon marked * (Scheme 13), which
was shifted upfield in the E isomer (dC =32.2 ppm, c.f.
38.5 ppm for the Z isomer) by steric compression (ꢀg-gauche
effect’).[29] Product 71b was obtained as a single E isomer
from (Z)-Et substituted diene 68b. The stereochemistry of
the ethyl substituent in 71b is consistent with the known
rapid epimerisation of the ethyl stereocentre in zirconacy-
clopentane 69b to the more stable isomer by reversible exo-
cyclic b-hydride elimination/re-addition.[12b] We found that
b-hydride elimination from 70 was a very fast process even
at low temperature (À788C), perhaps driven by a steric
clash between the Ph or Et substituent and the zirconocene
unit although we have no direct evidence that the zirconium
will be syn to R.
Insertion of lithiated 1,1-dibromoheptane and lithium
phenylacetylide into the tricyclic zirconacycle[12b] derived
from 72 was also studied. The carbenoid insertion occurred
very poorly and was followed by b-hydride elimination to
provide an inseparable mixture of the E and Z isomers (3:1
by 13C NMR spectroscopy) of 73 in very low yield
(Scheme 13). The main product (77%) was the protonated
zirconacycle 74.
Dienes 75 containing a nitrogen atom were examined
next. Co-cyclisation of N-allyl-N-benzylprop-2-en-1-amine
(75a) provided a mixture of 3.2:1 trans:cis ring junction zir-
conacycles 76a (ratio and stereochemistry determined by
GC analysis of the quenched zirconacycles and comparison
with the literature precedent) (Scheme 14, Table 2). The zir-
conocene-mediated co-cyclisation of this diene under similar
conditions is reported to give a 4:1 mixture of trans:cis ring
junction zirconacycles.[6c] Further insertion of lithiated 1,1-
dibromoheptane followed by lithium phenylacetylide addi-
quired warming to À258C rather than À558C before the
alkyne
was
incorporated.
Insertion
of
lithiated
(dichloromethyl)dimethylACTHNURTGNEU(GN phenyl)silane into the zirconacy-
cle 59 occurred efficiently, however, incorporation of lithium
phenylacetylide required warming to À208C and gave the
desired compound 63 in only 35% yield. The higher temper-
ature required is probably due to stability of the anion inter-
mediate 60 (M=Li) and it is significant that the protonated
compound 64 was formed in comparable amounts to 63
(GC) and isolated in 22% yield.
Zirconocene-mediated co-cyclisation of 1-allyl-2-vinylben-
zene (65) provided an 88:12 mixture of trans:cis ring junc-
tion zirconacycles 66 as determined by GC of the protonat-
ed products trans- and cis-1,2-dimethylindane. The stereo-
chemistries of the ring junction of these compounds were as-
1
signed by correlation of H NMR shifts of the methyl groups
with the reported spectra of trans- and cis-1,2-dimethylin-
dans.[28] Further insertion of lithiated 1,1-dibromoheptane
and lithium phenylacetylide, however, occurred exclusively
into trans-66 to give compound 67 (Scheme 12) as an insepa-
rable mixture of two diastereoisomers in the ratio of 1.4:1,
the relative identity of which have not been assigned. In the
final quenched reaction mixture GC showed that cis-1,2-di-
methyindene was still present in amounts comparable to
before carbenoid insertion, whereas trans-1,2-dimethylin-
dene was absent, indicating that it is the insertion of the car-
benoid derived from dibromoheptane, which fails for cis-66,
presumably due to steric hindrance.
Cyclisation of the terminally substituted dienes 68a and
68b with [Cp2ZrBu2] gave the a-substituted zirconacycles
69.[12b] Insertion of lithiated 1,1-dibromoheptane and lithium
phenylacetylide gave only compounds 71a and b, resulting
from b-hydride elimination (dehydrozirconation) from the
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4901

R. J. Whitby et al.
has not been proven, that the major isomer is the exo-addi-
tion product, the additional bulk on the exo face (c.f. the a
and b series) accounting for the small amount of endo-addi-
tion product formed. The major by-products were the exocy-
clic b-hydride elimination compounds 78c and d, in each
case contaminated with small amounts of what we believe to
be the endocyclic b-hydride elimination products 79c/d and/
or 80c/d, isolated in 44% and 23% combined yield, respec-
tively.
Variation in the ring size fused to the zirconacyclopen-
tanes was sought in respect of the multicomponent coupling
sequence. Zirconocene-mediated co-cyclisation of 1,7-octa-
diene provided cyclohexyl-fused zirconacycle 81 as an 82:18
mixture of cis:trans ring junction isomers, as determined by
hydrolysis of a sample to provide cis- and trans-1,2-dime-
thylcyclohexane.[32] Insertion of lithiated 1,1-dibromohep-
tane followed by lithium phenylacetylide gave compound 83
as an inseparable mixture of cis:trans ring junction isomers
in the ratio of 2:1 and 53% combined yield. The stereo-
chemistry of cis-83 followed from the symmetry demonstrat-
ed by 13C NMR spectroscopy, the number of signals for the
core carbons being halved. Only one cis isomer was isolated,
assumed to be the exo-addition product shown. An exocyclic
b-H elimination was also observed for this system to give
alkene 84 as the cis ring junction isomer in 40% yield. The
product yields show that the intermediate trans-82 goes ex-
clusively to the rearranged product trans-83, whereas the in-
termediate cis-82 partitions roughly equally between the
routes leading to cis-83 and cis-84. If the zirconium was on
the endo face of cis-82, the result would be consistent with
the very fast exocyclic b-hydride elimination induced by
steric compression observed from 70 (Scheme 13).
Co-cyclisation of 1,2-diallylbenzene 85 (Scheme 15) gave
zirconacycle 86 as a 57:43 cis-fused to trans-fused mixture as
determined from hydrolysis of a sample to give cis- and
trans-2,3-dimethyltetralin.[33] Zirconocene-mediated co-cycli-
sation of 85 has been reported to provide a 95:5 mixture of
cis:trans zirconacycles 86,[6c] but it is known that thermal
equilibration towards the trans-isomer may occur.[11b] Inser-
tion of lithiated 1,1-dibromoheptane and lithium phenylace-
tylide gave the cis ring-fused product 87 (40% yield) as well
as a 65:31:4 mixture of 89:88:90 (33% yield). The stereo-
chemistry of the product 87 was clear from the symmetry
demonstrated by the 13C NMR spectroscopy. The stereo-
chemistry of 88–90 has not been proven but 88 and 89 are
single isomers and, from the composition of 86, 89 at least is
likely to be trans-fused. The precedent from formation of 84
above is that 88 will be cis-fused. There are interesting con-
trasts with the cyclohexyl-fused systems described above in
that the trans-86 leads to an intermediate, which does not
undergo the rearrangement to alkenylidate, but instead
probably undergoes endocyclic b-hydride elimination to give
89, whereas trans-81 cleanly gives trans-83. Although the
result appears inconsistent with the selective conversion of
trans- over cis-66 into the rearranged product, in that case
the step that failed was different—the initial insertion of
halocarbenoid into the zirconacyclopentane.
Scheme 14. Pyrrolidine products.
Table 2. Pyrrolidine products.
R1
R2
76
Yield of
77 [%]
77
trans:cis
cis-77
exo:endo
trans:cis
a
b
c
Bn
Ph
Bn
Bn
H
H
Me
Ph
3.2:1
1:1.3
0:1
59
3.2:1
1:1.3
0:1
>50:1
>50:1
30:1
70
35[a]
14[b]
d
0:1
0:1
17:1
[a] Also 44% of a mixture of 78c–80c . [b] Also 23% of a mixture of
78d–80d.
tion and aqueous work-up gave 77a in 59% yield and the
same 3.2:1 ratio of trans:cis ring junction diastereoisomers,
which were separable by silica gel column chromatography.
Zirconocene-mediated co-cyclisation of N,N-diallylphenyla-
mine 75b gave zirconacycles 76b as a 1.3:1 cis:trans mixture
of ring junction isomers. It has been reported[30] that this
cyclisation gives a 2:1 mixture of cis:trans ring junction zir-
conacycles 76b, but we did not leave the zirconacycle as
long to equilibriate. Insertion of lithiated 1,1-dibromohep-
tane and lithium phenylacetylide into 76b provided the de-
sired adduct 77b as a mixture of cis:trans ring junction iso-
mers in the ratio of 1.27:1 and 70% yield. Noteworthy is
that the final products cis-77a and cis-77b were both
formed as single isomers with respect to the newly created
quaternary centre. Mechanistically we would expect the ad-
dition to bis(alkynyl zirconocene) to occur on the exo face
of the cis-bicycle to give exo-cis-77, but we have not proven
this to be the major isomer. The ring junction stereochemis-
try of 77a and b was determined on the basis of 13C NMR
analysis, the cis isomer showing degeneracy due to symme-
try.
Zirconocene mediated co-cyclisation of dienes with inter-
nally substituted double bonds, 75c and 75d gave exclusive-
ly cis-fused zirconacycles cis-76c and cis-76d, respectively
(Scheme 14, Table 2).[31] Subsequent addition of lithiated
1,1-dibromoheptane and lithium phenylacetylide gave the
required compounds cis-77c and cis-77d in 35% and 14%
yield, respectively, and contained 3% and 6%, respectively,
of the other isomer at the carbenoid derived quaternary
centre (determined by 13C NMR analysis). It is thought, but
4902
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Chem. Eur. J. 2011, 17, 4896 – 4904

Insertions into Zirconacycles
FULL PAPER
Experimental Section
General procedure for zirconacyclopentene or zirconacyclopentane for-
mation followed by insertion of a dihalocarbenoid and lithiated alkyne:
nBuLi (0.80 mL of a 2.5m solution in hexanes, 2.0 mmol) was added
dropwise (usually over 2 min) to
a solution of [Cp2ZrCl2] (0.293 g,
1.0 mmol) in dry, oxygen-free, THF (5 mL) cooled to À788C. After
25 min, a solution of the appropriate enyne or diene (1.0 mmol) in THF
(3 mL) was added dropwise. After 30 min at À788C the reaction mixture
was allowed to warm to room temperature and continued to stir for 2–
3 h. After re-cooling the reaction mixture to À788C, a solution of the ap-
propriate 1,1-dihaloalkane (1.1 mmol) in THF (1 mL) was added, fol-
lowed by dropwise addition of lithium diisopropylamide (LDA; 0.64 mL
of a 1.8m solution, 1.15 mmol) or freshly prepared LiTMP, [LiTMP pre-
pared from freshly distilled 2,2,6,6-tetramethylpiperidine (0.187 mL,
1.1 mmol) in dry THF (2 mL) and nBuLi (0.44 mL of a 2.5m solution in
hexanes, 1.1 mmol) at 08C over 20 min]. The reaction mixture was stirred
at À788C for 15 min before dropwise addition of the corresponding lithi-
um acetylide, [prepared from alkyne (3.0 mmol) in dry THF (3 mL) and
nBuLi (1.2 mL of a 2.5m solution in hexanes, 3.0 mmol) at À58C over
15 min]. The reaction mixture was allowed to warm slowly with monitor-
ing by GC, and quenched by the addition of MeOH (10 mL) and saturat-
ed aqueous solution of NaHCO3 (10 mL) when complete. For most reac-
tions this was after 0.5–1 h at a temperature < À558C. The quenched re-
action mixture was allowed to warm to room temperature and left stir-
ring for 12–16 h. The mixture was poured onto H2O (100 mL), and the
products were extracted with Et2O (3ꢃ75 mL). The combined organic
phases were washed with H2O (3ꢃ100 mL) and brine (100 mL), dried
over anhydrous MgSO4, filtered and concentrated in vacuo to give the
crude product. Purification was generally by chromatography on silica.
Scheme 15. Formation of cyclohexane-fused systems. Reagents and condi-
tions: a) [Cp2ZrBu2], THF À788C, 0.5 h, then 2 h at RT; b)
nC6H13CHBr2, LDA, À788C, 15 min; c) 3.0 equiv PhCCLi, À788C to
À508C over 0.5 h (83) or to À558C over 1 h (87); d) MeOH, aq.
NaHCO3, RT, 16 h.
Acknowledgements
We thank the Engineering and Physical Sciences Research Council and
Glaxo Smith Kline (GSK) for funding this work, and Dr. Tim Willson
(GSK) for his support.
Conclusion
1,1-Dihalo-1-lithio species insert into a wide range of zirco-
nacyclopentenes and zirconacyclopentanes to form a-halo-
zirconacyclohexenes or a-halozirconacyclohexanes, which
ring-contract to cyclopentenyl- or cyclopentylzirconium spe-
cies on addition of lithium alkynides. In most cases further
attack by the lithium alkynide leads to a rearrangement to
afford 1-alken-2-yl-substituted cyclopentenes or cyclopen-
tanes on protic work-up. The rearrangement probably pro-
ceeds by fragmentation to afford a bisACTHNUTRGNEUNG(alkynyl) zirconocene
and an organolithium species, which adds back to the b-
carbon of the alkynyl zirconocene to give a zirconocene al-
kenylidenate complex. For 3-zirconocenylbicycloACTHNUTRGNE[NUG 3.3.0]oct-1-
[1] a) R. D. Broene in Comprehensive Organometallic Chemistry II, Vol.
12 (Eds.: E. W. Abel, F. G. A. Stone, G. Wilkinson), Pergamon,
Oxford, 1995, pp. 323–347; b) E. Negishi in Comprehensive Organic
Synthesis, Vol. 5 (Eds.: B. M. Trost, I. Fleming), Pergamon, Oxford,
1991, pp. 1163–1184; c) E. Negishi, Dalton Trans. 2005, 827–848;
d) Z. Xi, Z. Li, Top. Organomet. Chem. 2005, 8, 27–56.
[2] E. Negishi, F. E. Cederbaum, T. Takahashi, Tetrahedron Lett. 1986,
27, 2829–2832.
[3] a) C. Copꢄret, E. I. Negishi, Z. F. Xi, T. Takahashi, Tetrahedron Lett.
1994, 35, 695–698; b) S. L. Zhou, B. Yan, Y. H. Liu, J. Org. Chem.
2005, 70, 4006–4012; c) C. Zhao, P. Li, X. Cao, Z. Xi, Chem. Eur. J.
2002, 8, 4292–4298.
[4] a) C. J. Zhao, J. Yan, Z. F. Xi, J. Org. Chem. 2003, 68, 4355–4360;
b) T. Takahashi, Y. Z. Li, T. Ito, F. Xu, K. Nakajima, Y. H. Liu, J.
Am. Chem. Soc. 2002, 124, 1144–1145.
ene systems, the fragmentation is driven by strain energy in
the system and is generally very efficient. For 8-
zirconocenylbicycloACHTUNGTRENNUNG[4.3.0]oct-6-ene, 3-zirconocenylcyclopen-
[5] a) T. Takahashi, M. Kotora, Z. F. Xi, J. Chem. Soc., Chem.
Commun. 1995, 1503–1504; b) B. H. Lipshutz, M. Segi, Tetrahedron
1995, 51, 4407–4420; c) M. Kotora, Z. F. Xi, T. Takahashi, J. Synth.
Org. Chem. Jpn. 1997, 55, 958–969; d) T. Takahashi, W. H. Sun,
Y. H. Liu, K. Nakajima, M. Kotora, Organometallics 1998, 17, 3841–
3843; e) T. Takahashi, M. Kotora, R. Hara, Z. F. Xi, Bull. Chem.
Soc. Jpn. 1999, 72, 2591–2602; f) Y. H. Liu, B. J. Shen, M. Kotora, T.
Takahashi, Angew. Chem. 1999, 111, 966–968; Angew. Chem. Int.
Ed. 1999, 38, 949–952; g) Z. F. Xi, Z. P. Li, C. Umeda, H. R. Guan,
P. X. Li, M. Kotora, T. Takahashi, Tetrahedron 2002, 58, 1107–1117;
h) T. Takahashi, Y. Li in Titanium and Zirconium in Organic Synthe-
sis (Ed. I. Marek), Wiley-VCH, Weinheim, 2002, pp. 50–85; i) C.
Chen, C. Xi, Y. Jiang, X. Hong, J. Am. Chem. Soc. 2005, 127, 8024–
8025; j) T. Takahashi, Y. Z. Li, J. H. Hu, F. Z. Kong, K. Nakajima,
tene, and zirconocenylcycopentane systems endo- or exocy-
clic b-hydride elimination competes with the fragmentation/
addition and the outcomes vary substantially with the struc-
tures. Overall, the process provides a highly convergent
ꢀone-potꢁ three-component synthesis of a wide range of car-
bocycles and heterocycles, which should find use in organic
synthesis and pharmaceutical discovery.
Chem. Eur. J. 2011, 17, 4896 – 4904
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4903

R. J. Whitby et al.
L. S. Zhou, K. I. Kanno, Tetrahedron Lett. 2007, 48, 6726–6730;
k) C. Chen, Y. H. Liu, C. Xi, Tetrahedron Lett. 2009, 50, 5434–5436;
l) C. Chen, C. J. Xi, Chin. Sci. Bull. 2010, 55, 3235–3247; m) C.
Chen, X. Yan, C. Xi, Synth. Commun. 2010, 40, 570–579.
[6] a) E. Negishi, S. J. Holmes, J. M. Tour, J. A. Miller, J. Am. Chem.
Soc. 1985, 107, 2568–2569; b) D. R. Swanson, C. J. Rousset, E. Ne-
gishi, T. Takahashi, T. Seki, M. Saburi, Y. Uchida, J. Org. Chem.
1989, 54, 3521–3523; c) C. J. Rousset, D. R. Swanson, F. Lamaty, E.
Negishi, Tetrahedron Lett. 1989, 30, 5105–5108; d) E. Negishi, S. J.
Holmes, J. M. Tour, J. A. Miller, F. E. Cederbaum, D. R. Swanson, T.
Takahashi, J. Am. Chem. Soc. 1989, 111, 3336–3346; e) T. Takaha-
shi, M. Kotora, K. Kasai, N. Suzuki, K. Nakajima, Organometallics
1994, 13, 4183–4185; f) J. Barluenga, R. Sanz, F. J. Fananas, Chem.
Eur. J. 1997, 3, 1324–1336; g) T. Takahashi, Z. F. Xi, Y. Nishihara,
S. Q. Huo, K. Kasai, K. Aoyagi, V. Denisov, E. Negishi, Tetrahedron
1997, 53, 9123–9134; h) T. Takahashi, S. Q. Huo, R. Hara, Y. Nogu-
chi, K. Nakajima, W. H. Sun, J. Am. Chem. Soc. 1999, 121, 1094–
1095.
[7] a) J. M. Davis, R. J. Whitby, A. Jaxa-Chamiec, Tetrahedron Lett.
1992, 33, 5655–5658; b) J. M. Davis, R. J. Whitby, A. Jaxa-Chamiec,
Synlett 1994, 110–112; c) J. M. Davis, R. J. Whitby, A. Jaxa-Chamiec,
Tetrahedron Lett. 1994, 35, 1445–1448; d) G. D. Probert, R. J.
Whitby, S. J. Coote, Tetrahedron Lett. 1995, 36, 4113–4116; e) R. J.
Whitby, S. Dixon, P. R. Maloney, P. Delerive, B. J. Goodwin, D. J.
Parks, T. M. Willson, J. Med. Chem. 2006, 49, 6652–6655.
[8] a) G. L. Closs, R. A. Moss, J. Am. Chem. Soc. 1964, 86, 4042–4053;
b) G. Kçbrich, Angew. Chem. 1972, 84, 557–570; Angew. Chem. Int.
Ed. Engl. 1972, 11, 473–485.
[9] E. Negishi, K. Akiyoshi, B. Oconnor, K. Takagi, G. Z. Wu, J. Am.
Chem. Soc. 1989, 111, 3089–3091.
[10] a) T. Luker, R. J. Whitby, Tetrahedron Lett. 1994, 35, 785–788;
b) G. J. Gordon, R. J. Whitby, Synlett 1995, 77–78.
[11] a) S. Dixon, R. J. Whitby in Titanium and Zirconium in Organic
Synthesis (Ed. I. Marek), Wiley-VCH, Weinheim, 2002, pp. 86–109;
b) S. Dixon, S. M. Fillery, A. Kasatkin, D. Norton, E. Thomas, R. J.
Whitby, Tetrahedron 2004, 60, 1401–1416.
[12] a) G. J. Gordon, T. Luker, M. W. Tuckett, R. J. Whitby, Tetrahedron
2000, 56, 2113–2129; b) E. Thomas, S. Dixon, R. J. Whitby, Tetrahe-
dron 2007, 63, 11686–11701.
[13] a) I. R. Baldwin, R. J. Whitby, Chem. Commun. 2003, 2786–2787;
b) S. Dixon, G. J. Gordon, R. J. Whitby, Chem. Commun. 2005,
4303–4305.
[14] P. Kocienski, C. Barber, Pure Appl. Chem. 1990, 62, 1933–1940.
[15] N. Vicart, R. J. Whitby, Chem. Commun. 1999, 1241–1242.
[16] a) G. Erker, R. Petrenz, C. Kruger, F. Lutz, A. Weiss, S. Werner, Or-
ganometallics 1992, 11, 1646–1655; b) K. Kasai, Y. H. Liu, R. Hara,
T. Takahashi, Chem. Commun. 1998, 1989–1990; c) T. Takahashi, Z.
Xi, R. Fischer, S. Huo, C. Xi, K. Nakajima, J. Am. Chem. Soc. 1997,
119, 4561–4562.
[18] a) T. Takahashi, Y. Nitto, T. Seki, M. Saburi, E. Negishi, Chem. Lett.
1990, 2259–2262; b) D. Kondakov, E. Negishi, Chem. Commun.
1996, 963–964; c) J. P. Majoral, M. Zablocka, New J. Chem. 2005,
29, 32–41.
[19] a) Y. Wakatsuki, J. Organomet. Chem. 2004, 689, 4092–4109;
b) M. I. Bruce, Chem. Rev. 1991, 91, 197–257.
[20] Calculations were carried out with the hybrid DFT/HF B3LYP
method using the 6–31G* basis set and core electron approximation
for zirconium, as implemented in the Spartan 06 progam: J. Kong,
C. A. White, A. I. Krylov, D. Sherrill, R. D. Adamson, T. R. Furlani,
M. S. Lee, A. M. Lee, S. R. Gwaltney, T. R. Adams, C. Ochsenfeld,
A. T. B. Gilbert, G. S. Kedziora, V. A. Rassolov, D. R. Maurice, N.
Nair, Y. H. Shao, N. A. Besley, P. E. Maslen, J. P. Dombroski, H. Da-
schel, W. M. Zhang, P. P. Korambath, J. Baker, E. F. C. Byrd, T. Van
Voorhis, M. Oumi, S. Hirata, C. P. Hsu, N. Ishikawa, J. Florian, A.
Warshel, B. G. Johnson, P. M. W. Gill, M. Head-Gordon, J. A. Pople,
J. Comput. Chem. 2000, 21, 1532–1548.
[21] D. Norton, R. J. Whitby, E. Griffen, Chem. Commun. 2004, 1214–
1215.
[22] R. B. Woodward, R. Hoffmann, Angew. Chem. 1969, 81, 797–869;
Angew. Chem. Int. Ed. Engl. 1969, 8, 781–853.
[23] Recently b-alkylation of alkynyl zirconate complexes has been re-
ported: X. Yan, Y. Zhou, C. Xi, Chem. Commun. 2010, 46, 7801–
7803.
[24] K. Takagi, C. J. Rousset, E. Negishi, J. Am. Chem. Soc. 1991, 113,
1440–1442.
[25] M. Karplus, J. Am. Chem. Soc. 1963, 85, 2870–2871.
[26] a) M. Topolski, M. Duraisamy, J. Rachon, J. Gawronski, K. Gawron-
ska, V. Goedken, H. M. Walborsky, J. Org. Chem. 1993, 58, 546–
555; b) M. Duraisamy, H. M. Walborsky, J. Am. Chem. Soc. 1984,
106, 5035–5037; c) H. M. Walborsky, M. Duraisamy, Tetrahedron
Lett. 1985, 26, 2743–2746; d) J. Rachon, V. Goedken, H. M. Walbor-
sky, J. Am. Chem. Soc. 1986, 108, 7435–7436.
[27] a) E. Negishi, T. Takahashi, Acc. Chem. Res. 1994, 27, 124–130;
b) T. Takahashi, T. Seki, Y. Nitto, M. Saburi, C. J. Rousset, E. Ne-
gishi, J. Am. Chem. Soc. 1991, 113, 6266–6268; c) Z. F. Xi, R. Hara,
T. Takahashi, J. Org. Chem. 1995, 60, 4444–4448.
[28] W. D. Crow, H. McNab, Aust. J. Chem. 1979, 32, 89–98.
[29] J. K. Whitesell, M. A. Minton in Stereochemical Analysis of Alicyclic
Compounds by C-13 NMR Spectroscopy, Springer, London, 1987,
pp. 37–55.
[30] W. A. Nugent, D. F. Taber, J. Am. Chem. Soc. 1989, 111, 6435–6437.
[31] Y. Yamaura, M. Hyakutake, M. Mori, J. Am. Chem. Soc. 1997, 119,
7615–7616.
[32] E. Negishi, C. J. Rousset, D. Choueiry, J. P. Maye, N. Suzuki, T. Ta-
kahashi, Inorg. Chim. Acta 1998, 280, 8–20.
[33] F. G. Morin, W. J. Horton, D. M. Grant, D. K. Dalling, R. J. Pugmire,
J. Am. Chem. Soc. 1983, 105, 3992–3998.
[17] E. Thomas, S. Dixon, R. J. Whitby, Angew. Chem. 2006, 118, 7228–
7230; Angew. Chem. Int. Ed. 2006, 45, 7070–7072.
Received: October 14, 2010
Published online: March 21, 2011
4904
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Chem. Eur. J. 2011, 17, 4896 – 4904