Highly selective barbier-type propargylations and allenylations catalyzed by titanocene(III)

Article abstract of DOI:10.1002/chem.201201720

The alkyne functional group is found in many bioactive natural products and is the key to many important chemical transformations developed over recent years. Moreover, allenes have recently gained relevance as versatile reagents in organic synthesis. Mild, catalytic methods to enable the selective introduction of either alkyne or allene motifs into organic molecules are very valuable but, as yet, quite scarce. We describe an extremely mild and selective method for either the propargylation or allenylation of carbonyl compounds catalyzed by the abundant, safe, and inexpensive metal titanium. These reactions can selectively provide homopropargylic alcohols from aldehydes and ketones or α-hydroxy-allenes from aldehydes. The mechanisms involved were also investigated. Copyright

Full text of DOI:10.1002/chem.201201720

FULL PAPER
DOI: 10.1002/chem.201201720
Highly Selective Barbier-Type Propargylations and Allenylations Catalyzed
by TitanoceneACTHUNTGRNEUNG(III)
Juan MuÇoz-Bascꢀn, Iris Sancho-Sanz, Enrique ꢁlvarez-Manzaneda,
Antonio Rosales,* and J. Enrique Oltra*[a]
Abstract: The alkyne functional group
or allene motifs into organic molecules
are very valuable but, as yet, quite
scarce. We describe an extremely mild
and selective method for either the
propargylation or allenylation of car-
is found in many bioactive natural
products and is the key to many impor-
tant chemical transformations devel-
oped over recent years. Moreover, al-
lenes have recently gained relevance as
versatile reagents in organic synthesis.
Mild, catalytic methods to enable the
selective introduction of either alkyne
bonyl compounds catalyzed by the
abundant, safe, and inexpensive metal
titanium. These reactions can selective-
ly provide homopropargylic alcohols
from aldehydes and ketones or a-hy-
droxy-allenes from aldehydes. The
mechanisms involved were also investi-
gated.
Keywords: allenes · carbonyl com-
pounds · homogeneous catalysis ·
propargylations · titanium
Introduction
er-type propargylations is often more convenient than the
two-step strategy (preparation of the propargylic organome-
tallic reagent and subsequent coupling with the carbonyl de-
rivative) characteristic of Grignard-type strategies. This is
especially so with propargylic halides because the Grignard
reagent may be difficult to prepare in high yields.
Due to the considerable synthetic importance of propar-
gylation reactions,[11] various transition metals, including
Mg,[12] Zn,[13] In,[14] Sn,[15] Ce,[16] Sm,[17] and Cr,[18] have been
assayed to achieve this reaction by the Barbier-type strategy.
Nevertheless, these metals are often required in stoichiomet-
ric proportions, which may be expensive and environmental-
ly unfriendly. Additionally, many of them work in the
The alkyne functional group is present in numerous natural
products, bioactive compounds, and interesting new materi-
als,[1] for example, metal–organic frameworks (MOFs).[2]
Moreover, in recent years, alkyne chemistry has received re-
newed interest in the field of organic synthesis, mainly due
to the development of efficient alkyne-based carbon–carbon
and carbon–heteroatom bond-forming processes, such as So-
nogashira couplings,[3] alkyne and enyne metathesis,[4] click
chemistry,[5] enyne cycloisomerizations,[6] and enantioselec-
tive additions to carbonyl groups,[7] among others.[8] On the
other hand, for many years allenes have been considered
unstable compounds or simple chemical curiosities but it is
now known that the allene motif is present in more than 150
natural products, especially terpenoids and carotenoids.[9]
Furthermore, allenes have proved themselves to be impor-
tant building blocks in organic synthesis, especially in cycli-
zation and cycloaddition reactions.[10] Nevertheless, methods
for the synthesis of allenes are still scarce and most are
based on alkyne isomerization reactions.[9,10] One of the
most straightforward ways of introducing alkyne motifs into
organic molecules is by the propargylation of carbonyl de-
rivatives. Within this context, the one-step strategy of Barbi-
heteroACHTNUTRGNEgNUG eneous phase, which compromises the reproducibili-
ty of results and affords, in many cases, mixtures of homo-
propargylic and allenic alcohols.
Within this context, we hypothesized that titanoceneACHTUNGTRENNUNG(III)
complexes might provide inexpensive catalysts to mediate
Barbier-type propargylations under safe, mild conditions. In
fact, titanium is the seventh most-abundant metal on Earth
and many titanium compounds are non-toxic and environ-
mentally friendly.[19] Moreover, in previous experiments
titaniumACTHNUTRGENUG(N III) catalysts had afforded excellent results for
Barbier-type allylations.[20] Additionally, preliminary results
obtained in our laboratory for propargylation reactions sup-
ported our hypothesis.[21]
[a] J. MuÇoz-Bascꢀn, I. Sancho-Sanz, Prof. E. ꢁlvarez-Manzaneda,
Dr. A. Rosales, Prof. J. E. Oltra
We report in detail the scope and limitations of the selec-
tive Barbier-type propargylation of aldehydes and ketones
and the allenylation of aldehydes catalyzed by [TiClCp2] at
room temperature under mild conditions compatible with
many functional groups. We also present the results of a
mechanistic study into these reactions and the catalytic cycle
involved.
Department of Organic Chemistry
University of Granada, Faculty of Sciences
Campus Fuentenueva s/n (Spain)
Fax : (+34)958248437
E-mail: a.rosales.martinez@gmail.com
joltra@ugr.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201720.
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Table 1. Barbier-type propargylation of aldehydes 6–17 catalyzed by
Results and Discussion
[TiClCp2].[a]
TitanoceneACHTUNGTRENNUNG(III)-catalyzed selective synthesis of terminal ho-
mopropargylic alcohols: Terminal alkynes have proved to be
very useful intermediates in organic synthesis, especially
after the development of the Sonogashira coupling and Car-
reiraꢃs enantioselective alkynylation of aldehydes, among
other reactions.[3,7] Therefore, we began this work by study-
ing the selective synthesis of terminal homopropargylic alco-
hols catalyzed by [TiClCp2].
Since its introduction by RajanBabu and Nugent as a
mild, single-electron-transfer reagent,[22] [TiClCp2] has
become a formidable tool in organic chemistry. In fact, this
complex is capable of catalyzing the homolytic ring opening
of epoxides,[23] the pinacol coupling of conjugated alde-
hydes,[24] stereoselective couplings between aldehydes and
conjugated alkenals,[25] Reformatsky-type processes,[26] diver-
Entry
Aldehyde
Product
Yield[b]
[%]
1
2
6
7
18
19
57
80[c]
3
8
20
87
4
5
6
7
9
21
22
23
24
90
66
57
83
ꢀ
gent C C bond-forming reactions with modulation by Ni or
Pd,[27] and other free-radical-based transformations.[28]
ACHTUNGTRENNUNG[TiClCp2] (1) can be easily generated in situ, simply by
10
11
12
stirring commercially available [TiCl2Cp2] with Mn dust in
deoxygenated THF. This method was chosen for the present
study because it is the most convenient procedure from a
practical point of view. When generated by this method, it
should be noted that the complex exists as an equilibrium
mixture of the monomer 1 and dimeric species [(TiClCp2)2]
(2), presumably with free coordination sites occupied by
THF molecules.[29] Moreover, with the aid of a titanocene-
regenerating agent, such as 3 (formed by the combination of
Me3SiCl and 2,4,6-collidine, developed in our laboratory),[23f]
Ti-catalyzed propargylations could be conducted with sub-
stoichiometric amounts of [TiCl2Cp2] (Scheme 1).
8
13
14
15
25
26
27
94
87
99
9
10
11
12
16
17
28
29
82
72
[a] Propargyl halide 4 afforded the best yield in all cases. [b] The homo-
propargylic alcohol was sometimes accompanied by a minor quantity of
the corresponding trimethylsilyl ether, which was easily transformed into
the alcohol. [c] syn/anti 9:1.
Scheme 1. Anticipated catalytic cycle for Ti-induced Barbier-type propar-
gylations.
To confirm this hypothesis, we treated aldehydes 6–17
with halides 4 and 5 in the presence of substoichiometric
[TiCl2Cp2] (0.2 equiv), relatively cheap Mn dust, and a com-
bination of Me3SiCl and 2,4,6-collidine.[30] Thus, we obtained
yields in the range of 57–99% for secondary homopropar-
gylic alcohols 18–29 (Table 1). Bromide 4 always afforded
better yields than chloride 5. A control experiment in the
absence of titanium did not lead to any coupling product.
The results summarized in Table 1 suggest that this Ti-cat-
alyzed procedure might become a general method for the
chemoselective synthesis of terminal alkynes from alde-
hydes, and thus avoid the formation of allene byproducts
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commonly found in previously described propargylation
methods.[11–18] Additionally, considerable stereoselectivity
was observed for the propargylation of a-substituted alde-
hyde 7 (Table 1, entry 2; syn/anti=9:1), which can be under-
stood by Cramꢃs rule.[31] It should be noted that the reac-
tions took place at room temperature under mild conditions
compatible with several functional groups, including conju-
gated alkynes, esters, fluorides, chlorides, and bromides.
Moreover, the reaction worked well with o-, m-, and p-sub-
stituted aromatic aldehydes.
ACHTUNGTRENNUNG
It should be noted that, once again, we did not detect the
formation of any allenic byproduct. Moreover, the regio-
and stereoselectivity of the Ti-catalyzed propargylation of
carvone (35) was noteworthy because it only produced prod-
uct 45, derived from axial 1,2-addition (Table 2, entry 6). It
is known that the transition state (TS) for the axial addition
of organometallic nucleophiles to cyclohexanones has lower
energy than the TS for equatorial addition.[32] This is espe-
cially so in the case of 35, which has additional steric bulk in
the equatorial plane by virtue of its methyl group.[33] There-
fore, the stereochemical outcome of our propargylation sug-
gests that the reaction took place by the 1,2-addition of a
bulky titanocene derivative (see mechanistic studies and dis-
cussion below).
The results summarized in Table 2 confirm that the Ti-cat-
alyzed procedure was also useful for the selective propargyl-
Table 2. Barbier-type propargylation of ketones 30–39 catalyzed by
[TiClCp2].[a]
As for aldehydes, Ti-catalyzed propargylation of ketones
took place at room temperature under mild conditions com-
patible with different functional groups, which included al-
kenes, conjugated alkenes and alkynes, and chlorides.
Selectivity is one of the most desirable properties for any
method in organic synthesis.[34] Therefore, we assayed the
capacity of our procedure for discrimination between alde-
hydes and ketones. The results of the competition experi-
ment depicted in Scheme 2 indicated that the propargylation
of decanal (50) was much faster than that of 2-decanone
(51). This phenomenon could be exploited to advantage in
the chemoselective propargylation of aldehydes in the pres-
ence of ketones.
Entry
Ketone
Product
Yield[b]
[%]
1
2
30
31
40
41
73
85
3
32
42
96
4
5
6
33
34
35
43
44
45
89
63
81
Scheme 2. Chemoselective Ti-catalyzed propargylation of decanal.
7
8
36
37
46
47
80
53
TitanoceneACTHNUTRGNEUG(N III)-catalyzed selective synthesis of internal ho-
mopropargylic alcohols and a-hydroxy-allenes: Once we
were confident about the utility of the Ti-catalyzed method
for the synthesis of terminal homopropargylic alcohols we
decided to explore the synthesis of the internal analogues.
For this purpose, we chose substituted propargyl halides 53–
57 as pronucleophiles. As expected, the reactions of ketones
51, 58, and 59 with halides 53 and 54, catalyzed by
[TiClCp2], gave rise mainly to internal alkynes 61–64, to-
gether with a minor quantity of allene 65 in the case of the
aromatic ketone 59 (Table 3, entries 1–4). In contrast, the re-
actions of aldehydes 7, 13, 50, and 60 with pronucleophiles
53–57, catalyzed by [TiClCp2], unexpectedly provided a-hy-
9
38
39
48
49
99
92
10
[a] Propargyl halide 5 provided the best yield in all cases. [b] The homo-
propargylic alcohol was sometimes accompanied by a minor quantity of
the corresponding trimethylsilyl ether, which was easily transformed into
the alcohol.
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A. Rosales, J. E. Oltra et al.
Table 3. Internal homopropargylic alcohols and a-hydroxy-allenes from
ketones and aldehydes, respectively.
Table 3. (Continued)
Entry Substrate
X[a] Product
Yield
[%]
73
74
75
76
77
52
20
61
20
84
11
60
53
Entry Substrate
X[a] Product
Yield
[%]
12
13
60
60
54
57
1
2
3
51
51
58
53
54
53
61[b] 77
62
79
63
64
71
58
[a] Propargyl halide. [b] A trace of an allene was detected. [c] A trace of
an internal alkyne was detected. [d] syn/anti 2:1.
4
59
53
65
20
droxy-allenes 66–71, 73, 75, and 77, accompanied by minor
quantities of alkynes 72, 74, and 76 in the cases of aromatic
aldehydes 13 and 60 (Table 3, entries 5–13).
5
6
7
8
9
50
50
50
50
7
53
54
55
56
53
66[c] 68
67[c] 91
68[c] 70
The unexpected difference in behavior between aldehydes
and ketones was intriguing and is discussed below. Aside
from mechanistic considerations, it should be noted that this
reaction constitutes a novel and convenient method for the
straightforward synthesis of a-hydroxy-allenes from alde-
hydes. Also notable is that the a-hydroxy-allene motif is
present in several bioactive natural products and its chemi-
cal synthesis is hard to achieve by any other method.[9]
Moreover, our procedure requires only catalytic proportions
of a non-toxic titanium complex, takes place at room tem-
perature under very mild conditions, and is compatible with
numerous functional groups.
69
73
70[d] 71
Finally, we assayed the Ti-catalyzed chemoselective alle-
nylation of 50 in the presence of 51 with internal chloroal-
kyne 54 as the pronucleophile (Scheme 3). We obtained an
71
72
66
19
10
13
53
Scheme 3. Competition experiment between decanal and 2-decanone.
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89% yield of a-hydroxy-allene 67 and 51 was recovered un-
changed. Moreover, homopropargylic alcohol 62, potentially
derived from ketone 51, was not detected.
hydes and consequently the activation energy (Ea) is also
lower, hence the reaction rate is faster than for ketones.
The mechanism depicted in Scheme 4, however, cannot
explain the formation of a-hydroxy-allenes 66–71, 73, 75,
and 77 from aldehydes (Table 3, entries 5–13). Therefore,
we decided to clarify the difference in behavior between al-
dehydes and ketones in reactions with the substituted pro-
Mechanistic studies and discussion: Ti-catalyzed reactions of
terminal propargyl halides 4 and 5 with aldehydes and ke-
tones (Tables 1 and 2) exclusively afforded homopropargylic
alcohols in all cases assayed. These results can be rational-
ized by the formation of an allenyl radical (78), which would
be trapped by a second [TiClCp2] species to give an organo-
metallic allenyltitanium species (79). Intermediate 79 would
eventually attack the carbonyl compound present via an in-
termediate such as 80 (Scheme 4), in a similar manner to
that previously proposed for Nozaki–Hiyama propargyl-
nucleophiles 53–57. Nevertheless, the titanoceneACTHNUTRGENUG(N III)-based
catalytic system involves many experimental variables and,
as a consequence, the experimental results might become
too complicated to be interpreted in a straightforward way.
So, we decided to assay the reactions of 50 and 51 with sub-
stituted pronucleophile 54 promoted by an excess of
[TiClCp2], thus simplifying the reaction system by removal
of the titanocene-regenerating agent 3 (Scheme 5). Gratify-
ACHTUNGTRENNUNG
ations.[18]
Scheme 4. Proposed mechanism for the selective formation of terminal
homopropargylic alcohols.
Scheme 5. Reactions between aldehyde 50 and ketone 51 with pronucleo-
phile 54 promoted by an excess of [TiClCp2].
To test this hypothesis, we reacted propargyl chloride 5
with [TiClCp2] in [D8]THF in the absence of any carbonyl
compound. After 1 h stirring, we analyzed the reaction mix-
ture by NMR spectroscopy. The 13C NMR spectrum showed
signals at d=78.2 (CH2), 122.2 (CH), and 218.1 ppm (C),
which could be assigned to 79, and supports the mechanism
proposed in Scheme 4. On the other hand, signals assignable
to a potential propargyltitanium species were not observed.
The above mechanism may also account for the observed
chemoselectivity towards aldehydes (Scheme 2). The transi-
tion state (TS) between 80 and 81 would presumably involve
six delocalized electrons in a flat arrangement (Figure 1).
ingly, the results obtained under these conditions were very
similar to those obtained with the catalytic system (Table 3).
These observations not only allowed us to use a simpler
system for our mechanistic studies but also suggested that
the relative proportions of the monomeric and dimeric
forms of the titanocene complex in the equilibrium 1Ð2[29]
do not substantially affect the reaction mechanism.
The formation of a-hydroxy-allenes from aldehydes might
be put down to the direct attack of an allenyl radical, similar
to the reaction between prenyl radicals and some alde-
AHCTUNGTRENNUNG
hydes.[20b] Therefore, we decided to trap these potential al-
lenyl radicals. It is known that radicals can be reduced by
hydrogen atom transfer (HAT) from [Ti
(OH2)ClCp2].[35] We
treated propargyl halide 54 with [Ti(OD2)ClCp2] (deep
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
blue) prepared in situ from [TiClCp2] (lime green) and D2O
(Scheme 6). In this way, we obtained a 9:1 mixture of iso-
topomers 82 and 83 with deuterium incorporation (DI)
Figure 1. TS formed between 80 and 81.
From a thermodynamic point of view, this could represent
quite a favorable situation and, in fact, reactions conducted
at room temperature afforded good-to-excellent yields. Nev-
ertheless, in ketones the R and R’ substituents are alkyl or
aryl groups and are both eclipsed by the adjacent hydrogen
atoms. However, in aldehydes either R or R’ is a hydrogen
atom and, therefore, only one bulky alkyl or aryl group is
eclipsed. Thus, steric hindrance in the TS is lower for alde-
Scheme 6. Reaction between 54 and [TiACTHNUTGRNEG(UN OD2)ClCp2].
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A. Rosales, J. E. Oltra et al.
greater than 95% in both compounds. It is noteworthy that
no dimerization products were detected (GC-MS analysis),
which indicated that, under the experimental conditions em-
ployed, deuterium atom transfer (DAT) was faster than the
potential radical–radical coupling that would have led to the
formation of dimers.
To the best of our knowledge this is the first evidence that
suggests that, in contrast to allylic and benzylic radicals,[35d]
propargylic radicals can be effectively reduced by HAT
from water in a reaction promoted by [TiClCp2]. Moreover,
the relative proportions of 82/83 indicate that the C1 radical
tautomer of 84 is more reactive than its C3 counterpart
These observations indicated that allenyltitanium species
86 was inert to neutral hydrolysis with D2O and, under these
conditions, was more robust than propargyltitanium species
85, 60% of which underwent neutral deuterolysis. Within
this context, we performed a series of six experiments by
treating 54 with [TiClCp2], followed by an acidic hydrolysis
quench after 0.5, 2, 10, 30, 60, and 120 min (Scheme 9).
against [TiACHTUNGTRENNUNG(OD2)ClCp2] (Scheme 7). In other words, the Ea
Scheme 9. Preparation and acidic hydrolysis of the 85/86 mixture.
of DAT to the C1 radical is substantially lower than the Ea
of DAT to the C3 radical.
At the respective time points we obtained 1:1, 1:1.5, 1:2,
1:3, 1:3, and 1:3 mixtures of products 87 and 88. These re-
sults suggest that 30 s after the addition of [TiClCp2], an
equimolar mixture of complexes 85 and 86 is formed. This
mixture gradually evolves to an equilibrium in which 85 pre-
dominates over 86 in a proportion of 3:1 (Scheme 10). This
Scheme 7. DAT from [TiACHTUNGTRENNUNG(OD2)ClCp2] to propargylic radical 84.
The above observations cast doubt on the possibility of
radical attack being responsible for the formation of a-hy-
droxy-allenes from aldehydes. Nevertheless, it is known that
conjugated aldehydes are more susceptible than non-conju-
gated aldehydes to attack by free radicals generated by
[TiClCp2][20b] and thus the possibility of minor propargylic
alcohols 72, 74, and 76 (Table 3, entries 10–12) derived from
attack by the propargylic radical 84 on aromatic aldehydes
cannot be ruled out.
A second way to explain the formation of a-hydroxy-al-
lenes 66–71, 73, 75, and 77 might be via propargyltitanium
intermediates. To study these potential organometallic inter-
mediates we treated propargyl chloride 54 with an excess of
[TiClCp2]. Once all of the starting material was consumed
we added D2O, stirred the reaction mixture for 16 h, and ob-
tained a 1:6 mixture of allene 87 and deuterium-labeled
alkyne 82 (60% DI) (Scheme 8).
Scheme 10. Equilibrium between complexes 85 and 86.
equilibrium is reached by 30 min after the addition of
[TiClCp2]. The equilibrium between propargyltitanium and
allenyltitanium species has been hypothesized previously[36]
but, to the best of our knowledge, our results represent the
first experimental evidence to support this hypothesis and
determine the relative proportions in the equilibrium.
In this scenario, it is not too far-fetched to suppose that a-
hydroxy-allenes 66–71, 73, 75, and 77 might be formed by
reaction between propargyltitanium complexes, such as 85,
and the appropriate aldehyde via intermediate 89, depicted
in Scheme 11.
Scheme 11. Proposed mechanism for the selective formation of a-hy-
droxy-allenes from aldehydes.
Nevertheless, reactions with ketones 51, 58, and 59 mainly
afforded internal homopropargylic alcohols 61–64 (Table 3,
entries 1–4). Formation of these alkynes cannot be justified
via an intermediate such as 89. Instead, these products could
derive from allenyltitanium species analogous to 86 via in-
termediates of type 91 (Scheme 12).
Scheme 8. Neutral D2O-promoted hydrolysis of propargyltitanium species
85.
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also present, for the first time, experimental evidence for
the chemical equilibrium between propargyltitanium and al-
lenyltitanium species. Additionally, the unprecedented re-
duction of propargyl radicals by hydrogen atom transfer
from water promoted by [TiClCp2] is shown.
Scheme 12. Proposed mechanism for the selective formation of internal
homopropargylic alcohols from ketones.
In the case of ketones, it would seem that TS 93
(Figure 2), derived from attack by a propargyltitanium spe-
cies, such as 85, would be energetically disfavored due to
steric hindrance from R’, which is in gauche orientation with
respect to both R and R’’ (in ketones, R and R’’ are bulky
alkyl or aryl groups). Thus, TS 94 (Figure 2)—formed be-
tween intermediates 91 and 92), in which this steric hin-
drance does not exist—would be energetically more favor-
Acknowledgements
We are grateful to the Spanish MICINN (Projects CTQ2009–09932 and
CTQ2011–24443) and the Regional Government of Andalucꢄa (Projects
P07.FQM.03212, P07.FQM.03101, and P10.FQM.6050) for financial sup-
port. We thank our English colleague Dr. J. Trout for revising our English
text. I.S-S. thanks the Spanish MICINN for a FPI grant.
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Figure 2. Potential transition states 93 and 94.
In summary, [TiClCp2] reacts with unsubstituted propargyl
halides (such as 4 and 5) to give allenyltitanium species 79,
which, in turn, attacks aldehydes and ketones to exclusively
provide terminal homopropargylic alcohols. On the other
hand, [TiClCp2] reacts with substituted propargyl halides
(such as 53–57) to give an equilibrium mixture of propargyl-
titanium and allenyltitanium complexes, such as 85 and 86.
Aldehydes react faster with propargyltitanium species than
with allenyltitanium species, thus mainly generate a-hy-
droxy-allenes. In contrast, ketones react faster with allenylti-
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Titanium is one of the most abundant, safe, and environ-
mentally friendly transition metals on Earth. We have de-
scribed a novel, selective method for either the propargyl-
A
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ACHTUNGTRENNUNG
under extremely mild conditions, compatible with numerous
functional groups, and provide terminal homopropargylic al-
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Chem. Eur. J. 2012, 00, 0 – 0
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Received: May 16, 2012
Published online: && &&, 0000
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ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Barbier-Type Propargylations
FULL PAPER
Effectiveness, selectivity, and sustaina-
bility are key properties of this novel
method for Ti-catalyzed Barbier-type
propargylation/allenylation. The reac-
tion proceeds at RT under mild condi-
tions compatible with numerous func-
tional groups and affords homopropar-
gylic alcohols from aldehydes and
ketones and a-hydroxy-allenes from
aldehydes (see scheme).
Synthetic Methods
J. MuÇoz-Bascꢀn, I. Sancho-Sanz,
E. ꢁlvarez-Manzaneda, A. Rosales,*
J. E. Oltra* .................... &&&& — &&&&
Highly Selective Barbier-Type Propar-
gylations and Allenylations Catalyzed
by TitanoceneACTHUNTGRNEUNG(III)
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!