Improved protocols for molybdenum- und tungsten-catalyzed hydrostannations

Article abstract of DOI:10.1002/adsc.200900093

A series of (isonitrile)tungsten carbonyl complexes of type W(CO) _m(CNR)_n has been synthesized and evaluated as hydrostannation catalysts. The results are compared with those obtained by the previously reported tri(tert-butylisonitri

Full text of DOI:10.1002/adsc.200900093

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DOI: 10.1002/adsc.200900093
Improved Protocols for Molybdenum- und Tungsten-Catalyzed
Hydrostannations
Alexander O. Wesqueta and Uli Kazmaiera,*
a
Institut fꢀr Organische Chemie, Universitꢁt des Saarlandes, Im Stadtwald, Geb. C4.2, 66041 Saarbrꢀcken, Germany
Fax : (+49)-681-302-2409; phone: (+49)-681-302-3409;; e-mail: u.kazmaier@mx.uni-saarland.de
Received: February 11, 2009; Published online: June 3, 2009
Abstract: A series of (isonitrile)tungsten carbonyl tions under microwave irradiation or when the reac-
complexes of type W(CO)mACHTNUGRTNE(NUG CNR)n has been synthe- tions are carried out under an atmosphere of carbon
sized and evaluated as hydrostannation catalysts. The monoxide. Based on these findings, a mechanistic ra-
results are compared with those obtained by the pre- tional is given, explaining the different pathways re-
viously reported tri(tert-butylisonitrile)molybdenum sponsible for the formation of hydrostannation or
tricarbonyl catalyst, Mo(CO)3ACHTNUTRGNEG(UN CN-t-Bu)3 (=MoBI3). distannation products.
The yields and selectivities strongly depend on the
isonitriles used, and with certain substrates better re-
sults are obtained compared to the molybdenum cat- Keywords: hydrostannation; isonitriles; microwave
alyst. No side products are observed in hydrostanna- heating; molybdenum; tungsten
Introduction
stance, in Pd-catalyzed allylic alkylations. This ap-
proach was used for the synthesis of stannylated
Vinylstannanes are interesting building blocks in or- amino acids,[7] which are ideal substrates for subse-
ganic synthesis, easily available, for example, by hy- quent Stille couplings.[8]
drostannations of alkynes, either under radical or
In general, not only the reactivity but also the re-
transition metal-catalyzed conditions.[1] The major gioselectivity towards the a-products increases with
drawbacks of these methods result from difficulties in the electron-withdrawing properties of the substitu-
controlling the regioselectivity of the tin hydride addi- ents adjacent to the triple bond. For example, prop-
tion to unsymmetrical alkynes, although the transition argyl esters are more reactive and more selective than
metal-catalyzed version has the advantage of a clean propargylic ethers,[9] sulfones[10] or phosphonates.[11]
cis addition, based on the reaction mechanism.[2] With To increase the performance of our catalyst with
terminal alkynes, catalytic hydrometallations in gener- these less reactive substrates, we undertook a intense
al give rise to the terminal (E)-vinyl metal com- metal and ligand screening, which resulted in the de-
pounds, species which can also be generated by stan- velopment of a new tungsten-based catalyst with a
nylcupration.[3] Interestingly, the corresponding (Z)- completely
different
reaction
behaviour.
isomers can be obtained in the presence of Lewis W(CO)5(CN-p-NO2Ph) (C2) gave only traces of hy-
acids, as reported by Yamamoto et al.[4] Hale et al. de- drostannated product, but an excellent yield of the
scribed regioselective radical hydrostannations of distannylated allyl compound 3 (Scheme 1) under the
propargyl alcohol derivatives, occurring preferentially same reaction conditions with which C1 provided the
at the a-position. If internal alkynes are used, also the a-stannylated product 2 nearly exclusively.[12] The dif-
(Z)-isomers are formed preferentially.[5]
ferent reaction behaviour of these two complexes
A few years ago our group developed a new cata- probably can be explained by the different electronic
lyst for regioselective hydrostannations, based on mo- properties of the isonitrile ligands, and this prompted
lybdenum. Mo(CO)3ACHTNUGRTNEUNG(CN-t-Bu)3 (=MoBI3, C1) was us to investigate these reactions in more detail.
found to be an excellent catalyst for highly selective
hydrostannations of functionalized alkynes, such as
propargylic esters or protected propargyl alcohols (1), Results and Discussion
affording the a-stannylated products (2) preferential-
ly.[6] Stannylated allylic esters of this type are excel- Although the MoBI3-catalyzed hydrostannation gives
lent building blocks which can be further used, for in- excellent results with electron-poor alkynes, with less
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Scheme 1. Hydro- and distannations of propargyl alcohol derivatives.
reactive substrates the reactions are relatively sluggish the isoelectronic new ligands. Based on our previous
in some cases, or the reactions stop before complete observations made with the molybdenum complexes,
conversion, even when Bu3SnH is left over in the re- we focused preferentially on the W(CO)
action mixture. In these slow reactions also another the most reactive W(CO)3A(CNR)3 complexes, because
problem becomes visible: the formation of side prod- the Mo(CO)5A(CNR) complexes showed no significant-
4ACHTUNGTERN(NUNG CNR)2 and
CTHUNGTRENNUNG
CTHUNGTRENNUNG
ucts such as distannylated alkenes (3). Furthermore, ly higher reactivity compared to Mo(CO)6. Therefore,
protodestannylation can be observed, resulting in an we used around three equivalents of the isonitrile in
overall reduction of the triple to a double bond. To the ligand exchange reaction to get the trisisonitrile
get rid of these side reactions we tried to optimize complexes preferentially (Table 1). Even when the bi-
our reaction conditions. The reactions can be acceler- sisonitrile complexes were obtained under these con-
ated dramatically under microwave irradiation,[9] ditions only as side products, their amount was
which allows the hydrostannation of thermally labile enough for us to investigate their catalytic properties.
substrates. Unfortunately, this protocol does not solve The complexes are stable and can be easily separated
the problem of the side reactions. Therefore, we eval- and purified by flash chromatography.
uated new catalysts based on tungsten to find better
The known tungsten complex C4, the analogue of
conditions for the hydrostannation and/or the distan- MoBI3, was prepared according to the literature,[14a] in
nation. We chose to synthesize mixed CO/isonitrile order to figure out the influence of the central metal
complexes to get comparable structures to our previ- atom. In the other new complexes (C5–C10) the
ous catalysts C1 and C2 (Table 1). We hoped to see steric and electronic properties of the ligands were
an influence of the isonitrile ligand on the selectivity varied. The complexes C9 and C10 were chosen to
in the addition step. The required isonitriles were evaluate the effect of the strong electron-donating
easily obtained from the corresponding formamides properties of the NMe2 group in comparison to the
by dehydration with POCl3.[13] With these isonitriles strong electron-withdrawing NO2 group in C2. As
in hand we performed ligand exchange reactions on
W(CO)6 according to a protocol developed by Albers
and Coville.[14] W(CO)6 was refluxed with the isoni-
trile in the presence of catalytic amounts of CoCl2.
Depending on the reaction time and the amount of
isonitrile used, one to three CO can be replaced by
Figure 1. Hydro- and distannations of terminal alkynes.
Table 1. Preparation of mixed tungsten CO/isonitrile complexes.
Entry Isonitrile
Equiv.
Reaction
time
Yield
[%][a]
Catalyst
Comment[b]
[14a]
1
2
3
4
5
6
7
8
t-BuNC
t-BuNC
i-BuNC
i-BuNC
c-HexNC
c-HexNC
p-Me2NPhNC
p-Me2NPhNC
3.0
3.0
3.2
3.2
2.9
2.9
3.3
3.3
160 min
160 min
150 min
150 min
150 min
150 min
90 min
55
34
39
38
26
54
21
52
C3
C4
C5
C6
C7
C8
C9
C10
W(CO)
W(CO)
W(CO)
W(CO)
W(CO)
W(CO)
W(CO)
W(CO)
4A
3A(CNtBu)3
4A(CNiBu)2
3A(CNiBu)3
4A(CNc-Hex)2
3A(CNc-Hex)3
4A(CNPhNMe2)2
3A(CNPhNMe2)3
Together with C4
Together with C3
Together with C6
Together with C5
Together with C8
Together with C7
Together with C10
Together with C9
90 min
[a]
[b]
Isolated yields of the separated catalysts.
The corresponding bis- and trisisonitrile complexes were synthesized in one reaction and separated afterwards by flash
chromatography.
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Table 2. Hydrostannations of propargylic substrates 4.
Entry
Alkyne
Catalyst
Time t [h]
Product ratio [%]
4
5
6
7
8
1
2
3
4
5
6
7
8
4a
4a
4a
4a
4a
4a
4a
4a
4b
4b
4b
4b
4b
4b
C1
C1
C4
C5
C6
C8
C9
C10
C1
C1
C6
C6
C10
C10
8
26
8
8
8
8
8
8
5
26
5
26
5
17
0
3
13
6
3
71
80
72
49
59
82
84
91
38
44
35
63
7
6
7
7
6
7
8
6
7
3
4
2
3
0
0
0
7
9
17
15
4
0
0
6
4
4
6
0
0
6
6
9
15
13
3
1
0
11
40
3
10
10
52
9
2
9
42
8
56
18
82
13
10
11
12
13
14
26
35
propargylic substrates we used two different candi- Table 3. Isolated yields of hydrostannations.
dates, 4a and 4b (Figure 1) which gave only moderate
Entry Alkyne Catalyst Time [h] Yield [%] Ratio 5:6
results (yield, selectivity) in our previous investiga-
tions of the MoBI3 catalyst, and therefore we saw
enough room for improvements.
To be able to compare the results obtained with the
different catalysts, the reaction conditions were stand-
ardized, and the reactions were run in parallel. In all
examples, 1 mmol of substrate was reacted with two
equivalents of Bu3SnH in the presence of 3 mol% cat-
1
2
3
4
4a
4a
4b
4b
C1
C10
C1
C6
10
10
10
24
68
75
20–40
59
92:8
92:8
95:5
95:5
We started our investigation with the p-nitrophenyl
alyst in THF. 10 mol% hydroquinone was added to ether 4a, which in general gives very reliable results
suppress radical side reactions. The reactions were and acceptable yields and selectivities. With catalyst
run under an argon atmosphere and samples were C1 (MoBI3) after 8 h still some starting material was
taken via syringe after certain times, which were ana- left, and the only side product was the alkene 8a
lyzed by NMR.
(entry 1). The yield of hydrostannation product (5a/
In principle, four different products can be expect- 6a) could be increased slightly by prolonging the reac-
ed (Table 2): The two regioisomeric hydrostannation tion time (entry 2), but under these conditions also
products 5 and 6, the distannylated product 7 and the the formation of distannane 7a was observed. The
alkene 8, resulting from protodestannylation. The side corresponding tungsten complex C4 seems to be
products 7 and 8 are formed preferentially in reac- slightly more reactive, because the substrate was con-
tions of unreactive substrates, which require long re- sumed nearly completely after 8 h (entry 3).
action times. The product distribution was calculated
The regioselectivity (ratio 5a:6a) was comparable,
from the H, 13C and 119Sn NMR spectra. The most but a higher ratio of distannane 7a was formed. The
significant results are summarized in Table 2.[15] Only formation of 7a was even more tremendous in the re-
the calculated product ratios are given, not the isolat- actions catalyzed by the isobutyl isonitrile complexes
ed yields (because samples were taken during the re- C5 and C6 (entries 4 and 5). The trisisonitrile complex
action). The reactions with the best catalysts were re- C6 showed a slightly higher reactivity compared to
peated and the isolated yields of these reactions are C5, but compared to the other complexes the results
summarized in Table 3. The results are compared with were significantly worse: more side products were
1
those obtained with MoBI3 (C1).
formed and the regioselectivity was lower. The
amount of side products dropped dramatically by
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Table 4. Hydrostannations under microwave irradiation.
switching to the more electron-donating ligands (en-
tries 6–8). Best results were obtained with the p-ami-
nophenyl isonitrile complexes, whereby even the bis-
ACHTUNGTRENNUNGisonitrile complex C9 showed good results. The best
catalyst for this reaction was C10 which gave also a
better isolated yield compared to C1 (Table 3, en-
tries 1 and 2), while the regioselectivity was not af-
fected.
Entry Alkyne Cat. Reaction Condi-
tions
Yield
[%]
Ratio
5:6
Next we focused on the propargylic sulfone 4b,
which is a relatively critical candidate, because in the
MoBI3-catalyzed reaction the yield was not only low,
but it also varied strongly from reaction to reaction
(Table 3, entry 3).[9] Monitoring the reaction by NMR
gave an explanation for this observation. The reaction
is relatively slow, but the hydrostannation products
are formed with high regioselectivity (Table 2,
entry 9). Unfortunately, prolonging the reaction time
does not increase the amount of 5b/6b, only the
amount of the alkene 8b (entry 10). Obviously proto-
destannylation is a significant problem with this sub-
strate. Switching to catalyst C6, which was the worst
with substrate 4a, gave a surprising result. Although
the reactivity was lower, compared to C1, less side
products were observed (entry 11). Hereby, the yield
could be increased by prolonging the reaction time
(entry 12). This allowed us to increase the isolated
yield reliably to close to 60% with excellent selectivi-
1
2
3
4
5
6
7
8
4a
4a
4a
4a
4a
4a
4b
4b
C1 150 W, 1258C,
15 min
C4 150 W, 1298C,
15 min
C6 150 W, 1338C,
15 min
C8 150 W, 1328C,
15 min
C9 150 W, 1368C,
15 min
C10 150 W, 1318C,
15 min
C1 100 W, 1108C,
5 min
C6 100 W, 1048C,
5 min
81
95
84
61
78
81
72
47
84:16
89:11
86:14
88:12
88:12
89:11
88:12
90:10
ty (Table 3, entry 4). Surprisingly, C10, the best cata- a slightly better regioselectivity, but a significantly
lyst in the previous studies, failed almost completely lower yield (entry 8). The results with the other cata-
with this substrate. Only traces of 5b were observed lysts were worse.
after 5 h (Table 2, entry 13), and longer reaction times
During these comparative studies we made an in-
resulted mainly in the formation of alkene 8b teresting observation. The tungsten catalyst C2 which
(entry 14). Similar results were obtained with the cat- was developed as a distannylation catalyst,[12a] gave
alysts C8 and C9. Definitely, for this kind of substrate, the expected distannane 7a preferentially. When we
C6 is the catalyst of choice.
ran out of catalyst we had to synthesize C2 again, but
Recently, we observed that the yields of “difficult” with this new charge, 7a was only the side product,
hydrostannations could be increased if the reactions while 5a was the major one (Scheme 2). The only dif-
are carried out under microwave irradiation.[9] This ference between the two catalysts was the purifica-
approach was especially successful with substrates tion. While the former charge was used in crystalline
such as 4b.
form (C2c), the new charge was purified by flash chro-
Unfortunately, under the microwave conditions the matography (C2fc) and used directly without further
regioselectivity slightly dropped compared to conven- purification.
tional heating. Therefore, we tested out new catalysts
In comparison to the other Mo and W complexes,
also under these conditions, again in comparison to C2 is a rather sensitive complex, undergoing fast de-
our MoBI3 catalyst (C1). With substrate 4a no side composition, for example, during work-up and flash
products (7/8) are formed in any reactions, and so the
isolated yields given in Table 4 were generally high.
Except for catalyst C8 (entry 4) all catalysts gave
yields comparable to C1 (80Æ4%) but better selectiv-
ities. The catalyst of choice under these conditions
was catalyst C4 which provided the expected product
in nearly quantitative yield (entry 2), even with only
1.1 equivalents of Bu3SnH (2 equivalents were used in
the heating reaction). With substrate 4b the reaction
had to be carried out under milder conditions (to
avoid decomposition) and C1 still seems to be the cat-
alyst of choice in this situation (entry 7). C6 provided Scheme 2. Hydro- and distannations of propargyl ether 4a.
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Table 5. Influence of CO on the hydrostannation of alkynes.
Entry Alkyne Cat. Atm. Product ratio [%]
chromatography. Therefore, we were not able to get
pure C2 by chromatography, it was always contami-
nated by traces of isonitrile (from decomposition).
Because this isonitrile is not volatile (in contrast to
the others used) these impurities could not be re-
moved under vacuum. Based on these findings, we
assume that those traces of isonitrile must be respon-
sible for the different reaction behaviours of our cata-
lyst. Probably the additional isonitrile can coordinate
to one of the intermediates in the catalytic cycle
changing the properties of our catalyst.
4
5
6
7
8
1
2
3
4
4a
4a
4a
4a
C1
C1
C4
C4
Ar
CO
Ar
0
0
0
2
80
92
67
91
8
8
7
7
4
0
11
0
8
0
15
0
CO
Table 6. Hydrostannations of alkynes under Ar or CO at-
Because distannation was a side reaction in several
reactions investigated so far, we decided to reinvesti-
gate our hydrostannations in the presence of addition-
al ligands. The easiest way to do so is to run the reac-
tion under a CO atmosphere (instead of Ar), because
CO is isoelectronic to the isonitriles. Therefore, if an
in situ coordination of this ligand can suppress the dis-
tannylation process, one might expect higher yields of
the hydrostannation products 5 and 6. The results ob-
tained under these conditions are summarized in
Table 5 (product distribution) and Table 6 (isolated
yields). And indeed, while in the hydrostannation of
4a in the presence of C1 under Ar the formation of
small amounts of 7 and 8 was observed, under a CO
atmosphere these side reactions could be suppressed
completely (Table 5, entries 1 and 2), and the yield
could be increased to 95% under these conditions
(Table 6, entries 1 and 2). To prove if this is a general
phenomenon, we repeated the reaction using catalyst
C4, which showed a good reactivity in previous inves-
tigations but a significant ratio of side products.
Under CO, also here these side reactions could be
completely suppressed (Table 5, entries 3 and 4).
It is worthy of mention that not only the yield in-
creased dramatically, but also the regioselectivity was
better under the new reaction conditions (Table 6, en-
tries 3 and 4). That this is a general trend is illustrated
with the further examples collected in Table 6. Even
complex, highly functionalized substrates such as 4f
can be hydrostannated in excellent yield and a highly
regioselective fashion (entry 16).
mosphere (5 bar).
Entry Alkyne
Cat. Atm. Time Yield Ratio
[h]
[%]
5:6
1
2
3
4
C1 Ar
20
82
95
65
89
91:9
92:8
90:10
93:7
C1 CO 20
C4 Ar 20
C4 CO 20
5
6
C6 Ar
C6 CO 26
26
59
64
95:5
95:5
7
8
9
10
11
12
C1 Ar
C1 CO
C6 Ar
C6 CO 20
C1 Ar 23
C1 CO 23
5
5
24
49
83
46
83
67
88
85:15
92:8
88:12
92:8
85:15
87:13
13
14
C1 Ar
C1 CO 23
23
74
78
82:18
87:13
15
16
C1 Ar
C1 CO
6
6
72
90
87:13
91:9
Based on these observations, we are now able to
postulate mechanistic pathways for the hydrostanna-
tion/distannation reaction which proceed via common
intermediates (Scheme 3). The different reaction be-
haviour of the complex C2 in contrast to the other
catalysts can probably be explained by the different
electronic properties of the isonitrile ligands. Replace-
17
18
C1 Ar
C1 CO 20
20
61
79
–
–
Therefore, the isonitrile is the ligand that dissoci-
ment of CO ligands, for example, by tert-butyl isoni- ates (L1) from the metal as weakest bound ligand
trile as electron-rich ligand results in a reduced metal- opening free coordination sites for the additions of
CO bond length (1.989–1.994 ꢃ)[16] in comparison to the tin hydride and the alkyne. This would explain the
Mo(CO)6 (2.053–2.065 ꢃ).[17] This clearly indicates higher reactivity of C1 and C4 in comparison to the
that tert-butyl isonitrile acts preferentially as a s- homoleptic carbonyl complexes M(CO)6 (M=Mo,
donor and less as a p-acceptor and enhanced so the W). On the other hand, the isonitrile stays in solution,
metal-CO bond by increasing of the electron density and after the reaction is finished it can recoordinate
at the metal.
to the metal, regenerating the catalyst. This is in good
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Scheme 3. Mechanistic proposal for the hydrostannation and distannation of alkynes using different types of catalysts.
agreement with the observation, that MoBI3 (C1) can catalyst C2, where no free ligand is available for coor-
be recovered in most cases in high yield without loss dination, the reductive elimination is slow, and the in-
of activity. The reaction probably starts with the disso- termediate complex D can undergo a second Bu3SnH
ciation of two weak-bonded ligands L1 from complex addition under liberation of H2 (path b).
C. The coordinative unsaturated complex formed can
The complex E formed undergoes reductive elimi-
undergo oxidative addition of Bu3SnH giving rise to nation giving rise to distannane 7 and an unsaturated
complex A. Coordination of the alkyne provides com- W fragment F, with free coordination sides for oxida-
plex B, and subsequent insertion of the alkyne into tive Bu3SnH addition, closing the catalytic cycle. Di-
À
the M Sn bond generates complex D, with the steri-
ACHTUNGTRENsNGNU tannane complexes similar to E were characterized
cally more demanding metal-group at the sterically by Schubert et al.[18] Reactions of this type are known
least hindered position. At this point the two competi- from palladium complexes,[19] and these complexes
tive pathways start. In case of the trisisonitrile ligands, also catalyze the addition of distannanes to alkynes[20]
where the dissociated isonitrile stays in solution, this and allenes.[21] Interestingly, in the tungsten-catalyzed
ligand can recoordinate towards the metal atom, version the distannylated products are only formed if
making the reductive elimination of the hydrostannat- Bu3SnH is used as tin source, but not with distan-
ed product 5 easier (path a). In principle, the catalyst nanes.
C is regenerated in this step and can undergo the next
catalytic cycle.
In the reactions where the impure catalyst C2fc was
used obviously the traces of isonitrile ligand were
Quite different is the situation in the case of the enough to skip from pathway b to pathway a. This is
tungsten complex C2. IR and Raman spectra show an also in good agreement with observations we made
increase of the C O bond energy in comparison to earlier[22] that, in principle, also Mo(CO)6 can be used
À
W(CO)6, which is equivalent to a reduced metal-CO as catalyst. Although the yields were low (because of
À
p-backbonding. The electron-poor isonitrile obviously the strong Mo CO bond) the distannylated product 7
is tightly bond to the tungsten, and in this case the was formed preferentially, while the complexes
CO probably is the ligand that dissociates from the Mo(CO)5ACHTUNGRTENNUG(CN-t-Bu) (MoBI1) and Mo(CO)4ACHTUNTGERN(NUGN CN-t-Bu)2
metal (L1). This is also in good agreement with the (MoBI2) which, in principle, should form the same in-
observation that the electron-poor isonitrile complex termediates B–D, gave rise to 5 (also in moderate
C2 cannot be recovered in contrast to C1. In case of yield).
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reaction was monitored by TLC. After the isonitrile addi-
tion the colour of the solution changed from light blue to
dark green. When the reaction was finished, the solvent was
removed under vacuum. Silica gel (300 vol%) was added to
the residue and the mixture was suspended in CH2Cl2. The
solids were filtered and extracted 5 times with CH2Cl2. The
combined organic layers were evaporated under vacuum
and the crude complexes were purified by flash chromatog-
raphy.
If the reactions are now run under a CO atmos-
phere, CO can also act as coordinating ligand (·····) in
path a, probably generating complex G which also
can regenerate A on oxidative addition of Bu3SnH.
With the huge excess of the CO ligand, pathway b
can be suppressed completely.
Conclusions
cis-Tetracarbonyl-bis(isovaleroisonitrile)tungsten
(C5)
and fac-tricarbonyl-tris(isovaleroisonitrile)tungsten (C6):
According to the general procedure for complex synthesis
(GP 1) C5 and C6 were obtained using isovaleroisonitrile
(800 mg, 9.60 mmol). After the isonitrile addition, the solu-
tion turned dark green at the beginning and pale green
during the reaction. After 2.5 h the two complexes were sep-
arated by flash chromatography (silica, hexanes/EtOAc 75/
25) giving rise to C5 as a grey solid; yield: 540 mg
(1.17 mmol, 39%) and C6 as a yellow solid; yield: 590 mg
(1.14 mmol, 38%). Rf (hexanes/EtOAc 8/2): C5 0.23; C6
0.18.
In conclusion, we could show that the previously re-
ported Mo catalyst MoBI3 (C1) is still one of the best
hydrostannation catalysts, giving good yields and re-
gioselectivites with various types of propargylic sub-
strates. In certain cases, the newly developed tungsten
catalysts give comparable or even better results, and
therefore these complexes should be considered for
substrates where MoBI3 gives only unsatisfying re-
sults. Detailed mechanistic investigations resulted in
an improved protocol for hydrostannations under a
CO atmosphere, which allows the suppression of side
reactions and better yields and selectivites.
C5: mp 508C; 1H NMR (500 MHz, CDCl3): d=1.03 (d,
J=6.7 Hz, 12H), 2.02, (qqd, J=6.7, 6.7, 6.0 Hz, 2H), 3.42
(d, J=6.0 Hz, 4H); 13C NMR (125 MHz, CDCl3): d=19.5,
29.0, 51.4, 197.3, 200.3; IR (cmÀ1): n=2967, 2935, 2876, 2182
(m, NC), 2153 (m, NC), 2012 (m, CO), 1938 (w, CO), 1844
(s, CO), 1517, 1464, 1439, 1390, 1373, 1339, 1296, 1261,
Experimental Section
1173 cmÀ1
;
HR-MS (CI): m/z=464.0732, calcd. for
C14H18N2O4186W [M]+: 464.0810.
General Remarks
C6: mp 878C; 1H NMR (500 MHz, CDCl3): d=1.02 (d,
J=6.7 Hz, 18H), 1.99, (qqd, J=6.7, 6.6, 6.3 Hz, 3H), 3.38
(d, J=6.3 Hz, 6H); 13C NMR (125 MHz, CDCl3): d=19.6,
29.1, 51.2, 151.3, 203.8; IR (cmÀ1): n=2964, 2931, 2874, 2177
(w, NC), 2134 (s, NC), 1910 (m, CO), 1817 (s, CO), 1515,
1462, 1441, 1389, 1260 cmÀ1; HR-MS (CI): m/z=519.1616,
calcd. for C18H27N3O3186W [M]+: 519.1596.
All air- or moisture-sensitive reactions were carried out in
oven-dried glassware (808C). Dried solvents were distilled
before use: THF was distilled from LiAlH4, CH2Cl2 was
dried with CaH2 before distillation. The products were puri-
fied by flash chromatography on silica gel columns (Ma-
cherey–Nagel 60, 0.063–0.2 mm). Analytical TLC was per-
formed on pre-coated silica gel plates (Macherey–Nagel,
Polygramꢄ SIL G/UV254). Visualization was accomplished
with UV-light, KMnO4 solution or iodine. Melting points
were determined with a MEL-TEMP II apparatus and are
cis-Tetracarbonyl-bis(cyclohexyl isonitrile)tungsten (C7)
and
fac-Tricarbonyl-tris(cyclohexyl
isonitrile)tungsten
(C8):[23] According to the GP 1 C7 and C8 were obtained by
using cyclohexyl isonitrile (924 mg, 8.79 mmol). After 2.5 h
the reaction mixture was purified by flash chromatography
(silica, hexanes/EtOAc 85/15) giving rise to C7 as a white
solid; yield: 404 mg (0.785 mmol, 26%) and C8 as a yellow
solid; Yield: 971 mg (1.63 mmol, 54%); Rf (hexanes/EtOAc
8/2): C7 0.32; C8 0.20.
1
uncorrected. H, 13C and 119Sn NMR spectra were recorded
with a Bruker AC-400 [400 MHz (1H), 100 MHz (13C) and
149 MHz (119Sn)] or Bruker DRX-500 [500 MHz (1H),
125 MHz (13C) and 186 MHz (119Sn)] spectrometer in CDCl3.
Chemical shifts are reported in ppm (d) with respect to
TMS, and CHCl3 was used as the internal standard. Selected
signals for the minor isomers are extracted from the spectra
of the isomeric mixture. Mass spectra were recorded with a
Finnigan MAT 95 spectrometer using the CI technique. Ele-
mental analyses were performed at the Saarland University.
1
C7:[23] mp 1058C; H NMR (500 MHz, CDCl3): d=1.30–
1.52 (m, 8H), 1.69–1.90 (m, 12H), 3.77 (m, 2H); 13C NMR
(125 MHz, CDCl3): d=22.5, 25.2, 32.8, 53.5, 203.8.
1
C8:[23] mp 1208C; H NMR (500 MHz, CDCl3): d=1.30–
1.62 (m, 12H), 1.69–1.94 (m, 18H), 3.77 (m, 3H); 13C NMR
(125 MHz, CDCl3): d=22.5, 25.2, 32.8, 53.5, 204.3; IR: n=
2836, 2857, 2162 (w, NC), 2116 (s, NC), 1904 (m, CO), 1820
(s, CO), 1447, 1360, 1347, 1310, 1261, 1244 cmÀ1; HR-MS
(CI): m/z=597,2071, calcd. for C24H33N3O3186W [M]+:
597.2066; elemental analysis: calcd. (%) for C24H33N3O3W
(595.38): C 48.42, H 5.59, N 7.06; found: C 48.52, H 5.34, N
6.84.
General Procedure (GP 1) for the Synthesis of
Tungsten Complexes
The tungsten complexes C3–C10 were prepared according
to a procedure developed by Albers and Coville and which
was used for the synthesis of C3 and C4 previously.[14] In
general, mixtures of bis- and trisisonitrile complexes were
obtained which could be separated by flash chromatography.
W(CO)6 (1.06 g, 3.00 mmol) and CoCl2 (28.6 mg,
0.22 mmol) were dissolved and refluxed in toluene (12 mL).
The corresponding isonitrile (~9 mmol) was added, and the
cis-Tetracarbonyl-bis(4-dimethylaminophenyl isonitrile)-
tungsten (C9) and fac-Tricarbonyl-tris(4-dimethylamino-
phenyl isonitrile)tungsten (C10): According to GP1 W(CO)6
(524 mg, 1.49 mmol), CoCl2 (14.3 mg, 110 mmol) and p-dime-
thylaminophenyl isonitrile[13] were refluxed in toluene for
Adv. Synth. Catal. 2009, 351, 1395 – 1404
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1401

Alexander O. Wesquet and Uli Kazmaier
FULL PAPERS
1.5 h. Flash chromatography (silica, 1) hexanes/CH2Cl2 1/1;
2) CH2Cl2) provided C9 as a pale yellow solid; yield: 184 mg
(313 mmol, 21%) and C10 as an achre solid; yield: 549 mg
(775 mmol, 52%); Rf (hexanes/EtOAc 50/50 to 0/100): C9
0.37; C10 0.18.
Hydrostannation of 1-phenylpropargyl acetate (4d): Ac-
cording to GP 4 alkyne 4d (432 mg, 2.48 mmol) was reacted
with Bu3SnH (1.46 g, 5.00 mmol) in the presence of C1
(22.8 mg, 53 mmol) and hydroquinone (30.4 mg, 280 mmol) in
THF (2 mL). Flash chromatography (hexanes/Et2O 95/5 to
85/15) gave rise to 5d/6d as an isomeric mixture (ratio 5d/6d
87/13) as a colorless liquid; yield: 1.02 g (2.18 mmol, 88%);
Rf (hexanes/EtOAc 95/5): 5d 0.37; 6d 0.33.
1
C9: mp 1418C; H NMR (500 MHz, CDCl3): d=2.98 (s,
12H), 6.61 (d, J=9.1 Hz, 4H), 7.20 (d, J=9.1 Hz, 4H);
13C NMR (125 MHz, CDCl3): d=40.4, 112.0, 127.3, 197.3,
200.7, 111.5, 149.8, 152.8; IR: n=2896, 2813, 2142 (w, NC),
2107 (m, NC), 2003 (m, CO), 1866 (s, CO), 1598, 1513, 1441,
1358, 1224, 1166, 809 cmÀ1; HR-MS (CI): m/z=590.1258,
calcd. for C22H20N4O4186W [M]+: 590.1028; elemental analy-
sis: calcd (%) for C22H20N4O4W (588.26): C 44.92, H 3.43, N
9.52; found: C 45.48, H 4.00, N 9.18.
5d: 1H NMR (500 MHz, CDCl3): d=0.74 (dt, JSn
=
51.7 Hz, J=8.2 Hz, 6H), 0.83 (t, J=7.3 Hz, 9H), 1.23 (tq,
J=7.3, 7.3 Hz, 6H), 1.28–1.40 (m, 6H), 2.09 (s, 3H), 5.35
(ddd, JSn =59.0 Hz, J=1.8, 1.8 Hz, 1H), 5.96 (ddd, JSn
=
122.6 Hz, J=1.8, 1.8 Hz, 1H), 6.36 (ddd, JSn =31.5 Hz, J=
1.8, 1.8 Hz, 1H), 7.23–7.38 (m, 5H); 13C NMR (125 MHz,
1
C10: mp 1738C (decomp.); H NMR (500 MHz, CDCl3):
CDCl3): d=9.9 (JSn =334.0 Hz), 13.6, 21.3, 27.3 (JSn
=
d=2.96 (s, 18H), 6.57 (d, J=9.1 Hz, 6H), 7.19 (d, J=9.1 Hz,
6H); 13C NMR (125 MHz, CDCl3): d=40.3, 111.9, 117.3,
127.2, 149.5, 156.9, 204.0; IR: n=2891, 2801, 2134 (w, NC),
2075 (s, NC), 1930 (w, CO), 1922 (m, CO), 1904 (m, CO),
1843 (s, CO), 1599, 1511, 1442, 1357, 1223, 1165 cmÀ1.
59.5 Hz), 28.9 (JSn =20.2 Hz), 81.6, 125.4 (JSn =19.2 Hz),
127.3, 127.9, 128.3, 139.5, 153.2, 169.8; 119Sn NMR
(149.2 MHz, CDCl3): d=À41.3.
6d (selected signals): 1H NMR (500 MHz, CDCl3): d=
2.10 (s, 1H), 6.07 (ddd, JSn =21.1 Hz, J=19.6, 4.7 Hz, 1H),
6.21 (ddd, JSn =67.2 Hz, J=19.6, 1.3 Hz, 1H), 6.22 (dd, J=
4.7, 1.3 Hz, 1H), 7.23–7.38 (m, 5H); 13C NMR (125 MHz,
CDCl3): d=9.5 (JSn =358.9 Hz), 13.7, 21.3, 27.2, 29.0, 78.3,
127.3, 128.0, 128.4, 131.2, 139.3, 145.0, 171.6; 119Sn NMR
(149.2 MHz, CDCl3): d=À46.3; HR-MS (CI): m/z=
409.1152, calcd. for C19H29O2120Sn [MÀBu]+: 409.1190; ele-
mental analysis: calcd. (%) for C23H38O2Sn (465.26): C
59.37, H 8.23; found: C 59.36, H 8.04.
General Procedure (GP 2) for Hydrostannations
under “Standard Conditions”
The acetylenic substrate (1.00 mmol) was dissolved in abso-
lute THF (2 mL) under argon in a Schlenk tube. A solution
of the corresponding catalyst (30 mmol, 3 mol%) and freshly
sublimed hydroquinone (11.0 mg, 100 mmol, 10 mol%) in
THF (2 mL) was added, followed by the tin hydride
(300 mg, 1.05 mmol). The Schlenk tube was heated in a
block heater to 608C. After 2 h further Bu3SnH (300 mg,
1.05 mmol) was added. The reaction was monitored by TLC.
After the reaction was complete, the solvent was removed
under vacuum and the crude product was purified by flash
chromatography.
Hydrostannation of 1-isopropylpropargyl acetate (4e): Ac-
cording to GP 4 alkyne 4e (1.42 mg, 1.00 mmol) and
Bu3SnH (582 mg, 20.0 mmol) were reacted in the presence
of C1 (13.1 mg, 30.6 mmol) and hydroquinone (10.1 mg, 92.0
mmol). After 23 h and flash chromatography (hexanes/Et2O
95/5) a mixture of 5e and 6e (ratio 87:13) was obtained as a
colorless liquid; yield: 340 mg (0.79 mmol, 78%); Rf (hex-
anes/EtOAc 95/5): 5e 0.38; 6e 0.38.
General Procedure (GP 3) for Hydrostannations
under Microwave Irradiation
1
5e: H NMR (400 MHz, CDCl3): d=0.82–1.00 (m, 21H),
1.30 (tq, J=7.3, 7.3 Hz, 6H), 1.42–1.52 (m, 6H), 1.71 (qqd,
In a pyrex reaction vessel the corresponding catalyst (50.0
mmol, 5 mol%) and freshly sublimed hydroquinone
(11.0 mg, 100 mmol, 10 mol%) was dissolved in THF (2 mL).
The alkyne (1.00 mmol) and Bu3SnH (310 mg, 1.10 mmol)
were added, the pyrex vessel was sealed with a teflon
septum, and the mixture was irradiated in a microwave oven
(100 W or 250 W) for 5–20 min. The work-up was carried
out as described in GP 2.
J=7.7, 7.7, 7.5 Hz, 1H), 2.03 (s, 3H), 5.06 (dddd, JSn
=
50.7 Hz, J=7.5, 1.1, 1.0 Hz, 1H), 5.28 (ddd, JSn =62.2 Hz,
J=2.2, 1.0 Hz, 1H), 5.78 (ddd, JSn =128.2 Hz, J=2.2, 1.1 Hz,
1H); 13C NMR (100 MHz, CDCl3): d=10.1 (JSn =331.6 Hz),
13.6, 17.9, 19.2, 21.2, 27.4 (JSn =58.0 Hz), 29.0 (JSn =19.8 Hz),
32.0, 86.4 (JSn =33.0 Hz), 127.1 (JSn =19.1 Hz), 153.0, 170.2;
119Sn NMR (149.2 MHz, CDCl3): d=À44.5.
1
6e (selected signals): H NMR (400 MHz, CDCl3): d=1.71
(qqd, J=6.5, 6.5, 6.5 Hz, 1H), 2.05 (s, 3H), 4.97–5.02 (m,
General Procedure (GP 4) for Hydrostannations
under a CO Atmosphere
1H), 5.82 (ddd, JSn =25.6 Hz, J=19.2, 6.2 Hz, 1H), 5.78
(ddd,
J
Sn =70.8 Hz, J=19.2, 1.3 Hz, 1H); 13C NMR
(100 MHz, CDCl3): d=9.5, 13.7, 18.0, 18.1, 21.2, 27.2, 29.1,
31.8, 81.7, 131.6, 144.2, 170.3; 119Sn NMR (149.2 MHz,
CDCl3): d=À47.8; HR-MS (CI); m/z=373.1361, calcd. for
C16H31O2118Sn [MÀBu]+: 373.1340; elemental analysis: calcd.
(%) for C20H40O2Sn (431.24): C 55.70, H 9.35; found: C
55.70, H 9.19.
Hydrostannation of propargylic amino acid derivative 4f:
According to GP 4, 4 f (81.1 mg, 250 mmol)[24] was subjected
to hydrostannation using Bu3SnH (146 mg, 500 mmol), C1
(4.4 mg, 10.3 mmol) and hydroquinone (4.70 mg, 42.7 mmol)
in THF (3 mL). After 6 h reaction time and work-up (silica,
hexanes/Et2O 9/1 to 8/2) 5f and 6f were obtained as a col-
The reactions were carried out in pyrex tubes with a special
constructed PTFE stopper, which allowed the addition or
removal of substrates via syringe. A solution of the alkyne
(1.00 mmol) in THF (2 mL) was added via syringe to a solu-
tion of the catalyst (30.0 mmol, 3 mol%) and hydroquinone
(11.0 mg, 100 mmol, 10 mol%) followed by Bu3SnH (570 mg,
2.0 mmol). The vessel was purged twice with CO, before the
CO pressure was increased to 5 bar and the mixture was
warmed up to 608C in a block heater. The work-up was car-
ried out as described for GP 3.
The hydrostannations and the products obtained from al-
kynes 4a–c were described previously.
1402
asc.wiley-vch.de
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 1395 – 1404

Improved Protocols for Molybdenum- und Tungsten-Catalyzed Hydrostannations
FULL PAPERS
ourless liquid; yield: 139 mg (226 mmol, 90%) (ratio 5f/6f: References
91/9); Rf (hexanes/EtOAc 8/2): 5f 0.46; 6f 0.46.
1
5f (major diastereomer): H NMR (400 MHz, CDCl3): d=
0.87 (t, J=7.3 Hz, 9H), 0.96 (dt, JSn =50.7 Hz, J=8.2 Hz,
6H), 1.30 (tq, J=7.3, 7.3 Hz, 6H), 1.42–1.54 (m, 6H), 1.45
(s, 1H), 2.06 (s, 3H), 4.73 (dd, J=8.2, 4.6 Hz, 1H), 5.40
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(ddd, JSn =56.7 Hz, J=1.6, 1.5 Hz, 1H), 5.63 (dddd, JSn
=
34.1 Hz, J=4.6, 1.5, 1.5 Hz, 1H), 5.87 (ddd, JSn =116.5 Hz,
J=1.6, 1.5 Hz, 1H), 6.99 (d, J=8.2 Hz, 1H); 13C NMR
(100 MHz, CDCl3): d=10.2 (JSn =317.7 Hz), 13.6, 20.9, 27.3
(JSn =60.9 Hz), 28.0, 28.9, 55.5, 79.6, 84.0, 115.6 (q, JF =
288.3), 128.1, 149.7, 156.9 (q, JF =38.2), 166.6, 169.8;
119Sn NMR (149.2 MHz, CDCl3): d=À38.7.
1
5f (minor diastereomer): H NMR (400 MHz, CDCl3): d=
0.88 (t, J=7.3 Hz, 9H), 0.98 (dt, JSn =50.7 Hz, J=8.3 Hz,
6H), 1.31 (tq, J=7.4, 7.4 Hz, 6H), 1.42–1.54 (m, 6H), 1.44
(s, 1H), 2.07 (s, 3H), 4.73 (dd, J=9.2, 3.1 Hz, 1H), 5.03
(ddd, JSn =56.7 HzJ=1.6, 1.5 Hz, 1H), 5.79 (dd, J=1.6,
1.5 Hz, 1H), 5.93 (dddd, JSn =19.8 Hz, J=3.1, 1.5, 1.5 Hz,
1H), 6.67 (d, J=9.2 Hz, 1H); 13C NMR (100 MHz, CDCl3):
d=10.0, 13.6, 20.6, 27.3 (JSn =60.9 Hz), 27.8, 28.8, 55.4, 77.7,
83.8, 126.1, 148.7, 166.8, 168.8; 119Sn NMR (149.2 MHz,
CDCl3): d=À41.4.
6f (major diastereomer, selected signals): 1H NMR
(400 MHz, CDCl3): d=4.84 (dd, J=8.3, 3.5 Hz, 1H), 5.56
(ddd, J=6.2, 3.5, 1.3 Hz, 1H), 6.37 (ddd, JSn =62.2 Hz, J=
19.1, 1.3 Hz, 1H), 6.92 (d, J=8.3 Hz, 1H).
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6f (minor diastereomer, selected signals): 1H NMR
4
(400 MHz, CDCl3): d=4.79 (dd, J=9.2, 2.6 Hz, 1H), J1,3
=
1.3 Hz, 1H), 6.23 (dd, J=20.3, 2.6 Hz, 1H), 6.75 (d, J=
9.2 Hz, 1H); HR-MS (CI): m/z=558.1489, calcd. for
C21H35F3NO5120Sn [MÀBu]+: 558.1489; elemental analysis:
calcd (%) for C25H44F3NO5Sn (614.33): C 48.88, H 7.22, N
2.28; found: C 48.66, H 6.97, N 2.61.
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Hydrostannation of diethyl 1,4-butynediol dicarbonate
(4g): According to GP 4 4g (932 mg, 4.05 mmol) was hydro-
stannated using Bu3SnH (2.33 g, 8.00 mmol), C1 (54.1 mg,
126 mmol) and hydroquinone (194 mg, 110 mmol) in THF
(4 mL). After 20 h the crude product was purified by flash
chromatography (hexanes/Et2O 9/:1 to 85/15) to give 5g as a
colourless liquid; yield: 1.67 g (3.20 mmol, 79%); Rf (hex-
1
anes/EtOAc 8/2): 5g 0.46. H NMR (400 MHz, CDCl3): d=
0.86 (t, J=7.3 Hz, 9H), 0.91 (dt, JSn =50.2 Hz, J=8.2 Hz,
6H), 1.22–1.33 (m, 12H), 1.40–1.50 (m, 6H), 4.16 (q, J=
7.3 Hz, 2H), 4.18 (q, J=7.3 Hz, 2H), 4.67 (ddt, JSn =9.4 Hz,
J=6.0, 1.0 Hz, 2H), 4.86 (ddt, JSn =32.9 Hz, J=2.1, 1.0 Hz,
2H), 5.74 (dtt, J3,Sn =60.5 Hz, J=6.0, 2.1 Hz, 1H); 13C NMR
2
(100 MHz, CDCl3): d=10.0 (JSn =338.2 Hz), 13.6, 14.2, 14.3,
27.3 (JSn =58.0 Hz), 28.9 (JSn =19.8 Hz), 63.9, 64.1, 64.3, 69.0
(JSn =16.8 Hz), 133.7 (JSn =20.5 Hz), 145.9, 154.9, 155.0;
119Sn NMR (149.2 MHz, CDCl3): d=À33.2; HR-MS (CI);
m/z=465.1206, calcd. for C18H33O6120Sn [M]+: 465.1299.
[9] A. O. Wesquet, S. Dçrrenbꢁcher, U. Kazmaier, Synlett
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Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie is gratefully ac-
knowledged.
Adv. Synth. Catal. 2009, 351, 1395 – 1404
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1403

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Adv. Synth. Catal. 2009, 351, 1395 – 1404