On the hydrostannylation of aryl propargylic alcohols and their derivatives: Remarkable differences in both regio- and stereoselectivity in radical- and nonradical-mediated transformations

Article abstract of DOI:10.1002/chem.201403459

Herein, we describe a highly regio- and stereoselective radical-mediated and molecular-oxygen (O₂)-dependent hydrostannylation of phenyl propargylic alcohols and their derivatives. There is a significant steric effect on the stereoselectivity of the tin-radical addition. Further, the uncatalyzed regio- and stereoselective hydrostannylation of aryl propargylic alcohols with nBu₃SnH and Ph₃SnH is also described and occurs with near titration kinetics. Although the uncatalyzed addition with nBu₃SnH gave a remarkable γ-regioselectivity irrespective of the electronic nature of the aryl moiety, addition with Ph₃SnH appears to be driven by the electronic nature of the aryl alkynes.

Full text of DOI:10.1002/chem.201403459

DOI: 10.1002/chem.201403459
Communication
&
Synthetic Methods
On the Hydrostannylation of Aryl Propargylic Alcohols and Their
Derivatives: Remarkable Differences in Both Regio- and
Stereoselectivity in Radical- and Nonradical-Mediated
Transformations
Martins S. Oderinde,[a] Robert D. J. Froese,[b] and Michael G. Organ*[a]
metal-catalyzed hydrometalation reactions have shown poor
Abstract: Herein, we describe a highly regio- and stereo-
selective radical-mediated and molecular-oxygen (O2)-de-
to good regiocontrol, better selectivity has been achieved with
radical-mediated hydrostannylation providing that a suitable
pendent hydrostannylation of phenyl propargylic alcohols
directing force is present, such as is the case with alkyl propar-
and their derivatives. There is a significant steric effect on
gylic alcohols and their derivatives (Scheme 1a).[4–6]
the stereoselectivity of the tin-radical addition. Further,
the uncatalyzed regio- and stereoselective hydrostannyla-
tion of aryl propargylic alcohols with nBu3SnH and Ph3SnH
is also described and occurs with near titration kinetics. Al-
though the uncatalyzed addition with nBu3SnH gave a re-
markable g-regioselectivity irrespective of the electronic
nature of the aryl moiety, addition with Ph3SnH appears to
be driven by the electronic nature of the aryl alkynes.
Hydrometalation is one of the most important sub-classes of
addition reactions involving alkenes, allenes, and alkynes.[1]
Scheme 1. Regioselective radical-mediated hydrostannylation of propargylic
alcohols and their derivatives.
Alkyne hydroboration, hydrozirconation, and hydrostannylation
are central transformations within this sub-class.[1] Of the or-
ganometallic products from these reactions, vinylstannanes are
arguably the most versatile due to the diverse array of reac-
tions that they are capable to undergo (e.g., cross-coupling,
metal–metal exchange, metal–halogen exchange).[2–3] Conse-
quently, the ability to control the regio- and stereochemical as-
pects of alkyne hydrostannylation is of great interest, particu-
larly with internal alkynes that give rise to increasingly com-
plex, and thus valuable products.[4]
We have studied the radical-mediated hydrostannylation of
alkyl propargylic alcohols and their derivatives and found cer-
tain aspects of the reaction very intriguing.[6] For example, we
reported that the regiochemical outcomes of hydrostannyla-
tion was unchanged when the hydroxyl group was adorned
with bulky groups, such as triisopropylsilyl (TIPS), which casts
doubt on proposals that suggest tin radical (or tin hydride) co-
ordination to the oxygen atom in the transition state controls
regioselectivity.[5]
The stereoselectivity in hydrostannylation is largely predicta-
ble depending on whether it is transition metal catalyzed,
which proceeds stereospecifically by syn addition of the tin hy-
dride across the triple bond, or radical mediated that provides
the trans addition product kinetically, although isomerization
to the syn product can occur over time if there is a thermody-
namic driving force.[5] However, the ability to control the regio-
selectivity of the addition has been the most challenging
aspect of all hydrometalation reactions.[1,4] Although transition-
In an effort to better understand alkyne hydrostannylation,
we investigated the reaction of aryl propargylic alcohols and
their derivatives. The products of such addition would be
highly valued, and the steric and electronic nuances of the aryl
systems can be systematically varied to provide key mechanis-
tic understanding (Scheme 1b). Surprisingly, we could not find
any prior reports of radical-mediated hydrostannylation of aryl
propargylic alcohols in the literature, and our initial reactions
may provide some understanding for why these substrates
have not been reported. Herein, we report on the radical-medi-
ated and uncatalyzed hydrostannylation of aryl propargylic al-
cohols and their derivatives.
[a] Dr. M. S. Oderinde, Prof. M. G. Organ
Department of Chemistry, York University
4700 Keele Street, Toronto, ON, M3J 1P3 (Canada)
E-mail: organ@yorku.ca
[b] Dr. R. D. J. Froese
Unlike their alkyl counterparts (Scheme 1a), the hydrostan-
nylation of the free propargylic alcohol 7a gave a mixture of
regio- and stereoisomers in low yield with either nBu3SnH or
The Dow Chemical Company, Midland, Michigan, 48674 (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403459.
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Ph3SnH (Table 1, entries 1 and 2). Although we have demon-
strated that Et3B leads to better stereoselectivity during the hy-
drostannylation of 1 and 2,[6] both Et3B and Azobisisobutyroni-
trile (AIBN) were unselective in the hydrostannylation of 7a
(entries 1 and 3). Remarkably, the reaction gave absolute regio-
control for the proximal b-8b without any of the distal g-8b
regioisomer in full conversion by using no more than
1.2 equivalents of nBu3SnH, albeit with little stereocontrol
when the alcohol was protected as an acetyl (Ac) group 7b
(Table 1, entry 4). Lowering the temperature from 80 to 238C
had no effect on the stereoselectivity of the addition, although
the reaction did go to completion (Table 1, entry 5). The ability
to carry out the hydrostannylation of aryl propargylic moieties
at RT by using nBu3SnH in benzene showed the higher reactivi-
ty of aryl propargylic moiety over its alkyl counterpart.[5–6]
Interestingly, increasing the size of the protecting group
from acetyl to tert-butyldimethylsilyl (TBS, 7c) slightly im-
proved the Z selectivity (i.e., trans addition) of the reaction
(Table 1, entry 6). This selectivity was further enhanced when
the bulk was increased to tert-butyldiphenylsilyl (TBDPS, 7d,
entry 7). Finally, when the triisopropylsilyl (TIPS) ether deriva-
tive (7e) was employed, absolute regio- and stereocontrol for
b-(Z)-8e was attained (entry 8).
Table 1. Radical-mediated hydrostannylation of phenyl propargylic alco-
hols and their derivatives.[a]
Entry
R
R1
T [8C]
b-(Z)-8/
b-(E)-8
Conv. of
b [%][b]
Conv. of
g-(E)-8 [%]
1 (a)
2 (a)
3 (a)
4 (b)
5 (b)
6 (c)
7 (d)
8 (e)
H
H
H
H
H
H
H
H
H
H
H
Ac
80
23
80
80
23
23
23
23
35:65
30:70
30:70
55:45
55:45
66:34
80:20
99:1
40[c]
30
40
20
–
–
–
35[d]
35[e]
100[c]
Ac
100[c]
TBS
TBDPS
TIPS
100 (97)[c]
100[c]
–
–
100 (96)[c]
[a] Percent conversion and isomer ratios were determined by 1H NMR
spectroscopy of the crude reaction mixture; >99:1 means that the other
isomer was not detected. [b] Isolated yield in parenthesis. [c] Alkyne
(0.5m in benzene), nBu3SnH (1.2 equiv), Et3B (0.5 equiv), 2 h. [d] Ph3SnH
(2.0 equiv) was used, 1 h. [e] Alkyne (0.5m in benzene), nBu3SnH
(1.2 equiv), AIBN (0.1 equiv), 2 h.
Armed with the knowledge that increased bulk on the prop-
argylic oxygen atom improves the regio- and stereoselectivity
of this radical-mediated hydrostannylation, we embarked on
scope studies that included primary, secondary, and tertiary
phenyl propargylic alcohols and their derivatives (Table 2). For
primary substrates, the use of nBu3SnH showed better stereo-
selectivity than Ph3SnH (e.g., 8e vs. f). Similarly, substituting
the hydrogen atom of some secondary propargylic alcohols
with tert-butyldimethylsilane (TBS) improved their stereoselec-
tivity (e.g., 8g vs. h and 8i vs. j). We also found that substitu-
ents on the oxygen were not necessary for high selectivity
when substituents were present at the a-position (8k–n). How-
ever, this change was accompanied by a reduced reaction rate
at 238C, in which case the reactions were carried out at 808C
by using AIBN with no adverse effect on regio- and stereose-
lectivity. In all cases, good to excellent yields with remarkable
regio- and stereocontrol were obtained. We rationalized the
trend in the above-described results on the basis of steric ef-
fects. As the bulk of the oxygen substituent increased from Ac
to TIPS, there is a significant decrease in the rate of the tin rad-
ical addition onto the starting alkyne. More importantly, addi-
tion of a second radical onto the kinetically formed b-(Z)-8
isomer was suppressed thus eliminating isomerization to the
b-(E)-8 isomer.[6a] With secondary and tertiary alcohols, an in-
crease in bulk at the a-carbon also showed similar steric effects
on selectivity.
Table 2. Scope studies of radical-mediated hydrostannylation of phenyl
propargylic alcohols and silyl ethers.[a]
For mechanistic considerations, we conducted the hydro-
stannylation of 7e by using AIBN (5 mol%) in the presence (re-
agents not degassed) and absence (fully degassed reagents
and the transformation was conducted inside of an argon-
filled glovebox) of O2 (Scheme 2). Interestingly, although the
reaction conducted in the presence of O2 (air) gave 100% con-
version to product within one hour, the one conducted inside
the glovebox with the rigorous exclusion of O2 gave only 5%
1
[a] Isomeric ratios were determined by H NMR spectroscopy of the crude
reaction mixture; >99:1 means that the other isomer was not detected
by 1H NMR spectroscopy. [b] Alkyne (0.5m in benzene), nBu3SnH
(1.2 equiv), Et3B (0.5 equiv), 238C, 2 h. [c] Alkyne (0.5m in benzene),
Ph3SnH (1.2 equiv), Et3B (0.5 equiv), 238C, 1 h. [d] Alkyne (0.5m in ben-
zene), nBu3SnH (2.0 equiv), AIBN (0.1 equiv), 808C, 3 h. [e] Alkyne (0.5m in
benzene), nBu3SnH (1.2 equiv), AIBN (0.1 equiv), 808C, 2 h.
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Table 3. Scope studies of uncatalyzed hydrostannylation of aryl propar-
gylic alcohols and silyl ethers with nBu3SnH.[a,b,c]
Scheme 2. Effect of molecular oxygen on hydrostannylation.
Figure 1. Proposed mechanism of radical-mediated hydrostannylation of
phenyl propargylic alcohols in the presence of O2.
1
[a] Isomeric ratios were determined by H NMR spectroscopy of the crude
reaction mixture. [b] The regioisomers were easily separated by silica-gel
chromatography giving rise to isomerically pure products in all cases.
[c] Alkyne (0.5m in THF), nBu3SnH (2 equiv), 5 min.
of conversion to product. As shown in Scheme 2, only a catalyt-
ic amount of O2 (10 mol%, purity>99.999%) is necessary for
radical-chain propagation.
To account for these results, we propose the mechanism
shown in Figure 1, which is consistent with our previous mech-
anistic studies on alkyl propargylic alcohols and that was sup-
ported by DFT studies.[7] Contrary to current belief in the hy-
drostannylation field, initial tin-radical addition to 7 need not
be regioselective to give rise to a single isomer of the hydro-
metalated alkene product. In fact, there may be little or no se-
lectivity in the initial tin-radical addition to give rise to an iso-
meric mixture of vinyl radicals 9 and 10. A single-electron
transfer (SET) from either 9 and 10 to O2 will coalesce to form
a more reactive benzylic/vinyl cation 11 and superoxide
hanced towards the major isomer in THF than benzene, in
which the reactions proceed at a slower rate. The regiochemi-
cal outcome of the additions was only slightly influenced by
the electronic nature of the substituents on the aromatic ring.
For example, although the uncatalyzed hydrostannylation of
aryl propargylic alcohols 12a–d containing electron-withdraw-
ing substituents (R=p-CH3(CO), p-NO2 and p-CN) gave g-re-
gioisomers 13a–d in good yields (g/bꢀ95:5), the selectivity
for the g-regioisomer only slightly decreased when the para-
position was unsubstituted 13e (g/bꢀ70:30) or contained
electron-donating groups (R=p-OMe and p-Me, 13 f and 13g,
g/b=80:20 and 85:15, respectively). Intriguingly, moving the
substituents from the para site to either the ortho or meta- po-
sitions did not affect the high selectivity for the g-regioisomer
(13h–k).[9] Perhaps most impressive is the fact that even or-
tho,ortho-disubstituted aryl rings (13l) failed to diminish the
high regioselectivity for what would appear to be the more
sterically congested regioisomer.
(O2 À).[8] Regioselective hydride transfer from nBu3SnH to the
C
polarized three-centered cation 11 will ultimately give the ki-
netic product b-(Z)-8 and nBu3Sn+, which is subsequently rap-
À
C
C
idly reduced by O2 to regenerate nBu3Sn and O2 thus com-
pleting the catalytic cycle.[7] Non-O2-dependent isomerization
C
of the (Z)-isomer with nBu3Sn would give rise to the (E)-
isomer.[5,7]
Puzzled by the origin of g-(E)-product in Table 1, entries 1–3,
we conducted the hydrostannylations of substituted aryl prop-
argylic alcohols and their silyl ethers in the absence of radical
mediators, transition metals,[4] or oxygen by simply adding
nBu3SnH to O2-free solutions of the alkynes and, shockingly,
the hydrostannylation rapidly gave products at RT (Table 3).[9]
Herein, not only the regiochemistry of the addition was com-
pletely reversed from the radical-mediated transformation, so
was the stereoselectivity (i.e., now exclusive syn addition). We
also found that the regioselectivities were significantly en-
The regiochemical outcomes when Ph3SnH was used instead
of nBu3SnH in these non-radical-mediated additions are in
stark contrast (Table 4). Although the hydride of Ph3SnH was
added to the b carbon of 12b, which has a para-electron-with-
drawing group, to give 14a, it added to the g carbons of 12 f
and g with para-electron-rich groups to give 14b and c, re-
spectively. Interestingly, Z-stereochemical outcomes were ob-
tained for the b-vinyltriphenylstannanes 14b and c. Assuming
that the initial tin hydride additions in Table 3 and 4 proceed
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are exothermic and in reality are just titrations. This encour-
aged us to revisit tolane hydrostannylation studies in the litera-
ture.[11] These Pd-catalyzed results that showed similar regio-
chemistry to those described in our study, that is, tin place-
ment closest to the substituted aryl ring (even ortho-substitut-
ed) when simple phenyl was at the other end of the alkyne.
We wondered if these reactions were, in fact, not catalyzed by
Pd at all given the very high reactivity that has been observed
with the aryl propargylic system in our study. Interestingly,
when we used 1-(2-methylphenyl)-2-phenylacetylene under
the conditions presented in Table 3, no reaction occurred, al-
though it did go to completion when the Pd catalyst was
added, as was described by Alami and co-workers[11a] and Ge-
vorgyan and co-workers.[11b] Clearly, there is something unique
about the aryl propargylic alcohol system that leads to this
high reactivity.
Table 4. Scope studies of uncatalyzed hydrostannylation of aryl propar-
gylic alcohols with Ph3SnH.[a,b]
1
[a] Isomeric ratios were determined by H NMR spectroscopy of the crude
reaction mixture; >99:1 means that the other isomer was not detected
by 1H NMR spectroscopy. [b] Alkyne (0.5m in THF), Ph3SnH (2 equiv),
5 min.
Finally, we carried out six diverse stereospecific transforma-
tions on b-(Z)-8h to illustrate the value of the hydrostannyla-
tion methodology developed in this report (Scheme 3). Both
the tin/iodide and tin/bromide exchange reactions of b-(Z)-8h
by using I2 and N-bromosuccinimide (NBS), respectively, pro-
ceeded quantitatively to give (Z)-15a and (Z)-15b selectivity.
Likewise, the protodestannylation of b-(Z)-8h with HCl in Et2O
gave the disubstituted internal alkene (E)-16 (Z/E 2:98) in high
by a stereospecific syn-addition,[10] the Z stereochemistries of
14b and c can only be explained by rapid E to Z-isomerization,
because any of the b-(E) isomer has been never observed, al-
though it remains unclear how this process proceeds
in the presumed absence of radicals. No such isomer-
ization was observed with b-vinyltributylstannanes as
for those shown in Table 3.
The fact that very similar substrates follow different
hydrostannylation mechanisms is striking and note-
worthy. From Table 1, simple phenylpropargyl alcohol
appears to undergo the trans radical-mediated addi-
tion at a similar rate as the direct (uncatalyzed) syn
hydrostannylation (Table 1, entries 1–3). Yet when dif-
ferent groups are placed on the oxygen moiety, this
pathway is completely shut down despite still giving
high conversion to the radical-addition product b8
(entries 4–8). This was further demonstrated in
Scheme 2, in which the reaction without O2 did not
give any syn hydrostannylation product despite that
only traces of b-E/Z-8e were formed after 1 h. More
striking is that the placement of seemingly any sub-
stituent at any position on the phenyl ring dramati-
Scheme 3. Stereospecific transformations of b-(Z)-8h. DCM=dichloromethane.
cally enhances direct hydrostannylation, which com-
pletes very rapidly at RT (Table 3). Indeed, when we
submitted substrates 12b and f to the radical-reaction condi-
tions given in Table 2, only the syn hydrostannylation products
13 were produced; none of the anti addition product resem-
bling 8 were ever observed. Thus, the direct hydrostannylation
mechanism with aryl propargyl alcohol derivatives does not
appear to be under electronic control imparted by the aryl
ring, at least when using tributylstannane. The reactivity pat-
tern being displayed between the two operating mechanisms
in this hydrometalation study appears to be without prece-
dent.
recovery. Pd-catalyzed cross-crossing of b-(Z)-8h with arylio-
dide 17 in the presence of CuI co-catalyst[10] gave (Z)-18 with-
out any isomerization of the double bond. Similarly, the Pd-cat-
alyzed cross-coupling of b-(Z)-8h with acid chloride 19 in the
presence of CuCN co-catalyst gave (Z)-20 as the major isomer.
Finally, the telescoped, two-step stereospecific conversation of
b-(Z)-8h to vinylcyanide (Z)-21 proceeded uneventfully giving
another versatile building block for further elaboration.
In conclusion, we have carried out the first studies on the
radical-mediated hydrostannylation of aryl propargylic alcohols
and their derivatives. Increasing the bulk on the propargylic
oxygen atom or at the a-carbon has two important roles. First,
The high reactivity of the direct hydrostannylation warrants
further comment. Additions involving the substrates in Table 3
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[1] For reviews see: a) B. M. Trost, Z. T. Ball, Synthesis 2005, 6, 853; b) N. D.
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shutz, J. Am. Chem. Soc. 1988, 110, 2641.
it ensures complete regioselective placement of the tin distal
to the phenyl ring, and second, it slows or completely prevents
the isomerization of the kinetically formed (Z)-isomer. All ex-
periments, including the dramatic effect of O2, strongly sug-
gest that a radical/cationic pathway is involved in the hydro-
stannylation of phenyl propargyl moieties and that O2 is serv-
ing as a redox catalyst in this process.[7,8]
[3] a) W. P. Neumann, The Organic Chemistry of Tin, Wiley, New York, 1970;
b) K. W. Kells, J. M. Chong, J. Am. Chem. Soc. 2004, 126, 15666.
[4] a) J. P. Betzer, F. Delaloge, B. Muller, A. Pancrazi, J. Prunet, J. Org. Chem.
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[5] a) M. Taddei, C. Nativi, J. Org. Chem. 1988, 53, 820; b) P. Dimopoulos, J.
George, D. A. Tocher, S. Manaviazar, K. Hale, Org. Lett. 2005, 7, 5377;
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Nozaki, K. Oshima, K. Utimoto, J. Am. Chem. Soc. 1987, 109, 2547.
[6] a) M. S. Oderinde, H. N. Hunter, M. G. Organ, Chem. Eur. J. 2012, 18,
10817; b) M. S. Oderinde, H. N. Hunter, R. D. J. Froese, M. G. Organ,
Chem. Eur. J. 2012, 18, 10821; c) M. S. Oderinde, M. G. Organ, Angew.
Chem. 2012, 124, 9972–9975; Angew. Chem. Int. Ed. 2012, 51, 9834–
9837; d) M. S. Oderinde, M. G. Organ, Chem. Eur. J. 2013, 19, 2615.
[7] M. S. Oderinde, R. D. J. Froese, M. G. Organ, Angew. Chem. 2013, 125,
11544; Angew. Chem. Int. Ed. 2013, 52, 11334.
[8] a) S. Zhu, A. Das, L. Bui, H. Zhou, D. P. Curran, M. Rueping, J. Am. Chem.
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[9] The inclusion of 2–50 mol% of either Pd(PPh3)2Cl2 or Ni(PPh3)2Cl2 did
not affect the reactivity or regiochemical outcomes of the addition.
See; F. Liron, P. Le Garrec, M. Alami, Synlett 1999, 2, 246.
We have also described the first uncatalyzed hydrostannyla-
tion protocols of aryl-substituted propargylic alcohols with
nBu3SnH and Ph3SnH. Although the uncatalyzed addition with
nBu3SnH proceeded with g-regioselectivity irrespective of the
electronic or steric nature of the aryl substituent, additions
with Ph3SnH demonstrated what appears to be an electronic
bias. This direct syn hydrostannylation is strongly promoted by
the presence of substituents on the aromatic ring, a result that
we cannot understand yet, despite studying the system by
computation methods. When these substituted arylalkynes are
presented with a choice of undergoing radical-mediated addi-
tion, or simple direct syn hydrometalation, only the product of
non-radical addition is obtained illustrating just how fast these
non-catalyzed additions are. As was demonstrated, the hydro-
stannylation protocols of aryl propargylic alcohols described
herein will be of value for the straightforward preparation of
functionalized building blocks and complex target molecules
in organic and medicinal chemistry.
[10] J. Chae, T. Konno, M. Kanda, T. Ishihara, H. Yamanaka, J. Fluorine Chem.
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Acknowledgements
This work was supported by NSERC (Canada) and The Ontario
Research Fund (ORF; Ontario).
Keywords: hydrostannylation · radicals · regioselectivity
stereoselectivity · vinylstannane
·
Received: May 8, 2014
Published online on && &&, 0000
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COMMUNICATION
&
Synthetic Methods
M. S. Oderinde, R. D. J. Froese,
M. G. Organ*
&& – &&
Electronic nature: The presence of
seemingly any substituent on the ring
of aryl propargyl alcohols led to ex-
tremely rapid hydrostannylation of the
triple bond with tributyltin hydride. The
addition is stereospecific (syn) and
highly regioselective to place the tin
moiety next to the aryl ring. This is in
stark contrast to the radical-mediated
hydrostannylation of simple phenyl
propargyl alcohols that lead to both the
opposite regio- and stereoselectivity
(see scheme; AIBN=2,2’-azobisisobutyro-
nitrile).
On the Hydrostannylation of Aryl
Propargylic Alcohols and Their
Derivatives: Remarkable Differences in
Both Regio- and Stereoselectivity in
Radical- and Nonradical-Mediated
Transformations
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