Mechanistic analysis of gold(I)-catalyzed intramolecular allene hydroalkoxylation reveals an off-cycle bis(gold) vinyl species and reversible C-O bond formation

Article abstract of DOI:10.1021/ja303844h

Mechanistic investigation of gold(I)-catalyzed intramolecular allene hydroalkoxylation established a mechanism involving rapid and reversible C-O bond formation followed by turnover-limiting protodeauration from a mono(gold) vinyl complex. This on-cycle p

Full text of DOI:10.1021/ja303844h

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Communication
Mechanistic Analysis of Gold(I)-Catalyzed Intramolecular
Allene Hydroalkoxylation Reveals an Off-Cycle Bis(gold)
Vinyl Species and Reversible C–O Bond Formation
Timothy J Brown, Dieter Weber, Michel R Gagné, and Ross A. Widenhoefer
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja303844h • Publication Date (Web): 23 May 2012
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Mechanistic Analysis of Gold(I)-Catalyzed Intramolecular Allene
Hydroalkoxylation Reveals an Off-Cycle Bis(gold) Vinyl Species
and Reversible C–O Bond Formation
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Timothy J. Brown,a Dieter Weber,b Michel R. Gagné,*b Ross A. Widenhoefer*a
a French Famil y Science Center, Duke University, Durham, NC 27708-0346.
b Caudill Laboratories, Department of Chemistry, University of North Carolina at Chapel Hill, NC
27599-3290.
Supporting Information Placeholder
tions and establ ishes the reversibil it y of C–O
ABSTRACT: Mechanistic in vestigation of gold(I)-
catal yzed intramolecular al lene hydroalkox y la-
tion establ ished a mechanism invol v ing rapid and
re versible C–O bond formation followed by turn-
over-limiting protodeauration from a mono(gold)
v in y l complex. This on-cycle pathway competes
w ith catal yst aggregation and formation of an off-
cyc le bis(gold) vin y l complex.
bond formation.8-10
(L)AuCl (5 mol %)
[L = P(t-Bu)2o-biphenyl]
AgOTs (5 mol %)
O
OH
eq 1
Ph
Ph
toluene, 24 °C, 3 min
91%
Ph
Ph
2
1
In an effort to intercept gold v in y l intermedi-
ates in the gold(I)-catal yzed conversion of 1 to 2, a
toluene suspension of 1, (L)AuCl [L = P(t-Bu)2o-
bipheny l], AgOTs, and Et3N (1:1:1:2) was stir red at
room temperature for 1.5 h (Scheme 1).11 Aqueous
work up and cr ystal l ization from warm hexanes
gave mono(gold) vin y l complex 3 in 87% yield as
an air- and thermal l y stable white solid that was
ful l y character ized by spectroscopy and X-ra y
cr ystal lograph y (Figure 1). Treatment of 3 (31
mM) with (L)AuOTs (1 equi v) in CD2Cl2 at 0 °C led
to immediate (≤5 min) formation of the bis(gold)
v in y l complex 4 in 98 ± 5% yield by 1H NMR
(Scheme 1). Comple x 4 persisted indefinite l y in
solution at this temperature but decomposed
when concentrated and was therefore character-
ized in solution. Notabl y, the 31P NMR spectrum
of 4 displa yed a 1:1 ratio of resonances at δ 61.7
and 60.9, which establ ished the presence of
chemical l y inequivalent (L)Au fragments, whi le
the large difference in the 1H NMR shifts of the
v in y l ic protons of 4 [δ 4.84 and 3.90] relati ve to
those of 3 [δ 5.48 and 4.45] establ ished interaction
of the v in y l moiety of 4 with both (L)Au+ frag-
ments.
Over the past decade, the appl ications of solu-
ble gold(I) complexes as catal ysts for organ ic
transformations, in particular the electrophil ic
activation of C–C multip le bonds, have increased
dramatical l y.1 In contrast to the extensive devel-
opment of the synthetic aspects of gold(I) π-
activation catal ysis, the mechanisms of these
transformations remain poorl y defined and are
derived largel y from computational anal yses.2
Howe ver, exper imental evidence pertain ing to
the mechanisms of gold(I) π-activation catal ysis,
notabl y the in situ detection and/or independent
synthesis of potential catal ytic intermediates,
has begun to emerge.3-5 Perhaps most intr iguing
among these potential intermediates are the
bis(gold) vin y l complexes formed via the addition
of nucleophiles to al lenes or alk ynes in the pres-
ence of gold(I).5,6 However, the extent to which
formation of bis(gold) species represent a genera l
phenomenon in gold π-activation catal ysis re-
mains unclear and lacking is information regard-
ing the behavior of bis(gold) vin y l species vis-à-
v is mono(gold) vin y l complexes4 under catal yt ic
conditions.
(L)AuCl (1 equiv)
OH
AgOTs (1 equiv)
Et3N (2 equiv)
O
AuL
Ph
Ph
toluene
25 °C, 1.5 h
87%
Ph
3
1
Ph
Herein we report the mechanistic investiga-
tion of the gold(I)-catal yzed intramolecular hy-
droalkox y lation of 2,2-dipheny l-4,5-hexadien-1-ol
(1) to form 2-vin y ltetrah ydrofuran 2 (eq 1),7 which
represents the first mechanistic anal ysis of gold-
catal yzed hydroalkox y lation. Importantl y, this
in vestigation delineates the catal ytic behavio r
and interp lay of the mono(gold) and bis(gold) vi-
ny l complexes generated under catal ytic condi-
OTs–
O
(L)AuOTs (1 equiv)
AuL
AuL
0 °C, ≤5 min
98 ± 5% (1H NMR)
Ph
4
Ph
Scheme 1
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equiv) at 25 °C led to immediate (≤10 min) forma-
tion of tetrahydrofuran 2 in 99 ± 5% yield (1H
1
2
1
NMR; Scheme 2).12 However, H NMR anal ysis of
the reaction of 3 with TsOH (1.3 equiv) at –80 °C
re vealed immediate (≤5 min) retroc ycl iza-
tion/aggregation to form a ~1:1 mixture of 1 and 4
in >90% combined yie ld, along with traces (~8%)
of 2 (Scheme 2). Warming this solution at 0 °C for
3
4
5
6
7
8
9
1
20 min led to complete (97 ± 5% yield by H NMR)
conversion to 2 and (L)AuOTs (Scheme 2). Inde-
pendent kinetic anal ysis of the reaction of 4 (40
mM) with HOTs at 5 °C in CD2Cl2 re vealed ~zeroth
order dependence on [HOTs] from 38 to 152 mM
w ith no signif icant deuterium KIE (kH/kD = 0.9)
for the reaction of 4 with DOTs (38 mM). Both ob-
servations point to a mechanism for the conver-
sion of 4 to 2 in vol ving rate-limiting dispropor-
tionation of 4 into 3 and (L)AuOTs fol lowed b y
rapid protodeauration of 3; however, it must be
noted that the concentrations of both 4 and HOTs
in these experiments exceed those present under
catal ytic conditions by an order of magnitude.
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Figure 1. ORTEP diagram of 3. Ell ipsoids are shown
at 50% probabil it y and hydrogen atoms have been
omitted for clarit y. Selected bond lengths (Å) and
angles (deg): Au1–C2 = 2.048(7), Au1–P1 = 2.3163(16),
C1–C2 = 1.333(11), C2–C3 = 1.518(10), C2–Au1–P1 =
174.3(2), C1–C2–Au1 = 120.5(6), C3–C2–Au1 = 122.4(5),
C1–C2–C3 = 117.0(7).
To evaluate the rele vance of vin y l gold com-
plexes 3 and 4 in the gold-catal yzed conversion of
1 to 2, we investigated the reaction of 1 with
(L)AuOTs unde r sto ic h iomet r i c and ca ta l y t i c con-
ditions. Reaction of a equimolar solution of 1 (55
mM) and (L)AuOTs in CD2Cl2 at –80 °C led to imme-
diate (≤10 min) formation of a 1:1 mixture of 1 and
4 without generation of detectable quantities of 3
or 2, which establ ished the facil ity of both C–O
bond formation and aggregation relati ve to pro-
todeauration (Scheme 2). Warming this solution to
–30 °C for 3 h led to formation of 2 in 94 ± 5% yield
(1H NMR) with concomitant regeneration of
(L)AuOTs. When a solution of 1 (120 mM) and a
catal ytic amount of (L)AuOTs (5 mol %) in CD2Cl2
at –30 °C was monitored periodical l y b y 31P NMR
spectroscopy, resonances corresponding to 4 ap-
peared immediatel y, persisted throughout ~95%
conversion as the onl y detectable organometall ic
species, and then disappeared with regeneration
of (L)AuOTs (δ 53.5). Kinetic anal ysis of the gold-
catal yzed conversion of 1 to 2 under similar con-
ditions re vealed a deuter ium KIE of kH/kD = 5.3
upon substitution of 1 with 1-O-d (94% d).12
The exper imental obser vations descr ibe here in
and elsewhere7,13 support a mechanism for the
gold-catal yzed conversion of 1 to 2 in vol ving
rapid and reversible outer-sphere C–O bond for-
mation7 from the unobser ved gold π-allene com-
plex I13 to form mono(gold) vin y l complex 3 with
re lease of HOTs (Scheme 3). Complex 3 undergoes
turnover-limiting protodeauration with HOTs to
form 2 and (L)AuOTs or, alternative l y, 3 is re-
versib l y sequestered by (L)Au+ to form 4 in an off-
cyc le pathwa y (Scheme 3). All available evi-
dence, notabl y the large deuter ium KIE of cata-
l y tic hydroalkox y lation, points to turnover-
l imiting protodeauration, while the kinetics and
KIE of the stoichiometr ic reaction of 4 with HOTs
argues strongl y against the direct protonation of
4.
OH
OTs–
OH
Ph
AuL
Ph
Ph
1
Ph
I
OH
(L)AuOTs (1 equiv)
LAuOTs
Ph
Ph
CD2Cl2, –80 °C, 5 min
HOTs
1
O
O
OTs–
OH
O
AuL
AuL
AuL
+
HOTs (1 equiv)
Ph
Ph
Ph
Ph
HOTs
2
3
Ph
Ph
O
CD2Cl2, –80 °C
5 min
Ph
4
1
Ph
(L)AuOTs
1:1
OTs–
O
≥ –30 °C
>95%
AuL
AuL
AuL
Ph
3
Ph
Ph
4
O
Ph
HOTs (1 equiv)
+ (L)AuOTs
Scheme 3
CD2Cl2, 25°C
5 min
Ph
2
Ph
W ith a work ing mechanism in place, we sought
to more ful l y delineate the interp lay between
mono(gold) and bis(gold) vin y l complexes during
catal ysis and to likew ise evaluate the reversib il-
it y of C–O bond formation under catal ytic condi-
Scheme 2
We likew ise in vestigated the protonol ysis be-
havior of vin y l gold complexes 3 and 4. Treat-
ment of mono(gold) vin y l species 3 with TsOH (2
2
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tions. To this end, we performed a deuter ium-
label ing exper iment that allowed the evolution
1
of bis(gold) vin y l complex 4 to be anal yzed inde-
pendentl y from on-cyc le catal ytic turnover. Spe-
cif ical l y, an equimolar mixture of 4 and HOTs (1.4
mM) was generated in situ from reaction of 1 and
(L)AuOTs (1:2) at –78 °C in CD2Cl2 (Scheme 4). This
solution was treated with excess 1,1-dideuter io-
2,2-dipheny l-4,5-hexadien-1-ol (1-d2; 29 mM) at –78
°C, warmed to –45 °C, and monitored per iodica l l y
b y 1H NMR spectroscopy (Scheme 4). Plots of [4],
[4 + 4-d2], [2], and [2 + 2-d2] versus time ear l y in
the reaction (2-7% conversion)14 provided initia l
rate values for the consumption of 4 (k4 = ‒3.8 ±
0.1 × 10–3 mM min–1) and 4 + 4-d2 (k4tot = ‒0.61 ± 0.02
× 10–3 mM min–1) and for the appearance of 2 (k2 =
0.74 ± 0.04 × 10–3 mM min–1) and 2 + 2-d2 (k2tot = 9.1
± 0.1 × 10–3 mM min–1) (Figure 2). These data re-
veal that the rate of total product formation [2-dx;
x = 0,2] was ~2.5 times greater than was the rate at
which 4 was consumed, which, in turn, was ~5
times greater than the rate of formation of 2 (Fig-
ure 2). Two important conclusions were drawn
from these obser vations. Firstl y, ~70% of catal yst
turnover bypasses the bis(gold) vin y l complex 4-
dx, which solidif ies assignment of 4 as an off-cyc le
catal yst reser voir. Secondl y, ~80% of mono(gold)
v in y l complex 3 generated via disproportiona-
tion of protio 4 undergoes cyc loreversion and
l igand exchange rather than protodeauration.15
Also worth noting was that over the same conver-
sion range noted above (2-7%), the total concentra-
tion of bis(gold) vin y l isotopomers [4-dx, X = 0,2]
decreased by ~8%, which presumabl y reflects the
approach to the steady-state distr ibution of (L)Au+
between 4-dx and on-cyc le mono(gold) complexes
(Scheme 3).
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4
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Figure 2. Concentration versus time plot for the
cycl ization of 1-d2 (40 mM) catal yzed by in situ gen-
erated 4 (2 mM) in CD2Cl2 at –45 °C from ~2% to ~7%
conversion: [2 + 2- d2] (♦); [2] (); [4 + 4-d2] (); [4]
().
In summary, we have eluc idated the mechanism
of the gold(I)-catal yzed conversion of 1 to 2, which
represents the first mechanistic anal ysis of gold-
catal yzed hydroalkox y lation. The two ke y con-
clusions drawn from these studies are that (1)
bis(gold) vin y l complex 4 is an off-cycle catal yst
reser voir and (2) C–O bond formation is rapid and
re versible under catal ytic conditions. The pres-
ence of an off-cyc le intermediate provides both a
target for improving catal ytic efficiencies and a
rationale for the enhanced reacti vity of ster i-
call y hindered phosphine and carbene supporting
l igands (vis-à-vis PPh3) in many gold(I)-catal yzed
reactions.1 Likew ise, reversible C–O bond forma-
tion has important impl ications regarding stere-
ochemical control in gold(I)-catal yzed enantiose-
lective al lene hydrofunctional ization.16 Specif i-
call y, the result suggests that extant mechanistic
models in vok ing stereochemical l y determining
C–Nuc bond formation may require re-evaluation
in the context of stereochemical l y-determining
protodeauration of an equil ib rating mixture of
diastereomeric gold vin y l complexes.
OTs–
O
AuL
AuL
Ph
4
Ph
OH
O
O
HOTs
AuL
Ph
Ph
HOTs
Ph
Ph
+
Ph
2
Ph
+
3
1
+
(L)AuOTs
(L)AuOTs
(L)AuOTs
1-d2
–HOTs
d-incorporation
D
ASSOCIATED CONTENT
OH
D
O
D
(L)AuOTs
O
HOTs
AuL
D
D
Supporting Information. Experimental procedures
and kinetic, spectroscopic, and X-ray cr ystal lo-
graph ic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
D
Ph
HOTs
Ph
Ph
Ph
3-d2
Ph
2-d2
1-d2
Ph
(L)AuOTs
D
O
AuL
AuL
AUTHOR INFORMATION
D
Ph
4-d2
Ph
Corresponding Author
Scheme 4. Pathways for the consumption of 4 (1.4
mM) in the presence of 1-d2 (29 mM) in CD2Cl2 at –
45 °C.
rwidenho@chem.duke.edu; mgagne@unc.edu
ACKNOWLEDGMENT
3
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14) Conversion defined by total product formation (2 + 2-
d2).
15) Because cycloreversion without ligand exchange (3 → I)
would not lead to dispersion of 1 into the reactant pool, the
relative rates of cycloreversion and protodeauration from 3 may
be significantly greater than the ~4:1 ratio determined from
this experiment.
16) (a) Li, H.; Lee, S. D.; Widenhoefer, R. A. J. Organomet.
Chem. 2011, 696, 316. (b) LaLonde, R. L; Wang, Z. J.; Mba,
M.; Lackner, A. D.; Toste, F. D. Angew. Chem. Int. Ed. 2010,
49, 598. (c) Zhang, Z.; Bender, C. F.; Widenhoefer, R. A. Org.
Lett. 2007, 9, 2887. (d) Zhang, Z.; Bender, C. F.; Widenhoe-
fer, R. A. J. Am. Chem. Soc. 2007, 129, 14148. (e) LaLonde, R.
L.; Sherry, B. D.; Kang, E. J.; Toste, F. D. J. Am. Chem. Soc.
2007, 129, 2452. (f) Hamilton, G. L.; Kang, E. J.; Mba, M.;
Toste, F. D. Science 2007, 317, 496-499. (g) Zhang, Z.; Wi-
denhoefer, R. A. Angew. Chem. Int. Ed. 2007, 46, 283.
Financial support is grateful l y acknowledged from
Duke Uni versit y for a C. R. Hauser fel lowship (TJB),
the Fulbr ight Foreign Student Program (DW), the
National Institute of General Medicine (GM-60578),
and the NSF (CHE-0555425).
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P.; Schädlich, J.; Wieteck, M.; Rudolph, M.; Rominger, F. An-
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6) Bis(gold) complexes were first proposed on the basis of
DFT calculations: Cheong, P. H.; Morganelli, P.; Luzung, M.
R.; Houk, K. N.; Toste, F. D. J. Am. Chem. Soc. 2008, 130,
4517.
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documented for gold-catalyzed allene hydrofunctionalization.
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noate to form a gold σ-vinyl allyl oxonium complex,9 while
Toste has provided evidence for the reversible conversion of a γ-
alkenyl urea to a σ-alkyl gold complex.10
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11) Cyclization of 1 catalyzed by either (L)AuOTs (5 mol %)
in dichloromethane or by a mixture of (L)AuCl/AgOTs in tolu-
ene formed 2 in >90% yield within 5 min at room temperature.
12) Both the cyclization of 1-O-d (≥90% d) catalyzed by
(L)AuOTs and the stoichiometric reaction of 4 with DOTs
(≥90%
d)
formed
4,4-diphenyl-2-vinyl(1-
deuterio)tetrahydrofuran (2-d1) with 85 and 79% deuterium
incorporation, respectively.
13) Brown, T. J.; Sugie, A.; Dickens, M. G.; Widenhoefer, R.
A. Organometallics 2010, 29, 4207.
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1
2
3
OH
4
5
HOTs
Ph
Ph
OTs–
O
6
7
8
9
O
LAuOTs
Ph
AuL
AuL
reversible C–O
bond formation
AuL
LAuOTs
O
Ph
Ph
turnover-limiting
protonolysis
Ph
off-cycle bis(gold) formation
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30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
H
HOTs
Ph
Ph
5
ACS Paragon Plus Environment