Terminating platinum-initiated cation-olefin reactions with simple alkenes

Article abstract of DOI:10.1002/anie.201100463

All for one: Phosphine-ligated platinum(II) electrophiles can activate polyalkenes for cyclase enzyme mimicking cascade cyclizations without the need for special terminating groups (see scheme). Like the cyclase enzymes, terminal alkenes that generate ter

Full text of DOI:10.1002/anie.201100463

Communications
DOI: 10.1002/anie.201100463
Cyclase Enzyme Mimics
Terminating Platinum-Initiated Cation-Olefin Reactions with Simple
Alkenes**
Joseph G. Sokol, Chandra Sekhar Korapala, Peter S. White, Jennifer J. Becker, and
Michel R. Gagnꢀ*
The en masse cyclization of polyolefins into polycyclic
terpenoids by cyclase enzymes (e.g. squalene to hopene), is
a biosynthetic reaction of particular fascination to chemists.[1]
Noteworthy recent additions to synthetic mimics[2] of the
cyclase enzymes are asymmetric methods that include
Brønsted–Lewis acids (BLA),[3] masked equivalents of Br+
and I+,[4] organocatalysts,[5] and electrophilic metal catalysts.[6]
With the exception of HgII reagents,[7] few electrophilic metal
catalysts cyclize polyenes with bio-like alkene terminators.[8]
The development of methods whose catalysts can initiate,
cyclize, and terminate polyenes under ligand control would
significantly advance the state of the art.
by the intramolecular addition of a protic trap [alcohol,
phenol, or sulfonamide, for example; Eq. (1)].[12] Computa-
tional analysis showed that when a base was hydrogen bonded
to the protic terminus and the alkene was in a suitable
geometry, the cyclization was highly favorable and virtually
barrierless.[13] In contrast, calculations under base-free con-
ditions were characterized by high-energy intermediates and
significantly less favorable thermodynamics.
This latter scenario most likely describes the early stages
of a polyene cascade that terminates with a nonprotic group,
and in the case of an alkene is not even acidic until the cation
is fully formed. The difficulty of productively engaging a
Brønsted base at an alkene terminus thus likely explains the
paucity of synthetic examples.[14,15]
The combination of a polar solvent (EtNO2) and either
Ph2NMe or, more conveniently, a resin N-bound piperidine
base led to an efficient and highly diastereoselective cycliza-
tion of triene 2 into 3 [Eq. (2)]. In contrast to protic
terminators, however, the reaction proceeds much more
Herein, we describe the development of an alkene-
terminated cation-olefin cascade reaction that is initiated by
the dicationic platinum complex [(PPP)Pt][BF4]2 (PPP = bis-
(2-diphenylphosphanylethyl)phenylphosphane), 1.[9] Com-
pound 1 is especially efficient at initiating cyclizations
wherein the polyene carries a monosubsti-
tuted alkene terminus.[10] In addition to dia-
stereoselectively forming polycyclic products
with a broad variety of terminating alkenes,
the reactions described herein contrast HgII
reagents by the lack of premature termination
processes.[11]
Our research group previously reported
that LnPt2+ sources will initiate the cation-
olefin cascade with subsequent termination
[*] J. G. Sokol, Dr. C. S. Korapala, Dr. P. S. White, Prof. Dr. M. R. Gagnꢀ
Department of Chemistry
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599-3290 (USA)
E-mail: mgagne@unc.edu
slowly [minutes for Eq. (1) vs. 36 hours for Eq. (2)], a
difference which we interpret as reflecting the kinetic cost
of generating a discrete tertiary cation.
Dr. J. J. Becker
U.S. Army Research Office
P.O. Box 12211, Research Triangle Park, NC 27709 (USA)
X-ray crystallographic characterization of 3[16] pointed to
a predictable initiation at the least-substituted alkene, a chair/
chair cyclization conformer, and the intermediacy of an
exocyclic tertiary cation that eliminates to the isopropenyl
group (Figure 1). Several features are notable in the solid-
[**] We thank the National Institutes of Health, General Medicine (grant
no. GM-60578), and the Army Research Office for generous
support.
À
state structure of 3. The first is the Pt CH orientation, which
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100463.
À
positions the C H vector in the square plane to minimize
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Angew. Chem. Int. Ed. 2011, 50, 5658 –5661

Table 1: Yields and scope of polyene cyclizations initiated by dicationic
platinum complex 1.
Entry Substrate[a]
Product[b]
Yield [%][c]
1
2
2
4
3 (X-ray)[16]
80
74
5
Figure 1. ORTEP plot of 3 showing the diagnostic interaction between
the axial methyl group and the Ph ring at phosphorus. Thermal
ellipsoids at 50% probability and hydrogen atoms are omitted for
clarity.
3
4
6: R=H
8: R=OMe
7
89
97
9 (X-ray)[16]
steric congestion. This rotamer positions the angular CH3
À
group near the face of one P Ph group, which causes an
upfield shifting of this CH3 group in the H NMR spectrum
1
(to ꢀ 0.1 ppm). This resonance proved to be diagnostic and
was observed in each of the described structures (see below).
A number of polyenes with terminating tertiary carboca-
tions were examined that varied in the number of rings
formed (two or three), the arrangement of the terminating
alkene (endo- versus exo-cyclic), and the ring size (Table 1).
Even more facile than the 6-exo termini were reactions
wherein the terminating alkene was arranged to react with the
6-endo geometry.[17] These reactions were 2–4 times faster
than the 6-exo analogue 2, and provided a number of carbon
skeletons. In the case of 5, the putative tertiary cation, formed
from a chair/chair/chair transition state, eliminates to give the
more stable C12/13 alkene isomer (Scheme 1). Products that
would have arisen from premature quenching of a putative
5
6
10
12
11 (X-ray)[16]
13 (X-ray)[16]
95
76
7
14
15 (X-ray)[16]
80
[a] Reaction conditions: (PPP)PtI2, 2 equivalents of substrate, 2.5 equiv-
alents of AgBF4, 3 equivalents of piperidine resin base, and EtNO2.
[b] [Pt]+ =[(PPP)Pt]+. [c] Yield of isolated product.
isoprene unit in the main chain does not significantly affect
the reaction barrier.
In the case of 6, extended reaction times led to a partial
conversion into the tetrasubstituted isomer at the B/C ring
junction (Scheme 2).[19] As reported by Surendra and Corey,[8]
Scheme 1. Chair/chair/chair cyclization with 6-endo termination.
cation at C5 or C9 were not observed (< 5%). In most cases,
the structure of the resulting platinum complex was con-
firmed by single-crystal X-ray analysis (see the Supporting
Information).[16]
Scheme 2. Reaction termination by selective deprotonation.
Even more reactive were compounds with conformation-
ally constrained dihydronaphthalene terminating groups (6, 8,
and 10), which efficiently converted to the tetra- and
pentacyclic products (Table 1, entries 3–5).[8] The conversion
of 8 to 9 was ꢀ 4 fold faster than the non-methoxy-substituted
example (Table 1, entry 3), thus suggesting that the nucleo-
philicity[18] and/or cation stability of the terminus plays a
significant role in the reaction kinetics. As judged by
comparing the cyclization rates of 8 and 10, an additional
this isomerization could be accelerated by acids, though the
sulfonic acids also caused partial protodemetalation of the
Pt.[20] By contrast, the tertiary cation formed on cyclizing 12
preferentially eliminates to the more stable tetrasubstituted
alkene product 13.[21]
When a 5-exo geometry was required for the formation of
a tertiary carbenium ion to terminate the cascade, an entirely
different path was followed. In these cases a clean Wagner–
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Communications
Meerwein rearrangement converted the tertiary cation into
the rearranged carbon skeleton of 15,[22] which was confirmed
by single-crystal X-ray analysis.[16]
To gain insight into the diverging behavior of 6-exo and 5-
exo terminated reactions, a computational analysis (DFT
B3LYP/6-31G*)[23] of the key 1,2-shifts was carried out on
simplified model systems (Scheme 3). Revealing was the
efficiently and diastereoselectively activates a single olefin
face.
In summary, we report the results of a platinum(II)-
mediated cyclization method that explores the boundaries of
polyalkene cation-olefin reactions. These data reinforce the
notion that the nucleophilicity/cation stability of the termi-
nating alkene is of paramount importance and the termina-
tion outcomes depend on structure. Electrophilic Pt dications
are also shown to be unique in their ability to activate and
mediate the cascade reactivity of polyene reactants. The
results pave the way to as of yet unknown catalytic
asymmetric cation-olefin cyclizations of polyalkenes.
Scheme 3. A comparison of the 6,5 (a) and 6,6 (b) energies (kcal
molÀ1) along the Wagner–Meerwein reaction coordinate.
differential activation energy for the initiating 1,2-hydride
shift, which was 7.3 kcalmolÀ1 more favorable for the 5-exo
terminated ring systems than for the 6-exo. The subsequent
steps were lower in energy, thus suggesting that it is the slower Experimental Section
Standard cyclization reaction: To 30 mg of [(PPP)PtI2] was added
initiating 1,2-hydrogen transfer in the 6,6 case which diverts
the reaction towards a competitive base-induced elimination.
Compound 1 was additionally investigated for its ability to
cyclize a squalene analogue that lacks the terminal methyl
groups [Eq. (3)]. Although the complexity of the spectra was
15 mg of AgBF4 followed by 0.75 mL of EtNO2. The mixture was then
stirred for 1 h in the dark. The contents were filtered through a 0.2 mm
PTFE syringe filter, washing out the flask and syringe with 0.25 mL
EtNO2, into a flask containing 2 equiv of substrate and 3 equiv of
piperidine resin. The reaction mixture was stirred in the dark until the
reaction was complete (3–48 h, verified by 31P NMR spectroscopy).
The reaction mixture was passed through a 0.2 mm PTFE syringe
filter, washing out the flask and syringe filter with 0.25 mL EtNO2.
Solvent was then removed under a stream of N2. The complex was
twice reconstituted in a minimum amount of CH2Cl2 and force
precipitated with cold tBuOMe. The mixture was centrifuged and the
solvent was decanted off. The crude residue was purified by flash
column chromatography on silica gel.
Received: January 19, 2011
Published online: May 5, 2011
significant and more than one isomer was formed, similarities
to 15 suggested that the cyclization followed a 6,6,5-exo
pathway to give a cation at C14, which nonselectively
rearranged akin to 4. Unlike cyclase enzymes, the environ-
ment of the terminating cation is not conducive to ring
expansion/D-ring annulation.[24,25] van Tamelen made similar
observations in Brønsted acid mediated reactions on squalene
oxide.[26]
The viability of performing an asymmetric cascade
cyclization was investigated using the chiral [P2PPt]2+ com-
plex (P2 = DTBM-SEGPHOS, P = PMe3) 19. The combina-
tion of a chiral P2 ligand and an achiral monodentate
phosphine has been previously shown to catalyze cyclo-
rearrangment reactions with high enantiomeric excess.[27]
When 19 was treated with 8 under the standard conditions
(Table 1), NMR spectroscopy indicated that a single stereo-
isomer was obtained (1H, 31P), i.e. the chiral initiator
Keywords: biomimetic synthesis · cascade cyclization ·
electrophilic activation · platinum
.
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Angew. Chem. Int. Ed. 2011, 50, 5658 –5661
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www.angewandte.org
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