Replacement of HMPA in samarium diiodide promoted cyclizations and reactions of organolithium compounds

Article abstract of DOI:10.1002/ejoc.201101830

Tripyrrolidinophosphoric acid triamide (TPPA) can replace carcinogenic HMPA as a Lewis basic additive in many reactions involving samarium ketyls. In most cases, yields and selectivities of cyclizations of (het)aryl, alkenyl, and alkynyl ketones are simil

Full text of DOI:10.1002/ejoc.201101830

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201101830
Replacement of HMPA in Samarium Diiodide Promoted Cyclizations and
Reactions of Organolithium Compounds
Mathias Berndt,[a] Alexandra Hölemann,[a] André Niermann,[a] Christoph Bentz,[a]
Reinhold Zimmer,[a] and Hans-Ulrich Reissig*[a]
Dedicated to Professor Dr. Gerhard Erker on the occasion of his 65th birthday
Keywords: Lewis bases / Phosphorus / Samarium / Radicals / Cyclization / Lithium
Tripyrrolidinophosphoric acid triamide (TPPA) can replace
carcinogenic HMPA as a Lewis basic additive in many reac-
tions involving samarium ketyls. In most cases, yields and
selectivities of cyclizations of (het)aryl, alkenyl, and alkynyl
ketones are similar. TPPA is also a good substitute of HMPA
in the O-silylation of an ester enolate and in reactions of
lithiated 1,3-dithiane. All these results clearly demonstrate
that in many cases the use of HMPA can be avoided.
Due to the positive chemical effects of HMPA in the
above-mentioned transformations, a search for substitutes
of this useful additive started long ago. For the chemistry
of organolithium compounds, including enolates, DMPU
(Scheme 1) was found to be a good replacement in many
cases,[3] but there are still reactions where HMPA is supe-
rior. In samarium diiodide promoted processes alternative
solvent systems often provided good results, sometimes
even with surprising chemoselectivities. The growing use of
alcohols, water, or water/amine mixtures as Lewis basic and
protic activators for carbonyl reductions with SmI2 should
be emphasized.[5i,11] Curran also reported on DMPU as an
HMPA substitute, but the scope of this system is limited.[12]
When samarium ketyls are involved, strong Lewis bases
seem to be unavoidable. Recently, McDonald et al. reported
on the use of the commercially available pyrrolidino ana-
logue of HMPA, tripyrrolidinophosphoric acid triamide
(TPPA), and carefully characterized the complexes with
SmI2 by electrochemical measurements.[13] Similar to
HMPA, four equivalents of TPPA are required to take full
advantage of reactivity enhancement. The McDonald
group also performed a few typical samarium diiodide in-
duced reactions: the reductions of 1-bromodecane and of
2-octanone as well as an intermolecular and an intramolec-
Introduction
Hexamethylphosphoric acid triamide (HMPA) is known
to be an excellent polar aprotic solvent (ET = 40.9).[1] Its
high Lewis basicity is also exploited in organocatalysis.[2]
HMPA is frequently employed as a powerful ligand for
organolithium compounds,[3] but more recently it was
mainly applied in reactions promoted by samarium di-
iodide, also known as Kagan’s reagent.[4] This electron-
transfer reagent has been used for many interesting selective
transformations,[5] including the syntheses of complex natu-
ral products.[6] It has been demonstrated that in THF as
solvent, four equivalents of HMPA[7] provide a species
[Sm(HMPA)4(THF)2]2+ 2I– which is much more reactive
than SmI2 itself.[8] The standard potential of ca. –1.3 V in
the absence of ligands is shifted to ca. –2.1 V in the presence
of four HMPA ligands, hence leading to a strongly in-
creased reducing power. Unfortunately, HMPA is known as
a carcinogenic, antispermatogenic, and mutagenic com-
pound and its use has thus been banned in many laborato-
ries.[9] Very severe safety rules have strictly to be followed
when HMPA is employed. It has been demonstrated that
the N-methyl groups of HMPA are responsible for these
deleterious biological activities, as they are metabolized to
provide formaldehyde, which then undergoes subsequent re-
actions.[10] As a consequence, compounds without the di-
methylamino groups may serve as potential HMPA substi-
tutes.
[a] Institut für Chemie und Biochemie, Freie Universität Berlin,
Takustr. 3, 14195 Berlin, Germany
Scheme 1. HMPA and possible substitutes such as DMPU, DMI,
or TPPA.
Fax: +49-30-83855367
E-mail: hans.reissig@chemie.fu-berlin.de
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H.-U. Reissig et al.
SHORT COMMUNICATION
ular samarium ketyl olefin addition. These reactions pro-
ceeded equally well when compared to the HMPA-pro-
moted transformations.
Our long-standing interest in intramolecular couplings
of samarium ketyls to alkenes,[14] alkynes,[15] and (het)-
arenes[16,17] also caused us to search for a replacement of
HMPA during the last 10 years. In singular examples, a
mixture of DMI and lithium bromide served as a good sub-
stitute for HMPA,[16f,16g] but in most cases it was observed
that this method of samarium diiodide activation failed.
Actually, we also studied TPPA some time ago,[18] as it is
to be expected that a pyrrolidino moiety is an even slightly
stronger donor substituent[19] at the phosphorus than the
dimethylamino group. We found that TPPA can replace
HMPA often with good success, but there were also limita-
tions. Here we present typical results of samarium diiodide
induced coupling reactions comparing the use of TPPA
with HMPA. We also include two transformations demon-
strating that TPPA may probably replace HMPA in many
reactions of lithium enolates or other organolithium species.
Results and Discussion
As typical samarium diiodide promoted transformations,
the reductive cyclizations of several γ-aryl ketones to bi-
or polycyclic products were studied (Scheme 2). This novel
reductive dearomatizing process was discovered by our
group in 1998 and subsequently investigated in many varia-
tions.[16,17] A strong Lewis base such as HMPA is required
to generate a sufficiently reactive samarium ketyl which is
able to undergo the addition to the aromatic moiety. Model
substrate 1 was converted into bicyclic product 2[16e] in
moderate yield in the presence of HMPA, whereas the use
of TPPA even slightly increased the yield. Substrate 3 with
geminal dialkyl groups underwent cyclization to regioiso-
mers 4a/4b[16i] in good yield under standard conditions,
whereas with TPPA the regioselectivity and the yield were
moderate; secondary alcohol 5 was isolated in 26% yield as
a byproduct. In contrast, the transformation of tertiary
amine 6 proceeded with higher yield and selectivity by em-
ploying TPPA as ligand. Isoquinoline derivative 7a[16a] was
formed almost exclusively. The reductive cyclizations of
naphthalene derivatives 8 and 10 furnished expected tri-
and tetracyclic compounds 9 and 11,[16d] respectively, in ex-
cellent yields with both additives. The conversion of indole
Scheme 2. Samarium diiodide induced cyclizations of γ-(het)aryl
ketones to bi- and polycyclic compounds in the presence of HMPA
or TPPA as additive. Conditions: SmI2 (2.2–3.0 equiv.), tBuOH
(2 equiv.), additive (18 equiv.). [a] Reaction in the absence of
tBuOH; after decoloration of the solution, bromoacetonitrile (1.0–
3.0 equiv.) was added.
derivative 12 into tetracyclic product 13 – the crucial inter- of propargylamine derivative 17 leading to eight-membered
mediate in our short strychnine synthesis[17g] – was much heterocycle 18[15b] is even more efficient with TPPA. With
less efficient in the presence of TPPA, providing the product HMPA, 9% of starting material 17 was recovered. These
only in 45% yield. The separation of relatively polar 13 two experiments and those in Scheme 2 indicate that it is
from TPPA was fairly difficult.
difficult to decide which additive is actually more efficient.
We also compared the efficacy of the two Lewis bases Because samarium diiodide is a fairly sensitive reagent
HMPA and TPPA in typical intramolecular couplings of (trace amounts of oxygen or other impurities can interfere),
samarium ketyls to alkene or alkyne moieties (Scheme 3). single experiments for one substrate may not be sufficient
The first example shows that the yield for the TPPA-sup- for a final statement about the influence of an additive.
ported cyclization of 14 to 15 is almost as high as that of
HMPA is also an important additive that strongly in-
the HMPA-promoted reaction;[20] traces of secondary fluences the structure and reactivity of organolithium com-
alcohol 16 were isolated by using TPPA. The cyclization pounds.[21] It converts lithium enolates into solvent-sepa-
1300
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Samarium Diiodide Promoted Cyclizations
most identical ratios of 22/23 with the first additive being
only slightly more selective in favor of 1,4-addition product
23. These preliminary results indicate that TPPA may re-
place HMPA also in reactions of other organolithium com-
pounds.[23]
Conclusions
The examples presented in this communication reveal
that TPPA is indeed a very good substitute of HMPA in
many samarium diiodide induced cyclization reactions and
in representative transformations of organolithium com-
pounds. Nevertheless, it cannot be regarded as a general
substitute, as it was considerably less efficient than HMPA
in several reactions for unknown reasons. In a few cases
examined, larger amounts of samarium diiodide could in-
Scheme 3. Samarium diiodide induced cyclizations of alkenyl
ketone 14 and propargylamine derivative 17 in the presence of
HMPA or TPPA as additive. Conditions: SmI2 (2.2–2.5 equiv.),
tBuOH (2–3 equiv.), additive (18–20 equiv.). [a] Substrate was reco- crease the conversion to the products, but this was not al-
vered in 9%.
ways the case. Differences in the regioselectivities have also
to be considered. It should be mentioned that TPPA is more
viscous and higher boiling than HMPA and hence its trans-
rated ion pairs and thus considerably increases its reactivity
fer by syringe is less convenient. TPPA is also less polar and
as nucleophiles, often connected with a change in selectivity.
often needs higher efforts to remove this additive required
In Scheme 4 we present an example demonstrating that
in fairly high amounts. This may have led to lower yields in
TPPA can substitute HMPA in lithium enolate chemistry
singular cases due to the more difficult separation of prod-
(or related carbanions). By treatment with LDA, methyl
ucts from TPPA. Nevertheless, the first reagent of choice
acetate 19 was converted into the corresponding ester enol-
ate which was trapped with tert-butyldimethylsilyl chloride
to give desired ketene silyl acetal 20. Without additive this
should be TPPA in reactions such as those reported here,
and if it fails, the second choice may be HMPA.
process is slow and also provides the C-silylated compound.
The literature-reported method employed HMPA as addi-
Experimental Section
tive and furnished 20 in 72% yield.[22] We used TPPA in a
similar procedure and isolated product 20 in 65% yield
without formation of the C-silylated methyl acetate. As a
second example, we investigated the influence of additives
on the 1,2- vs. 1,4-addition selectivity of lithiated 1,3-di-
thiane. Two equivalents of HMPA[3] or TPPA provided al-
SmI2-Induced Cyclization of 4-Benzyl-5-methoxy-4-(methoxymeth-
yl)pentan-2-one: Degassed TPPA (726 μL, 3.16 mmol) was added
to a solution of SmI2 in THF (0.1 m, 5.27 mL, 0.53 mmol), and the
solution was stirred for 15 min. In a second flask, ketone 3 (44 mg,
0.18 mmol) and tBuOH (26 mg, 0.35 mmol) were dissolved in THF
(5 mL) and argon was bubbled through the solution for 20 min.
Then, the substrate solution was transferred to the SmI2 solution
at room temperature by syringe. After stirring overnight, sat. aq.
NaHCO3 solution was added, the organic phase was separated,
and the aqueous phase was extracted with diethyl ether (4ϫ). The
combined ether extracts were washed once with brine, dried with
MgSO4, and filtered. After evaporation of the solvent, the residue
was filtered through a short silica gel plug with hexane/EtOAc (1:1)
for removal of TPPA. The solvent was evaporated, and the residue
was purified by column chromatography on aluminum oxide
(activity III; hexane/EtOAc, 4:1) to afford 32 mg (72%) of a
37:63 mixture of (1S*,8aS*)-3,3-bis(methoxymethyl)-1-methyl-
1,2,3,4,6,8a-hexahydronaphthalen-1-ol (4a) and (1S*,8aS*)-3,3-bis-
(methoxymethyl)-1-methyl-1,2,3,4,8,8a-hexahydronaphthalen-1-ol
(4b). In addition, 4-benzyl-5-methoxy-4-(methoxymethyl)pentan-2-
1
ol (5; 12 mg, 26%) was isolated. Analytical data of 4b: H NMR
(700 MHz, CDCl3): δ = 1.19 (s, 3 H, CH3), 1.52, 1.63 (2 d, J =
14.3 Hz, 1 H each, 2-H), 1.70 (br. s, 1 H, OH), 1.98 (d, J = 13.6 Hz,
1 H, 4-H1), 2.06 (tdd, J = 2.8, 12.8, 17.4 Hz, 1 H, 8-H1), 2.26 (dddd,
J = 1.3, 4.9, 9.7, 17.4 Hz, 1 H, 8-H2), 2.32–2.37 (m, 1 H, 8a-H),
2.54 (br. d, J ≈ 13.6 Hz, 1 H, 4-H2), 3.14, 3.17 (AB system, JAB
=
9.6 Hz, 1 H each, OCH2), 3.26 (mc, 2 H, OCH2), 3.29, 3.38 (2 s, 3
H each, OCH3), 5.64 (mc, 1 H, 5-H), 5.69 (td, J = 4.1, 9.1 Hz, 1
H, 7-H), 5.80 (mc, 1 H, 6-H) ppm. 13C NMR (176 MHz, CDCl3):
δ = 24.4 (t, C-8), 26.0 (q, CH3), 36.0 (t, C-4), 39.9 (s, C-3), 42.2 (t,
Scheme 4. Synthesis of ketene silyl acetal 20 and addition of li-
thiated 1,3-dithiane 21 to cyclohexenone in the presence of HMPA
or TPPA as additives.
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1-Methoxy-1-[tert-butyl(dimethyl)silyloxy]ethene (20): A solution of
diisopropylamine (1.92 g, 19.0 mmol) in THF (20 mL) was treated
with nBuLi (2.3 m in hexanes; 7.9 mL, 18.1 mmol) at –78 °C. After
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additional 10 min, a solution of TBSCl (2.73 g, 18.1 mmol) in hex-
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–78 °C. The mixture was quenched with water (10 mL), the phases
were separated, and the aqueous layer was extracted with hexane
(3ϫ 30 mL). The combined organic layers were washed with brine
(1ϫ 10 mL), dried with Na2SO4, filtered, and concentrated. The
crude product was purified by kugelrohr distillation (70 °C, 6–
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Support of this work by the Deutsche Forschungsgemeinschaft, the
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Received: December 21, 2011
Published Online: January 25, 2012
1302
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