Allenylphosphine oxides as simple scaffolds for phosphinoylindoles and phosphinoylisocoumarins

Article abstract of DOI:10.3762/bjoc.10.99

A range of phosphinoylindoles was prepared in one-pot from functionalized propargyl alcohols and a suitable P(III) precursor via a base-mediated reaction. The reaction proceeds via the intermediacy of allenylphosphine oxides. Similarly, phosphinoylisocoumarins were prepared from allenylphosphine oxides in a trifluoroacetic acid-mediated reaction; the latter also acts as a solvent. Interestingly, in the presence of wet trifluoroacetic acid, in addition to phosphinoylisocoumarins, phosphorus-free isocoumarins were also obtained. Key products were characterized by single crystal X-ray crystallography.

Full text of DOI:10.3762/bjoc.10.99

Allenylphosphine oxides as simple scaffolds for
phosphinoylindoles and phosphinoylisocoumarins
G. Gangadhararao, Ramesh Kotikalapudi, M. Nagarjuna Reddy
and K. C. Kumara Swamy*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 996–1005.
School of Chemistry, University of Hyderabad, Hyderabad 500 046,
A. P., India. Fax: (+91)-40-23012460
doi:10.3762/bjoc.10.99
Received: 23 January 2014
Accepted: 09 April 2014
Published: 02 May 2014
Email:
K. C. Kumara Swamy* - kckssc@uohyd.ac.in
* Corresponding author
This article is part of the Thematic Series "Organophosphorus chemistry".
Guest Editor: P. R. Hanson
Keywords:
allenes; indoles; isocoumarins; organophosphorus;
phosphinoyl-heterocycles; propargyl alcohols
© 2014 Gangadhararao et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
A range of phosphinoylindoles was prepared in one-pot from functionalized propargyl alcohols and a suitable P(III) precursor via a
base-mediated reaction. The reaction proceeds via the intermediacy of allenylphosphine oxides. Similarly, phosphinoyliso-
coumarins were prepared from allenylphosphine oxides in a trifluoroacetic acid-mediated reaction; the latter also acts as a solvent.
Interestingly, in the presence of wet trifluoroacetic acid, in addition to phosphinoylisocoumarins, phosphorus-free isocoumarins
were also obtained. Key products were characterized by single crystal X-ray crystallography.
Introduction
Allenes, by virtue of cumulative double bonds that facilitate It may also be noted that organophosphonates in addition have
reactions with diverse classes of substrates, are versatile wide applications in medicinal chemistry [3,18,19] and as
building blocks from a synthetic perspective [1,2]. They are reagents in organic synthesis [20]. In our previous reports, we
also found in many natural products, pharmaceuticals [3] and described the utility of phosphorus-based allenes in various
molecular materials [4]. Thus, over the last decade, allenes have cyclization reactions involving heteroatoms that could lead to
attained a prominent position in organic transformations like phosphono/phosphinoyl hetero-/carbocycles [21-30]. The
cycloaddition, cycloisomerization, base or metal-catalyzed reac- reported series include phosphonobenzofurans/indenones
tions [5-7]. In particular, cyclization reaction of allenes has [21,22], -pyrazoles [23], -chromenes/thiochromenes [24,25],
emerged as a valuable tool in developing different methods -pyrroles [26], multiply substituted furans [27], indolopyran-1-
leading to various carbo-/heterocycles [8-14]. Allenylphospho- ones [28], N-hydroxyindolinones [29], and oxindoles [30]. In
nates and allenylphosphine oxides, as a subclass of allenes, the reaction shown in Scheme 1a, for the formation of the phos-
have also been utlized in several novel transformations [15-17]. phinoylindolinone, one of the oxygen atoms of the nitro group
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Beilstein J. Org. Chem. 2014, 10, 996–1005.
has been moved to a carbon [29]. The reaction shown in routes to phosphinoylindoles, and -isocoumarins utilizing func-
Scheme 1b led to rather previously unsuspected and unex- tionalized allenylphosphine oxides/allenylphosphonates.
pected benzazepines as products. After the elimination of a CO2
molecule, this reaction also features an unprecedented Results and Discussion
rearrangement involving the interemdiate allene [29]. Many In order to achieve the anticipated phosphinoylindoles/
other unusual transformations have also been reported recently isocoumarins, we prepared a variety of functionalized propargyl
[31]. In another reaction leading to phosphinoylindenone alcohols 1a–m and 2a–j containing an acetamide, benzamide or
depicted in Scheme 1c, an intramolecular ene-reaction is an ester group at the ortho position (Figure 1) [35-37]. Some of
possibly involved and in Scheme 1d the reaction led to phosphi- the propargyl alcohols 1a–c, 1m and 2a–j were transformed to
noylisochromenes via deprotection of an allene intermediate allenylphosphine oxides 3a–c, 3m and 4a–j (Scheme 2) by
under Lewis acid mediation [22]. In this context it was of following known methods [38,39].
interest to see, in a reaction like that shown in Scheme 1c,
whether the introduction of an amide or a carboxylate ester in After having several functionalized allenes in hand, initially we
place of the –CHO group could lead to phosphinoyl-subtsti- chose allenes 3a and 3m to achieve intramolecular cyclization.
tuted indoles/isocoumarins via allenic intermediates or not. It is These were treated with 0.5 mol equivalents of base (K3PO4)
pertinent to note that indoles and isocoumarins are core struc- since the substrates contain active hydrogen. This reaction
tures found in many natural and pharmacological products [32- afforded the N-substituted phosphinoylindoles 5 and 7, 8.
34]. Thus in this paper, we wish to report simple synthetic Essentially a single isomer 5 (a dihydroindole), in which the
Scheme 1: Reaction of P(III)-Cl precursors with propargyl alcohols leading to phosphorus based (a) N-hydroxyindolinone, (b) benzazepine, (c) inde-
none and (d) isochromene via allenic intermediates.
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Figure 1: Functionalized propargyl alcohols 1a–m and 2a–j used in the present study.
Scheme 2: Synthesis of functionalized allenes 3a–c, 3m and 4a–j.
N–H proton moves only to the α-carbon resulting in an the =CHMe allene 3m, two isomers in which the N–H proton
exocyclic double bond, was formed (Scheme 3). The presence moves to either the α-carbon (7) or the γ-carbon (8), are
of a doublet for PCH carbon at δ 48.3 with a 1J(P–C) value of obtained. These two isomers can be readily distinguished by the
62.0 Hz reveals that the phosphorus moiety is attached to an corresponding δ and 1J values for the P–C carbon (for 7, δ 47.3
sp3-hybridized carbon. On the other hand, in the reaction using and J = 62.0 Hz; for 8, δ 106.5 and J = 120.0 Hz). Overall, the
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Beilstein J. Org. Chem. 2014, 10, 996–1005.
Scheme 3: Reaction of functionalized allenes 3a and 3m leading to phosphinoylindoles. Conditions: (i) K3PO4 (0.5 equiv), THF, 80 °C, 12 h,
(ii) NaOH (2 equiv), EtOH/H2O (4:1), 80 °C, 8 h.
yields of the isolated products were excellent in both cases. The
structure of compound 7 was further confirmed by X-ray
crystallography (Figure 2). The C=CHMe distance of
1.317(2) Å clearly indicates a double bond between these two
carbon atoms. The other stereoisomer in which the methyl
group is trans to the nitrogen was not observed. Interestingly
though, the removal of the acyl/benzoyl group on the nitrogen
in compounds 5 or 7, 8 in aq NaOH afforded the 2,3-disubsti-
tuted NH-indoles 6 or 9, respectively, in excellent yields. The
NH band (3156 cm−1) in the IR spectrum and a doublet for PC
carbon at δ 98.4 (1J(PC) = 128.0 Hz) reveal the identity of com-
pound 9. Its structure was further confirmed by X-ray crystal-
lography (Figure 3).
Subsequently, we used aq sodium hydroxide as the base instead
of K3PO4 (cf. conditions (ii) in Scheme 3) to perform the reac-
tion on allene 3a. To our delight, only phosphinoyl-NH-indole 6
was the sole product with not even traces of 5 (Scheme 4). This
shows that a strong base like sodium hydroxide effectively
performs both deprotection and cyclization in a single step.
Figure 2: Molecular structure of compound 7. Hydrogen atoms (except
PCH) are omitted for clarity. Selected bond distances (Å): P1–C13
1.8402(15), C13–C20 1.5161(19), C13–C14 1.512(2), C20–N1
1.4508(18), C20–C21 1.317(2).
With the above conditions in hand, we then performed the reac-
tion in one pot starting from propargyl alcohol 1a without
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Beilstein J. Org. Chem. 2014, 10, 996–1005.
Scheme 4: Synthesis of phosphinoylindole from allene 3a in a single
step.
isolating the intermediate allenylphosphine oxide 3a. Gratify-
ingly, the method furnished the desired product 6 in 80% yield.
Inspired by this, functionalized propargyl alcohols 1b–l were
also subjected to the same one-pot conditions (Scheme 5). This
one-pot strategy furnished the desired phosphinoylindoles 9–19
in good to excellent yields without any difficulty in isolation.
Analogous products could also be isolated using the P(III)
precursor (OCH2CMe2CH2O)PCl (see Supporting Information
Figure 3: Molecular structure of compound 9. Hydrogen atoms (except
NH) are omitted for clarity. Selected bond distances (Å): P1–C13
1.771(2), C13–C20 1.385(3), C13–C14 1.450(3), C20–N1 1.360(3),
C20–C21 1.489(3).
Scheme 5: One-pot preparation of substituted phosphinoylindoles 6 and 9–19 from functionalized alcohols.
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Beilstein J. Org. Chem. 2014, 10, 996–1005.
File 1 for details). In our attmept to obtain phosphorus-free After succeeding in generating phosphinoylindoles, we then
2-alkylindole from 17 in the presence of triflic acid (as a concentrated on synthesizing phosphinoylisocoumarins. To
solvent; 100 °C) led to a mixture of products in which the achieve this, we treated the functionalized allene precursors
benzyl group also was cleaved (NMR evidence). Such a reduc- 4a–j that are tethered with a methyl ester group, with an excess
tive cleavage of the P–C bond from phosphinoyl indoles is a of trifluoroacetic acid at room temperature for 6 h. Gratifyingly,
reaction that we are still exploring.
this readily leds to the phosphinoylisocoumarins 20–29
(Scheme 7) in good yields. In the case of compound 25, as
A plausible pathway for the formation of phosphinoylindoles 6 expected, both the E and Z isomers are present in a ratio of
and 9–19 is shown in Scheme 6. As depicted above in 1:0.65 (close Rf values). Very subtle energy differences seem to
Scheme 2, the normal reaction of propargyl alcohol with be prevalent between the dihydroisocoumains 22, 24, 25, 28, 29
chlorodiphenylphosphine is expected to lead to the allenylphos- and the normal isocoumarins 20, 21, 23, 26, 27. The former set
phine oxide. We believe that there is a subtle difference shows a doublet in the 1H NMR spectra at δ ~ 4.78 (2J(P–H) =
between the use of K3PO4 and aq NaOH. K3PO4 abstracts the 18.0 Hz, PCH) which is absent in the latter set; the difference in
NH proton from allenylphosphine oxide leading to intermediate the value of 1J(P–C) in the two sets is also consistent with the
I which is followed by attack of the nitrogen lone pair on the hybridization at the corresponding α-carbon (to phosphorus).
β-carbon [24] of the allene forming addition product II or III. Finally, the X-ray structure was determined for 20 (Figure 4).
This upon treating with aq NaOH leads to the deacylated/deben-
zoylated phosphinoylindoles. In the one-pot reaction, though, The above reaction is believed to proceed by the initial inter-
the in situ generated allenylphosphine oxide first undergoes action of H+ with the α,β-allenic double bond to lead to V
deacylation/debenzoylation with aq NaOH resulting in –NH2 (Scheme 8) which on subsequent attack of oxygen of the ester
functionalized allene IV; the lone pair on nitrogen will then group onto the β-position of allene forms VI. Intermediate VI
attack the β-carbon of the allene intramolecularly leading to on demethylation leads to phosphinoylisocoumarin VII. This
phosphinoylindoles 6 or 9–19.
product VII further involves the double bond isomerization to
Scheme 6: Possible pathway for the formation of phosphinoyl indoles 6 and 9–19.
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Beilstein J. Org. Chem. 2014, 10, 996–1005.
Scheme 7: Synthesis of phosphinoylisocoumarins from functionalized allenes.
When the above reaction was performed in wet trifluoroacetic
acid (TFA/H2O = 20:1) at 70 °C, phosphinoylisocoumarins
were formed in all cases, but additionally, phosphorus-free
isocoumarins 30–35 (Scheme 9) [37] are also formed in the
reaction using terminally substituted allenes 4b–d and 4h–j. We
have also determined the X-ray structure of compound 33
(Figure 5) for final confirmation. It is possible that
isocoumarins 30–35 are formed via the intermediates VIII–IX
(Scheme 10) [40]. The phosphorus moiety of IX may then be
cleaved as Ph2POOH to form the phosphorus-free
isocoumarins. Since this was not the interest in the present
study, we did not proceed further.
Figure 4: Molecular structure of 20. Selected bond lengths [Å] with
estimated standard deviations are given in parentheses: O3–C21
1.386(2), C21–C22 1.486(3).
lead to phosphinoyl isocoumarins 20, 21, 23, 26 and 27. The
isomerization is not observed in the case of 22, 24, 25, 28 and
29. Alternatively, the cyclization may also proceed after the
hydrolysis of ester group to –COOH due to the presence of
adventitious moisture in trifluoroacetic acid.
Figure 5: Molecular structure of 33. Selected bond lengths [Å] with
estimated standard deviations are given in parentheses: O2–C8
1.377(6), C8–C10 1.492(6).
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Beilstein J. Org. Chem. 2014, 10, 996–1005.
Scheme 8: Possible pathway for the formation of phosphinoylisocoumarins.
Scheme 9: Reaction of allenes in wet trifluoroacetic acid.
Conclusion
981067-981070. Copies of the data can be obtained free of
A fairly simple route to phosphinoylindoles and phosphi- charge, on application to CCDC, 12 Union Road, Cambridge
noylisocoumarins starting from functionalized propargyl alco- CB2 1EZ, UK [fax: +44(1223)336033 or e-mail:
hols via allenyl phosphine oxide is developed. The first reac- deposit@ccdc.cam.ac.uk]. The structures were solved and
tion involves base-mediated deprotection and cyclization while refined by standard methods [41-43].
the latter methodology involves acid mediation in which tri-
fluoroacetic acid acts as the reagent as well as the solvent.
7: Colorless block, C29H24NO2P, M = 449.46, monoclinic,
space group P21/c, a = 9.9427(15), b = 16.894(3), c =
14.819(2) Å, α = 90.00, β = 109.195(2), γ = 90.00o, V =
Experimental
Details on the synthesis of the compounds 1a–1m, 2a–2j, 2350.8(6) Å3, Z = 4, µ = 0.143 mm−1, data/restrains/parameters:
3a–3c, 3m, 4a–4j and 5–35 are given in Supporing Information 4141/0/299, R indices (I> 2σ(I)): R1 = 0.0408, wR2 (all data) =
File 1.
0.1101. CCDC no. 981067.
Crystallographic data for the structures of 7, 9, 20 and 33 9: Colorless block, C22H20NOP, M = 345.36, orthorhombic,
have been deposited with the Cambridge Crystallographic space group Pccn, a = 11.2497(6), b = 21.1287(9), c =
Data Centre as supplementary publication number CCDC 15.2880(6) Å, α = 90, β = 90, γ = 90o, V = 3633.8(3) Å3, Z = 8,
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Beilstein J. Org. Chem. 2014, 10, 996–1005.
Scheme 10: Possible pathway for the formation of isocoumarins 30–35 (along with 21–23 and 27–29).
µ = 0.160 mm−1, data/restrains/parameters: 3205/0/231, R Acknowledgements
indices (I> 2σ(I)): R1 = 0.0430, wR2 (all data) = 0.1076. CCDC We thank the Department of Science and Technology (DST,
no. 981068.
New Delhi) for financial support and the Single Crystal X-ray
diffractometer facility, and the University Grants Commission
(UGC, New Delhi) for equipment under UPE and CAS
programs. G.G., R.K., M.N.R. thank the Council of Scientific
and Industrial Research (CSIR, New Delhi) for fellowship.
K.C.K.S. thanks DST for the J. C. Bose fellowship.
20: Colorless block, C22H17O3P, M = 360.33, triclinic, space
group , a = 9.7440(19), b = 9.9918(17), c = 10.2864(18) Å, α
= 84.229(14), β = 76.556(16), γ = 66.323(18)o, V = 892.0(3) Å3,
Z = 2, µ = 0.173 mm−1, data/restrains/parameters: 3647/0/236,
R indices (I> 2σ(I)): R1 = 0.0455, wR2 (all data) = 0.1114.
CCDC no. 981069.
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