Silicon-induced phenanthrene formation from benzynes and allenylsilanes

Article abstract of DOI:10.1002/chem.201203738

Now directly feasible: The silicon atom in allenylsilanes enabled their reaction with two equivalents of benzynes to generate (α-phenanthrenyl) vinylsilanes in good to excellent yields through an unprecedented [2+2+2] pathway (see scheme). Copyright

Full text of DOI:10.1002/chem.201203738

DOI: 10.1002/chem.201203738
Silicon-Induced Phenanthrene Formation from Benzynes and Allenylsilanes
Jih Ru Hwu*[a, b] and Sharada P. Swain[a]
As an “angular” tricyclic aromatic hydrocarbon, phenan-
threne appears as a white crystalline powder. Compounds in
this family can be prepared by various methods that often
require catalysts such as Pd and Ni salts.[1,2] For example,
Cheng and co-workers[3] reported a new method for the syn-
thesis of 10-methylene-9,10-dihydrophenanthrenes by Ni-
catalyzed chemoselective cocyclization of arynes with al-
lenes. Applicability of these metal-catalyzed reactions may
Scheme 1. Valuable-entity-containing moieties of phenanthrene and vi-
nylsilane.
be retarded by many disadvantages, such as the high cost of
catalysts,[4] necessary prevention of air[5] or moisture,[6] re-
quirement of multiple steps,[7,8] the resulting environmental
issues, or the production of mixtures.[9] Compounds contain-
ing a phenanthrene moiety are found to play an essential
role in the development of new materials. Examples include
the use of a (phenanthrenyl)indole as a fluorescent probe
for peptides and lipid membranes, as reported by Castan-
heira and co-workers;[10] tetraalkoxyphenanthrenes as pre-
cursors for luminescent conjugated polymers, as reported by
MacLachlan and co-workers;[11] copolymers containing a
phenanthrene group inserted along a polymer backbone
with photoluminescence properties, as reported by Akcelrud
and co-workers;[12] poly(phenanthrylene vinylene) as a light-
emitting material for applications in electroluminescent de-
vices, as reported by Bazan and co-workers;[13,14] and phen-
Our research group has been able to use the electronic
effect of silicon to control different types of reactions. In
2001, a silicon-directed hydroxymethylation of b-silylcycloal-
kanone enol acetates by an electrochemical method was re-
ported.[18] Silicon also directs the fragmentation of b-silylcy-
cloalkanone radical cations by its b-stabilizing effect.[19]
Moreover, silicon induces an ene-type reaction in the ther-
mal conversion of enolates to b-silyl enones with molecular
dioxygen.[20] By insertion into the backbone of polymers, sili-
con can induce photo-degradation of polyureas.[21]
To continue the study of silicon-directed, -induced, and
-promoted reactions,[22] we found a way to directly obtain
(phenanthrenyl)vinylsilanes 4 by reacting two equivalents of
2-(trimethylsilyl)phenyl triflates 2 with allenylsilanes 3[23] in
the presence of CsF and dry air at 08C for 8.0–10 h
(Scheme 2). Various reactants were employed for the exami-
nation of the nature and character of the reaction (Table 1).
The benzene nuclei of silylphenyl triflates 2 were attached
to methyl groups or fused with 1,3-dioxole and cyclopentene
rings (i.e., 2b–d); the allenylsilanes[24] were attached with a
methyl, ethyl, n-propyl, isopropyl, and n-butyl group at the
ACHTUNGTRENNUNGanthrene-based conjugated oligomers used as p-channel
semiconductors in field-effect transistors, as reported by
Geng and co-workers.[15] On the other hand, vinylsilanes
react readily with a wide range of electrophiles to give prod-
ucts of addition or substitution.[16] The regiochemical out-
come is normally unambiguous.[17] The combination of these
two moieties working together (i.e., 1 in Scheme 1) would
provide new avenues for the synthesis of aromatic com-
pounds and polymers. Herein, we report the development of
a new reaction that is induced by silicon and its application
to the synthesis of (a-phenanthrenyl)vinylsilanes (1) with ef-
ficiency.
[a] Prof. Dr. J. R. Hwu, S. P. Swain
Department of Chemistry and Frontier Research Center on
Fundamental and Applied Sciences of Matters
National Tsing Hua University, Hsinchu 30013 (Taiwan)
Fax : (+886)3-5721594
E-mail: jrhwu@mx.nthu.edu.tw
[b] Prof. Dr. J. R. Hwu
Department of Chemistry
National Central University, Jhongli City 32001 (Taiwan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203738.
Scheme 2. Reaction of a-silylphenyl triflates with allenylsilanes to gener-
ate (a-phenanthrenyl)vinylsilanes.
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COMMUNICATION
Table 1. Reactants 2 and 3, products 4, and the isolated yields of the new reaction.[a]
2
3
4
Yield Time
2
3
4
Yield Time
[%]
[h]
[%]
[h]
92
8
2a
78
8
3a
3a
3a
90
82
85
10
10
10
2a
2a
2a
82
81
92
8
8
8
83
80
8
8
2a
2a
73
66
8
8
2a
[a] A solution containing a sulfonate 2 (ꢀ0.38 mmol, 1.0 equiv), an allenylsilane 3 (ꢀ0.31 mmol, 0.8 equiv), and CsF (ꢀ0.57 mmol, 1.5 equiv) in CH3CN
(2.5 mL) was stirred at 08C in the presence of dry air for 8.0–10 h.
a- or the g-position (i.e., 3a–i). Furthermore, the three li-
gands on the Si atom in 3 (i.e., SiMe3, SiMe2Ph, SiMePh2, or
SiPh3) were altered. Sufficient yields between 66–92% were
obtained for all of these reactions. In contrast, when the tri-
flate 2a was treated with the norsilyl compound n-butylal-
lene instead of an allenylsilane 3 under the same conditions,
as shown in Scheme 2, a mixture containing three major
fractions was produced. None of them was detected by GC-
MS to have an exact mass around 246.14, which would cor-
relate with the desired norsilyl phenanthrene.
The g and b’ positions of allenylsilanes 3 must both con-
tain a hydrogen atom to make the annulation possible.
These two hydrogen atoms were sacrificed during the aro-
matization and the subsequent formation of the conjugated
carbon–carbon double bond, respectively.
rivative 7 through a [2+2] cycloaddition and an acetylene 8
through an ene-like reaction (see pathway A). Many novel
metal free carbon–carbon bond forming methods involving
aryne chemistry have also been developed and applied in or-
ganic syntheses.[28]
In contrast to the pathway A involving norsilylallenes,
arylated zwitterionic adducts 9 are generated as intermedi-
ates through pathway B by use of allenes 6 that are attached
directly to an SiL3 group. The driving force is that the vinyl
cationic intermediates 9a can be stabilized by the b-silyl
group through the “sSiC!pz empty hyperconjugation.”[29] As a
result, it may offer a considerable length of time[30] for 9
(also 9a) to react with the second molecule of benzynes 5.
Annulation then takes place on the resultant biphenyl inter-
mediates 10 to form the tricycles 11. Its formation is sup-
ported by the performance of the same reaction in a spin-
ning NMR tube containing triflate 2a and allenylsilane 3b
in deuterated acetonitrile without oxygen gas. A quartet
with coupling of J=7.2 Hz was detected at d=3.45 ppm for
the C(10) proton of the dihydrophenanthrene 11 and a dou-
blet with coupling of J=7.2 Hz at d=1.57 ppm for three
A plausible mechanism, illustrated in Scheme 3, takes into
account the essential roles played by the Si atom in allenyl-
silanes. A benzyne (5) is first generated by 1,2-elimination
of a silylphenyl triflate (2) with CsF.[25,26] Wasserman and
Keller[27] reported that reaction of benzyne with allenes 6
(without a silyl group) gives a mixture of a cyclobutane de-
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J. R. Hwu and S. P. Swain
tal X-ray diffraction analysis
(see Figure 1a). The monoclinic
crystals (m.p. 191–1928C, di-
chloromethane) possessed the
space group C2/c with a=
36.560(12) ꢁ,
b=7.831(3) ꢁ,
c=15.907(5) ꢁ, a=908, b=
103.960(6)8, and g=908. The X-
ray data indicate that the SiMe3
group therein was trans to the
1
n-propyl group. In the H NMR
spectrum, a characteristic triplet
with coupling of J=6.8 Hz oc-
curred at d=6.216 ppm for the
SiC=CH proton and a singlet at
d=7.015 ppm for the phenan-
threne-C(10)H proton. Further-
more, the SiC=CH resonance
was saturated in
a
Nuclear
Overhauser
Enhancement
(NOE) experiment, which led
to a 6.08% enhancement of the
Si
ACHTUNGTRENNUNG
uration of the SiAHCTUNGTRENNUNG
nance led to a 0.85% enhance-
ment of SiC=CH peak. These
NOE results confirmed the cis
relationship between the vinylic
Scheme 3. A plausible mechanism for the oxidative annulation to form phenanthrenes.
proton and the SiACHTUNTRGNEUNG(CH3)3 group
in compound 4c. Meanwhile,
protons of the methyl group (i.e., Z) attached to the C(10)
position in the H NMR spectrum was also observed. These
two sets of doublets of triplets (coupling of J2 =4.8 and J3 =
12.6 Hz) were detected in the H NMR spectrum of 4g for
1
1
chemical shifts and the coupling constants are consistent
with those of related compounds reported in the literature.[3]
The intermediates 11 then undergo an oxidative aromati-
zation by reacting with molecular oxygen. Once again, the
silyl group plays a stabilization role; it exerts the “s!p hy-
perconjugation”[31] on the b-carboradical center in the inter-
mediates 12 (cf. 12a). Subsequently, an intramolecular hy-
drogen transfer takes place from a benzylic carbon atom to
an oxygen atom through a six-membered ring transition
state. Monodeoxygenation[32] occurs in the resultant perox-
ides 13 to afford benzylic alcohols 14. Finally, compounds 14
undergo dehydration in situ to give the final products 15.
The intrinsic stabilizing ability of the silyl groups greatly
contributes to the arene formation.
the two protons of the CH2 attached to the phenanthrene-
C(10) position. As shown in Figure 1b, obtained by the con-
sistent-valence force field (CVFF) computation of com-
pound 4g, steric congestion existed between the two propyl
groups attached to the phenACTHNUTRGENNGaU nCAHTUNGTRNEtNUGN hrenyl and the vinyl groups.
Resulting from the vinyl-C(9) single bond rotation (cf. struc-
ture 15), the dihedral angle was found 82.048 between the
plane of phenanthrene and the plane defined by the SiC=
CH moiety in 4g. It might be the cause of the non-equiva-
lency of these two protons.
Four advantages are associated with this newly invented
reaction: 1) The starting materials were versatile; benzynes
and allenylsilanes with various substituents and ligands were
adaptable to generation of the desired products; 2) (a-Phen-
In a control experiment, the same arene formation reac-
tion was performed under nitrogen gas. This reaction mix-
ture was monitored by TLC, which showed several spots
after 10 h. Further exposure of the reaction mixture to air
gradually strengthened the spot associated with the desired
products 4. These results provide evidence to support the
oxidative aromatization mechanism shown in Scheme 3, in
which the O2 gas was essential.
ACHTUNGTRENaNUNG nthrenyl)vinylsilanes were produced exclusively as the E-
isomers; often the methods for the generation of vinylsilanes
lead to a mixture of E- and Z-isomers;[33,34] 3) No catalysts
were required in our reaction for the phenanthrenes forma-
tion; other examples usually need catalysts such as CuBr2,[35]
2ACTHNUTRGNEUNG
CuI,[36] NiBr (dppe),[3] and Pd with various ligands;[37,38]
4) The new reaction provided products in good yields (66–
92%), which were higher than those associated with the
metal-catalyzed[3,39] cyclizations in the formation of phenan-
threnes from benzynes and (norsilyl)allenes (29–77%).
To confirm the structure and regiochemistry of (a-phe-
nanthrenyl)vinylsilanes 4, we unambiguously determined the
molecular framework of compound 4c by using single-crys-
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Silicon-Induced Phenanthrene Formation
COMMUNICATION
Keywords: allenylsilanes
phenanthrenes · silicon
·
arenes
·
benzynes
·
AHCTUNGTRENNUNG
[1] R. Jana, T. P. Pathak, M. S. Sigman, Chem. Rev. 2011, 111, 1417–
1492.
[2] K. V. Radhakrishnan, E. Yoshikawa, Y. Yamamoto, Tetrahedron
Lett. 1999, 40, 7533–7536.
[3] J.-C. Hsieh, D. K. Rayabarapu, C.-H. Cheng, Chem. Commun. 2004,
532–533.
[4] Z. Liu, R. C. Larock, Angew. Chem. 2007, 119, 2587–2590; Angew.
Chem. Int. Ed. 2007, 46, 2535–2538.
[5] Y. H. Kim, H. Lee, Y. J. Kim, T. K. Bum, J.-N. Heo, J. Org. Chem.
2008, 73, 495–501.
[6] A. Matsumoto, L. Ilies, E. Nakamura, J. Am. Chem. Soc. 2011, 133,
6557–6559.
[7] C. A. Johnson, M. M. Haley in Carbon-rich Compounds: From Mol-
ecules to Materials, (Eds.: M. M. Haley, R. R. Tykwinski), Wiley-
VCH, Weinheim, 2006, pp. 2–3.
[8] A. Iuliano, P. Piccioli, D. Fabri, Org. Lett. 2004, 6, 3711–3714.
[9] D. PeÇa, D. Perez, E. Guitian, L. Castedo, J. Am. Chem. Soc. 1999,
121, 5827–5828, and references cited therein.
[10] B. F. Hermenegildo, G. Pereira, A. S. Abreu, E. M. S. Castanheira,
P. M. T. Ferreira, M.-J. R. P. Queiroz, J. Photochem. Photobiol. A
2011, 221, 47–57.
[11] B. N. Boden, K. J. Jardine, A. C. W. Leung, M. J. MacLachlan, Org.
Lett. 2006, 8, 1855–1858.
[12] A. M. Machado, M. Munaro, T. D. Martins, L. Y. A. Davila, R. Giro,
M. J. Caldas, T. D. Z. Atvars, L. C. Akcelrud, Macromolecules 2006,
39, 3398–3407.
[13] Y.-J. Miao, W. G. Herkstroeter, B. J. Sun, A. G. Wong-Foy, G. C.
Bazan, J. Am. Chem. Soc. 1995, 117, 11407–11420.
[14] For a recent review, see: A. C. Grimsdale, K. L. Chan, R. E. Martin,
P. G. Jokisz, A. B. Holmes, Chem. Rev. 2009, 109, 897–1091.
[15] H. Tian, J. Shi, S. Dong, D. Yan, L. Wang, Y. Geng, F. Wang, Chem.
Commun. 2006, 3498–3500.
[16] E. W. Colvin, Silicon in Organic Synthesis, Butterworths, London,
1981, p. 62.
[17] M. J. Curtis-Long, Y. Aye, Chem. Eur. J. 2009, 15, 5402–5416.
[18] J. R. Hwu, S.-C. Tsay, L. C. Lin, L. L. Chueh, J. Am. Chem. Soc.
2001, 123, 5104–5105.
Figure 1. a) ORTEP diagram of 4c obtained by X-ray analysis. b) Severe
steric congestion existed in compound 4g, of which the thermodynamical-
ly most stable conformation was obtained by the CVFF calculation. Its
minimization process with quantified energy is shown in the left bottom
corner. CCDC-884621 (4c) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.[a]
[19] J. R. Hwu, S.-S. Shiao, S.-C. Tsay, J. Am. Chem. Soc. 2000, 122,
5899–5900.
[20] J. R. Hwu, C. H. Chen, C.-I. Hsu, A. R. Das, Y. C. Li, L. C. Lin,
Org. Lett. 2008, 10, 1913–1916.
[21] J. R. Hwu, K. Y. King, Chem. Eur. J. 2005, 11, 3805–3815.
[22] For examples, see: H. Vorbrꢂggen, Silicon-Mediated Transforma-
tions of Functional Groups, Wiley-VCH, Weinheim, 2004, p. 28, 84,
138, 161, 166, 280, 281, and 285.
[23] For their preparation from silylated propargylic alcohols and orga-
nocuprates, see: a) H. Westmijze, P. Vermeer, Synthesis 1979, 390–
392; b) L. Carroll, M. C. Pacheco, L. Garcia, V. Gouverneur, Chem.
Commun. 2006, 4113–4115.
[24] For applications of allenylsilane, see: a) J. A. Marshall, J. Org.
Chem. 2007, 72, 8153–8166; b) E. Colvin, Silicon in Organic Synthe-
sis, Butterworths, London, 1981, p. 165.
In conclusion, an efficient method for the direct synthesis
of various phenanthrenes with a vinylsilane moiety starting
from two equivalents of benzynes and one equivalent of al-
lenylsilanes was developed. It involved a new silicon-in-
duced reaction, which proceeded through a [2+2+2] path-
way. The success was attributed to the stabilizing capability
of a silicon on a b vinyl carbocation by the sSiC!pz empty hy-
perconjugation and, subsequently, on a b carboradical by
the s!p hyperconjugation. Without the need for a catalyst,
this reaction led to the desired products in good-to-excellent
yields with high regioselectivity under mild conditions.
[25] For recent reviews on benzyne, see: a) P. M. Tadross, B. M. Stoltz,
Chem. Rev. 2012, 112, 3550–3577; b) H. H. Wenk, M. Winkler, W.
Sander, Angew. Chem. 2003, 115, 518–546; Angew. Chem. Int. Ed.
2003, 42, 502–528.
[26] For generation of benzyne by the same method and its recent appli-
cations, see: a) J. R. Hwu, Y. Chang Hsu, Chem. Eur. J. 2011, 17,
4727–4731; b) Y. Chang Hsu, J. R. Hwu, Chem. Eur. J. 2012, 18,
7686–7690.
[27] H. H. Wasserman, L. S. Keller, J. Chem. Soc. D. Chem. Commun.
1970, 1483–1484.
Acknowledgements
For financial support, we thank the National Science Council (Grant No.
99-2113M-007-008-MY3) and Ministry of Education (Grant Nos.
102N2011E1 and 102N2018E1) of R.O.C.
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J. R. Hwu and S. P. Swain
[28] For recent articles, see: a) F. R. Leroux, A. Berthelot, L. Bonnafoux,
A. Panossian, F. Colobert, Chem. Eur. J. 2012, 18, 14232–14236;
b) S. S. Bhojgude, A. T. Biju, Angew. Chem. 2012, 124, 1550–1552;
Angew. Chem. Int. Ed. 2012, 51, 1520–1522; c) K. Mohanan, Y. Co-
querel, J. Rodriguez, Org. Lett. 2012, 14, 4686–4689; d) J. Li, N.
Wang, C. Li, X. Jia, Org. Lett. 2012, 14, 4994–4997; e) K. Okuma,
R. Itoyama, A. Sou, N. Nagahora, K. Shioj, Chem. Commun. 2012,
48, 11145–11147; f) K. M. Allan, C. D. Gilmore, B. M. Stoltz,
Angew. Chem. 2011, 123, 4580–4583; Angew. Chem. Int. Ed. 2011,
50, 4488–4491; g) T. Truong, O. Daugulis, J. Am. Chem. Soc. 2011,
133, 4243–4245; h) D. A. Candito, J. Panteleev, M. Lautens, J. Am.
Chem. Soc. 2011, 133, 14200–14203.
Syha, J. Am. Chem. Soc. 2005, 127, 10852–10860; e) A. Klaer, Y.
Syha, H. R. Nasiri, T. Mꢂller, Chem. Eur. J. 2009, 15, 8414–8423.
[31] A. R. Bassindale, S. J. Glynn, P. G. Tylor in The Chemistry of Organ-
ic Silicon Compounds, Vol. 2, Part 1, (Eds.: Z. Rappoport, Y. Ape-
loig), Wiley, New York, 1998, Chapter 7.
[32] G. B. Shulꢃpin, G. S. Fink, J. R. L. Smith, Tetrahedron 1999, 55,
5345–5358.
[33] J. McNulty, P. Das, Chem. Commun. 2008, 1244–1245.
[34] P. Jankowski, K. Plesniak, J. Wicha, Org. Lett. 2003, 5, 2789–2792.
[35] F. Ye, L. Zhou, Q. Xiao, Y. Zhang, J. Wang, Org. Lett. 2011, 13,
5020–5023.
[36] M. Saifuddin, P. K. Agarwal, B. Kundu, J. Org. Chem. 2011, 76,
10122–10128.
[37] S. A. Worlikar, R. C. Larock, J. Org. Chem. 2009, 74, 9132–9139.
[38] E. Yoshikawa, K. V. Radhakrishnan, Y. Yamamoto, J. Am. Chem.
Soc. 2000, 122, 7280–7286.
[29] J. B. Lambert, Y. Zhao, R. W. Emblidge, L. A. Salvador, X. Liu,
J. H. So, E. C. Chelius, Acc. Chem. Res. 1999, 32, 183–190.
[30] For evidence of vinylic cations stabilized by a b-silyl group with sig-
nificant lifetime, see: a) H.-U. Siehl, Pure Appl. Chem. 1995, 67,
769–775; b) M. A. Brook, Silicon in Organic, Organometallic, and
Polymer Chemistry, Wiley, Toronto, 2000, p. 485; c) T. Mꢂller, M.
Juhasz, C. A. Reed, Angew. Chem. 2004, 116, 1569–1572; Angew.
Chem. Int. Ed. 2004, 43, 1543–1546; d) T. Mꢂller, D. Margraf, Y.
[39] Y.-L. Liu, Y. Liang, S.-F. Pi, X.-C. Huang, J.-H. Li, J. Org. Chem.
2009, 74, 3199–3202.
Received: October 19, 2012
Revised: March 4, 2013
Published online: April 9, 2013
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Chem. Eur. J. 2013, 19, 6556 – 6560