Cyclobutene Formation by Borane Catalyzed [2+2] Cycloaddition of a Vinylphosphane with Conjugated Ynones

Article abstract of DOI:10.1002/ejic.202000293

Dimesityl(vinyl)phosphane reacts with benzoyl(phenyl)acetylene under B(C₆F₅)₃ catalysis to give the formal [2+2] cycloaddition product 1-benzoyl-2-phenyl-3-dimesitylphosphinocyclobutene.

Full text of DOI:10.1002/ejic.202000293

European Journal of Inorganic Chemistry
Accepted Article
Title: Cyclobutene formation by borane catalyzed [2+2] cycloaddition of
a vinylphosphane with conjugated ynones
Authors: Xin Tao, Maurice M. Gruber, Constantin G. Daniliuc, Christian
Mück-Lichtenfeld, Gerald Kehr, and Gerhard Erker
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202000293
Link to VoR: https://doi.org/10.1002/ejic.202000293
01/2020

10.1002/ejic.202000293
European Journal of Inorganic Chemistry
COMMUNICATION
Cyclobutene formation by borane catalyzed [2+2] cycloaddition
of a vinylphosphane with conjugated ynones
Xin Tao,[a,b] Maurice M. Gruber,[a] Constantin G. Daniliuc,[a] Christian Mück-Lichtenfeld,[a] Gerald Kehr,[a]
and Gerhard Erker*[a]
[a]
M. M. Gruber, Dr. C. G. Daniliuc, Dr. G. Kehr, Prof. Dr. G. Erker
Organisch-Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstraße 40, 48149 Münster (Germany)
E-mail: erker@uni-muenster.de
[b]
New address: Prof. Dr. X. Tao
School of Chemistry
Northeast Normal University
Renmin street 5268, Changchun 130024, PR China
Supporting information for this article is given via a link at the end of the document.
Abstract: Dimesityl(vinyl)phosphane reacts with benzoyl(phenyl)-
acetylene under B(C6F5)3 catalysis to give the formal [2+2] cyclo-
addition product 1-benzoyl-3-dimesitylphosphino-2-phenylcyclo-
butene 11a.
We had recently described a phosphane analogue of the enamine
Stork reaction: treatment of the “enphosphane” 4 with chalcone
(5) gave the respective cyclobutane derivative 6 in a B(C6F5)3
catalyzed reaction (10 mol% cat., 50 oC) [eqn. (3) in Scheme 1].[6]
In view of the reported variants of the enamine Stork reaction with
acetylenic esters [eqn. (2) in Scheme 1],[4,5] it was tempting to also
react the enphosphane 4 with carbonyl substituted alkynes. In this
sense we had reacted the bulky vinylphosphane 4 with a small
series of conjugated ynones. We reported that in the presence of
a stoichiometric quantity of the strong borane Lewis acid B(C6F5)3
this formally resulted in the generation of a [3+2] cycloaddition
product 9 that had become trapped by B(C6F5)3 to form the stable
derivative 10, which we isolated (Scheme 2).[7] We have now
carried out the reaction of the Mes2P-vinyl phosphane (4) with a
small series of conjugated ynones in the presence of a catalytic
amount of B(C6F5)3 and found a different preferred product
formation, namely of substituted cyclobutene derivatives. This will
be described in this account.
Enamines are important reagents in organic synthesis. They
undergo carbon-carbon coupling reactions with a variety of
organic carbonyl compounds and related systems (Stork
reaction).[1,2] Enamine carbon nucleophiles add to the Michael
position of ,-unsaturated ketones.[1b] Ultimately, this leads
either to 1,5-dicarbonyl products or to substituted cyclobutane
derivatives, depending on the substituent pattern of the system.[3]
There are also a few reports about the reaction of enamines with
electron deficient acetylenes.[4] The reaction of pyrrolidino-cyclo-
hexene with dimethyl acetylenedicarboxylate is a typical example.
It yields the cyclobutene derivative 3, which undergoes
subsequent ring opening upon heating to give the isomeric
cyclooctadiene derivative.[5]
Scheme 2. Formal enephosphane plus ynone [3+2] cycloaddition in the
presence of stoichiometric B(C6F5)3.
Thermolysis of a mixture of the compounds 4 and 7a (in CD2Cl2
solution, 60 oC, 24 h) in an autoclave did not result in any reaction.
The compound mixture remained unchanged under these
conditions. However, in the presence of 10 mol% of B(C6F5)3 a
catalyzed reaction between the bulky vinylphosphane 4 and the
conjugated ynone 7a took place under otherwise the same
conditions. Workup gave the cyclobutene derivative 11a that we
isolated as a solid in 85% yield. The product 11a was
Scheme 1. Variants of the Stork reaction.
1
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10.1002/ejic.202000293
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characterized by spectroscopy and by X-ray diffraction. Single
crystals suited for the X-ray crystal structure analysis were
obtained from dichloromethane/pentane at -35 oC by the diffusion
method. The structure (Figure 1) shows the close to planar
cyclobutene core. The carbon-carbon double bond bears the
benzoyl substituent at its carbon atom C3 and the phenyl
substituent at C4. This phenyl substituent is oriented close to
coplanar with the four-membered ring plane. The C5=O1 carbonyl
group in compound 11a is markedly rotated away from
conjugation with the adjacent C3=C4 carbon-carbon double bond
( C4-C3-C5-O1: 38.1(4)o). Carbon atom C1 has the -PMes2
substituent bonded. The coordination geometry at the
phosphorus atom is trigonal pyramidal.
isolated amorphous solid contained small amounts of as yet
unidentified fluoro-containing impurities. Both the compounds 11b
and 11c were characterized spectroscopically and by X-ray
crystal structure analysis (see the Supporting Information for the
respective data and the depicted NMR spectra and molecular
structures).
In an attempt to further purify the cyclobutene derivative 11a (R1
= R2 = phenyl) by column chromatography (silica gel, dichloro-
methane/pentane ca. 1:2) we isolated its isomer 12a in 80% yield.
We assume an acid catalyzed isomerization of compound 11a to
12a on the column.
Scheme 3. B(C6F5)3 catalyzed cyclobutene formation [with DFT calculated
relative Gibbs energies ΔG(298)solvin dichloromethane solution in kcal mol-1].
Figure 2. Molecular structure of compound 12a (with unsystematic atom
numbering scheme, thermal ellipsoids are shown at 15% probability). Selected
bond lengths (Å) and angles (o): C1C2 1.523(4), C2-C3 1.568(4), C3-C4
1.522(4), C4-C1 1.359(4), C3-C5 1.513(4), C5-O1 1.223(3), P1-C1 1.803(3),
P1-C11 1.843(3), P1-C21 1.846(3), P1CCC 320.0, C4-C1-P1-C11 -82.2(3), C4-
C1-P1-C21 163.2(3), C1-C4-C41-C42 9.8(5), C1-C2-C3-C4 -0.5(2).
Compound 12a was characterized by an X-ray crystal structure
analysis. In the solid state compound 12a shows a central four-
membered carbocycle (Figure 2). It contains a carbon-carbon
double bond that has the phosphanyl group and the phenyl
substituent attached. Consequently, now a sp3-carbon atom of the
ring bears the benzoyl substituent. The Mes2P-substituent has the
P1C21 vector oriented close to in plane with the four-membered
ring, whereas the other (P1C11) is oriented toward an
orthogonal arrangement. The coordination geometry at
phosphorus is trigonal-pyramidal. The phenyl substituent at the
sp2-C ring carbon atom is oriented close to in conjugation with the
C1=C4  system.
Figure 1. A view of the molecular structure of the cyclobutene derivative 11a in
the crystal. (with unsystematic atom numbering scheme, thermal ellipsoids
shown at 30% probability). Selected bond lengths (Å) and angles (o): C1-C2
1.571(3), C2-C3 1.522(3), C3-C4 1.352(3), C4-C1 1.528(3), C3-C5 1.484(3),
C5-O1 1.230(3), P1-C1 1.869(2), P1-C11 1.847(2), P1-C21 1.852(2), P1CCC
319.3, C3-C4-C41-C46 13.0(4).
In solution (CD2Cl2, 299 K) compound 12a shows a 31P NMR
resonance at  43.4. It features 13C NMR signals of the four-
membered core at  154.1 (2JPC = 27.6 Hz),  139.4 (1JPC = 32.6
Hz),  47.4 (3JPC = 12.9 Hz) and  37.1 (2JPC = 4.8 Hz) and the
carbonyl carbon resonance at  198.1. The P-coupled ABX
pattern of the ring -CH2-CHCO unit is found at  2.88/2.50 (CH2)
and  4.89. A DFT analysis (PW6B95-D3/def2-TZVP+COSMO-
RS)[8] has confirmed that compound 12a is thermodynamically
favored over its precursor 11a by ΔG(298K) = 4.7 kcal/mol in
CH2Cl2 (see the Supporting Information for details of the
computational analysis).
In solution compound 11a shows the single [P]-CH 1H NMR signal
at  4.74 and the respective resonances of the adjacent pair of
methylene hydrogen atoms at  3.38 and 3.00. The 13C NMR
spectrum shows the signals of the olefinic ring carbon atoms at 
157.1 and  136.1 [CH at  38.2 (1JPC = 26.7 Hz), CH2 at  37.1
(2JPC = 16.4 Hz)]. The carbonyl carbon resonance was located at
1
 190.6. There are the H/13C NMR signals of the pair of phenyl
groups. Compound 11a features a 31P NMR signal at  12.3.
The substituted ynones 7b (R1 = p-tolyl, R2 = phenyl) and 7c (R1
= phenyl, R2 = p-tbutylphenyl) reacted with dimesityl(vinyl)-
phosphane (4) under analogous conditions to give the substituted
cyclobutene derivatives 11b and 11c, respectively (Scheme 3).
Both were isolated in ca. 70% yield after removal of the solvent
and washing with pentane. We found for the example 11c that the
We note that the phospha-Stork reaction between a bulky
vinylphosphane and a conjugated enone yields cyclobutane
derivatives that have the carbonyl functionality and the
2
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10.1002/ejic.202000293
European Journal of Inorganic Chemistry
COMMUNICATION
[5]
[6]
K. C. Brannock, R. D. Burpitt, V. W. Goodlett, J. G. Thweatt, J. Org.
Chem. 1963, 28, 1464.
phosphanyl substituent 1,2-positioned at the four-membered
carbocyclic framework. In contrast, the here described borane
a) Y. Hasegawa, C. G. Daniliuc, G. Kehr, G. Erker, Angew, Chem. Int.
Ed. 2014, 53, 12168; b) Y. Hasegawa, G. Kehr, S. Ehrlich, S. Grimme,
C. G. Daniliuc, G. Erker, Chem. Sci. 2014, 5, 797.
catalyzed
carbon-carbon
coupling
reactions
between
vinylphosphane Mes2P-CH=CH2 and the conjugated ynones 7
give rise to the formation of cyclobutene derivatives with the
Mes2P- and the -COAr substituents being 1,3-positioned at the
central ring system.[6a] The products 11 were, consequently, not
formed by means of a variant of the phospha-Stork reaction.
[7]
[8]
J. Li, C. G. Daniliuc, G. Kehr, G. Erker, Eur. J. Inorg. Chem. 2020, 1096.
a) A. Klamt, J. Phys. Chem. 1995, 99, 2224 (COSMO-RS); b) F. Weigend,
R. Ahlrichs. Phys. Chem. Chem. Phys. 2005, 7, 3297 (def2-TZVP basis
set); c) Y. Zhao, D. G. Truhlar, J. Phys. Chem. A 2005, 109, 5656
(PW6B95); d) S. Grimme, J. Antony, S. Ehrlich, H. Krieg, J. Chem. Phys.
2010, 132, 154104; e) S. Grimme, S. Ehrlich, L. Goerigk, J. Comput.
Chem. 2011, 32, 1456 (D3 dispersion correction).
[9]
A. W. Johnson and J. C. Tebby, J. Chem. Soc. 1961, 2126.
[10] See for a comparison: a) I. F. Wilson, J. C. Tebby, J. Chem. Soc., Perkin
Trans. 1 1972, 2830; b) I. Yavari, M. Adib, L. Hojabri, Tetrahedron 2002,
58, 7213; c) R. A. Aitken, S. J. Costello, A. M. Z. Slawin, N. J. Wilson,
Eur. J. Org. Chem. 2003, 623; d) M. R. Islami, Z. Hassani, K. Saidi, Synth.
Commun. 2003, 33, 65; e) V. Nair, A. Deepthi, P. B. Beneesh, S.
Eringathodi, Synthesis 2006, 1443; f) M. Adib, E. Sheibani, A. Abbasi, H.
R. Bijanzadeh, Tetrahedron Lett. 2007, 48, 1179; h) K. Nagao, H.
Ohmiya, M. Sawamura, J. Am. Chem. Soc. 2014, 136, 10605; i) A.
Ramazani, F. Z. Nasrabadi, K. Ślepokura, T. Lis, S. W. Joo, J. Heterocycl.
Chem. 2017, 54, 55.
Scheme 4. Mechanistic rationalization of the B(C6F5)3 catalyzed formation of the
cyclobutene derivatives 11.
We assume a pathway that is initiated by a reaction sequence as
it is taking place in the reaction between 4 and 7 with
stoichiometric B(C6F5)3, namely by a reaction that is governed by
attack of the phosphane nucleophile at the “Michael” position of
the ynone.[9] Subsequent ring closure will then lead to the ylidic[10]
intermediate 13. In the stoichiometric reaction (Scheme 2) this is
then trapped by addition of the B(C6F5)3 electrophile to the
nucleophilic ylide carbon atom to form 10. In the catalytic reaction
this pathway is apparently precluded. The P-ylide nucleophile
may then attack the adjacent enone moiety intramolecularly. Ring
opening in the resulting reactive intermediate 14 by cleavage of
the proximal P-C bond directly leads to the observed products 11
(Scheme 4). It seems that under the influence of a strongly
electrophilic borane vinylphosphanes can react either as carbon
or as phosphorus nucleophiles, depending on the detailed overall
conditions. In any case these reactions are taking place under
frustrated Lewis pair conditions which precludes deactivating P/B
Lewis pair adduct formation.
Keywords: phosphorus • boron • cyclobutene • ynone •
catalysis
CCDC numbers: 1985880-1985883
[1]
[2]
[3]
[4]
a) G. Stork, R. Terrell, J. Szmuszkovicz, J. Am. Chem. Soc. 1954, 76,
2029; b) G. Stork, S. R. Dowd, J. Am. Chem. Soc. 1963, 85, 207.
a) P. W. Hickmott, Tetrahedron 1982, 38, 1975; b) P. W. Hickmott,
Tetrahedron 1982, 38, 3363.
K. C. Brannock, A. Bell, R. D. Burpitt, C. A. Kelly, J. Org. Chem. 1961,
26, 625.
a) C. F. Huebner, L. Dorfman, M. M. Robison, E. Donoghue, W. G.
Pierson, P. Strachan, J. Org. Chem. 1963, 28, 3134; b) G. A. Berchtold,
G. F. Uhlig, J. Org. Chem. 1963, 28, 1459; c) D. Becker, L. R. Hughes,
R. A. Raphael, J. Chem. Soc., Chem. Commun. 1974, 430; d) N. Tunoglu,
N. Uludag, Org. Prep. Proced. Int. 1997, 29, 541; e) N. Uludag, O. Asutay,
M. Yakup, Asian J. Chem. 2014, 26, 971.
3
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Entry for the Table of Contents
Insert graphic for Table of Contents here.
Key topic: Cyclobutene formation
The tris(pentafluorophenyl)borane catalyzed reactions of the bulky dimesityl(vinyl)phosphane with conjugated ynones yield
cyclobutene derivatives. The reaction is probably initiated by phosphane attack at the activated carbon-carbon triple bond followed by
ring-closure and rearrangement to give the conjugated acyl-cyclobutene products. Isomerization to the non-conjugated isomer takes
place during chromatography.
4
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