Formation of a cyclobutylidene ring: Intramolecular [2 + 2] cycloaddition of allyl and vinylidene C=C bonds under mild conditions

Article abstract of DOI:10.1021/ja027209s

Intramolecular [2 + 2] cycloaddition of two C=C bonds in vinylidene complexes [Ru(η⁵-C₉H₇){=C=C(R)H}(PPh₃){κ^1-(P)-PPh₂(C₃H₅)][BF₄] affords cyclobutylidene complexes [Ru(η⁵-C₉H₇){κ²-(P,C)-(=(CC(R)HCH₂CHCH₂PPh₂)}(PPh₃)]-[BF₄]. which can be also obtained by reaction of terminal alkynes with [Ru(η⁵-C₉H₇)(PPh₃){κ³-(P,C,C)-PPh₂(C₃H₅)}][PF₆]. The reaction proceeds under mild conditions via vinylidene complexes, and the activation parameters were determined by kinetic studies. Copyright

Full text of DOI:10.1021/ja027209s

Published on Web 02/05/2003
Formation of a Cyclobutylidene Ring: Intramolecular [2 + 2] Cycloaddition of
Allyl and Vinylidene CdC Bonds under Mild Conditions
Patricia Alvarez,† Elena Lastra,† Jose´ Gimeno,*,† Mauro Bassetti,*,‡ and Larry R. Falvello§
Departamento de Qu´ımica Orga´nica e Inorga´nica, Instituto de Qu´ımica Organometa´lica “Enrique Moles”
(Unidad Asociada al C.S.I.C.), Facultad de Qu´ımica, UniVersidad de OViedo, 33006 OViedo, Spain,
Istituto CNR di Chimica dei Composti Organometallici, Sezione di Roma, Dipartimento di Chimica,
UniVersita´ La Sapienza, 00185 Roma, Italy, and Departamento de Qu´ımica Inorga´nica, UniVersidad de Zaragoza,
Plaza San Francisco s/n, 50009 Zaragoza, Spain
Received June 7, 2002 ; E-mail: jgh@sauron.quimica.uniovi.es
The [2 + 2] cycloaddition process is the most versatile and
efficient method for synthesizing four-membered rings containing
both heterocyclic and isocyclic skeletons.1 Nevertheless, and despite
the rich and versatile reactivity of the vinylidene group in metal
complexes [M]dCdCR1R2, [2 + 2] cycloadditions of the vinyl-
idene CRdCâ bond are limited to a few examples involving coupling
with the triple bond of alkynes and metal alkynyls or the CdN
bond of azabutadienes and imines.2 Metal vinylidene complexes
are well-known active substrates in a series of stoichiometric and
catalytic C-C and C-heteroatom coupling reactions3 which
proceed with high selectivity and atom economy. In particular,
Figure 1. One unit of complex 3, showing the atom naming scheme. Some
ruthenium(II) vinylidenes have proven to be efficient catalysts in
hydrogen atoms and phenyl carbon atoms have been excluded for clarity.
Non-hydrogen atoms are represented by their 50% probability ellipsoids.
RCM of olefins, with the key step in the catalytic process being
the [2 + 2] coupling of the olefin with the RudC bond of the
vinylidene moiety.4 However, [2 + 2] cycloaddition of the
vinylidene CRdCâ bond with an olefin has not previously been
described. We report here an unprecedented diastereoselective [2
+ 2] intramolecular cycloaddition of two CdC bond systems which
proceeds readily under mild conditions to give a cyclobutylidene
ring. We also present rate studies of the coupling reaction.
The process takes place by heating the vinylidene complexes
[Ru(η5-C9H7){dCdC(R)H}{κ1-(P)-PPh2(C3H5)}(PPh3)][BF4] (R )
Ph (1), p-MeC6H4 (2)),5 yielding the bicyclic alkylidenes [Ru(η5-
as air-stable yellow solids and fully characterized by elemental
analysis and spectroscopic means.
The formation of the transient vinylidene species is assessed by
the 31P{1H} NMR spectrum (CH2Cl2) of the reaction mixture (R
) Ph), which shows, after a few minutes in refluxing CH2Cl2, two
doublet resonances (δ 43.4/32.4) of the corresponding vinylidene
complex 1. After ca. 30 min at room temperature, only the
resonances of the cycloaddition product 3 are observed.
The 31P{1H} NMR spectra of 3, 4, and 6 reveal that the reaction
proceeds stereoselectively because only one diastereoisomer is
formed. The 1H and 13C{1H} NMR spectra are also consistent with
the formation of the bicycle.6,8 In particular, the 13C{1H} NMR
spectra show the low field carbene resonances as a broad singlet
at δ 366.0 (3) and δ 371.0 (4) or doublets at δ 360.6 (JCP ) 19.6
Hz) (6) along with the expected resonances for the rest of the
carbocycle.
C9H7){κ2-(P,C)-(dCC(R)HCH2CHCH2PPh2)}(PPh3)][BF4] (R ) Ph
(3), p-MeC6H4 (4))6 each as the only product of its respective
reaction. Thus, complexes 3 and 4 have been prepared (70%) by
refluxing a dichloromethane solution of 1 or 2 for 5 min (eq 1).
The crystal structure of complex 3 has been determined by X-ray
diffraction (Figure 1).9 The most remarkable feature is the presence
of a metallaphosphabicycle resulting from the [2 + 2] cycloaddition.
The bicycle contains a cyclobutylidene ring attached to the
ruthenium atom (the bond length Ru-C(8) of 1.864(5) Å is typical
of a ruthenium-carbon double bond) and a five-membered ruthena-
phospha ring. Figure 1 also shows the absolute configuration of S
at C(7) and S at C(10).
These complexes can also be obtained by the one-pot reaction
of [Ru(η5-C9H7){κ3-(P,C,C)-PPh2(C3H5)}(PPh3)][PF6] (5)7 with an
excess of the corresponding terminal alkyne (eq 2). Similarly,
complex 5 reacts with Me3SiCtCH in the presence of NH4PF6 to
afford [Ru(η5-C9H7){κ2-(P,C)-(dCCH2CH2CHCH2PPh2)}(PPh3)]-
[PF6] (6)8 (85%) (eq 2). Complexes 3 , 4, and 6 have been isolated
† Universidad de Oviedo.
‡ Universita´ La Sapienza.
§ Universidad de Zaragoza.
The molecule shown in the figure is present in the crystal in
equal numbers with its enantiomer, as the crystal belongs to the
9
2386
J. AM. CHEM. SOC. 2003, 125, 2386-2387
10.1021/ja027209s CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. This work was supported by the Ministerio
de Educacio´n y Ciencia (PB98-1593, MCT-00-BQU-0227) and by
the COST Chemistry Project “Ruthenium catalysts for Fine
Chemistry” (D12/0025/99).
Supporting Information Available: Crystallographic data of 3,
kinetic data, and experimental details for 1-6 (PDF and TXT). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
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(5) Complexes 1 and 2 have been obtained by protonation of the corresponding
alkynyl derivatives [Ru(η5-C9H7)(CtCR){κ1-(P)-PPh2(C3H5)}(PPh3)]. The
latter complexes were prepared (85-90%) by reaction of [Ru(η5-C9H7)-
Cl{κ1-(P)-PPh2(C3H5)}(PPh3)] with the correspondent alkyne and KOtBu
in CH2Cl2 (see Supporting Information). Selected spectroscopic data for
vinylidene complexes 1 and 2. (1): 31P{1H} NMR (CDCl3) δ 43.4 (d, JPP
) 23.6 Hz), 32.4 (d, JPP ) 23.6 Hz); 1H NMR (CDCl3) δ 2.81 (m, 2H,
PCH2), 4.82 (m, 1H, dCH), 4.32 (m, 1H, dCH2), 4.58 (m, 1H, dCH2),
5.34 (s br, 1H, dCdC(Ph)H); 13C{1H} NMR (CDCl3) δ 360.1 (s br,
dCdC(Ph)H). (2): 31P{1H} NMR (CDCl3) δ 43.6 (d, JPP ) 25.4 Hz),
32.2 (d, JPP ) 25.4 Hz); 1H NMR (CDCl3) 2.31 (s, Me), 2.82 (m, 2H,
CH2), 4.29 (m, 1H, dCH2), 4.57 (m, 1H, dCH2), 5.12 (m, 1H, dCH),
5.31 (s a, dCdCH); 13C{1H} NMR (CDCl3) δ 369.2 (s a, dCdC(p-
MeC6H4)H) (see Supporting Information).
Figure 2. Graphical representation of the reaction of complex 5 (12 mg,
0.0147 mmol, 0.027 M) with PhCtCH (0.040 mL, 0.36 mmol, 0.674 M)
at 45.6 °C in CDCl3, showing the evolution of complexes 5, 1, and 3. The
solid line applied to complex 1 represents the fitting with eq 3.
centric space group P21/n. The other two possible diastereoisomers
and their enantiomers are absent.
To gain kinetic information on the reaction pathway, the reaction
of complex 5 with an excess of PhCtCH in CDCl3 was monitored
in situ by 31P{1H} NMR spectroscopy. A reaction profile obtained
from the experiment at 45.6 °C is shown in Figure 2.
The concentration of complex 5 decreases as the reaction
proceeds and is fitted conveniently with a first-order rate equation
to obtain the observed rate constant for its disappearance, kobs
)
1.20 × 10-3 s-1 (45.6 °C). With the assumption that the back
reactions from the vinylidene intermediate 1 to complex 5 and from
the bicyclic product 3 to the vinylidene are negligible, the overall
process can be simplified to a sequence of two consecutive first-
order reactions, from 5 to 1 (k1), via a fast π-alkyne/vinylidene
tautomerization, and from 1 to 3 (k2).10 Fitting the concentration
values of complex 1 with eq 3 yields the values of k1 (1.28 × 10-3
s-1) (comparable to the value given by the first-order rate equation)
and of k2 (2.12 × 10-4 s-1), the rate constant of the [2 + 2] coupling
reaction.
(6) Selected spectroscopic data. (3): 31P{1H} NMR (CDCl3) δ 81.5 (d, JPP
) 32.6 Hz), 42.3 (d, JPP ) 32.6 Hz); 1H NMR (CDCl3) δ 1.31 (m, 2H,
PCH2), 3.01 (m, 1H, dCCHPh); 13C{1H} NMR (CDCl3): δ 366.0 (s br,
dC). (4): 31P{1H} NMR (CDCl3) δ 78.5 (d, JPP ) 31.2 Hz), 45.7 (d, JPP
) 31.2 Hz); 1H NMR (CDCl3) 1.23 (m, 2H, P-CH2), 3.21 (m, 1H, dC-
CHPh); 13C{1H} NMR (CDCl3) δ 371.0 (s br, dC) (see Supporting
Information).
(7) Complex 5 was prepared (90%) by reaction of [Ru(η5-C9H7)Cl{κ1-(P)-
PPh2(C3H5)}(PPh3)] with NaPF6 in refluxing methanol (see Supporting
Information).
k1
[1] ) [5]
(exp(-k1t) - exp(-k2t))
(3)
0 k2 - k1
(8) Selected spectroscopic data. (6): 31P{1H} NMR (CDCl3) δ 88.4 (d, JPP
) 31.6 Hz), 42.3 (d, JPP ) 31.6 Hz); 1H NMR (CDCl3): δ 1.71 (v t, JHH
) 13.0 Hz, 1H, PCH2), 1.91 (m, 1H, PCH2), 2.57 (m, 1H, CH); 13C{1H}
NMR (CDCl3): δ 360.6 (d, JCP ) 19.6 Hz, dC) (see Supporting
Information).
Experiments at different temperatures in the range 38-59 °C
allow us to obtain the activation parameters for the coupling step,
which are ∆Hq ) 19 ((2) kcal mol-1 and ∆Sq ) -16 ((4) cal
(9) [RuC50H43P2](PF6)‚CH2Cl2, monoclinic, P21/n, yellow-orange crystal, a
) 11.073(3), b ) 25.216(6), c ) 16.324(4) Å, â ) 99.21(2)°, V ) 4499(2)
Å3, T ) -123 ( 1 °C, Z ) 4, R1 ) 0.0556, wR2 ) 0.1517, GOF )
1.044.
(10) Maskill H. The Physical Basis of Organic Chemistry; Oxford University
Press: New York, 1985; p 282.
mol-1 K-1 11
. The value of the activation entropy is consistent with
an ordered structure of the transition state, essentially due to the
loss of conformational freedom of the three single bonds of complex
1.12
(11) The value of ∆Hq is, as expected, smaller than the activation enthalpy
involved in the dimerization of ethylene (43.2 kcal mol-1): Lowry, T.
H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 2nd
ed.; Harper & Row Publishers: New York, 1981; p 829. ∆Hq and ∆Sq
(16 kcal mol-1 and -20 cal mol-1 K-1, respectively) have been reported
for [2 + 2] cycloaddition of dimethylketene with ethylvinyl ether: Isaacs,
N. S.; Stanbury, P. J. Chem. Soc., Perkin Trans, 2 1973, 166.
(12) Galli, C.; Mandolini, L. Eur. J. Org. Chem. 2000, 3117.
(13) Smith, M. B.; March, J. March’s AdVanced Organic Chemistry: Reaction,
Mechanisms and Structure, 5th ed.; John Wiley & Sons: New York, 2001;
p 1077.
In summary, in this work, an unprecedented stereospecific [2 +
2] cycloaddition of two CdC bonds under mild thermal conditions
is reported. It is worth emphasizing that only allenes or activated
alkenes (bearing electron-donating or -withdrawing substituents)13
are able to undergo [2 + 2] cycloadditions under mild conditions.
Otherwise, reaction at high temperature (>100 °C) is required.
Theoretical calculations aimed at shedding light on the stereospecific
cycloaddition mechanism are in progress.
JA027209S
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