Thermodynamics and mechanism of the reversible tin-to-palladium transmetalation of the furyl group [21]

Full text of DOI:10.1021/ja980901w

11016
J. Am. Chem. Soc. 1998, 120, 11016-11017
Thermodynamics and Mechanism of the Reversible
Tin-to-Palladium Transmetalation of the Furyl
Group
W. Donald Cotter,* Lauren Barbour, Kristin L. McNamara,
Rachel Hechter, and Rene J. Lachicotte
Department of Chemistry, Mount Holyoke College
South Hadley, Massachusetts 01075
ReceiVed March 17, 1998
Transmetalation, the transfer of an organic moiety from one
metal center to another, is an elementary organometallic process
of importance in organic cross-coupling reactions, notably the
palladium-catalyzed coupling of organostannanes with organic
halides or triflates (Stille coupling).1 We report here the first direct
observation and spectroscopic characterization of an intermediate
en route to transmetalation, using a cationic catalyst model. We
also demonstrate that the transfer is reversible and report, for the
first time, thermodynamic parameters relating the starting materi-
als, products, and a key intermediate in a carbon transmetalation
reaction.
Figure 1. ORTEP drawing of 3. Thermal ellipsoids are drawn at 50%.
Hydrogen atoms are omitted for clarity.
Scheme 1
Square planar complexes of the 2,6-bis(diphenylphosphino)-
methylphenyl ligand2 (1; see Scheme 1) are useful in the study
of processes involving cationic palladium intermediates. The
tridentate ligand inhibits both phosphine dissociation and reductive
elimination of the aryl ring. The benzylic methylene protons
1
resonate in an unobstructed region of the H NMR spectrum (δ
≈ 4.0 ppm), and when the ligand is coordinated to palladium,
they appear as a diagnostic virtual triplet.3
1 and the triflatopalladium complex 2 were prepared according
to literature methods.2 Treatment of 2 with excess tributylstan-
nylfuran in cold, anhydrous acetone leads to rapid, quantitative
precipitation of the furyl complex 3 (Scheme 1).4 The structure
of 3 was determined by X-ray analysis of crystals grown by vapor
diffusion of pentane into a methylene chloride solution (Figure
1),5 confirming the transmetalation of the furyl group. Formation
of 3 is reversible.6 Treatment of 3 with tributylstannyl triflate in
CD2Cl2 leads to rapid regeneration of tributylstannylfuran and
2.7
Transmetalation is preceded by coordination of 2-tributylstan-
nylfuran to palladium via the tin-substituted double bond. Thus,
when 2 is treated with 2-tributylstannylfuran in CD2Cl2 and
observed in a cold (205-230 K) NMR probe, a small amount of
3 accumulates along with a larger amount of the furan adduct 4.7
Formation of 3 and 4 is reversible upon warming the sample.
The molecular identity of 4 was established by a series of H,
1
31P, and 119Sn NMR experiments.4 The furyl Hâ signals8 (see
Scheme 1) are similar to those of the only structurally character-
ized η2-furan complex, [Os(NH3)5(C4H4O)]2+.9 The strong upfield
shift of Hâ′ indicates dihapto coordination of the furan moiety.10
A series of magnetization-transfer experiments unambiguously
connects each of the furyl protons in 3 and 4 to a corresponding
resonance in 2-tributylstannylfuran, confirming the regiochemistry
of 4. Thus, saturation of the Hâ or Hâ′ signal in 2-tributylstan-
nylfuran leads to suppression of the corresponding resonance in
3 and 4.11
(1) (a) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 98, 504. (b) Mitchell,
T. N. Chem. Soc. ReV. 1992, 803.
The 119Sn signal in 4 is, at +102 ppm, shifted strongly
downfield from 2-tributylstannylfuran (δ -52 ppm). The 119Sn
chemical shift of tetrahedral R3SnX molecules is a sensitive
indicator of the polarization in the Sn-X bond (cf. Bu3SnOTf, δ
172;12 Me3SnCl, δ 160; Me3SnBr, δ 128; Me3SnI, δ 39).13 The
Sn-C bond in 4 is polarized by complexation of the cationic
(2) (a) Rimml, H.; Venanzi, L. M. J. Organomet. Chem. 1983, 259, C6.
(b) Rimml, H.; Venanzi, L. M. J. Organomet. Chem. 1984, 260, C52. (c)
Rimml, H. Dissertation, ETH No. 7562, 1984. (d) Rimml, H.; Venanzi, L.
M. Phosphorus Sulfur 1987, 30, 297.
(3) Verkade, J. G. Coord. Chem. ReV. 1972, 9, 1.
(4) Selected NMR data for 3 at 293 K. HR: δ 7.52, d, JRâ ) 1.7 Hz, 1 1H.;
Hâ: δ 6.17, d of d, 1 1H Hâ′ δ 5.79, d, Jââ′ ) 2.9 Hz, 1 1H -CH2-: δ 4.12,
2
t, JPH ) 9.0 Hz, 4 1H. 31P: δ 33.1 (vs 85% H3PO4). Selected NMR data for
1
1
4 at 220 K. Hâ: δ 6.19, br, 1 H Hâ′: δ 4.38, d, J ) 2.1 Hz, 1 H -CH2-:
δ 3.8 - 3.9, v br. 31P: δ 34 (vs 85% H3PO4), v br. 119Sn: δ 102 (vs SnMe4),
br.
(8) The HR signal is lost in the crowded aryl region.
(9) (a) Cordone, R.; Harman, W. D.; Taube, H. J. Am. Chem. Soc. 1989,
111, 5969. (b) Chen, H.; Hodges, M.; Liu, R.; Stevens, W. C., Jr.; Sabat, M.;
Harman, W. D. J. Am. Chem. Soc. 1994, 116, 5499.
(5) Selected bond lengths (Å): Pd(1)-C(33), 2.089(2); Pd(1)-C(1), 2.073-
(2); Pd(1)-P(1), 2.2709(5); Pd(1)-P(2), 2.2921(5); C(33)-C(36), 1.331(3);
C(35)-C(36), 1.446(4); C(34)-C(35), 1.341(4); C(34)-O(1), 1.367(3);
C(33)-O(1), 1.433(3). Selected angles and dihedrals (deg): P(1)-Pd(1)-
P(2), 160.58(2); C(1)-Pd(1)-C(33), 176.91; O(1)-C(33)-Pd(1)-C(1),
-54.85(1.53); C(33)-Pd(1)-C(1)-C(2), 15.87(1.55). Sum of angles about
Pd, 359.95°.
(10) Transition metal complexes of furan oxygen are unknown, except
where the furan ring is part of a macrocyclic ligand. See: (a) Crescenzi, R.;
Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Inorg. Chem. 1996, 35,
2413. (b) Chmielewski, P. J.; Latos-Grazynski, L.; Olmstead, M. M.; Balch,
A. L. Chem. Eur. J. 1997, 3, 268.
(6) Reversible transfer of alkynyl groups among Pd(II), Pt(II), and Cu(I)
centers has recently been described: Osakada, K.; Sakata, R.; Yamamoto, T.
J. Chem. Soc., Dalton Trans. 1997, 1265.
(7) A small amount of another, unidentified organopalladium species is
also formed, the concentration of which is temperature invariant and which
does not contain a furyl moiety.
(11) A reviewer has suggested an alternative structure for 4, in which the
furan moiety adopts a zwitterionic, allyl(CR-Câ-Câ′)/oxonium structure. We
currently disfavor this alternative because of the very weak perturbation of
the Hâ resonance in 4 relative to 3 and free 2-tributylstannylfuran.
(12) Arshadi, M.; Johnels, D.; Edlund, U. Chem. Commun. (Cambridge)
1996, 1279.
10.1021/ja980901w CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/08/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 42, 1998 11017
palladium fragment, with the tin atom acquiring substantial
positive charge. The ability of tin to stabilize adjacent charge is
well-known,14 and it undoubtedly contributes to the stability of
the Pd(olefin) bond in 4. Coordination of Pd to the more hindered
double bond is surprising, but as Scheme 1 illustrates, this
produces a sterically favorable isomer. Coordination of the
unsubstituted double bond would bring the tributyltin moiety into
closer contact with the phosphine phenyl groups.
exothermic (∆H1 ) -5(1) kcal/mol); subsequent transfer of the
furyl group to palladium is endothermic (∆H2 ) +3(1) kcal/mol).
Substrate binding requires a substantial loss of entropy (∆S1 )
-28(4) eu) as would be expected for an associative event. Some
entropy is regained after transmetalation (∆S2 ) +12(5) eu). The
overall reaction is weakly endothermic (∆Hrxn ) -1.6(0.2) kcal/
mol) and proceeds with a modest loss of entropy (∆Srxn
)
-16(1) eu), rendering it somewhat endergonic at room temper-
ature (K298,calc ≈ 10-3). Although these values must be interpreted
with caution, they draw a clear distinction between the thermo-
dynamic requirements of the reaction steps.
These results constitute the most detailed sketch presently
available for a carbon transmetalation pathway.18 The observation
of 4 establishes that substrate precoordination is an important
activating step when transmetalation occurs to an electrophilic
Pd fragment and that reversible η2 coordination is viable for
aromatic heterocycles. These experiments do not reveal the
process by which the adduct 4 collapses to product. We are
continuing our investigations into the details of cationically
promoted transmetalation.
Complex 4 is unsymmetrical, with inequivalent 31P and
methylene resonances expected from the two sides of the ligand.
These diastereotopic sites undergo rapid exchange, even at low
temperatures. Thus, both the 1H methylene and the 31P resonances
for 4 at 205 K are very broad, while the same signals for 2 are
sharp. Loss of furan followed by coordination of the opposite
face provides the most obvious means of exchange. Exchange
must occur without diffusion of the furan away from the palladium
center and regeneration of 2.15
The observation of 4 in equilibrium with 2 and 3 does not
demonstrate that 4 is a true intermediate. Better evidence is found
in a further magnetization-transfer experiment. In an equilibrating
mixture at 205 K, saturation of Hâ′ in 4 leads to suppression of
Hâ′ in 3. Transfer of magnetization between the two minor
components of the system, in the presence of a ∼10-fold excess
of 2-tributylstannylfuran (with respect to 4), strongly suggests
that 4 is indeed an immediate precursor to 3. Decoordination of
the furan ligand in 4 prior to formation of 3 would result in loss
of magnetization in 3.
Acknowledgment. This work was supported by a Camille and Henry
Dreyfus Start-up Grant for Undergraduate Faculty, a Research Corp.
Cottrell Grant, the Petroleum Research Fund, and Mount Holyoke College.
L.B. is the recipient of an AIURP Fellowship from the Council on
Undergraduate Research, sponsored by Merck and Co. W.D.C. thanks
Professor Guillermo Bazan and his research group for many helpful
discussions and for their gracious hospitality during a sabbatical leave.
The temperature regime in which 2, 3, and 4 can be simulta-
neously observed by NMR is narrow.16 Despite this limitation, a
quantitative treatment of the equilibria connecting these species
is informative.17 The thermodynamic parameters for the two
processes are significantly different. Substrate binding to 2 is
Supporting Information Available: Details of the preparation and
crystallographic study of 3, procedures for spectroscopic experiments and
representative spectra, and equilibrium constant data (16 pages, print/
PDF). This material is contained in many libraries on microfiche,
immediately follows the article in the microfilm version of the journal,
and can be ordered from the ACS. See any current masthead page for
ordering information. See any current masthead page for ordering
information and Web access instructions.
(13) Many 119Sn chemical shifts and a discussion of chemical shift
correlations in compounds of group 14 elements appear in: Mitchell, T. N. J.
Organomet. Chem. 1983, 255, 279.
(14) Lambert, J. B.; Wang, G.-t.; Teramura, D. H. J. Org. Chem. 1988,
53, 5422.
(15) Other mechanisms are possible, for example, formation of an η1-allyl/
oxonium intermediate (see ref 11) and rotation about Pd-C.
(16) Below 205 K, precipitation within the sample tube results in loss of
signal.
JA980901W
(18) Hartwig recently described a mechanism for the activation of tin-
sulfur bonds by neutral palladium intermediates (Louie, J.; Hartwig, J. F. J.
Am. Chem. Soc. 1995, 117, 11598). In that case, the stannane clearly behaves
as an electrophile, and the palladium center proceeds partly toward oxidative
addition in a concerted process.
(17) Concentrations of palladium species were determined from integration
of the appropriate methylene and/or furyl signals relative to an internal toluene
standard. Concentrations of the tin reagents were calculated stoichiometrically.