Deoxygenations of (silox)3WNO and R3PO by (silox)3M (M = V, Ta) and (silox)3NBL (silox = tBU3SiO): Consequences of electronic effects

Full text of DOI:10.1021/ja004329w

J. Am. Chem. Soc. 2001, 123, 6419-6420
6419
Deoxygenations of (silox)3WNO and R3PO by
(silox)3M (M ) V, Ta) and (silox)3NbL (silox )
tBu3SiO): Consequences of Electronic Effects
Adam S. Veige,† Lee M. Slaughter,† Peter T. Wolczanski,*,†
Nikita Matsunaga,‡ Stephen A. Decker,§ and
Thomas R. Cundari*,§
HNbOSitBu2CMe2CH2 (5-Nb, 23 °C, <5 min)14 competed with
deoxygenation; 5-Nb then slowly deoxygenated 2, presumably
via reversible formation of 1-Nb. The thermodynamics of
deoxygenation15 were investigated by high-level quantum calcula-
tions,16 with (HO)3M serving as the model of respective tris-silox
centers in 1-M and 3-M. In each case the reaction was extremely
Cornell UniVersity
Department of Chemistry and Chemical Biology
Baker Laboratory, Ithaca, New York 14853
ReceiVed December 27, 2000
exoergic (25 °C: M ) V, ∆G°
) -66 kcal/mol; M ) Nb,
rxn
Ta, -100 kcal/mol). With favorable thermodynamics, the un-
competitive (1-Ta) and relatively slow (1-V, 1-Nb) deoxygen-
ations are puzzling.
Since (silox)3V (1-V, S ) 1) binds various L (L ) THF, py,
etc.), while (silox)3Ta (1-Ta, S ) 0) does not,17 the singlet and
triplet states of 1-M were examined via quantum calculations.16
Figure 1 reveals that 1-V is a triplet at the optimized geometries
Oxygen atom transfers involving terminal metal-oxo func-
tionalities are central to many biological transformations,1 promi-
nent in applications to organic synthesis,2-4 and of increasing
importance in inorganic systems as synthetic tools,5-7 objectives
in biomimicry,1,8,9 and targets of fundamental studies.5-13 As a
2
1
2
1
2
for S ) 0 ((dz ) ) and S ) 1 ((dz ) (dxz/dyz) ), and the TfS barrier
is 17 kcal/mol, assuming a facile intersystem crossing. 1-Ta is a
singlet at the optimized S ) 0 and S ) 1 geometries and its
intersystem crossing barrier is 17 kcal/mol. 1-Nb is a singlet, but
the conversion barrier to a triplet of nearly the same energy is 2
kcal/mol. If the approach of (silox)3WNO (2) to the 1-M center
is linear because of intermolecular silox/silox interactions, then
a 4e- repulsion will result in the case of 1-Ta, but successful
docking to an S ) 1 intermediate (silox)3MONW(silox)3 (1-
M-2) will occur for M ) V, Nb. The additional S-T barrier
forced on 1-Ta allows unimolecular cyclometalation to compete
with the bimolecular deoxygenation of 2.
t
synthetic route to (silox)3WN (4, silox ) Bu3SiO), the deoxy-
genation of (silox)3WNO (2) by (silox)3Ta (1-Ta) was attempted
without success, despite ample precedent in cleavages of ep-
oxides,10 N2O, NO,11 CO2, and CO.12 A comparison study
involving sources of M(silox)3 (1-M; M ) V, Nb, Ta) revealed
that features of deoxygenations of 2 and R3PO (R ) Me, Ph,
tBu) are the consequences of electronic effects enforced by a
limiting steric environment.
Table 1. summarizes the deoxygenation studies, and shows
that (silox)3Ta (1-Ta) preferred to cyclometalate to (silox)2-
HTaOSitBu2CMe2CH2 (5-Ta, 87%, 14 d)13 rather than deoxy-
genate (silox)3WNO (2)14 to (silox)3WN (4, 12%),14 whereas the
Table 1 lists the results of R3PO deoxygenations by (1-V, Ta)
and 1-NbL (L ) 4-pic, PMe3), which are predicted by quantum
calculations to be exothermic for V (-15 kcal/mol) and Nb or
Ta (-45 kcal/mol) with Me3PO. Curiously, 1-Ta and 1-NbL both
smaller (silox)3V (1-V)14 slowly (85 °C, ∼1.4 × 10-4 M-1 s-1
)
converted 2 to the nitride. (silox)3Nb(η2-N,C-4-picoline) (1-Nb-
(4-pic), S ) 0)11 and (silox)3NbPMe3 (1-NbPMe3, S ) 1)14
deoxygenated 2 and formed 4 and (silox)3NbO (3-Nb) swiftly at
first, then more slowly as the released 4-picoline and PMe3
inhibited the reactions, respectively. With a 4-picoline scavenger
(1-Ta) present in the former, swift cyclometalation to (silox)2-
t
deoxygenated Me3PO and Ph3PO, but failed with Bu3PO; 1-Ta
cyclometalated to 5-Ta, 1-Nb(4-pic) converted to (silox)3Nbd
NCHCHCMeCHCHdNb(silox)3 (6; 85 °C, 35 d) and 4-picoline,18
t
and 1-NbPMe3 decomposed. The inability to deoxygenate Bu3-
PO is not steric in origin, as an X-ray crystal structure of (silox)3V-
OPtBu3 (1-VOPtBu3) attests. R3PO deoxygenation attempts with
† Cornell University.
t
1-V led to (silox)3V-OPR3 (1-VOPR3; R ) Me, Ph, Bu),14 and
‡ Long Island University, Dept. of Chemistry, Brooklyn, New York 11201.
§ University of Memphis, Dept. of Chemistry, Memphis, Tennessee 38152.
(1) (a) Cytochrome P450, Structure, Mechanism and Biochemistry, 2nd
ed.; Ortiz de Montellano, P. R., Ed., Plenum: New York, 1995. (b) Enemark,
J. H.; Young, C. G. AdV. Inorg. Chem. 1993, 40, 1-88.
(2) (a) Palucki, M.; Finney, N. S.; Pospisil, P. J.; Gu¨ler, M. L.; Ishida, T.;
Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120, 948-954. (b) Finney, N. S.;
Pospisil, P. J.; Chang, S.; Palucki, M.; Konsler, R. G.; Hansen, K. B.; Jacobsen,
E. N. Angew. Chem., Int. Ed. Engl. 1997, 36, 1720-1723.
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1994, 94, 2483-2547. (b) Norrby, P.-O.; Rasmussen, T.; Haller, J.; Strassner,
T.; Houk, K. N. J. Am. Chem. Soc. 1999, 121, 10186-10192.
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Chem. Soc. Chem. Commun. 1991, 762-764. (b) Vivanco, M.; Ruiz, J.;
Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1993, 12, 1802-
1810.
prolonged thermolysis (100 °C, >20 d) of (silox)3VO (3-V) with
PMe3 afforded some 1-VOPMe3, consistent with calculations that
portray the phospine oxide adducts as the most stable species in
the vanadium system.19,20
The S-T energetics of Figure 1 do not explain the slow rates
of deoxygenation of (silox)3WNO (2) by 1-NbL and 1-V, nor do
t
they rationalize the disparate R3PO (R ) Me, Ph) and Bu3PO
results with 1-Ta and 1-NbL. Is there an intrinsic problem to
t
O-atom transfer for 2 and Bu3PO?
The smaller substrates Me3PO and Ph3PO may attack (silox)3M
(1-M; M ) Nb, Ta) at the side of the PO bond, whereas O-atom
t
transfer from (silox)3WNO (2) and Bu3PO may be sterically
restricted to occur linearly.17 With substantial thermodynamic
impetus, the deoxygenations are swift as long as (silox)3M-OE
(6) Odom, A. L.; Cummins, C. C.; Protasiewicz, J. D. J. Am. Chem. Soc.
1995, 117, 6613-6614.
(7) (a) Crevier, T. J.; Mayer, J. M. J. Am. Chem. Soc. 1997, 119, 8485-
8491. (b) Hall, K. A.; Mayer, J. M. J. Am. Chem. Soc. 1992, 114, 10402-
10411.
(8) Lim, B. S.; Sung, K.-M.; Holm, R. H. J. Am. Chem. Soc. 2000, 122,
7410-7411 and references therein.
(9) Jin, N.; Bourassa, J. L.; Tizio, S. C.; Groves, J. T. Angew. Chem., Int.
Ed. 2000, 39, 3849-3851.
(10) Bonanno, J. B.; Henry, T. P.; Neithamer, D. R.; Wolczanski, P. T.;
Lobkovsky, E. B. J. Am. Chem. Soc. 1996, 118, 5132-5133.
(11) Veige, A. S.; Kleckley, T. S.; Chamberlin, R. L. M.; Neithamer, D.
R.; Lee, C. E.; Wolczanski, P. T.; Lobkovsky, E. B.; Glassey, W. V. J.
Organomet. Chem. 1999, 591, 194-203.
(12) Neithamer, D. R.; LaPointe, R. E.; Wheeler, R. A.; Richeson, D. S.;
Van Duyne, G. D.; Wolczanski, P. T. J. Am. Chem. Soc. 1989, 111, 9056-
9072.
(13) Miller, R. L.; Toreki, R.; LaPointe, R. E.; Wolczanski, P. T.; Van
Duyne, G. D.; Roe, D. C. J. Am. Chem. Soc. 1993, 115, 5570-5588.
(14) Spectroscopic information, magnetic measurements (Evans’ method),
and elemental analyses are available as Supporting Information.
(15) Holm, R. H.; Donahue, J. P. Polyhedron 1993, 12, 571-593.
(16) Calculated energetics were determined at the CCSD(T)/SBK(d)//
B3LYP/SBK(d) level of theory. (a) Becke, A. D. J. Chem. Phys. 1993, 98,
5648-5652. (b) Krauss, M.; Stevens, W. J.; Basch, H.; Jasien, P. G. Can. J.
Chem. 1992, 70, 612-630. (c) Bartlett, R. J.; Stanton, J. F. In ReViews in
Computational Chemistry; Boyd, D. B., Lipkowitz, K. B., Eds.; VCH
Publishers: New York, 1994; Vol. 5, pp 65-169.
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Schaller, C. P.; Wolczanski, P. T. Inorg. Chem. 1991, 30, 2494-2508.
(18) (a) Kleckley, T. S.; Bennett, J. L.; Wolczanski, P. T.; Lobkovsky, E.
B. J. Am. Chem. Soc. 1997, 119, 247-248. (b) Kleckley, T. S. Ph.D. Thesis,
Cornell University, 1998.
(19) Quantum calculations suggest ∆G° ≈ -20 kcal/mol for (HO)3V +
OPMe3 f (HO)3VOPMe3, and ∆G° ≈ -6 kcal/mol for (HO)3VO + PMe3 f
(HO)3VOPMe3.
(20) Similar intermediates have recently been identified in transferases:
Smith, P. D.; Millar, A. J.; Young, C. G.; Ghosh, A.; Basu, P. J. Am. Chem.
Soc. 2000, 122, 9298-9299.
10.1021/ja004329w CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/06/2001

6420 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Communications to the Editor
Table 1. (silox)3M (1-M; M ) V, Ta)/(silox)3NbL (L ) 4-pic, PMe3) + EO f (silox)3MO (3-M) + E and Related Reactions (C6D6 or C7D8)
(silox)3M/(silox)3NbL
1-M; M ) V, Ta/1-NbL
(silox)3MO + other products
3-M; M ) V, Nb, Ta
EO
E
T(°C)
qualitative rate
1-V
(silox)3WNO (2)
2
2
2
3-V
(silox)3WN (4)
4
4
4
4 (12%)
-
85
85
23
slowa
1-Nb(4-pic)
1-NbPMe3
1-Nb(4-pic) + 1-Ta
1-Ta
3-Nb + 4-pic
3-Nb + PMe3
3-Nb + 5-Nb + 1-Ta(4-pic)
5-Ta (87%), 3-Ta (12%)
(silox)3VOPR3 (1-VOPR3)
3-Nb + 4-pic
3-Nb + 4-pic
6h
5-Nb + 1-Ta(4-pic)
3-Nb + PMe3
no reactionj
3-Ta
5-Ta
fast then slowb
fast then slowc
23
fast then slowd
2
23e
100f
23
-
1-V
R3PO (R ) Me, Ph, tBu)
Me3PO
-
1-Nb(4-pic)
1-Nb(4-pic)
1-Nb(4-pic)
1-Nb(4-pic) + 1-Ta
1-NbPMe3
1-NbPMe3
1-Ta
Me3P
Ph3P
-
fast
Ph3PO
23
fast then slowg
tBu3PO
85h
23
-
-
tBu3POi
-
R3PO (R ) Me, Ph)
R3P
-
23
23
23
85
fast
-
tBu3POi
R3PO (R ) Me, Ph)
tBu3PO
R3P
-
fast
-
1-Ta
a k ≈ 1.4 × 10-4 M-1 s-1
.
b Inhibition by released 4-picoline. c Inhibition by PMe3; 61% conversion after 2 d and 86% after 9 d; with 8 equiv
of PMe3, 10% conversion after 2 d. d Swift competitive deoxygenation and cyclometalation to 5-Nb; 5-Nb then deoxygenates 2 slowly. e At 85 °C
and 11 h, 23% deoxygenation and 77% 5-Ta. f No deoxygenation after 75 d. For R3P + 3-V f 1-VOPR3: R ) Me, 70% conversion after 86 d
(100 °C), k ≈ 2 × 10-7 M-1 s-1
.
g Inhibition by released 4-pic; 50% conversion at t ≈ 0 and 85% conversion at t ≈ 15 h. h After 35 d at 85 °C,
sparingly soluble 6 produced. i 10 equiv. j Thermal degradation of 1-NbPMe3 affords multiple products.
on O and E, and the MO σ-bond. Consequently, correlation of a
reactant σ* orbital with a product σ-orbital is required in a linear
O-atom transfer, but as the M-O-E angle decreases, σ-character
can be mixed into low-lying π-type orbitals. Intersystem crossing
must occur at a maximum for a linear O-atom transfer, because
significant mixing with an 1A1 excited-state derived from popula-
tion of the σ*-orbital is needed to ensure conversion from the
1
3A2 (reactant) state to the A1 (product) state;21 the greater the
degree of bending in the M-O-E angle, the greater the σ/π-
mixing, and intersystem crossing becomes more facile.
The mismatch in the numbers of occupied reactant and product
σ-orbitals and its effect on intersystem crossing in O-atom transfer
are related to several findings: (1) despite a ∆H°rxn of -82 kcal/
mol, N2 scission in [{tBu(3,5-Me2C6H3)N}3Mo](µ-N2) has an
appreciable barrier (∆Hq ) 23.3 (3) kcal/mol) and requires a kink
in the MoNNMo linkage to facilitate triplet reactant/singlet
product intersystem crossing;22 (2) enantioselective epoxidations
using Jacobsen’s catalyst require an olefin to approach the Mn-
(oxo) at a low Mn-O-E angle;2,3,23 (3) O-atom transfers in
bioinorganic systems have been calculated by DFT to occur via
transition states with M-O-E near 90°;24 (4) phosphine oxide
chelation may enable the necessary geometry for O-atom trans-
fer;25 and (5) O-atom transfers from MnVO are subject to spin-
state crossing effects.26
Figure 1. Energetics (kcal/mol, 25 °C) of (HO)3M (M ) V, Nb, Ta) as
models for (silox)3M (M ) V, 1-V; Nb, 1-Nb; Ta, 1-Ta). S and T refer
to the singlet and triplet energies at those optimized geometries. S@T
refers to the singlet energy at the optimized triplet geometry. Intersystem
crossing barriers are indicated by the middle vertical lines.
Acknowledgment. We thank the National Science Foundation (CHE-
9528914 (P.T.W.) and CHE-9983665 (T.R.C.)) for financial support.
Supporting Information Available: Spectral and analytical data for
all new compounds; experimental procedures; and computational details
(PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
JA004329W
Figure 2. Orbital correlation diagram for generic M-O-E a MO + E;
energetics are based on EHMO calculations of M ) (HO)3V. Reactant
(S ) 1) states efficiently intersystem cross to product S ) 0 states when
reactant excited states with populated σ* orbitals are mixed in effectively;
the reduction in symmetry upon bending M-O-E facilitates this process.
(21) Shaik, S.: Filatov, M.; Schroder, D.; Schwarz, H. Chem. Eur. J. 1998,
4, 193-199.
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E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem. Soc. 1996,
118, 8623-8638.
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592. (b) Linde, C.; A° kermark, B.; Norrby, P.-O.; Svensson, M. J. Am. Chem.
Soc. 1999, 121, 5083-5084. (c) Linde, C.; Arnold, M.; Norrby, P.-O.;
A° kermark, B. Angew. Chem., Int. Ed. Engl. 1997, 36, 1723-1725.
(24) (a) Pietsch, M. A.; Hall, M. B. Inorg. Chem. 1996, 35, 1273-1278.
(b) Pietsch, M. A.; Couty, M.; Hall, M. B. J. Phys. Chem. 1995, 99, 16315-
16319.
(1-M-OE, S ) 1) can be accessed and an additional electronic
factor revealed in the molecular orbital diagram in Figure 2 can
be overcome. Orbitals of the reactant 3A2 (C3ν) (HO)3M-OE
1
complex are shown correlating with the A1 product (HO)3MO
+ E orbitals. There are only two σ-type orbitals on the reactants
the MO and OE bond pairssbut three on the products; lone pairs
(25) Brock, S. L.; Mayer, J. M. Inorg. Chem. 1991, 30, 2138-2143.
(26) Jin, N.; Groves, J. T. J. Am. Chem. Soc. 1999, 121, 2923-2924.