Pd-Catalyzed Formylation of Aryl Bromides
A R T I C L E S
rapidly transforms into 17 in CO atmosphere, the stability of 1
under the reaction conditions is the same as that of 17. Therefore,
it is not surprising that hydrogenation of complexes 8 and 9
carried out in the presence of TMEDA at 100 °C yielded
palladium black and free phosphine. The same result was
obtained for complex 10 when the reaction was carried out in
the presence of 2 equiv of TMEDA or PtBu3 (eq 17).
the other hand, the reaction of [Pd(PtBu3)2] with TMEDA·HBr
in toluene-d8 at 70 °C did not proceed to any extent (eq 20).
These results imply that the reactivity of these hydrido
complexes toward the base is governed by thermodynamics,
but not kinetics. Therefore, the reason for the pronounced
difference in the reactivity between 14, 15, and 18 may be
revealed by comparison of the ground states of the [PdL2] and
[Pd(Br)(H)L2] complexes. It is evident that PtBu3 is more
sterically demanding than PtBu2 Bu, and probably P(1-Ad)2 Bu.
Since the hydridobromide complexes are more sterically en-
cumbered as compared to the corresponding bisphosphine
palladium(0) complexes, the increased steric bulk of PtBu3
facilitates reductive elimination from 18. In addition, the
divergent reactivity may be caused by the different electron-
donating properties of the phosphines. Clearly, electron-rich
phosphines stabilize a hydrido palladium(II) complex and
increase the basicity of the respective palladium(0) complex,
thus making dehydrobromination less favorable. To evaluate
the σ-donor ability of the phosphines, we compared phosphorus-
selenium (1JPSe) coupling constants of the corresponding phos-
n
n
A completely different reaction pattern was observed when
the hydrogenation of acyl complexes 8 and 9 was effected in
the presence of additional phosphine and TMEDA (catalytic
conditions). Here, the corresponding hydride complexes 14 and
15 are formed as main products. This suggests that hydride
species 14 and 15, unlike the respective carbonyl clusters, are
stable at 100 °C for 16 h under synthesis gas. Moreover, it is
rather surprising that these complexes were observed even when
an excess of base was used (eq 18)!
1
phine selenides. The magnitude of JPSe is known to increase
with decrease in basicity of the phosphine.56 We prepared
n
n
selenides of PtBu2 Bu and P(1-Ad)2 Bu by stirring of the
phosphines with 2 equiv of Se in CDCl3 at 60 °C for 40 min.
The resulting phosphine selenides formed quantitatively, and
1
the coupling constants JPSe were measured without isolation.
Interestingly, 1JPSe values obtained for PtBu2 Bu (690 Hz) and
n
P(1-Ad)2 Bu (680 Hz) are lower than that of PtBu3 (712 Hz),57
n
indicating that the first two phosphines are more basic. This
may imply that these ligands form more stable hydrido
complexes 14 and 15 with respect to the PtBu3-ligated com-
1
plexes. However, care must be taken in correlating JPSe
1
constants with electronic properties of phosphines since JPSe
also may depend on steric factors.58 Thus, the poor reactivity
Intrigued by these results, we examined the reactivity of
n
n
of PtBu2 Bu- and P(1-Ad)2 Bu-ligated hydrobromide complexes
toward the amine bases as compared to 18 may be explained
by both steric and electronic factors.
n
hydrido palladium complexes [Pd(H)(Br)L2], L) P(1-Ad)2 Bu
n
(14), PtBu2 Bu (15), and PtBu3 (18), toward TMEDA in detail.
When 14 or 15 was mixed with a 2-fold excess of TMEDA in
toluene-d8 at room temperature, the 1H and 31P{1H} NMR
spectra showed no appreciable reaction. Moreover, neither
heating of 14 with 2 equiv of TMEDA at 100 °C nor use of a
100-fold excess (!) of base at room temperature showed any
formation of palladium(0) complex 1 in the 31P{1H} NMR
spectra (eq 19).
Palladium hydride complexes are assumed to be crucial
intermediates in many palladium-catalyzed coupling reactions
of aryl halides.59 In general, the main role of base in these
reactions consists of trapping the hydrogen halide from [Pd(H)-
(54) Most phosphine-containing palladium carbonyl clusters possesses from
3 to 69 palladium atoms: (a) Cavell, K. J. In ComprehensiVe
Organometallic Chemistry III; Mingos, D. M., Crabtree, R. H., Canty,
A., Eds.; Elsevier: Oxford, 2007; Vol. 8, pp 206-210. (b) Kudo, K.;
Hidai, M.; Uchida, Y. J. Organomet. Chem. 1971, 33, 393. (c) Yoshida,
T.; Otsuka, S. J. Am. Chem. Soc. 1977, 99, 2134. (d) Mednikov, E. V.;
Eremenko, N. K.; Mikhalkov, V. A.; Gubin, S. P.; Slovokhotov, Y. L.;
Struchkov, Y. T. J. Chem. Soc., Chem. Commun. 1981, 989. (e)
Goddard, R.; Jolly, P. W.; Kru¨ger, C.; Schick, K.-P.; Wilke, G.
Organometallics 1982, 1, 1709. (f) Manojloviæ-Muir, L.; Muir, K. W.;
Lloyd, B. R.; Puddephatt, P. J. J. Chem. Soc., Chem. Commun. 1985,
536. (g) Klein, H.-F.; Mager, M. Organometallics 1992, 11, 3915.
(h) Tran, N. T.; Kawano, M.; Dahl, L. F. J. Chem. Soc., Dalton Trans.
2001, 2731.
In contrast to 14 and 15, the reaction of the PtBu3-ligated
hydrido palladium complex 18 with TMEDA proceeded smoothly
within 10 min at room temperature to give the respective
palladium(0) complex quantitatively (eq 20).
(55) At room temperature reaction does not occur due to very low solubility
of TMEDA ·HBr in toluene.
(56) (a) Allen, D. W.; Taylor, B. F. J. Chem. Soc., Dalton Trans. 1982,
51. (b) Socol, S. M.; Verkade, J. G. Inorg. Chem. 1984, 23, 3487. (c)
Sua´rez, A.; Me´ndez-Rojas, M. A.; Pizzano, A. Organometallics 2002,
21, 4611.
(57) Du Mont, W.-W.; Kroth, H.-J. J. Organomet. Chem. 1976, 113, C35.
(58) Pinnell, R. P.; Megerle, C. A.; Manatt, S. L.; Kroon, P. A. Inorg.
Chem. 1973, 95, 977.
The same result was obtained when the reactions depicted in
esq 19 and 20 were effected from opposite directions. For
example, heating of 1 with TMEDA·HBr in toluene-d8 at 70
°C55 gave the hydrido complex 14 in 93% yield (eq 19). On
(59) Reviews on palladium hydride complexes: (a) Grushin, V. V. Chem.
ReV. 1996, 96, 2011. (b) Hii, K. K. In Handbook of Organopalladium
Chemistry for Organic Synthesis; Negishi, E. I., Ed.; Wiley-Inter-
science: New York, 2002; Vol. 1, pp 81-90.
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