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The role of organopalladium (IV) species in carbon-oxygen and carbon-selenium bond forming reactions

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Denney, Melanie Claire (2003) The role of organopalladium (IV) species in carbon-oxygen and carbon-selenium bond forming reactions. PhD thesis, University of Tasmania.

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Abstract

The role of palladium in carbon-oxygen and carbon-selenium bond forming
processes has been investigated with particular emphasis on the postulated
intermediacy of organopalladium(IV) species.
Reactions of the diorganopalladium(II) complexes PdMeR(L 2) [R = Me, 4-toly1
(Tol); L2 = 2,2'-bipyridirle (bPY), N,N,NcN'-tetramethylethylenediamine (tmeda)]
with diaroyl peroxides (ArCO2)2 [Ar = Ph, 4-CF3C6114 (ArF)] were investigated using
variable temperature 1 1-1 NMR spectroscopy, revealing a complex series of reactions.
The complexes Pd1IMeR(L2) are predicted to undergo initial oxidative addition of
(ArCO2)2 to give an unobserved Pd(IV) intermediate, P d1v(02CAr)2MeR(L2) ' ,
which immediately takes part in an exchange reaction with the reagents Pd IIMeR(1,2)
to form the observed intermediates Pd 11(02CAr)R(L2) and Pdiv(02CAr)Me2R(L2)
(except in the case of R = Tol and L2 = tmeda, where no Pd(W) intermediate was
observed). Decomposition of Pdiv(02CAr)Me2R(L2) then occurred via reductive
elimination of carbon-carbon bonds with absence of carbon-oxygen coupling.
Carbon-oxygen coupling to form R-02CAr occurred from the reaction of
monoorganopalladium(II) species Pd 11(02CAr)R(L2) with (ArCO2)2, with concurrent
formation of Pd11(02CAr)2(L2). The exception was the reaction of
Pd11 (02CAr)Tol(tmeda) with (ArCO2)2, which proceeded in a 2:1 ratio of reactants to
form 4,4'-bitoly1 and Pd11 (02CAr)2(tmeda). Observed HQ° and Pd(IV)
intermediates and products of the reactions were also prepared by independent
syntheses. Attempts to prepare Pdiv(02CAr)2MeR(bpy) were unsuccessful.
The use of palladium and platinum complexes incorporating an intramolecularly
coordinating `NCN-pincer' ligand, [C6H3(CH2NMe2)2-2,6] -, as a potential means of
preparing stable models for Pd(IV) intermediates has been investigated. No reaction
was observed between Pdil(02CPh)(NCN) and (PhCO2)2, while the reaction with the
platinum analogue resulted in formation and isolation of the stable complex
mer-Ptiv(02CPh)3(NCN). The reactions of MI1 (02CPh)(NCN) (M = Pd, Pt) with
iodomethane resulted in formation of M 11 I(NCN) and Me-02CPh in a 1:1 ratio. Low.
temperature 1H NMR spectroscopic studies revealed the intermediacy of a Pt(IV)
species, cis-PtIv(02CPh)2Me(NCN), but Pd(IV) intermediates were not observed.
The complex cis-Ptiv(02CPh)2Me(NCN) has been isolated from the reaction of
Pt11 (02CPh)(NCN) with excess Ag[02CPh] and iodomethane, and was found to
decompose via first-order kinetics to form Pt11 (02CPh)(NCN) and Me-02CPh.
The reaction of PtllTol(NCN) with (PhCO2)2 led to the formation of
cis-Ptiv(02CPh)2Tol(NCN). This species has been isolated from the reaction of
PtI2Tol(NCN) with Ag[02CPh] and found to be resistant to decomposition. Studies
of the analogous palladium chemistry were not feasible due to the instability of
PdliTol(NCN).
Carbon-selenium bond-forming reactions at palladium were studied as potential
models for carbon-oxygen coupling. The complexes PdilMeAr(bpy) [Ar = Tol,
4-anisyl (Anis)] were found to react with bis(4-chlorophenyl) diselenide
[(C1C6H4Se)2] to form trans-Pdw(SeC6H4C1)2MeAr(bPY), in a rare example of
reversible oxidative addition. The Pd(IV) intermediate decomposed by both carboncarbon
and methyl-selenium coupling, with the absence of aryl-selenium coupling.
Reactions of Pd11MeR(tmeda) (R = Me, Tol) with (C1C6H4Se)2 led to the formation
of products arising from both carbon-carbon and carbon-selenium coupling.
Palladium(IV) species are postulated as intermediates but have not been
observed. Reactions of PdIlMeR(dmpe) [R = Me, Tol; dmpe =
1,2-bis(dimethylphosphino)ethane] with (C1C6H4Se)2 resulted in carbon=selenium
coupling only. The reaction of PdIlMeTol(dmpe) with one equivalent of (C1C6H4Se)2
gave Me-SeC6H4C1 and Pd i1(SeC6H4C1)Tol(dmpe) as the major products. However,
addition of a second equivalent of (C1C6H4Se)2 resulted in aryl-selenium coupling,
from the reaction of Pd il(SeC6H4C1)Tol(dmpe) with (C1C6H4Se)2.
X-ray structural studies are reported for Pd(02CP1)2(1)PY), Pd(02CAr)2(tmeda)
(Ar = Ph, ArF), Pd(02CPh)Tol(bpy), M(02CPh)(NCN) (M = Pd, Pt),
Pt(02CPh)3(NCN), Pt(02CPh)2R(NCN) (R = Tol, Me) and Pd(SeC6H4C1)Me(dmpe).
Observations of the reactivity of M(II) and M(IV) (M = Pd; Pt) complexes have
led to a better understanding of the possible role of Pd(IV) in carbon-oxygen bond
forming processes in reactions catalysed by palladium complexes.

Item Type: Thesis (PhD)
Keywords: Palladium catalysts, Organopalladium compounds
Copyright Holders: The Author
Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2003. Includes bibliographical references

Date Deposited: 09 Dec 2014 00:08
Last Modified: 11 Mar 2016 05:54
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