Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes Page: 19,144
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Journal of the American Chemical Society
Scheme 13. Comparison of calculated Gibbs free energies (kcal/mol, THF) for ethylene insertion and ethylene C-H activation
by [(bpy)Pt(Ph) (q2-C2H4)]+ (7')Reductive1
EliminationPt /
-NFVinyl C-H Activation
K /\
Pt;
-NOlefin Inse(8')rtion
Olefin InsertionScheme 14. Comparison of the Calculated AGt (kcal/mol)
for Ethylene Insertion vs Ethylene C-H Activation between
Several Reported Ethylene Hydroarylation Catalysts, with the
AAG* between the Two Transformations Highlighted[M]
[(tbpy)Pt]+
TpRu(CO)
TpRu{P(OCH3)CEt}
TpRu(PMe3)
Ethylene]
InsertionJ
AAG
16.3
8.6
74 [M]
3.1[M]
C-H activation to produce [(tbpy)Pt(ethylbenzene)(Ph)]+ (A
in Scheme 15). Dissociation of ethylbenzene from A completes
the catalytic cycle for ethylene hydrophenylation. However,
aromatic C-H activation of ethylbenzene generates [(tbpy)Pt-
(C6H4Et)(C6H6)]+ (B, 3 isomers are possible due to regioselec-
tivity of ethylbenzene C-H activation). The conversion of A toB requires the conversion of alkyl-coordinated ethylbenzene to
an arene-coordinated system. Such a rearrangement of C-H
coordinated alkyl arene to a tr-arene complex has been observed
for late transition metals.86 From complex B, coordination and
insertion of a second equivalent of ethylene with subsequent
benzene C-H activation leads to diethylbenzene. At 100 OC and
0.1 MPa of ethylene, the ratio of ethylbenzene to diethylbenzenes
ranges from 4.4 to 4.8, which indicates that the rate of ethylben-
zene dissociation is only 4.5 times faster than diethylbenzene
formation. Hence, we suggest that relatively slow dissociation of
ethylbenzene, regardless of whether this occurs by a net associa-
tive or dissociative process, allows opportunity for a second
C-H activation and, hence, formation of diethylbenzene. It is
also feasible that diethylbenzene formation begins with an
initial cyclometalation of the phenethyl ligand, as reported by
Goldberg et al. for a related Pt system. However, in this
pathway (Scheme 16) the formation of 1,3-diethylbenzene and
1,4-diethylbenzene would likely require that the Pt(II) complex
coordinate ethylbenzene with sufficient lifetime to allow a second
C-H activation (and ethylene insertion) to compete with net
dissociation of ethylbenzene; hence, complex A is still implicated
as an intermediate.
Catalysis using a 1:1 ratio (v:v) of C6H6 and C6D6 with C2H4
was anticipated to produce ethylbenzene-dn with n = 0, 1, 5 and 6,
which would arise from ethylene insertion into either Pt-Ph or
Pt-Ph-ds followed by activation of either C6H6 or C6D6
(Scheme 17), as observed for the same experiment using TpRu-
(CO) (NCMe)Ph as the catalyst.18 However, all isotopologues of
ethylbenzene-dn with n = 0-6 (m/z = 106-112) were detected
by GC/MS using complex 2. The observation of substantial
H/D exchange is consistent with our proposed mechanism for
diethylbenzene formation in which relatively slow net dissocia-
tion of ethylbenzene from Pt(II) (regardless of the mechanism ofdx.doi.org/10.1021/ja206064v IJ. Am. Chem. Soc. 2011, 133, 19131-19152
-1
Ethylene C-H
Activation
[M] --H19144
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McKeown, Bradley A.; Gonzalez, Hector Emanuel; Friedfeld, Max R.; Gunnoe, T. Brent; Cundari, Thomas R., 1964- & Sabat, Michal. Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes, article, November 8, 2011; [Washington, DC]. (https://digital.library.unt.edu/ark:/67531/metadc107788/m1/14/: accessed May 6, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.