Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes Page: 19,146
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Journal of the American Chemical Society
Scheme 18. Proposed Mechanism for H/D Scrambling that
Occurs during the Thermolysis of [(tbpy)Pt(CH2CH2Ph)-
(?]2-C2H4)] [BAr'4 ] in C6D6
tBu 7+ tBu +
PtC /Pt
tB tB d6
tBu dn + tBu dn\
I t-dEt-d
N'\H/D Exchange N /
Pt Pt
N N
tB tB d5-n
d6-n
Et-di
dn
isotopologues of ethylbenzene is similar to that observed for
the thermolysis of 4 in C6D6 with H/D exchange occurring
predominantly into the phenyl ring of ethylbenzene. Similar to
4 and C6D6, the addition of the Lewis base increases the
prevalence of the higher mass ions (m/z > 108) compared to
the reaction without its addition.
Benzene coordination and C-H activation by [(bpy)Pt-
(CH2CH2Ph)]+ forms [(bpy)Pt(Ph)(y/2-C,C-C6HsEt)]+ (A').
Simulations were used to assess the potential for A' to (a)
undergo dissociation of ethylbenzene, (b) undergo a second
aromatic C-H activation event and (c) undergo sp3 C-H
activation. The last process (i.e., c), which forms [(bpy)Pt-
(CH CH2Ph)(y2-C,C-C6H6)]+, has a high calculated barrier
(AG = 27.5 kcal/mol), consistent with the absence of extensive
H/D scrambling into the ethyl fragment. The free energy
for dissociation of ethylbenzene from A' is calculated to be
12.5 kcal/mol (Scheme 19; these calculations are gas phase and
thus the free energy of ethylbenzene dissociation is different from
that in Scheme 8), while arene C-H activation barriers are
calculated to be 16.2 kcal/mol (para C-H activation), 16.7 kcal/
mol (meta C-H activation) and 18.7 (ortho C-H activation).
Thus, the calculations suggest that the energetics ofethylbenzene
dissociation and a second C-H activation are of similar magni-
tude. As a benchmark, the energetics of ethylbenzene dissocia-
tion were compared with that for the charge-neutral complex
(dmpp)Pt(Ph) (2-C,C-ethylbenzene) [dmpp =3,5-dimethyl-
2-(2-pyridyl)pyrrolide], which exhibits less extensive formation
of diethylbenzenes during catalytic hydrophenylation of ethy-
lene. The barrier to ethylbenzene dissociation from (dmpp)Pt-
(Ph)(y2-C,C-ethylbenzene) was calculated to be only 0.6 kcal/
mol, and ethylbenzene dissociation from TpRu(CO)(ethyl-
benzene)(Ph) is calculated to be exergonic by 8 kcal/mol(Scheme 19).
PhCH2CH2D
2.5 2.3 2.1 1.9 1.7 1.5 1.3 1.1 0.9 0.7 0.5
Figure 15. 2H NMR spectrum for the product of thermolysis of 4 in
C6D6, highlighting the resonance (6 = 1.03 ppm) of the ethylbenzene
isotopologues possessing an ethyl fragment with a terminal -CH2D
group. Incorporation of deuterium into the benzylic position is not
observed.
Scheme 19. Comparison of the Calculated AG (kcal/mol;
gas phase) for Ethylbenzene Dissociation from Several
Reported Ethylene Hydroarylation CatalystsEt
[M] +
12.5
0.6
-8.0
[M]
Et[M]
[(tbpy)Pt]+
(dmpp)Pt
TpRu(CO)A striking observation is the regioselectivity for diethylben-
zene formation as a function of aromatic substrate. For example,
the ortho:meta:para (o:m:p) selectivity for the production of
diethylbenzenes from 1 with ethylene and benzene is 1:2.6:1.6
after 4 h (Table 1). In contrast, for the reaction of ethylene with
ethylbenzene under identical conditions, the o:m:p selectivity is
1.0:34.8:24.0 (eq 7). Notably, the m/p selectivity ratio is
approximately the same as the reaction with benzene while the
ratio of o to m/p changes dramatically. The different selectivity
suggests distinct pathways for diethylbenzene formation from
C2H4/C6H6 and C2H4/ethylbenzene reactions (Scheme 20).
For the reaction of ethylene and benzene, the formation of
diethylbenzene is proposed to originate via benzene C-H
activation to give A, followed by sp2 C-H activation of the
coordinated ethylbenzene (Scheme 15). In this scenario,
Pt(II) migrates from the methyl group of coordinated ethylben-
zene (C in Scheme 20) to an yr2-C,C intermediate involving
the ortho carbon (D in Scheme 20). If the rate of aromatic
C-H activation is competitive with migration around the phenyl
ring, it is expected that ortho C-H activation, ultimately
resulting in 1,2-diethylbenzene, will be competitive. For thedx.doi.org/10.1021/ja206064v 1. Am. Chem. Soc. 2011, 133, 19131-19152
19146
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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/m1/16/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.