cs9b05328_si_001.pdf (1.09 MB)
Download fileRoles of Base in the Pd-Catalyzed Annulative Chlorophenylene Dimerization
journal contribution
posted on 17.02.2020, 16:14 authored by Li-Ping Xu, Brandon E. Haines, Manjaly J. Ajitha, Kei Murakami, Kenichiro Itami, Djamaladdin G. MusaevThe
detailed mechanism of the Pd-catalyzed annulative chlorophenylene
dimerization (ACD) has been elucidated and the roles of the base have
been identified. It is shown that the initial steps of this reactionthe
active catalyst formation and the C–Cl bond activationproceed
via the “base-assisted oxidative addition” mechanism
and require only a 16.2 kcal/mol barrier. The following steps of the
reaction are palladacycle and Pd–aryne formations, among which
the former is favorable: it proceeds with a moderate C–H activation
barrier and is slightly exergonic. Although the Pd–aryne formation
requires a slightly lower energy barrier, it is highly endergonic.
It is shown that the base plays important roles also in the palladacycle
formation and facilitates the driving of the reaction forward by removing
a proton to the solution via the bicarbonate-to-carbonate exchange
mechanism. Addition of the second C–Cl bond to the thermodynamically
favorable palladacycle, i.e., Pd(II)/Pd(IV) oxidation, is a rate-limiting
step of the entire Pd-catalyzed and Cs-carbonate-mediated ACD reactions:
it occurs with a 35.8 kcal/mol energy barrier and is exergonic by
25.1 kcal/mol. The following polycyclic aromatic hydrocarbon (PAH)
formation is a multistep process and requires a lesser energy barrier.
An alternative pathway, namely, cyclooctatetraene (COT) formation,
requires a higher energy barrier and is not feasible. This finding
is consistent with experiments that show no COT product in the utilized
conditions. The calculations also indicate that the observed diminishing
of the yield of the Pd-catalyzed ACD reaction upon the use of Na2CO3 instead of Cs2CO3 is
the result of not only the poor solubility of Na-carbonate in the
used experimental conditions but also a prohibitively large free-energy
barrier required for the second C–Cl activation.