reaction was half order in  (fig. S51). Further
NMR scale reactions performed in protio-benzene
with a minimum amount of C6D6 as an internal
locking source demonstrated that this reactivity
could be extended to the synthesis of the non-deuterated alkylbenzenes, which were formed
through the activation of a single C(sp2)–H bond
and the generation of compound 6 (figs. S52
To provide further insight into the nature of
these processes, the reaction of compound 9 with
benzene was assessed with DFT (B3PW91) calculations (Fig. 5). Consistent with the half-order
dependence of the reaction on , the dimeric
calcium alkyl must first dissociate to a monomeric
form (G). As implied by the reaction temperature
(60°C), this process is substantially endothermic
(DH = +23.2 kcal mol−1). The resultant coordinatively unsaturated calcium center interacts via an
h6 contact with the aromatic electron density of
a molecule of benzene (H). The subsequent barrier toward the nucleophilic attack of the n-hexyl
a-methylene carbon on a benzene C(sp2)-H bond
via TSHI is negligible. At this transition state, the
n-hexyl group acts as a charge-separated external
nucleophile and attacks the benzene molecule at
a C–H bond from the opposite face to that engaged
with the calcium center. This process enforces an
interaction between calcium and the hydrogen
bonded to the now four-coordinate carbon, so
that the negative charge (−0.9) is relocalized on
the remaining five carbon atoms of the benzene
ring, in a manner analogous to a nonstabilized
Meisenheimer complex. Whereas the maximum
negative charges are located at the ortho, ortho′,
and para positions, a positive charge is found on
the newly four-coordinate carbon (+1.2), and the
reactive hydrogen accumulates a charge (−0.3)
that is consistent with incipient hydridic character before the C–H bond–breaking process.
The overall reaction, therefore, is not a classical
s-bond metathesis in which both Ca–C bond–
breaking and Ca–H bond formation ensue simultaneously, but is best described as an effective
nucleophilic (SN2) displacement of hydride from
the benzene C–H bond (40). The breaking of the
benzene C–H bond results in the generation of a
further p complex of the as-formed calcium hydride and n-hexylbenzene (I), while arene dissociation and dimerization of the monomeric
hydride ensures the overall exothermicity of
the reaction (DH = −30.1 kcal mol−1). The reaction is thus heavily dependent on a sequence of
monomer-dimer equilibria of both the initial calcium n-hexyl and ultimate calcium hydride species. In support of this latter hypothesis, further
examination of the in situ 1H NMR spectra recorded during the experimental monitoring of
the reactions between compounds 8 and 9 and
C6D6 revealed an additional upfield triplet resonance at ~d −1.1 ppm, which persisted in low but
steady-state concentrations until the complete
consumption of the calcium n-alkyl derivatives
and which we ascribe to the presence of dimeric
hydrido(n-alkyl)dicalcium compounds analogous
to species C shown in fig. S55.
The reactions of compounds 7 through 9 with
benzene show that nucleophilic alkylation of benzene may be achieved through the use of sufficiently potent alkylcalcium nucleophiles. This
reactivity also achieves the net hydroarylation of
terminal alkenes. The simultaneous reformation
of the calcium hydride (6) therefore indicates
that this chemistry holds the potential for elaboration to catalysis.
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We thank the University of Bath and the Engineering and Physical
Sciences Research Council for funding of a Doctoral Training
Partnership Ph.D. studentship (A.S.S. W.). The crystallographic data
for compounds 6, 7, 8, 9, and 6-d have been deposited with
the Cambridge Crystallographic Data Centre as entries 1565865 to
1565869, respectively. All other experimental data are presented
in the supplementary materials. Data file S1 contains the Cartesian
coordinates for the calculated structures described in this study.
Data file S2 contains the Cartesian coordinates of the transition state
for the alternative s-bond metathesis pathway that was located
+29.7 kcal mol−1 higher in energy than the metathesis transition
state (TSHI) illustrated in Fig. 5.
Materials and Methods
Figs. S1 to S56
Data Files S1 and S2
6 August 2017; accepted 12 October 2017
Fig. 5. Computed (DFT, B3PW91) energy profile for the reaction between compound 9