ACS Catalysis
Research Article
and found that the energy of the aminoalkene adduct species is
CONCLUSIONS
■
71
4.3 kcal/mol higher than that of its precursor, in accord with
The entire range of rare-earth metal bisalkyl complexes
stabilized by the pyridine methylene fluorenyl ligand was,
respectively, used to catalyze the IAHA of 1-amino-2,2-
diphenyl-4-pentene and IEHA of styrene and pyrrolidine. As
observed for all the reported rare-earth metal-based catalytic
systems, the rate of the IAHA reaction increased almost
linearly with the metal ion radius. The reactivity pattern in the
IEHA reaction with respect to the ionic radius size followed
the opposite trend to that of the IAHA reaction. DFT
calculations suggest the prevailing mechanism to be a stepwise
σ-insertive pathway that involves the insertion of CC into
the Ln−N bond and protonolysis of the Ln−C bond,
regardless of intramolecular or intermolecular hydroamination.
The activation barrier of protonolysis is higher than that of the
insertion step, suggesting that the former reaction should be
the turnover-limiting step. The protonolysis step was facilitated
by an additional molecule of amine. Thus, the coordination of
the amine to the insertion product is one of the key factors that
control the hydroamination reaction. Two key aspects can be
derived: (1) either the formation of the amine adduct is
endothermic due to the crowded coordination sphere around
the metal center, increasing the transition-state energy of the
protonolysis step and a first-order dependence on the
concentration of amine, such as the catalytic system reported
by Sadow; (2) or the amine coordination is thermodynami-
cally preferred, resulting in lowering the transition-state energy
of the protonolysis step. In the latter, a zero-order dependence
on the amine is observed, such as IAHA catalyzed by yttrium
and praseodymium base complexes. Since the coordination of
aminoalkene to the scandium insertion product is nearly
thermoneutral, the population of the aminoalkene adduct is
rather low to limit the protonolysis reaction, resulting in the
lowest activity of scandium species toward IAHA. Moreover,
the enthalpy of protonolysis is another key factor that controls
the hydroamination reaction. For the IEHA, with the increase
of the metal radius, the aminolysis tends to be thermoneutral,
leading to the left shift of the equilibrium and therefore a
decrease of the catalytic activity following a trend of Sc > Y >
Pr. The above systematic investigation fundamentally resolved
the controversial issues toward hydroamination mediated by
rare-earth metal catalysts and will be helpful to rationally
design catalysts.
a first-order dependence on the concentration of aminoalkene
30
observed by Sadow.
i ΔG(III·A)y
i ΔG(III)y
j
z
j
z
z
n
/nIII = expj−
z/expj−
III·A
j
k
z
{
j
k
z
{
RT
RT
where nIII·A and n represent the populations of III·A and III,
respectively, ΔG
of III·A and III, respectively, R is the gas constant, and T is the
absolute temperature. T = 298.15 K is used here as the free
energy is calculated at this temperature, which is also
consistent with the experimental temperature (298 K).
III,
and ΔG denote the Gibbs free energies
III·A
III
For the IEHA (Figure 12), the noninsertive mechanism
involves N−C ring closure triggered by concomitant amino
proton delivery onto the adjacent CC linkage, thereby
affording a cyclamine product through a concerted single-step
transformation. This process is seen to be energetically
comparable to the σ-insertive pathway, but the order of the
barrier for the N−C ring closure (Sc: 21.0 kcal/mol > Y: 20.1
kcal/mol > Pr: 19.1 kcal/mol) is opposite to the experimental
result that the catalytic activity decreased with the increasing
metal size. This precluded the proton-assisted concerted N−
C/C−H bond-forming pathway. Thus, a detailed analysis was
carried out for the classic σ-insertive pathway (Figure 12,
right). The insertion starts from the coordination of styrene to
the metal center of II’, which is endothermic. The population
ratio of styrene adduct II’·S′ to its precursor species II’ is much
lower than 1, suggesting that the formation of the styrene
adduct is sensitive to the styrene concentration. This is
consistent with the experimental result that the reaction has a
first-order dependence on styrene concentration. The N−C
bond formation occurs through a four-center planar structure
30
TS’ involving a metal-mediated migratory insertion of the
1
styrene CC linkage into the Ln−N pyrroline σ bond.
Following the reaction, TS’1 decays to a metal alkyl
intermediate III’. The olefin insertion step is endothermic,
suggesting that the migratory olefin insertion is reversible. The
additional coordination of pyrrolidine to III’, generating III’·
A’, drives the thermodynamics of the reaction (exothermic
step) and is hardly affected by the pyrrolidine concentration
(
Table 4). Protonolytic cleavage of the metal carbon bond
occurs through a metathesis-type transition state TS’ . The
The authors declare no competing financial interest.
2
associated activation barriers (Sc: 16.4 kcal/mol, Y: 16.3 kcal/
mol, and Pr: 16.2 kcal/mol) are almost the same and higher
than those of their corresponding olefin insertion steps (Sc:
ASSOCIATED CONTENT
sı Supporting Information
■
*
1
2.1 kcal/mol, Y: 15.6 kcal/mol, and Pr: 9.1 kcal/mol),
suggesting that protonolysis should be the turnover-limiting
step. This result is consistent with that observed in IEHA
catalyzed by an alkaline-earth metal complex. It is worth
noting that the equilibrium constant K298 for this protonolysis
step is only 1.965 for the Pr-based catalyst calculated using the
following equation
1
Experimental section, H NMR spectra of intramolecular
72
details of DFT calculation (PDF)
Corresponding Authors
■
i ΔG≠ y
j
z
K = expj−
z
j
k
z
RT {
suggesting that it is difficult to generate the product. With
the decrease of the metal radius, the equilibrium constant
Bo Liu − State Key Laboratory of Polymer Physics and
Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China;
University of Science and Technology of China, Hefei
4
becomes larger (Y: 311.61 and Sc: 4.17 × 10 ), indicating that
the reaction degree was dramatically improved. Thus, the
IEHA is under thermodynamic control.
3
797
ACS Catal. 2021, 11, 3790−3800