3. Table S1. Commentary on the Stationary
points on the potential surfacea for the
overall mechanism, illustrated using the RR,SS
stereoisomer. |
Storyboard:
-
In this terminology, RR (of RR,SS) refers to the
configuration of the two chiral centres of the
previously ring-opened lactide unit whilst SS refers to the newly
co-ordinated lactide unit which is about to be ring-opened (see
Int1 window).
-
The lowest energy of the first lactide (RR) is
that of a 5-ring coordinated species: .
-
The reactant is solved by one molecule of THF
, but the solvent is then displaced by a
second (SS) lactide monomer. The coordination of the THF solvent is
closer to equatorial rather than axial.
- The geometry of the resulting intermediate (Int1)
is mediated by a weak O...C=O electrostatic
interaction (3.01Å).
- The first transition state involves O...C=O bond
formation in which the weak O...C=O electrostatic
interaction is strengthened (2.88Å).
- The geometry of this transition state is
controlled by the stereoelectronic antiperiplanar
orientation of the incoming O...C bond and the C-C
bond
- The original alkyl-oxygen coordination to Mg is
preserved in this transition state
- as it also is in the resulting tetrahedral
intermediate.
- The tetrahedral intermediate undergoes a
(rocker-switch like) reorganisation of the Mg-O
coordination, accompanied by a conformational change.
Thus the original alkyl-oxygen...Mg bond breaks
and a new alkyl-oxygen...Mg bond forms
- This change in conformation now sets up the final
C-O bond cleavage
- which after further Mg...O de-coordination,
and re-coordination by the terminal C=O
group
- results in the product with extruded polymer
chain
- The rate-determining transition state has an
imaginary normal mode which reveals both O-C cleavage and
concomitant C=O...Mg de-coordination with a computed wave number of 49
cm-1. This unusually low value for a
transition mode arises in part because of the highly
correlated nature of the vibration, in which most
atoms of the reaction centre participate, and hence
the relatively large mass-weighting of this
vibration.
- TS2 has Wiberg bond indices in the NAO
basisb; C-Oacyl 0.15 , Oacyl-Mg 0.09 , Ocarbonyl-Mg 0.03, Ocarbonyl-C 1.65) with considerable ionic nature (natural
charge on Mg +1.68) .
- The product re-coordinates a solvent THF molecule
in such a manner as to minimise repulsions
to the methyl group . This means in effect that the THF solvent
and the polymer chain have to be coordinated
di-equatorial to the Mg. This methyl group thus
avoids clashing with the isopropyl group . The extruding (and conformationally
flexible) polymer chain folds back across the same
face as the THF solvent. The energy reported here is
considered an upper bound, since not all the
conformational space of this chain has been explored.
Alternative modes of coordination such as having the
THF and the polymer chain effectively di-axial at the
Mg centre are significantly higher in energy .
-
The thermochemistry of the overall mechanism is
(respectively, as difference in total energy gas
phase, with IEFPCM solvation correction for
THF,b and for ΔG gas phase):
- (TS2,THF) - (RR-reactant+THF,lactide): 17.1,
16.5, 18.9 kcal mol-1.
- TS2 - Int1(RR-reactant+lactide): 18.3, 13.7,
20.2 kcal mol-1
- (RR-reactant,lactide) - (Product): -16.9,
-7.7c, -0.9,
- (RR-reactant+thf,lactide) - (Product+thf):
-11.4, -2.6, +2.3
|
Total energies (Hartree)/Relative
energies (kcal-1)
[Corrected for ΔG298
(Hartree)/relative ΔG298,
kcal-1], {IEFPCM solvation
model}b |
RR-Reactant |
SS-Lactide |
-2077.83601 [-2077.06000] {-2077.78925} |
-534.35128 [-534.24493] {-534.35364} |
Total: 2612.18729
[2611.30493] {2612.14289} |
|
|
RR-Reactant+THF |
THF |
-2310.29905 [-2309.40424] {-2310.25064} |
-232.44540 [-232.35690] {-232.44402} |
Total: 2612.18729
[2611.30493] {2612.14289} |
|
|
Int1 (=Reactant + lactide - thf) |
TS1 |
-2612.20735/0.0 [-2611.29448/0.0]
{-2612.155852} |
-2612.18938/11.3 [-2611.27550/11.9] |
|
|
TI1 |
TI2 |
-2612.18950 [-2611.27569] |
-2612.19235 [-2611.27650] |
|
|
TS2 |
Product |
-2612.17810/18.4 [-2611.26220/20.2]
{-2612.134079} |
-2612.21424 [-2611.30626/-7.4]
{-2612.155228} |
|
|
Product.thf |
Product.thf isomer |
-2844.66852 [-2843.645407] |
2844.65185 [-2843.63036] |
|
|
|
aThese calculations are all based on the B3LYP
density functional procedure, employing a 6-311G(3d) basis for
Mg, 6-31G(d) for the lactide and core ligand units, and STO-3G
for the 2,6-di-isopropyl aryl ligand substituents. The Gaussian
98 and 03 programs were used; a) Gaussian 03, Revision C.02,
Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven,
T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.;
Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.;
Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.;
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.;
Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J.
E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala,
P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J.
J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M.
C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox,
D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.;
Wong, M. W.; Gonzalez, C.; and Pople, J. A.; Gaussian, Inc.,
Wallingford CT, 2004. b Reed, A. E.; Curtiss, L. A.;
Weinhold, F., Chem. Rev., 1988, 88, 899 - 926.
b M. T. Cances, B. Mennucci, and J. Tomasi, J.
Chem. Phys., 1997, 107, 3032. c Errors in
convergence may render this value suspect.