Fig. 4 Orientation of the carbonyl groupin (a) B@A2 and (b)C@A2 from
molecular dynamics calculations (GROMACS, OPLS-AA forcefield).
has its origin not on the conformational control but on rota-
tional restriction of the reactive part of the molecule18,19 within
the nanoscopic reaction cavity of octa acid. In examples dis-
cussed here the lack of free space within a well-defined reaction
cavity reduced the rotational mobility, which is translated into
product selectivity.20 Unexpected observations made with the
two ketones capable of undergoing classic Norrish type I and II
reactions have brought out yet another facet of control on
molecular motions in confined spaces thus opening new oppor-
tunities for manipulating photoreactions in confined spaces.
VR is grateful to the National Science Foundation, USA,
for generous financial support (CHE-0848017). The authors
thank A. Barman for help with MD simulations.
Fig. 3 Partial 1H DQF COSY NMR (500 MHz) spectra of (i) C in
CDCl3, (ii) C@A2. ([C] = 2.5 mM and [A] = 5 mM in 50 mM borate
buffer in D2O). (* Peak is the residual water signal). Green arrow
shows the correlation between proton 4a and 3a. Dotted circle with an
arrow shows the absence of correlation between 4a and 3e.
Notes and references
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5 N. J. Turro, V. Ramamurthy and J. C. Scaiano, Modern Molecular
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ketone C with the same conformation both inside and outside
the capsule yielded different products.
We next turned our attention to explore the rotational
restriction of the axial phenylacyl group through 40 ns molecular
dynamics (MD) simulations in aqueous solution. These simula-
tions were performed using the OPLS-AA force field13,14 utilizing
the GROMACS program.15,16 The details of these simulations
are provided in ESI.w The root-mean-square-deviation (rmsd)
of the MD trajectories indicate that both B@A2 and C@A2
complexes are well equilibrated (Fig. S11 in ESIw), which justify
the time scale used for these simulations. The most representative
structures of these complexes derived from the MD simulations
are shown in Fig. 4. Initial structures used for MD simulations
are provided in Fig. S12 and S13 in ESI.w These structures reveal
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presence of the phenylacyl group in the axial position. The
carbonyl group in these structures is clearly turned away from
the g-hydrogens and the tight encapsulation most likely prevents
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state to face the g-hydrogens.17
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We initiated the investigation with the postulate that one
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structural and photochemical studies revealed that the control
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2841–2843 2843