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Organic & Biomolecular Chemistry
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COMMUNICATION
Journal Name
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Dion, J. Chem. Soc., Chem. Comm., 198D4O, 1I:2160.010. 39/C9OB01241A
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DI, and subsequent consumption of DI in the reduction of
intermediates 3 and 4 (Scheme 4), the catalytic role of I2 for the
transfer hydrogenation of PCP with water is supported (see ESI
p.S31-S38 for full analysis).
On the basis of the above results and the mechanistic
understandings from the group 9 metalloporphyrin catalysed
transfer hydrogenation of PCP with water,7-9,23 Scheme 3
illustrates the proposed mechanism for the I2 catalysed transfer
hydrogenation of PCP with water (D2O is used for clarity).24
Initially, disproportionation of I2 with D2O produces an
equilibrium mixture with DI, DOI and DIO3 (Scheme 4a).18 The
DIO3 generated undergoes consecutive decomposition to give
O2 and regenerates I2.25 These species do not react with C6D6 at
6
7
8
9
C. T. To, C. M. Tam and K. S. Chan, ACS Catal., 2015, 5, 4333.
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Luo, CRC Press: Boca Raton, FL, 2007
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o
200 C as I2 could be quantitatively recovered (Table 2, eq 10,
entry 3). Concurrently, homolysis of the weak I-I bond (36.4
kcal/mol)14 produces iodine radicals with an estimated forward
rate of 1.8 x 10-5 s-1 and equilibrium constant of 2.4 x 10-5 M at
200 oC.26 The iodine radical then cleaves the benzylic C-C bond
in PCP to yield 4,4’-diiodomethylbibenzyl 3 through homolytic
radical substitution in a radical chain process (Scheme 4b). The
DI generated undergoes hydrodeiodination with 3 where the
benzylic C-I bonds in 3 are consecutively reduced to give the
final product 1 via intermediate 4 (Scheme 4c). The reduction of
Bn-I with HI to give Bn-H and I2 is estimated to be exergonic by
9.9 kcal/mol.14,19b Alternatively, intermediate 3 is hydrolyzed
with water to generate DI and a benzylic alcohol intermediate,
which is subsequently converted to the observed aldehyde
product 5 as a minor parallel process (Scheme 4d). As the
reaction proceeds, I2 starts to regenerate and build up again in
the reaction mixture and mediates the dehydrogenation of 1 to
give the observed (E)-4,4’-dimethylstilbene 2. Concurrently, 3
undergoes disproportionation to yield 1 and 2 without the
incorporation of deuterium at the terminal benzylic positions.
In summary, we have successfully made use of the isolobal
concept to realize the I2 catalysed transfer hydrogenation of
PCP with H2O, which combines the homolytic radical
substitution of carbon-carbon σ-bond with iodine radical and
the reaction of I2 with H2O to generate HI as a facile reductant.
Conflicts of interest
There are no conflicts to declare.
19 (a) R. A. Ogg, J. Am. Chem. Soc., 1934, 56, 526. (b) W. S.
Graham, R. J. Nichol A. R. Ubbelohde, J. Chem. Soc., 1955, 115.
(c) N. C. Deno, N. Friedman, J. D. Hodeg, F. P. MacKay and G.
Saines, J. Am. Chem. Soc., 1962, 84, 4713.
Notes and references
‡
We thank the Research Grant Council of Hong Kong SAR
(14301717) and the Hong Kong PhD Fellowship (WY) for
financial support. We are thankful for the help in the
computation of molecular orbitals from Prof. Zhen Shen
group at Nanjing University.
(a) A. Sen, Acc. Chem. Res., 1998, 31, 550. (b) W. D. Jones,
Nature, 1993, 364, 676. (c) B. Rybtchinski and D. Milstein,
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Org. Chem., 2012, 2683. (e) L. Souillart and N. Cramer, Chem.
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20 (a) M. Dobmeier, J. M. Herrmann, D. Lenoir and B. Kőnig,
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Karthikeyan and G. Sekar, RSC Adv., 2015, 5, 58790.
21 P. S. Nangia and S. W. Benson, J. Am. Chem. Soc., 1964, 86,
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22 The preferential internal benzylic H/D exchange was
attributed to the weaker internal benzylic C-H bond.
23 C. M. Tam, C. T. To and K. S. Chan, Organometallics, 2016, 35,
2174.
1
24 Alternative mechanistic possibilities such as I2 as radical trap
or single electron transfer prior to C-C cleavage are unlikely.
See ESI for details.
2
(a) J. A. Evans, G. F. Everitt, W. D. W. Kemmitt and D. R. Russell,
J. Chem. Soc., Chem. Comm., 1973, 158. (b) C. F. H. Tipper, J.
Chem. Soc., 1955, 2045.
4 | J. Name., 2012, 00, 1-3
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