Please do not adjust margins
ChemComm
Page 4 of 5
ARTICLE
Journal Name
metal center, while the hydrogen bonding between phosphine
oxides and benzyl alcohol is persistent. Specifically, addition of
tBuOK (2 equiv. to Ru) and BnOH (3.5 equiv.) followed by heating
at 90 °C for 2 h, new signals were detected (δ 31P: 37.4, 35.4 ppm;
Figure 1d). This suggests that hydrogen bonding is sustained
during the reaction and presumably plays promoting and
discriminating role for alcohol substrates in cross-coupling
dehydrogenations.
DOI: 10.1039/C8CC10315A
3 M. A. Ogliaruso and J. F. Wolfe, Synthesis of Carboxylic Acids, Esters and
Their Derivatives; John Wiley & Sons, Hoboken, New Jersey, 1991.
4 (a) M. Rueping, H. Sunden, L. Hubener and E. Sugiono, Chem. Commun.,
2012, 48, 2201-2203; (b) T. Tana, X.-W. Guo, Q. Xiao, Y. Huang, S.
Sarina, P. Christopher, J. Jia, H. Wu and H. Zhu, Chem. Commun., 2016,
52, 11567-11570.
5 L. Wu, Q. Liu, I. Fleischer, R. Jackstell and M. Beller, Nat. Commun.,
2014, 5, 3091.
6 S. Werkmeister, J. Neumann, K. Junge and M. Beller, Chem. Eur. J.,
2015, 21, 12226-12250.
7 (a) K.-i. Fujita, N. Tanino and R. Yamaguchi, Org. Lett., 2007, 9, 109-111;
(b) R. Kawahara, K.-i. Fujita and R. Yamaguchi, Angew. Chem. Int. Ed.,
2012, 51, 12790-12794; (c) S. Musa, I. Shaposhnikov, S. Cohen and D.
Gelman, Angew. Chem. Int. Ed., 2011, 50, 3533-3537; (d) S. Amanda
and M. Robert, Organometallics 2011, 30, 6044-6048; (e) M. Nielsen,
H. Junge, A. Kammer and M. Beller Angew. Chem. Int. Ed., 2012, 51,
5711-5713.
8 C. Guan, D.-D. Zhang, Y. Pan, M. Iguchi, M. J. Ajitha, J. Hu, H. Li, C. Yao,
M.-H. Huang, S. Min, J. Min, Y. Himeda, H. Kawanami and K.-W. Huang,
Inorg. Chem., 2017, 56, 438-445.
9 (a) D. H. Nguyen, X. Trivelli, F. Capet, J.-F. Paul, F. Dumeignil and R. M.
Gauvin, ACS Catal., 2017, 7, 2022-2032; (b) Z.-H. Shao, Y.-J. Wang, Y.-
Q. Liu, Q. Wang, X.-L. Fu and Q. Liu, Org. Chem. Front., 2018, 5, 1248-
1256.
10 (a) J. Cheng, M. Zhu, C. Wang, J. Li, X. Jiang, Y. Wei, W. Tang, D. Xue, J.
Xiao, Chem. Sci., 2016, 7, 4428-4434; (b) E. A. Bielinski, M. Förster, Y.
Zhang, W. H. Bernskoetter, N. Hazari, M. C. Holthausen, ACS Catal.,
2015, 5, 2404-2415; (c) D. Srimani, E. Balaraman, B. Gnanaprakasam,
Y. Ben-David and D. Milstein, Adv. Synth. Catal., 2012, 354, 2403-2406.
11 (a) T. Steiner, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2000,
56, 1033; (b) R. C. Castells, L. M. Romero and A. M. Nardillo, J.
Chromatogr. A, 2000, 898, 103.
1
During H NMR studies, significant amounts of Ru-H hydride
species were detected (δ 1H: -13.38 ppm; insert in Figure 1d). This
signal corresponds to the formation of active hydride species (eg.
Ru[L4]H(OBn)) responsible for the slow generation of
benzaldehyde via β-hydride elimination.
Scheme 3. Proposed reaction pathway.
H
Cl
[RuH2]
R1CH2OH, base
O
L
(
)Ru
L
(
)Ru
R1
R2 OH
OH
H
R1
O
Cl
activation
O
[Ru]
- H2
other esters
+
R1
O
R2
R1
O
R2
Consistently, trace amounts of benzaldehyde were detected by
GC-MS and observed during monitoring the reaction by in situ 1H
NMR (10.50 ppm). We further carried out controlled experiments
showing considerable reactivity for ester product formation by
using aldehydes and alcohols as starting materials. Hence, we
propose that dehydrogenation of alcohols to aldehydes via
Ru[L4]H(OBn) is a slow step while the formation of ester products
is through further dehydrogenation of hemiacetal intermediates
(Scheme 3).
12 Milstein et al reported the in situ formed aldehyde trapped by the
catalyst through C-C coupling with the PNP pincer ligand, see: M.
Montag, J. Zhang and D. Milstein, J. Am. Chem. Soc., 2012, 134, 10325-
10328.
Conclusions
13 For reaction details of the oxygen-transfer alkylation procedure, see
the Supporting Information.
In conclusion, we have discovered that phosphine-phosphine
oxide ligands can be synthesized via a novel intramolecular
oxygen-transfer alkylation procedure. Preliminary mechanistic
studies imply that the hydrogen bonding between phosphine
oxide moiety and alcohol substrates plays an important role for
the challenging selective cross-coupling dehydrogenations.
Further use of phosphine oxide in ligand design for other types of
selective transformations is currently pursued.
14 In a related report, N-alkylation of PN(H)P ligands makes catalytic
hydrogenations more effective, see: L. Zhang, Z. Han, X. Zhao, Z. Wang
and K. Ding, Angew. Chem. Int. Ed., 2015, 54, 6186-6189.
15 Gas phase pKa (intrinsic acidity) values for 1-butanol, 1-pentanol and
1-hexanol are 368.7, 367.3 and 366.5, respectively (from iBond
consistent with the trend of alcohol acidity.
16 Efficiently differentiating various alcohol molecules (both the
substrate and reaction intermediate) is believed to be important for
the desired cross-coupling dehydrogenations.
17 (a) C. Liu, J. Wang, L.-K. Meng, Y. Deng, Y. Li and A.-W. Lei, Angew.
Chem. Int. Ed., 2011, 50, 5144-5148; (b) (b) T. Song, J. E. Park and Y. K.
Chung, J. Org. Chem., 2018, 83, 4197-4203.
Conflicts of interest
There are no conflicts to declare.
18 For chemical shift changes of phosphine oxides caused by alcohols,
see: Y. E. Türkmen, Turk. J. Chem., 2018, 42, 1398-1407.
Acknowledgements
We are grateful for the financial supports from the Program
for Innovative Research Team of the Ministry of Education, the
Program for Liaoning Innovative Research Team in University,
and NSFC (21633013, 21101109, 21602228).
Notes and references
1 J. Otera, Esterification: Methods, Reactions, and Applications, Wiley-
VCH, Weinheim, 2003.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins