(507e) Operando Raman of the Nitroldol Reaction over Pore-Expanded Fdu-12 – Dendron Hybrids

Many labs have used the Nitroaldol reaction to probe the accessibility and reactivity of supported organic ligands.¹ Our lab has reported prior work on simple ligands and branched melamine-dendrons as catalysts for this reaction. The current work describes our effort on using pore-expanded FDU-12² as a novel support for growing melamine dendrons and their use as catalysts for this reaction. Kinetics measurements were obtained using a suite of dendrons with different diamine spacers and peripheral groups to determine how the diamine identity and density influences reactivity. To understand the chemistry at a more basic level operando Raman spectroscopy was used to monitor the various intermediates formed.

A library of samples of first through third generation dendrons supported on pore expanded FDU-12 were synthesized, characterized, and tested for the Nitroaldol reaction. Consistent with prior work the conversion/rate decreases with increasing dendron generation for all samples. We ascribe this finding to mass transfer limitations. We observe that diamines with secondary amines give higher conversions than primary amines. In the case of the G1 samples these samples all have within +/-5% the same number of functional groups.

On a per site basis the dendrons with bulky diamines such as trimethylenedi(piperidien) have a rate a factor 2-3 times higher than the dendrons made with 4-aminomethylpiperidine. Our work shows the effect of primary versus secondary amines, and the effect of the free volume on reactivity, wherein high free volumes are advantageous for reactivity. In all cases we see high (>80%) selectivity to the alcohol product except for the amino-silica.

We have also performed operando Raman³ measurements to monitor the reaction. A band at 1715 cm^-1 (C=O) can be seen to systematically decrease with conversion, and the appearance of the C=C double bond of the nitroalkene at 1650 cm^-1 is clearly observed. Finally a feature at 1350 cm^-1 assigned to C=N intermediates is observed to increase during reaction, illustrating the utility of Raman as a tool to probe this reaction.

In summary our results show that 1) while we can synthesize more uniform dendrons on pore expanded FDU-12 than SBA-15 the windows between the cages impose severe transport restrictions which are clearly discernible in our results, and 2) operando Raman can be used to easily monitor the formation of the alkene product (C=C bond) as well as to observe the imine (C=N) intermediate. The results show these hybrid materials are interesting catalytic materials for this reaction and that they are amenable to analysis using operando methodologies which have not been widely used prior in this sub-community.

References

1. Bass, J. D., Solovyov, A., Pascall, A. J., and Katz, A. J. Am. Chem. Soc. 128 373 (2006).

2. Fan, J, Yu, C., Gao, F., Lei, J., Tian, B., Wang, L., Luo, Q., Tu, B., Zhou, W. and Zhao, D. Angew. Chem. Int. Ed. 42 3146 (2003).

3. Grabow, K., and Bentrop, U. ACS Catal. 4 2153 (2014).