The reaction would be slower than that with p-dimethoxybenzene itself (due to the deactivating effect of the halide) but it would be fater than with benzene itself (due to the activating effects of the methoxy groups).
Rhodium, I don’t agree with that. According to A guidebook to mechanism in organic chemistry. (Peter Sykes, 6th edition): “…the influence of electronic affects alone [on the reactivity of benzaldehides toward the nucleophiles] may be seen in the series of compounds ( p-X-C6H4-CHO ).
Relative rates: X= NO2 > H > MeO.”
Then it goes on explaining how electronwithdrawing groups on the bezaldehide increase the electrophilicity of the carbonyl group, while the electrondonating groups (R-O- etc.) reduce it. Therefore a halogen para to the aldehido group increase its reactivity toward nucleophiles while the methoxies reduce it.
As an example, here are the cianohydrine formation equilibrium constants (K) for the three above-mentioned benzaldehides:
p-NO2-C6H4-CHO K=1420
C6H5-CHO K=210
p-MeO-C6H4-CHO K=32
Well, luckily the Darzen rxn is not as reversible as cianohydrine rxn.
Psyx, god luck with your synt.