  This is the first of a series of three papers on clustered star formation in
the Rosette Molecular Complex. Here we investigate star formation in the
interfacing layers between the expanding Rosette Nebula and its surrounding
cloud, based on an analysis of the spatially complete and unbiased 2MASS data.
Two medium-mass infrared clusters with ages of around 1 Myr are identified in
the south and south-east arcs of the fragmented shell. The majority of the
candidate cluster members in these radiation and pressure-confined regions are
found to be almost uniformly distributed, roughly following the compression
layers traced by the distribution of optical depth at 100 $\mu$m, and may well
develop into gravitationally unbound systems upon their emergence from the
parental cloud. These expanding shells are believed to be playing important
roles in impeding the emerging young open cluster NGC 2244 from intruding
immediately and deeply into the ambient molecular cloud, where sequential
formation of massive clusters is taking place.
