  A measurement is reported of differential top quark pair
($\mathrm{t\bar{t}}$) production cross sections, where top quarks are produced
at large transverse momenta. The data collected with the CMS detector at the
LHC are from pp collisions at a center-of-mass energy of 13 TeV corresponding
to an integrated luminosity of 35.9 fb$^{-1}$. The measurement uses events
where at least one top quark decays as t $\to$ Wb $\to$ qq'b and is
reconstructed as a large-radius jet with transverse momentum in excess of 400
GeV. The second top quark is required to decay either in a similar way, or
leptonically, as inferred from a reconstructed electron or muon, a bottom quark
jet, and a missing transverse momentum due to the undetected neutrino. The
cross section is extracted as a function of kinematic variables of individual
top quarks or of the $\mathrm{t\bar{t}}$ system. The results are presented at
the particle level, within a region of phase space close to that of the
experimental acceptance, and at the parton level, and are compared to various
theoretical models. In both decay channels the observed absolute cross sections
are significantly lower than the predictions from theory, while the normalized
differential measurements are well described.
