  We study the quantum phase transition from an insulator to a metal realized
at t'=t'_c > 0.5t in the ground state of the half-filled Hubbard chain with
both nearest-neighbor (t) and next-nearest-neighbor (t') hopping. The study is
carried out using the bosonization approach and density matrix renormalization
group calculations. An effective low-energy Hamiltonian that describes the
insulator-metal transition is derived. We find that the gross features of the
phase diagram are well-described by the standard theory of
commensurate-incommensurate transitions in a wide range of parameters. We also
obtain an analytical expression for the insulator-metal transition line
t'_c(U,t). We argue that close to the insulator-metal transition line, a
crossover to a regime corresponding to an infinite-order transition takes
place.We present results of density-matrix-renormalization-group calculations
of spin and charge distribution in various sectors of the phase diagram. The
numerical results support the picture derived from the effective theory and
give evidence for the complete separation of the transitions involving spin and
charge degrees of freedom.
