Hierarchical RNNs are used for various NLP tasks such as machine translation (Luong and Manning, 2016), document modeling Lin et al., 2015), dialog generation (Serban et al., 2017), image captioning (Krause et al., 2016), and video captioning (Yu et al., 2016).
 and Luong and Manning (2016) use hierarchical encoder-decoder models, but not for the purpose of learning syntactic structures of target sentences.