products-with-no-categories

{% set excludedIds = [] %}
{% set allProducts = craft.entries.section('products') %}
{% for product in allProducts %}
    {% if product.productCategories|length %}
        {% set excludedIds = excludedIds|merge(['not '~product.id]) %}
    {% endif %}
{% endfor %}

{% set products = craft.entries.section('products').id(excludedIds).limit(null) %}