How to create an IIS7 re-write rule for a single category

I would strongly suggest leaving IIS alone and implement your requirements in WordPress:

/**
 * Remove category base from "newcat" permalink.
 *
 * @param   string  $link
 * @param   object  $term
 * @return  string
 */
function wpse_175424_term_link( $link, $term ) {
    if ( $term->taxonomy === 'category' && $term->slug === 'newcat' )
        $link = home_url( user_trailingslashit( $term->slug ) );
    return $link;
}

add_filter( 'term_link', 'wpse_175424_term_link', 10, 2 );

/**
 * Add our custom rewrite rule to the top of category rules.
 *
 * @param   array   $rules
 * @return  array
 */
function wpse_175424_category_rewrite_rules( $rules ) {
    global $wp_rewrite;

    $matches_1 = $wp_rewrite->preg_index( 1 );
    $new_rules = array(
        'newcat/feed/(feed|rdf|rss|rss2|atom)/?$' => "$wp_rewrite->index?category_name=newcat&feed=$matches_1",
        'newcat/(feed|rdf|rss|rss2|atom)/?$' => "$wp_rewrite->index?category_name=newcat&feed=$matches_1",
        'newcat/page/?([0-9]{1,})/?$' => "$wp_rewrite->index?category_name=newcat&paged=$matches_1",
        'newcat/?$' => "$wp_rewrite->index?category_name=newcat",
    );

    return $new_rules + $rules;
}

add_filter( 'category_rewrite_rules', 'wpse_175424_category_rewrite_rules' );

You’ll need to flush your rewrite rules after adding the code (simply visiting the Settings > Permalinks page is sufficient).

Note that I’ve hardcoded newcat throughout the two functions – if you needed this to be flexible & pulled by ID/option it’s easy enough.