WP_Query how to add a thumbnail to the first post from the last 5 posts?

Because you’re not specifying order and orderby parameters within your WP_Query query, the latest added post will be shown in the first place.

Having that in mind, you can do the following:

<ul>
    <?php $the_query = new WP_Query( array( 'cat' => '1', 'posts_per_page' => 5 ) ); ?>
    <?php $iterator = 0; ?>
    <?php while ($the_query -> have_posts()) : $the_query -> the_post(); ?>
    <li>
        <?php if ( has_post_thumbnail() && $iterator == 0 ) :  ?>
            <?php the_post_thumbnail(array(230,163)); ?>
            <?php $done_last_post_thumbnail = true; ?>
        <?php endif; ?>

        <a href="https://wordpress.stackexchange.com/questions/313245/<?php the_permalink() ?>"><?php the_title(); ?></a>
        <?php $iterator++; ?>
    </li>
    <?php
        endwhile;
        wp_reset_postdata();
    ?>
</ul>

Of course, this will work only if the latest post has thumbnail.