You can not enable comments in future posts. Those posts are not public and are not private,  so they verify the conditional in line 61 of `wp-comments-post.php` file; that conditional is (WP 4.1.1):
   
   
    } elseif ( ! $status_obj->public && ! $status_obj->private ) {
    	/**
    	 * Fires when a comment is attempted on a post in draft mode.
    	 *
    	 * @since 1.5.1
    	 *
    	 * @param int $comment_post_ID Post ID.
    	 */
    	do_action( 'comment_on_draft', $comment_post_ID );
    	exit;
    }

To allow comments in scheduled posts you could hook in `comment_on_draft` action and copy the insert comment code from `wp-comments-post.php` inside the callback function.

    add_action( 'comment_on_draft', function( $comment_post_ID ) {
 
	    do_action( 'pre_comment_on_post', $comment_post_ID );

        $comment_author       = ( isset($_POST['author']) )  ? trim(strip_tags($_POST['author'])) : null;
        $comment_author_email = ( isset($_POST['email']) )   ? trim($_POST['email']) : null;
        $comment_author_url   = ( isset($_POST['url']) )     ? trim($_POST['url']) : null;
        $comment_content      = ( isset($_POST['comment']) ) ? trim($_POST['comment']) : null;

        // If the user is logged in
        $user = wp_get_current_user();
        if ( $user->exists() ) {
            if ( empty( $user->display_name ) )
                $user->display_name=$user->user_login;
            $comment_author       = wp_slash( $user->display_name );
            $comment_author_email = wp_slash( $user->user_email );
            $comment_author_url   = wp_slash( $user->user_url );
            if ( current_user_can( 'unfiltered_html' ) ) {
		        if ( ! isset( $_POST['_wp_unfiltered_html_comment'] )
			|| ! wp_verify_nonce( $_POST['_wp_unfiltered_html_comment'], 'unfiltered-html-comment_' . $comment_post_ID )
                ) {
                    kses_remove_filters(); // start with a clean slate
                    kses_init_filters(); // set up the filters
                }
            }
        } else {
            if ( get_option( 'comment_registration' ) || 'private' == $status ) {
                wp_die( __( 'Sorry, you must be logged in to post a comment.' ), 403 );
            }
        }

        $comment_type = '';

        if ( get_option('require_name_email') && !$user->exists() ) {
            if ( 6 > strlen( $comment_author_email ) || '' == $comment_author ) {
                wp_die( __( '<strong>ERROR</strong>: please fill the required fields (name, email).' ), 200 );
            } else if ( ! is_email( $comment_author_email ) ) {
                wp_die( __( '<strong>ERROR</strong>: please enter a valid email address.' ), 200 );
            }
        }

        if ( '' == $comment_content ) {
            wp_die( __( '<strong>ERROR</strong>: please type a comment.' ), 200 );
        }

        $comment_parent = isset($_POST['comment_parent']) ? absint($_POST['comment_parent']) : 0;

        $commentdata = compact('comment_post_ID', 'comment_author', 'comment_author_email', 'comment_author_url', 'comment_content', 'comment_type', 'comment_parent', 'user_ID');

        $comment_id = wp_new_comment( $commentdata );
        if ( ! $comment_id ) {
	        wp_die( __( "<strong>ERROR</strong>: The comment could not be saved. Please try again later." ) );
        }

        $comment = get_comment( $comment_id );

        /**
        * Perform other actions when comment cookies are set.
        *
        * @since 3.4.0
        *
        * @param object $comment Comment object.
        * @param WP_User $user   User object. The user may not exist.
        */
        do_action( 'set_comment_cookies', $comment, $user );

        $location = empty($_POST['redirect_to']) ? get_comment_link($comment_id) : $_POST['redirect_to'] . '#comment-' . $comment_id;

        /**
        * Filter the location URI to send the commenter after posting.
        *
        * @since 2.0.5
        *
        * @param string $location The 'redirect_to' URI sent via $_POST.
        * @param object $comment  Comment object.
        */
        $location = apply_filters( 'comment_post_redirect', $location, $comment );

        wp_safe_redirect( $location );
        exit;

    });

Additionally, we could check for "future" status to exclude "draft" posts, but in this case we can end up with conflicts with other plugins that use `comment_on_draft` action hook:

    add_action( 'comment_on_draft', function( $comment_post_ID ) {

        if( get_post_status( $comment_post_ID ) != 'future' ) {
            exit;
        }
 
	    //Rest of the code

    });