Really it's not a problem, is a new requirement to allow Joomla K2 component to send notifications when a new comment has been published on frontpage; i still don't understand how that functionality has not been implementd on standar release of K2.
Solution:
we need to edit the code that allow to save new comments, you can find that on components/com_k2/models/
1.- edit item.php and search for function comment()
2.- search string "...K2_COMMENT_ADDED_
3.- before that line add the next code:
$mainframe = &JFactory::getApplication();
$mail = &JFactory::getMailer();
$senderEmail = $mainframe->getCfg('mailfrom');
$senderName = $mainframe->getCfg('fromname');
$mail->setSender(array($senderEmail, $senderName));
$mail->setSubject(JText::_('K2_COMMENT_REPORT'));
$mail->IsHTML(true);
$body = "
<strong>".JText::_('K2_NAME')."</strong>: ".$name." <br/>
<strong>".JText::_('K2_REPORT_REASON')."</strong>: ".$reportReason." <br/>
<strong>".JText::_('K2_COMMENT')."</strong>: ".nl2br($row->commentText)." <br/>
";
$mail->setBody($body);
$mail->ClearAddresses();
$mail->AddAddress($params->get('commentsReportRecipient', $mainframe->getCfg('mailfrom')));
$mail->Send();