Drupal load form with ajax callback in Bootstrap modal

I need to load a form into a 'bootstrap modal dialog' which will submit data via ajax callback to callback function.

The form display properly when the modal dialog is launched:

<a data-toggle='modal' href='?q=document/share' data-target='#Modalshare' class='ModalTrigger'>
 <div class='' style='float:right;'><span class=''><span title='' class='glyphicon glyphicon-share-alt'></span></span></div>
<a/>

The menu 'document/share call the function 'modal_share()' that display the modal content

<div>...
content
...<div>

Within this function modal_share(), a form is called for render:

function modal_share($fid) {

$f = drupal_get_form('modal_share_form',$fid);
....
....
echo <div> .... ".drupal_render($f) .".....</div>

}

So far so good and the modal is correctly output with the form 'modal_share_form' as per below structure:

function modal_share_form($form,&$form_state,$fid) {
form['share_list'] = array(
  '#type' => 'select',
  '#multiple' => TRUE,
  '#options' => $option ,
  '#title' => t(""),
  '#description' => "",
  '#required' => false,
  '#default_value' => $default

);

$form['share'] = array (
        '#type' => 'button',
        '#value' => "<span class='glyphicon glyphicon-share-alt'></span><span id=''> ".t('record')."</span>",
        '#ajax' => array(
          'callback' => "modal_share_form_submit", 
          'wrapper' => 'shareMessage',
          'effect' => 'fade',
          'method'=>'html'
        ),

        '#id' => 'modalsharebutton',
);
return $form;
}

Now the problem is the the #ajax function in the button is not rendered by drupal as it should.

the html rendering is:

<button id="modalsharebutton" class="btn btn-primary form-submit" type="button" value="<span class='glyphicon glyphicon-share-alt'></span><span id=''>record</span>" name="op">

without the "ajax-processed" class attached

The thing is that if I load the form on a main page ajax function is rendered properly.

So my conclusion is that when building the form for modal, it is not going through the proper ajax build process...

But I can't see the solution.

Thanks for help.