Microsoft Word 2011 does not, as far as I can tell, offer a means of assigning a keyboard shortcut to close the sidebar. The AppleScript described here provides that functionality, and sets a keyboard shortcut that will trigger it.
Name the following Applescript "MicrosoftWordCloseSidebar\mosH.scpt" (notice the reversed slash) and save it in ~/Library/Application Support/Microsoft/Office/Word Script Menu Items:
do shell script "osascript -e '
tell application \"System Events\" to tell process \"Microsoft Word\"
set {frontmost, itemFound} to {true, false}
repeat with w in windows
tell w to repeat with g in groups
tell g to repeat with c in checkboxes
tell c to if value of its attribute \"AXHelp\" = \"Close the Sidebar\" then
set itemFound to true
click
exit repeat
end if
end repeat
if itemFound then exit repeat
end repeat
if itemFound then exit repeat
end repeat
end tell
' >/dev/null 2>&1 &"
Mac OS X Hints
http://hints.macworld.com/article.php?story=20130217172318170