The American Stock Exchange has asked the Securities and Exchange Commission to approve development of an electronic market for placement of unregistered foreign securities with U.S. institutional investors. The plan envisions the creation of a closed network of investors who could trade among themselves. Kenneth Leibler, president of the AMEX, said private placements, now a $40-billion to $50-billion-a-year market, would grow rapidly if the present manual system were replaced by an organized market. Earlier this year the National Assn. of Securities Dealers said it was working on an "electronic bulletin board" for the private placement market.