how to generate excel report as asynchronous

we have one excel report is there ;it will take time to generate that report from frond end.normally it will take 30 to 45 mins to generate the report and currently it will ask the user the to open or save as the excel sheet.now its working fine.

now my requirement is we want to generate that report as asynchronous because we are waiting the user for that much time.i want to generate this report as follows.

1.whenever user clicks that report we will send a mail to that user saying that you have clicked that report successfully and same message will displayed in the frond end also.but in the back end whatever method is calling currently and the process will go on.

2.we will place this report in the folder once it is generated and again send a information to that user this file has been placed so and so place.