How can binding value with [data] is Observable | async

[VanNNT]

Html:

<select2 [data]="listMajor | async"
                 [options]="options"
                 [value]="valueMajor"></select2>

Component:

public listMajor: Observable<Select2OptionData[]>;
this.listMajor = this.searchService.getMajor();
 this.options = {
      multiple: true
    };
 this.valueMajor = [4307, 4308];

Service:

 getMajor(): Observable<Select2OptionData[]> {
    return this._http.get(this.constant.MAJOR)
      .map((response: Response) => response.json())
      .map((majors) => {
        majors.unshift({id:'0',name:''});
        return majors.map((major) => {
          return {id: major.id, text: major.majorName};
        });
      })
  }

I try to binding [value] but not work. When list Major is Array, it can binding [value]. I also try to convert Observable<Select2OptionData[]> to Array, but I don't know how do it. How can binding [value]? Should I take in mind any issue? Thank you.