I'm not clear why what you have isn't working (perhaps someone can enlighten us) but this works:
UIColor *confirmButtonColor = self.btnConfirm.currentTitleColor;
self.btnConfirm.layer.shadowOffset = CGSizeZero;
self.btnConfirm.layer.masksToBounds = NO;
self.btnConfirm.layer.shadowColor = confirmButtonColor.CGColor;
self.btnConfirm.layer.shadowRadius = 6.0f;
self.btnConfirm.layer.shadowOpacity = 1.0f; // Note: You need the final value here
CABasicAnimation* animation = [CABasicAnimation animationWithKeyPath:@"shadowOpacity"];
animation.fromValue = [NSNumber numberWithFloat:.0];
animation.toValue = [NSNumber numberWithFloat:1.0];
[self.btnConfirm.layer addAnimation:animation forKey:@"shadowOpacity"];