R1 is (i) -y-B-y-R2 and R2 is -x-(CH2)p???W, wherein x is as defined above and W is hydrogen, phenyl optionally substituted with up to three substituents selected from the group consisting of halogen, C1???C4 alkyl, C1???C4 alkoxy, and carboxyl, N-maleimide, or V as defined below, and p is as defined below; or, (ii) R1 and R2 together are a single bond;