Based on these results, we propose that the reaction does not proceed through carbenes H or J, as these intermediates would give 5a-D2 with reagent 1a-D2. Likewise, a concerted hydride shift and elimination from I is ruled out. The results from Schemes 5c-d indicate that the base is involved in protonation, which aligns well with pathways V-VIII proceeding through intermediates I and K. These pathways were analyzed in more detail to rationalize the observed stereoselectivity in formation of 5a-D (Scheme 6). The reaction of phosphine oxide 4a with 1a-D would generate anionic intermediate I-D, which could either be protonated from above or below by + DBU-H to reach two diastereomeric intermediates. Rotation around the single bond to obtain antiperiplanar alignment followed by b-elimination yields (Z)-5a-D and (E)-5a-D, respectively. With the assumption that antielimination (E2) is favored over syn-elimination in this system, the observed high (Z)-selectivity in Scheme 5a indicates that protonation of I-D preferentially takes place from the same face as the phosphorous addition. This is also in agreement with the opposite stereoselectivity observed in Scheme 5d, where + DBU-D is used in the protonation step. The formation of 5a as the major product in that reaction indicates that protonation with + DBU-H, which forms in the E2elimination, is much faster than with + DBU-D. This is expected as a competitive proton/deuterium transfer should display a significant primary kinetic isotope effect. shift (Figure ) was found to be higher in energy than baseassisted conversion of I¢ to 5a. Protonation by + DBU-H preferentially takes place from above, to yield syn-K¢ (Figure ), which rotates to anti-K¢. As expected, subsequent E2-elimination is favored over the corresponding b-elimination from syn-K', (Figure ) and provides 5a and 2-iodobenzoate.