Table 5.

pK_a values of HO₂X and first order k_obs values for conversion of 2•O₂X to 3•O₂X and 2•O₂X to 3′•′ O₂X.

O2X	pKa of HO2X	2 → 3 a Kobs (s−1)	2 → 3′b Kobs (s−1)
O2AsMe2	6.27	-- c	-- d
O2PMe2	3.08 e	-- c	1.8(4) × 10−4 (−40 °C)
O2PPh2	2.32 e	3.1 × 10−4 f (−40 °C)	4.3(4) × 10−3 (−40 °C)
O2P(OPh)2	1.85 e	1.2(1) × 10−4	3.1(1) × 10−2
O2CC6H2-3,4,5-(OMe)3	4.24	4.2(6) × 10−4	0.13(5)
O2CC6H3-3,4-(OMe)2	4.36	6.6(9) × 10−4	0.12(1)
O2CCPh3	3.96	1.7(2) × 10−3	2.6(3) × 10−2
O2CC6H3-3,5-(OMe)2	3.97	4.8(2) × 10−3	0.20(1)
O2CC6H4-4-OMe	4.50	4.7(4) × 10−3	0.12(1)
O2CCMe3	5.03	1.8(1) × 10−2	8.0(3) × 10−2
O2CPh	4.19	5.3(1) × 10−2 g (−80 °C)	-- h

^aAll rates measured at −90 °C except where noted.

^bAll rates measured after addition of 20 equivalents of OPPh₃ at −90 °C except where noted.

^cConversion to 3•O₂X did not take place at any temperature.

^dConversion to 3′•O₂X did not take place at any temperature even with addition of 100 equivalents of OPPh₃.

^eMeasured in 7% EtOH (Reference 53).

^ft −40 °C in CH₂Cl₂, 3•O₂PPh₂ starts to decay before complete conversion from 2•O₂PPh₂ occurs. For this reason, k_obs was calculated from the y-intercept of the OPPh₃ concentration dependence plot for the conversion of 2•O₂PPh₂ to 3′•O₂PPh₂ (Figure S2).

^gRate measured at −80 °C using stopped-flow techniques.

^hBy the time enough 2•O₂CPh had formed to allow for addition of OPPh₃, significant conversion to 3•O₂CPh had occurred, preventing accurate rate determination for the conversion of 2•O₂CPh to 3′•O₂CPh.