Addition of H2O (1%) provided a clear solution; however, conversion to product 3a decreased in yield to 55%. This was probably due to the decomposition of S-chlorocysteine species in the presence of H2O (entry 13, Figure ). Using an excess of 1 (1. Method A, entries 1-6 (Figures ) for 2c and S11 for 2b-f, 4)). Trp selective sulfenylation was confirmed by trypsin digestion of each product followed by peptide mapping (Figure ). As compared to reactions run in AcOH, the sulfenylation of 2b-f (0.1 mmol) with 1 (1.5 mM) in 1% H2O/BMPy•OTf containing 30 mM MgCl2 and 0.1% TFA at 50 °C exhibited decreased reaction efficiency to give 3b-f in moderate yield (73-85%), with the exception of the disulfide-containing ) or in BMPy•OTf ((b) entry 14 in Table ). Analytical HPLC conditions: linear gradient of 0.1% TFA/CH3CN in 0.1% TFA/H2O, 5% to 65% over 30 min. UV detection at 220 nm. *materials derived from the use of excess amount of 1 in the BMPy•OTf system, see SI (Figure ). substrate 4 (Table Method B, entries 7-12, Figures and) for 2c and 4, and Figure ). Attempted sulfenylation of biologically active peptides 6 in AcOH solvent encountered difficulties related to peptide solubility, with reactions containing 6a, 6b, or 6d affording a suspension of incompletely dissolved solids. The suspensions became homogeneous solutions upon adding 2% BMPy•OTf at 25 °C. However, the increase in concentration of BMPy•OTf led to a decrease in the conversion efficacy (Figure ). The sulfenylation of 6c (1.0 mM) with 1 (1.5 mM) in 0.1% TFA/AcOH in the presence of 30 mM MgCl2 at 25 °C for 6 h (Condition A) gave the modified Octreotide (7c) in 94% yield (Figure ). As mentioned above, addition of BMPy•OTf, decreased the reaction efficiency. Therefore, sulfenylation of 6a, b or d (1.0 mM) with 1 (1.5 mM) was performed with the reaction temperature elevated to 37 °C from 25 °C in 2% BMPy•OTf/AcOH Table . Substrate versatility a Conversion (%) were determined by HPLC analysis with UV detection at 220 nm and calculated using the equation: percent formation = 100 [integ. 3a-3f or 5 / (integ. 2a-2f or 4 + 3a-3f or 5)], where integ. = integration of peak area of the UV absorption.