In the process of synthesizing B-12, Woodward & Co. produce an intermediate thioether they refer to as Type I. "Type I because we found early in working with these substances that they were very labile, and undergo quite a plethora of changes. [Type I is prone] to an isomerization in which the C-N double bond is transformed into a C-C double bond to give thioether Type II." I may be missing something basic here, but what's behind the ready conversion of Type I into Type II? (I haven't taken o-chem in a while and am reading out of curiosity.) Also, does he mean that as soon as he produces Type I it starts converting into Type II, almost as if, practically, Type I doesn't exist? 
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3$\begingroup$ Generally imines are a bit more favorable then enamines. It's technically thioimidoester, (imine+thioether at one carbon), though, and apparently tautomerisation equilibrium is somewhat shifted. $\endgroup$– MithoronAug 22 at 22:40
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