Mono- and Dialkyne Insertion Reactions of Cyclopalladated <i>N</i>,<i>N</i>′,<i>N</i>″‑Triarylguanidines [κ²(<i>C</i>,<i>N</i>)Pd(μ-Br)]₂ and <i>cis‑</i>/<i>trans</i>-[κ²(<i>C</i>,<i>N</i>)Pd(Lewis Base)Br]. Scaffolds for Enlarged, Rearranged, and Zwitterionic Palladacycles through Ring Contraction cum Amine–Imine Tautomerization

Insertion reactions of six-membered cyclopalladated <i>N</i>,<i>N</i>′,<i>N</i>″-triarylguanidines, [κ²(<i>C</i>,<i>N</i>)­Pd­(μ-Br)]₂ (<b>III</b>–<b>V</b>), with various alkynes in CH₂Cl₂ under ambient conditions afforded diinserted eight-membered palladacycles, [(κ²(<i>C</i>,<i>N</i>):η²(<i>C</i><i>C</i>)-PdBr] (<b>1</b>–<b>11</b>), in high yield (76–96%), while insertion reactions of six-membered cyclopalladated <i>N</i>,<i>N</i>′,<i>N</i>″-triarylguanidines, [κ²(<i>C</i>,<i>N</i>)­Pd­(Lewis base)­Br] (<b>VI</b>–<b>XI</b>), with various alkynes under the aforementioned conditions afforded monoinserted six-membered palladacycles, [κ²(<i>C</i>,<i>N</i>)­Pd­(Lewis base)­Br] (<b>12</b>–<b>21</b>), in high yield (81–91%) except for <b>14</b> (23%). The insertion reaction of <b>VI</b> with 2 equiv of dimethyl acetylenedicarboxylate (DMAD) and the insertion reaction of <b>12</b> with 1 equiv of DMAD in CH₂Cl₂ under ambient conditions resulted in the formation of a diinserted zwitterionic five-membered palladacycle, [κ²(<i>C</i>,<i>C</i>)­Pd­(2,6-lutidine)­Br] (<b>22</b>), in 76% and 70% yields, respectively. Palladacycle <b>22</b> upon reaction with AgOTf in wet MeCN afforded the ionic palladacycle [κ²(<i>C</i>,<i>C</i>)­Pd­(2,6-lutidine)­(H₂O)]­[OTf] (<b>23</b>) in 78% yield. The ring size of the “[κ²(<i>C</i>,<i>N</i>)­Pd]” unit in the structurally characterized diinserted palladacycles (<b>1</b>·2CH₂Cl₂·H₂O, <b>2</b>, <b>5</b>, and <b>7</b>), and monoinserted palladacycles (<b>17</b>, <b>18</b>, and <b>20</b>·C₇H₈·H₂O) is smaller than that anticipated for mono- and diinserted palladacycles, and this feature is mainly ascribed to the proclivity of <b>III</b>–<b>XI</b> to undergo ring contraction cum amine–imine tautomerization upon alkyne insertion. Palladacycle <b>22</b> represents the first diinserted product obtained in alkyne insertion reactions of [κ²(<i>C</i>,<i>N</i>)­Pd­(Lewis base)­X] type palladacycles. The molecular structure of <b>22</b>·H₂O determined by X-ray diffraction indicates that the positive charge on the guanidinium moiety is balanced by the negative charge on the palladium atom and thus represents the first structurally characterized zwitterionic palladacycle to be reported in alkyne insertion chemistry. Plausible mechanisms of formation of <b>12</b>–<b>21</b> and <b>22</b> have been outlined. The presence of more than one species in solution for some of the palladacycles in the series <b>1</b>–<b>7</b> and <b>12</b>–<b>21</b> was explained by invoking the C–N single-bond rotation of the CN₃ unit of the guanidine moiety, while this process in conjunction with Pd–N­(lutidine) bond rotation was invoked to explain the presence of four isomers of <b>15</b>, as studied with the aid of variable-concentration ¹H NMR experiments carried out for <b>14</b> and <b>15