Addition of N-nucleophiles to gold(III)-bound isocyanides leading to short-lived gold(III) acyclic diaminocarbene complexes

Addition of hydrazone to gold(iii)–isocyanides led to the generation of rare short-lived gold(iii) acyclic diaminocarbene complexes.</p

Tetrazol-5-ylidene Gold(III) Complexes from Sequential [2+3] Cycloaddition of Azide to Metal-Bound Isocyanides and N4 Alkylation

The reaction between equimolar amounts of the isocyanide complexes [AuCl₃(<u>C</u>NR)] [R = 2,6-Me₂C₆H₃ (Xyl), <b>1a</b>; 2,4,6-Me₃C₆H₂ (Mes), <b>1b</b>; Cy, <b>1c</b>; <i>t-</i>Bu, <b>1d</b>] and tetrabutylammonium azide (<b>2</b>) proceeds in CH₂Cl₂ at room temperature for ∼10 min to furnish the gold­(III) tetrazolates [<i>n</i>-Bu₄N]­[AuCl₃(<u>C</u>N₄R)] (<b>3a</b>–<b>d</b>), which were obtained in 89–95% yields after purification. Subsequent reaction between equimolar amounts of <b>3a</b>–<b>d</b> and methyl trifluoromethanesulfonate (MeOTf) proceeds in CH₂Cl₂ at −70 °C for ∼30 min to give the corresponding gold­(III) complexes [AuCl₃(C^<i>a</i>N­(Me)­N₂N^<i>b</i>R)]^<i>a–b</i> (<b>5a</b>–<b>d</b>) bearing 1,4-disubstituted tetrazol-5-ylidene ligands (69–75%). Complexes <b>3a</b>–<b>d</b> were obtained as pale-yellow solids and characterized by elemental analyses (C, H, N), HRESI^–-MS, FTIR, and ¹H and ¹³C­{¹H} NMR spectroscopies. Complexes <b>5a</b>–<b>d</b> were obtained as colorless solids and characterized by elemental analyses (C, H, N), HRESI⁺-MS, and 1D (¹H and ¹³C­{¹H}) and 2D (¹H,¹³C-HMBC) NMR spectroscopies. In addition, the structures of <b>3a</b>, <b>3b</b>, <b>3c</b>, and <b>5a</b> were established by single-crystal X-ray diffraction. Analysis of the Wiberg bond indices (WI) for gas phase-optimized model structures of <b>3a</b>–<b>c</b> and <b>5a</b> computed using the natural bond orbital (NBO) partitioning scheme disclosed a higher degree of electron density delocalization in the CN₄ moiety of carbene <b>5a</b> when compared to tetrazolate <b>3a</b>–<b>c</b>. Results of DFT calculations for a model system reveal that the mechanism for the cycloaddition of an azide to the isocyanide ligand in [AuCl₃(CNMe)] is stepwise and involves nucleophilic attack of N₃^– on the N atom of CNMe followed by ring closure. The addition is both kinetically and thermodynamically favorable and occurs via the formation of an acyclic NNNCN intermediate, whereas the cyclization is the rate-determining step

1,3-Dipolar Cycloaddition of Nitrones to Gold(III)-Bound Isocyanides

Treatment of gold­(III)-isocyanides [AuCl₃(CNR¹)] (R¹ = Xyl <b>1</b>, Cy <b>2</b>, Bu^<i>t</i> <b>3</b>) with an equimolar amount of 5,5-dimethyl-1-pyrroline-<i>N</i>-oxide (<b>4</b>) in CH₂Cl₂ at −74 °C leads to the generation of the heterocyclic aminocarbene species [AuCl₃{C­(ON^<i>a</i>CMe₂CH₂CH₂C^<i>b</i>H)N^<i>e</i>R¹}­(N^<i>a</i>–C^<i>b</i>)­(C^<i>b</i>–N^<i>e</i>)] <b>8</b> (for R¹ = Bu^<i>t</i>) or gold­(III) complexes <i>cis</i>-[AuCl₂{N^<i>a</i>(CMe₂CH₂CH₂C^<i>b</i>N^<i>e</i>R¹)­C^<i>d</i>O}­(N^<i>a</i>C^<i>b</i>)­(N^<i>e</i>–C^<i>d</i>)] <b>9</b> and <b>10</b> (for R¹ = Xyl and Cy) in good isolated yields (75–87%). DFT calculations show that deprotonation of the endocyclic CH group in the carbene ligand leads to spontaneous N–O bond cleavage, and acidity of this group is a factor controlling the different chemical behavior of <b>1</b>–<b>3</b> depending on the nature of substituent R¹. The reaction of equimolar amounts of the aldonitrone <i>p</i>-TolCHN⁺(Me)­O^– (<b>5</b>) or the ketonitrones Ph₂CN⁺(R²)­O^– (R² = Ph <b>6</b>, CH₂Ph <b>7</b>) with <b>1</b>–<b>3</b> in CD₂Cl₂ at −70 °C in air (or under N₂) revealed the formation of the carbene complexes [AuCl₃{C­(ONMeC^<i>a</i>H-<i>p</i>-Tol)N^<i>b</i>R¹}­(C^<i>a</i>–N^<i>b</i>)] (R¹ = Cy <b>11</b>, Xyl <b>12</b>, Bu^<i>t</i> <b>13</b>), [AuCl₃{C­(ONPhC^<i>a</i>Ph₂)N^<i>b</i>R¹}­(C^<i>a</i>–N^<i>b</i>)] (R¹ = Cy <b>14</b>, Bu^<i>t</i> <b>15</b>), or [AuCl₃{C­(ON­(CH₂Ph)­C^<i>a</i>Ph₂)N^<i>b</i>R¹}­(C^<i>a</i>–N^<i>b</i>)] (R¹ = Cy <b>16</b>, Xyl <b>17</b>), as studied by ¹H NMR. The reaction of <b>6</b> with <b>1</b> and of <b>7</b> with <b>3</b> did not furnish carbene products. Compounds <b>8</b>–<b>10</b> were characterized by ESI-MS, IR, 1D (¹H, ¹³C­{H}) and 2D (¹H,¹H–COSY, ¹H,¹³C-HSQC, ¹H,¹³C-HMBC) NMR spectroscopic techniques, and, only for <b>8</b>, elemental analyses (C, H, N), while compounds <b>11</b>–<b>17</b> were characterized by 1D (¹H, ¹³C­{H}) and 2D (¹H,¹³C-HSQC) NMR. Structures of compounds <b>8</b>, <b>9</b>, and <b>13</b> were additionally established by single-crystal X-ray diffraction