Stannatrane
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A stannatrane (IUPAC: 1-aza-5-stannabicyclo[3.3.3]undecane) is a tin-based atrane belonging to the larger class of organostannanes. Though the term stannatrane is often used to refer to the more commonly employed carbastannatrane, azastannatranes have also been synthesized (prefix refers to the identity of the atom bound directly to tin center).[1] Stannatrane reagents offer highly selective methods for the incorporation of "R" substituents in complex molecules for late-stage diversification. These reagents differ from their tetraalkyl organostannane analogues in that there is no participation of dummy ligands in the transmetalation step, offering selective alkyl transfer in Stille Coupling reactions.[2][3][4] These transmetalating agents are known to be air- and moisture-stable, as well as generally less toxic than their tetraalkyl counterparts.
The first carbastannatrane was reported in 1984 by Jurkschat and Tzschach.[5] By reaction of an amino-triGrignard reagent with tin(IV) chloride to yield the stannatrane chloride, which was treated with methyl lithium to yield the corresponding methyl stannatrane. Based on a very small methyl J(119Sn–13C) coupling constant of 171 Hz, it was determined that the tin center of methyl stannatrane was indeed pentacoordinate, indicating nitrogen coordination.
The crystal and molecular structure was explained by X-ray crystallography.[6] The X-ray diffraction study confirmed the tricyclic ring structure and gave insight toward the geometry of the complex. With a tin-nitrogen distance of 2.624 Å, the formal bond order was calculated to be about 0.46. The presence of the tin-nitrogen interaction, albeit weaker than anticipated, led to a few key discoveries: (1) the distortion from ideal trigonal bipyramidal toward monocapped tetrahedron geometry;[7] (2) the lengthening of the apical tin-methyl bond by ~ 0.1 Å (largest known value for any existing tetraorganotin compounds); (3) the observation of unusual hybridization at the apical tin-methyl bond.
