Si‐Promoted Norbornyl–Norpinyl Rearrangements

Abstract

defining a side of the cube. Alternatively, the cluster framework of 7 can be described as a polyhedron formed by joining six Pd, rings and bridging the faces with p5-As atoms (Fig. 3, bottom).The As-Pd distances are significantly shorter for the atoms of the Pd, cube than for the outer Pd atoms. These peripheral Pd atoms are each coordinated in a distorted tetrahedral fashion by three As ligands and one PPh, ligand.The electron balance for clusters with n > 13 is often difficult to explain by using the usual models. If the As ligands are treated as three-electron donors and the PPh, ligands as two-electron donors, then 7 contains 260 valence electrons, that is, 28 electrons fewer than required by the 18-electron rule and 12 electrons more than expected according to the 16-electron rule. According to the concept proposed by Mingos (n, = 12, Ai = 120),*41 264 valence electrons are obtained, that is, only four electrons more than the number calculated for 7. Similarly large Pd clusters containing CO and PR, ligands have been synthesized and structurally characterized by Mednikov et al.: for example, [Pd,o(CO),,(PBu,),J, [Pd, 3(Co)2 2 (PEtd I 01, and [Pd3,(CO)m(PEt3) I 21 .I9'Experimental Procedure 5 : 4 (670 mg, 0.44 mmol) was suspended in 50 mL of THF and then treated with As(SiMe,), (0.26 mL, 0.88 mmol). Compound 4 dissolved rapidly with formation of a dark brown solution, which afforded 110 mg of a yellow microcrystalline precipitate of unknown composition after 7 d. Cluster 5 was obtained as black crystals by addition of a layer of heptane to the filtrate (yield 30% based on Pd). 6 and 7: IPdCI2 (PPh,),] (2.12 g, 2.97 mmol) was suspended in 100 mL o f T H F and then treated with As(SiMe,), (2.97 g, 0.87 mL). Upon dissolution of the [PdCl,(PPh,),] over 30-40 min, the solution turned dark brown. After about 1 h, the Pd complex had dissolved completely. The solution afforded small dark yellow crystals of 6 in small yield (15 based on Pd) when allowed to stand for 14d. The solvent was removed from the still dark colored filtrate and the residue was washed with 50 mL of pentane and dissolved in 100 mL of toluene. After 2-3 weeks, a few large and uniform dark crystals of 7 had formed (yield 30%) together with crystals of 5 (yield 10%).

The β Effect of Silicon and Related Manifestations of σ Conjugation

“…The interaction also was examined in the 1,3-diexo geometry of norbornane by Kirmse and co-workers. 6 The γ effect of tin has been examined by Shiner, Adcock, and others. 5 We found that the acceleration (k Sn / k H ) in the cyclohexyl system 14, in which the electrofuge and nucleofuge are both cis, is at least 2.0 × 10 3 in 97% trifluoroethanol, whereas the trans system 15 had essentially no acceleration (a factor of 8).…”

“…The latter term was intended to highlight that a σ bond in 13a is replaced by another σ bond in 13c . The interaction also was examined in the 1,3-diexo geometry of norbornane by Kirmse and co-workers …”

“…This Account focuses on the contributions from our research group since the 1990 Tetrahedron Report. During this period, the field saw excellent contributions from numerous other research groups. − Some of this work is mentioned herein briefly. This Account is divided into four sections: (1) progress on understanding the mechanism of formation of saturated β-silyl carbocations through kinetic and stereochemical studies; (2) longer range effects analogous to the β effect, in which the number of bonds between silicon (or germanium or tin) and the cationic carbon is increased; (3) β effects from outside group 14; (4) stable, isolable cations with σ conjugation.…”