Variable temperature NMR studies

The solvated sulfenamides [Li2( BuNSC6H4Me-4)2(THF)n] (n = 2,4) have dimeric structures with a central Li2N2 ring. The coordination mode is determined by the extent of solvation of the Li" ions monosolvation allows for rj -N,S coordination whereas disolvation restricts the coordination mode to // -M Variable temperature NMR studies indicated that a dynamic exchange between these two structural types occurs in THF solution (Scheme 10.10). The dihapto coordination mode is observed exclusively in transition-metal complexes and the... 

Variable temperature NMR studies of [IrCl2(PMe2Ph)4]+ (XXV L = PMe2Ph) show a broad line in the 31P spectrum at room temperature but on cooling to 80°C a mass of sharp lines are observed, owing to three to four rotational isomers (Figure 2.89) the predominant ABCD pattern is... 

The unsolvated complexes Li[P(NR)(NR )] In may adopt monomeric, dimeric or tetrameric structures as indicated by the examples (4) [7], (5) [6] and (6) [7]. Variable temperature NMR studies reveal that the tetramer (6) dissociates into the corresponding dimer in solution [7]. The dimeric cubic structure is stabilized in the THF-solvated complex THF-Li [P(N Bu)2] 2 (7) [8]. 

In the case of neutral systems the geometry corresponds to a bicapped tetrahedron or, in other words, to a tetrahedron which undergoes two nucleophilic coordinations. They have been observed [5] to be stable in solution. A careful H variable temperature NMR study shows that these compounds undergo an intramolecular isomerization which takes place without cleavage of Si-N bonds. The AGt of this so-called "Baylar Twist" isomerization has been estimated at between 12 and 18 kcal/mole depending on the substituents at the silicon. We can conclude that this work supports strongly hexacoordinated silicon structures as possible intermediates in the usual course of nucleophilic reactions at silicon. 

Evans suggests that the catalyst resting state in this reaction is a 55c Cu alkene complex 58, Scheme 4 (35). Variable temperature NMR studies indicate that the catalyst complexes one equivalent of styrene which, in the presence of excess alkene, undergoes ready alkene exchange at ambient temperature but forms only a mono alkene-copper complex at -53°C. Addition of diazoester fails to provide an observable complex. These workers invoke the metallacyclobutane intermediate 60 via a formal [2 + 2] cycloaddition from copper carbenoid alkene complex 59. Formation of 60 is the stereochemistry-determining event in this reaction. The square-planar S Cu(III) intermediate 60 then undergoes a reductive elimination forming the cyclopropane product and Complex 55c-Cu, which binds another alkene molecule. 

Oxidative addition of hydrogen to mononuclear Cp Ru-diborolyl complexes formed the dihydride species 24, which could not be isolated, and readily lose H2 in vacuum 30 detailed variable-temperature NMR studies were unable to distinguish between the possible classical Ru(H)2 and non-classical Ru(H2) structures. Acetonitrile, CO, and other electron donors add similarly and in some cases reversibly, leading to bent structures such as 25.29,30... 

The structures of several other symmetrical organoaluminum compounds, which have important implications with regard to electron-deficient bonding, have been determined from X-ray data. The first of these (VI) shows the structure of tricyclopropylaluminum dimer (84, 99). In this compound, the A1—C—A1 bridge is symmetrical and the cyclopropyl rings are oriented so that the p orbital on the bridging carbon atom may interact with the appropriate vacant orbitals on the metal atoms. This interaction appears to increase the stability of the bridge bond as indicated from variable-temperature NMR studies. The... 

The acid-catalysed hydrolysis of the cyclic diazothiolactone, 4-diazoisochroman-3-one (159) to a ring-contracted product involves protonation of the diazo carbon in the slow step.133 A variable-temperature NMR study has been undertaken to study the reaction of the pyrazine (160) giving (161a R = OH, Me, H). Spectral evidence has been obtained supporting the involvement of the dihydroimidazopyrazine ring system (161b).134 Work by Edward and co-workers on the alkaline hydrolysis of strychnine and some of its derivatives was dealt with earlier.21... 

F and 119Sn variable-temperature NMR studies of [Bu3SnF2][Bu4N] show the presence of two distinct tributyltin species in equilibrium, the traws-BujSnl - and trans-(Bu3SnF2)2F anions222. 

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