Allyls fluxional

Scheme 12), but absence of it in 18, where 18 can be considered as based on a non-fluxional tetrahedron or octahedron. A theoretical analysis of the allyl fluxionality in 17 and 18 has recently been published. Formation of an 7] -allyl allows an alternative to rotation for interconversion of exo- and endodi.ovneK and characterized by interchange of syn- and /(-protons. This is a well-documented process which is particularly important in asymmetric catalysis using Pd allyls (Scheme J3) 1 >16,76,77... 

The structures of jj3-allyl complexes of platinum have been fully summarized by Hartley in Chapter 39 of Comprehensive Organometallic Chemistry . This article also details the NMR methods used to investigate fluxionality of the r/3-allyl ligand. 

Scheme 7.15] or S -type mechanism [Equation (7.9)]. Depending on the nature of the nucleophile and catalyst employed, the subsequent nucleophilic substitution of the metal can follow either via a-elimination [path A, Equations (7.8) and (7.9), Scheme 7.15], via an SN2 reaction (path B) or via an SN2 -type reaction (path C). For reasons of clarity, only strictly concerted and stereospecific SN2- or SN2 -anti-type mechanistic scenarios are shown in Scheme 7.15. The situation might, however, be complicated if, e.g., the initial S l -anti ionization event is competing with an Sn2 -syn reaction. Erosion in stereo- and regioselectivity can be the result of these competing reactions. Furthermore, fluxional intermediates such as 7t-allyl Fe complexes are not shown in Scheme 7.15 for reasons of clarity. These intermediates are known for a variety of late transition metal allyl complexes and will be referred to later. Moreover, apart from these ionic mechanisms, radicals might also be involved in the reaction. So far no distinct mechanistic study on allylic substitutions has been published.

The resonances due to unsaturated 29Si nuclei of cyclotetrasilenylium cation 70 appear at + 77.3 (terminal Si) and +315.7 ppm (central Si). The chemical shifts are independent of the solvent such as dichloromethane, benzene, and toluene, implying no covalent interaction between the cation part and solvent molecules. Rather unusually, the central silicon in the cation part is more deshielded than the terminal silicons.42 The terminal and central 29Si resonances of anion 71 are -31.5 and + 273.0 ppm in toluene-terminal silicon atoms are equivalent, indicating that the lithium cation is fluxional and 71 adopts an allylic anion-type structure in solution. Similarly to allyllithium, the central nucleus of 71 is deshielded, while the terminal nuclei are highly shielded. 

Fluxional behaviour, 73-allyl palladium complexes, 8, 371 Formamidinates, in chromium complexes, 5, 355 Formates, with Ru and Os half-sandwiches, 6, 483... 

The iron complex of 1,2-cycloheptadiene was synthesized to study the relationship between 312 and a valence isomeric form (316) that has been referred to as the allyl cation form (Scheme 39).119 Because of ring strain, the latter had been predicted by EHMO calculations to lie rather close in energy to an allene ground state.126 The NMR of 312 confirmed the allene form as the ground state and fluxionality experiments on the terminal allene... 

Transition-metal complexes of the more highly unsaturated seven-mem-bered allene 1,2,4,6-cycloheptatetraene have been isolated in two forms depending on the metal and its ligands an allene form (329), which is a complex of the parent cycloheptatetraene 327,134 and a carbene form (330/331), which is a complex of the controversial cycloheptatrienylidene 328.4135136 The carbene form corresponds to the allyl cation (320) that was suggested as an intermediate/transition state for fluxionality of 318. Scheme 42 lists all such complexes that have been prepared to date. The allene form is the ground state for all Pt(0) complexes (343, 344, 345, 347)82,83,130 137 138 and one dibenzannelated W(II) complex139 (346), whereas the carbene form is the ground state for all Fe(II)+ complexes (332-336),140-143 all Ru(II)+ complexes (337-340),144 one Pt(II)+ complex (342),137 and one W(0) complex (341).139... 

Organic isocyanates and isothiocyanates as well as nitric oxide insert similady (412). Carbon monoxide inserts to yield very stable acyltitamums (412,413). Miscellaneous Reactions of CpfTi Derivatives. Coupling of fluxional pentadienide ion with allyl bromide is regiospecificatly catalyzed by Cp2TiCl2 (414,415). In contrast, cuprous chloride gives the linear triene ... 

II, C,3) have established that t -triazenide ligands, like their j/ -allyl counterparts, display fluxional behavior. 

Allyl complexes are characteristically fluxional, the principal pathway being the rf-rf-rf process shown below. 

Chemical shifts have been reported for [Me3CCH2HgX] 203) (see Table XII), tr-allylic derivatives (see Table XIII), o-cyclopentadienyl derivatives (see Table XIV), and miscellaneous compounds (see Table XV). In the case of fluxional a-cyclopentadienyl derivatives, it has been shown that chemical shifts can be used to differentiate between fluxional o-cyclopentadienyl derivatives and w-cyclopentadienyl derivatives 103). The NMR spectrum of [Sb(cyclopropyl)s] has only two carbon resonances, showing that the molecule is fluxional (56) and the NMR spectrum of the naturally occurring organometallic compound, 5 -deoxyadenosylcobalamin, has been reported (65). For a number of norbornane and allylic derivatives of palladium and platinum, a number of linear relationships have been found between chemical shifts of various carbon atoms. It was suggested that a term due to paramagnetic shielding by the metal was dominant 52b). 

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