Group 4 metal substituents reactivity effects

The C-M a bonds (where M = Si, Ge, Sn, Pb) in the fragment C-MR3 (R = alkyl, aryl) are powerful a donor orbitals the electrons in the C-M bond are weakly held and easily ionized, and, importantly, the C-M bonding electrons are polarized towards the carbon. As a result, the presence of group 4 metal substituents in organic compounds has long been recognized to have very profound effects on both the ground-state properties and reactivities of the compounds.10-13 The most well known of these effects is the silicon (3-effect,10 12 which is the... 

Organoaluminum compounds featuring simple c-bonded aliphatic or aryl substituents [21, 22]. By virtue of the polarity in the metal-carbon bond, these groups render the metal highly reactive to both moisture and oxygen and discussion will focus on the synthetic procedures and precautions necessary to prevent decomposition as well as on the ability to harness insertion reactions for benefit. Focus will then shift to the effects of introducing Lewis base and the observation of 4- and 5-coordinate complexes. 

Some of the vinyl monomers polymerized by transition metal benzyl compounds are listed in Table IX. In this table R represents the rate of polymerization in moles per liter per second M sec-1), [M]0 the initial monomer concentration in moles per liter (M) and [C]0 the initial concentration of catalyst in the same units. The ratio i2/[M]0[C]0 gives a measure of the reactivity of the system which is approximately independent of the concentration of catalyst and monomer. It will be observed that the substitution in the benzyl group is able to affect the polymerization rate significantly, but the groups that increase the polymerization rate toward ethylene have the opposite effect where styrene is concerned. It would also appear that titanium complexes are more active than zirconium. The results with styrene and p-bromostyrene suggests that substituents in the monomer, which increase the electronegative character of the double bond, reduces the polymerization rate. The order of reactivity of various olefinically unsaturated compounds is approximately as follows ... 

The ability to harness alkynes as effective precursors of reactive metal vinylidenes in catalysis depends on rapid alkyne-to-vinylidene interconversion [1]. This process has been studied experimentally and computationally for [MC1(PR3)2] (M = Rh, Ir, Scheme 9.1) [2]. Starting from the 7t-alkyne complex 1, oxidative addition is proposed to give a transient hydridoacetylide complex (3) vhich can undergo intramolecular 1,3-H-shift to provide a vinylidene complex (S). Main-group atoms presumably migrate via a similar mechanism. For iridium, intermediates of type 3 have been directly observed [3]. Section 9.3 describes the use of an alternate alkylative approach for the formation of rhodium vinylidene intermediates bearing two carbon-substituents (alkenylidenes). 

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