Alkyls ligands

Migration of a hydride ligand from Pd to a coordinated alkene (insertion of alkene) to form an alkyl ligand (alkylpalladium complex) (12) is a typical example of the a, /(-insertion of alkenes. In addition, many other un.saturated bonds such as in conjugated dienes, alkynes, CO2, and carbonyl groups, undergo the q, /(-insertion to Pd-X cr-bonds. The insertion of an internal alkyne to the Pd—C bond to form 13 can be understood as the c -carbopa-lladation of the alkyne. The insertion of butadiene into a Ph—Pd bond leads to the rr-allylpalladium complex 14. The insertion is usually highly stereospecific. 

Hie tliernial instab dity of litliium dialkylcuprate reagents and tlieir ability F transfer only one alkyl ligand prompted searclies for effective non-transferable o... 

The best titanium mediator appears to be methyltitanium triisopropoxide, yet good yields are also obtained with titanium tetraisopropoxide and chlorotitanium triisopropoxide. The methyl group on titanium serves as a dummy alkyl ligand which is eliminated as methane after hydride transfer from the... 

The relative reactivities of alkyltin compounds towards tert-butoxyl radicals, ketone triplets, and succinimidyl radicals are dominated by the steric effect of the alkyl ligands (R" > R"), but that towards bromine atoms follows the reverse sequence (R" < R ). 

In the case of BAE and salen complexes with simple alkyl ligands these different configurations can usually be distinguished by their color and... 

It is possible that the general instability of secondary alkyl ligands such as isopropyl and cyclohexyl (see Section B,2) may also be due to homolytic fission at room temperature, but here also other mechanisms, such as the elimination of Co—H, are possible. 

Qualitative Comparison of the Stability of Pentacyanides [RCo(CNj)] " Containing Primary, Secondary, and Tertiary Alkyl Ligands"... 

The decomposition of 4-pyridylmethyl- and a-(2-pyridyl)ethylpenta-cyanide, which probably involves homolytic fission (see Section B,l,a), occurs only after the loss of one cyanide to give the (presumably trans) organotetracyanoaquo complexes 100,102), i.e., in this case we observe the order H2O > CN . The decomposition of corrinoids possessing secondary alkyl ligands is accelerated by the addition or presence of bases and cyanide. Isopropylcobalamin is more unstable than the cobinamide 61) cyclohexyl-... 

Catalytic hydrogenation with platinum liberates the hydrocarbon from methylcobalamin (57) and from alkyl-Co-DMG complexes (161), but not from pentacyanides with primary alkyl, vinyl, or benzyl ligands, though the cr-allyl complex yields propylene (109). Sodium sand gives mixtures of hydrocarbons with the alkyl-Co-salen complexes (64). Dithioerythritol will liberate methane from a variety of methyl complexes [cobalamin, DMG, DMG-BF2, G, DPG, CHD, salen, and (DO)(DOH)pn] (156), as will 1,4-butanedithiol from the DMG complex (157), and certain unspecified thiols will reduce DMG complexes with substituted alkyl ligands (e.g., C0-CH2COOH ->CH3C00H) (163, 164). Reaction with thiols can also lead to the formation of thioethers (see Section C,3). 

Based on our observation in these two systems, it would appear that Cp Cr -alkyls, if rendered electrophilic and/or sufficiently coordinatively unsaturated, will both bind and insert a-olefins. However, the more heavily substituted alkyl ligands thus formed (i.e. CrBl-CH2-CH(R)-P vs. Crni-CH2-CH2-P resulting from ethylene insertion) seem to be very susceptible to facile 3-hydrogen elimination. Rapid chain transfer and very low molecular weights are the results of this tendency. Whether the latter is an innate property of all chromium alkyls or reflects the particular chemical nature of the Cp Cr-moiety remains to be established. To this end, investigation of chromium alkyls with a variety of other ancillary ligands are needed. 

Prakash, A. S. Denny, W. A. Gourdie, T. A. Valu, K. K. Woodgate, P. D. Wakelin, L. P. G. DNA-directed alkylating ligands as potential antitumor agents sequence specificity of alkylation by intercalating aniline mustards. Biochemistry 1990, 29, 9799-9807. 

In the case of terminal C=C (1,2 addition units), i.e. when R=R =H and R" (or R111) = polymer chain, two types of hydride migration are possible, namely (i) The Markownikoff s addition which would lead to the formation of B type repeating units and (ii) The anti Markownikoff s addition which would result in the formation of the observed repeating units C. In the case of Markownikoff s type addition the hydride transfer occurs to Ca and results in the formation of branched alkyl-rhodium intermediate complex shown by Structure 2. Whereas when anti Markownikoff s addition occurs, the resulting intermediate alkyl-rhodium complex has linear alkyl ligand as shown by Structure 3. 

Theoretical studies of catalytic alkane-dehydrogenation reactions by [(PCP )IrH2], PCP rf-C6H3(CH2P112)2-l, 3 and [cpIr(PH3)(H)]+, suggest that they proceed through similar steps in both cases namely (i) alkane oxidation, (ii) dihydride reductive elimination, (iii) /3-II transfer from alkyl ligand to metal, (iv) elimination of olefin.402 The calculated barriers to steps (i), (ii), and (iv) are more balanced for the PCP system than for cp(PH3). 

Fig. 35. (a) Correlation between the photolysis rate of halomethyl-cobalamins and the electronegativity of the alkyl ligand. (The electronegativity values are shown as the sums of the Pauling electronegatives of the atoms attached to the methyl carbon.) (b) Correlation between pKa for displacement of benzimidazole and alkyl group electronegativity for a series of halomethyl-cobalamins... 

---