Alkyl-transfer

Even more important is the stereoregular catalytic polymerization of ethene and other alkenes to give high-density polyethene ( polythene ) and other plastics. A typical Ziegler-Natta catalyst can be made by mixing TiCU and Al2Eti in heptane partial reduction to Ti " and alkyl transfer occur, and a brown suspension forms which rapidly absorbs and polymerizes ethene even at room temperature and atmospheric pressure. Typical industrial conditions are 50- 150°C and 10 atm. Polyethene... 

Sn2 reactions can be thought of as alkyl transfer reactions, and Sn2 characteristics can be anticipated by examining analogous proton transfer reactions. 

The counterion strongly affected the relative rates of elimination vs. alkylation (transfer vs. termination). 

It is also well known that alkyl groups can be tran.sferred intramolecularly from one position to another on the same ring and intermoiccularly from one aromatic ring to another through dealkylation reactions catalyzed by Lewis acid. The intramolecular alkyl-transfer is called reorientation or isomerization and the intermolecular alkyl transfer is referred to as disproportionation. Reorientation processes arc normally faster than disproportionation. 

In the case of zirconiimi hydride, the hydrogenolysis of propane into a 1 1 mixture of methane and ethane is in good agreement with a /1-alkyl transfer as a key step for carbon-carbon bond cleavage (Scheme 21) [90,93]. 

Comparing the product selectivity at low conversion in the hydrogenolysis of 2,2-dimethylbutane for the two catalysts is noteworthy. Zirconium hydride supported on siUca does not produce neopentane, but only isopentane (10%) as a Cs product in agreement with a /1-alkyl transfer as a key step for the carbon-carbon cleavage (no neopentane can be formed through this mechanism, Scheme 25). 

In contrast, tantalum hydride supported on silica gives mainly neopentane (31%), which indicates that the mechanism of carbon-carbon bond cleavage must involve the removal of one carbon at a time (in contrast to /3-alkyl transfer, which involves the removal of at least two carbons). 

It was assumed that C—C bond cleavage passes through an elementary step of p-alkyl transfer. The mechanism of hydroisomerization passes also by a p-alkyl transfer step, but in this case the P-H elimination-olefin reinsertion occurs rapidly and a skeletal isomerization also occurs. 

Intramolecular alkyl transfer is a fundamental problem with this reaction this problem can be addressed with modification in structure of the reagents. Neutral trivalent phosphorus reagents do react with carbonyl compounds at much lower temperatures, but lead to several types of pentacoordinated phosphorus products.190-198 More will be noted about the use of such pentacoordinated phosphorus species for carbon-phosphorus bond formation in Chapter 5. 

Trivalent phosphorus-halogen reagents have been noted to be of use in obtaining simple Abramov-type products with chloral199 200 and with aldimines.201 Similarly, phosphorus-carboxylate anhydrides have been useful in overcoming the stereochemical difficulties associated with alkyl transfer for obtaining Abramov-type products in a direct manner.202 205... 

Several approaches have been used to overcome the stereochemical difficulties for intramolecular "alkyl transfer." One of these is to use a "trapping agent" in the reaction mixture with which the oxyanion site of the intermediate can react. A silyl halide works nicely for this purpose the halide anion facilitates the required dealkylation.206-210... 

The other major approach toward overcoming the "alkyl transfer" difficulty of the Abramov reaction involves the use of silyl esters of the trivalent phosphorus acids. Unlike carbon, silicon does not have the stereochemical restraints associated with ordinary alkyl groups for intramolecular transfer.211 The preparation of mixed alkyl—silyl esters of trivalent phosphorus acids paved the way for the Abramov reaction to be of general utility.204 208 212 An example is shown in Equation 3.14. 

Ph-CH2+, whereas during ethyl transfer it is a more stable secondary cation, - h-CH3/ which is easier to form. It is also apparent, that ethylbenzene is a better acceptor than xylene. We suggest that this is largely a consequence of the larger steric requirement of the bulky diphenylmethane intermediate for alkyl transfer to xylene vs to ethylbenzene. 

C(3)-Zn-N(21) [133.1(3)°] in the molecular structure of 15 and implies a possible stereo-selective induction for future alkyl transfer reactions. 

The use of copper catalysts based on chiral phosphorus ligands to assist 1,4-additions of dialkylzinc reagents has in recent years produced major breakthroughs, with excellent enantioselectivities. A number of monodentate and bidentate phos-phoramidites, phosphites, phosphonites, and phosphines are now available as chiral ligands for alkyl transfer to a variety of cyclic and acyclic enones. So far. 

P-alkyl transfer has been suggested to occur especially to account for the fact that ethane was not cleaved. Note that this P-alkyl transfer is the reverse process of an olefin insertion into a metal-carbon bond (Scheme 3.3). 

In fact, the C-H bond activation by the zirconium or tantalum hydride on 2,2-dimethylbutane can occur in three different positions (Scheme 3.5) from which only isobutane and isopentane can be obtained via a P-alkyl transfer process the formation of neopentane from these various metal-alkyl structures necessarily requires a one-carbon-atom transfer process like an a-alkyl transfer or carbene deinsertion. This one-carbon-atom process does not preclude the formation of isopentane but neopentane is largely preferred in the case of tantalum hydride. 

Since Zr-H is able both to (i) activate the C-H bonds of alkanes (via cr-bond metathesis) [15, 48] and to carry out their hydrogenolysis (transfer of a least two carbons via a P-alkyl transfer) and (ii) polymerize olefins (via insertion), the ability of such supported Zr-H was tested in the homologation of propane. 

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