P-alkyl transfer

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. 

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. 

However, this hypothesis does not explain why Cj, is not equal to C, j, forp = 1-6. If the mechanism described in Scheme 3.14 is considered, a successive P-alkyl transfer should occur. When zirconium is in position (P-rl) ethane and C, j 2 (instead of C j,) hydrocarbons are produced. When zirconium is in position (P+2) propane and C, j 3 hydrocarbon are produced, and so on. This hypothesis involves only an overproduction of ethane and of propane, but no overproduction of C4 to Cis. These results imply a secondary cleavage for p = 1-6. 

P-Alkyl transfer (R = CH3, CH3CH2) produces polypropene and poly(l-butene) with vinyl end groups ... 

The degree of polymerization is obtained by dividing the propagation rate by the sum of all chain-breaking (transfer) reactions. For the simple situation where the only P-hydride transfer is that described by Eqs. 8-37 and 8-38 (which produce vinylidene end groups in polypropene) and no P-alkyl transfer occurs, the degree of polymerization is... 

In the absence of H2 and other transfer agents, polymer molecular weight is limited by various P-hydride transfers—from normal (1,2-) and reverse (2,1-) propagating centers, before and after rearrangement [Lehmus et al., 2000 Resconi et al., 2000 Rossi et al., 1995, 1996 Zhou et al., 2001] (Sec. 8-4i-2). Vinylidene, vinylene, and trisubstituted double-bond end groups are formed in 1-alkene polymerizations, vinyl and vinylene in ethylene polymerization. [Vinyl groups are also produced in some 1-alkene polymerizations, not by P-hydride transfer, but by P-alkyl transfer (Sec. 8-4i-2).]... 

Finally, note that it is possible to cleave a C-C bond of a ligand by p-alkyl transfer (extrusion of an olefin from the alkyl ligand), bnt only with early transition-metal and rare-earth complexes. This reaction, that parallels the more common P-H elimination, is the reverse of the insertion of an olefin into a metal-alkyl complex and is consequently discussed in Chap. 6 dealing with insertion and extrusion reactions. 

A possible mechanism for the P-alkylation of secondary alcohols with primary alcohols catalyzed by a 1/base system is illustrated in Scheme 5.28. The first step of the reaction involves oxidation of the primary and secondary alcohols to aldehydes and ketones, accompanied by the transitory generation of a hydrido iridium species. A base-mediated cross-aldol condensation then occurs to give an a,P-unsaturated ketone. Finally, successive transfer hydrogenation of the C=C and C=0 double bonds of the a,P-unsaturated ketone by the hydrido iridium species occurs to give the product. 

NMR analysis of the polyketone end groups and in situ NMR studies have shown that two transfer mechanisms in MeOH may occur simultaneously (a) methano-lysis of Pd-acyl and (b) protonolysis of Pd-alkyl (Scheme 7.15). The eventual presence of water in MeOH, even in trace amounts, gives rise to two similar terminations, yielding different end groups (-COOH) and metal re-initiator (Pd-OH) [5e-g, 13, 36]. Termination by P-H transfer (c) is typical for reactions performed in organic solvents. 

Since Corey s group first reported 0(9)-allyl-N-(9-anthracenylmethyl) cinchonidi-nium bromide as a new phase-transfer catalyst [13], its application to various asymmetric reactions has been investigated. In particular, this catalyst represents a powerful tool in various conjugated additions using chalcone derivatives (Scheme 3.2). For example, nitromethane [14], acetophenone [15], and silyl eno-lates [16] produce the corresponding adducts in high enantioselectivity. When p-alkyl substrates are used under PTC conditions, asymmetric dimerization triggered by the abstraction of a y-proton proceeds smoothly, with up to 98% ee [17]. 

Under certain circumstances, the conjugate addition and cycloaddition reaction pathways overlap for a,P-unsaturated electrophiles. For example, when the 2,5-dimethylpyrrole complex 22 is combined with 1 equiv of MVK in acetonitrile, a 1 1 ratio of the p-alkylated 3//-pyrrole complex 96 and the a-alkylated 2Z/-pyrrole complex 98 is isolated (Figure 18). When the reaction is monitored by H NMR in CD3CN, an intermediate 7-azabicyclo[2.2.1]heptene complex (97) is observed at early reaction times, and eventually converts (tm 1 h) to compound 98 via a retro-Mannich reaction followed by proton transfer. 

P-4, pump (positive displacement) for alkylate transfer from T-3 to C-l 10 gpm (150 psi) Cast iron ... 

Release of the unsaturated chain end of a polyolefin can occur by fi-H transfer to the metal or to a monomer molecule (see Appendix 1 for backgound material). A metal-alkyl species, i.e. the starting unit for a new polymer chain, arises from the metal-hydride species formed in the first case by insertion of an olefin, or it can be formed directly by f-H transfer to a monomer (Figure 20). While the results are thus identical, the two reaction paths differ in their respective kinetics In the first case, the rate-limiting p-H transfer is independent of the olefin concentration, while the rate of p-H transfer to a monomer requires the formation of an olefin-containing reaction complex and will thus increase linearly with olefin concentration. 

Other processes also contribute to chain growth termination under special conditions. In particularly crowded catalysts, fi-methyl transfer to the metal centre can occur instead of p-H transfer. When other reaction paths are blocked, a-bond metathesis, i.e. transfer of an H atom from a monomer to the metal-bound alkyl C atom can release a polymer with a saturated chain end with formation of a new unsaturated metal-bound chain start. Saturated chain ends will also result when H2 gas is added to a catalyst system thus leading to the production of shortened polymer chains. Such an H2 addition will often also cause an increase in overall catalyst activity, since H2 will predominantly react with species - such as occasional 2,1-inserted units - which are rather... 

Complexes of the type CpCo(PR3)2 are alkylated at the metal with small alkyl halides to give CpCo(PR3)2R (Scheme 25). Bulky halides produce ring-substituted hydrido cations instead, explained by attack of the electrophile from the exo site followed by ring-to-metal proton transfer. This reaction could be electrophilic addition (see Electrophilic Reaction), 5ei, or more probably radical addition initiated by electron transfer similar to the RX reaction of cobaltocene (Section 7.1). Since the oxidation potential of CpCo(P(alkyl)3)2 is more negative than that of cobaltocene, this latter mechanism is very plausible. 

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