Synthesis from metal alkyls/hydrides

There are now a number of quite stable Pt(IV) alkyl hydride complexes known and the synthesis and characterization of many of these complexes were covered in a 2001 review on platinum(IV) hydride chemistry (69). These six-coordinate Pt(IV) complexes have one feature in common a ligand set wherein none of the ligands can easily dissociate from the metal. Thus it would appear that prevention of access to a five-coordinate Pt(IV) species contributes to the stability of Pt(IV) alkyl hydrides. The availability of Pt(IV) alkyl hydrides has recently allowed detailed studies of C-H reductive elimination from Pt(IV) to be carried out. These studies, as described below, also provide important insight into the mechanism of oxidative addition of C-H bonds to Pt(II). 

Many synthetic methods have been reported for the pyrrolidine alkaloids, including procedures based on the Hofmann-Loffler reaction 132,412), the metal hydride reduction of pyrrolines 413,414), the a-alkylation of N-nitro-sopyrrolidine 412,415), the catalytic hydrogenation of pyrroles 133), the reductive amination of 1,4-diketones 25,138), the direct alkylation of 1-methoxy-carbonyl-3-pyrroline 416), the versatile synthesis from the Lukes-Sorm dilac-... 

Other methods for obtaining complexes of ethylene and other alkenes include ligand substitution reactions, reduction of a higher valent metal in the presence of an alkene, and synthesis from alkyl and related species [reductive elimination, of an allyl or hydride, for example hydride abstraction from alkyls protonation of sigma-allyls from epoxides (indirectly)] [74a],... 

The ease of reversal of alkene insertion is evident from the numerous syntheses of transition metal-hydride complexes using main group metal alkyls as the source of hydride. The hydride in the products of such reactions usually arises from -hydride abstraction or elimination from intermediate unstable transition metal alkyls. This idea is reinforced by the greater effectiveness of secondary alkyls such as isopropyl or cyclohexyl compounds. However, it has been shown that in at least one case the hydride results from hydrolysis of a Pt-Mg bond, not from the alkyl formed from reaction of a Pt-Cl bond with a Grignard reagent. Several of the reactions listed in Table 1 are spontaneously reversible. Reactions where -hydride elimination has been used in the synthesis of hydrides are listed in Table... 

Transition-metal-silyl complexes are also formed by the reactions of metal-alkyl complexes with silanes to form free alkane and a metal-silyl complex. Two examples are shown in Equations 4.114 and 4.115. ° The synthesis of silyl complexes by this method has been accomplished with both early and late transition metal complexes. The formation of metal-silyl complexes from late-metal-alkyl complexes resembles the hydrogenolysis of metal-alkyl complexes to form metal hydrides and an alkane. The mechanisms of these reactions are discussed in Chapter 6. In brief, these reactions with late transition metal complexes to form silyl complexes typically occur by a sequence of oxidative addition of the silane, followed by reductive elimination of alkane. An example of this is shown in the coupling of 1,2-bis-dimethylsilyl benzene with a dimethyl platinum(II) complex (Equation 4.114). Similar reactions occur with d° early metal complexes by a a-bond metathesis process that avoids these redox events. For example, the reaction of Cp ScPh with MesSiH, has been shown to proceed through this pathway (Equation 4.115). ... 

The Williamson ether synthesis is the most widely used method to produce ethers. It occurs by an Sj 2 reaction in which a metal alkoxide displaces a halide ion from an alkyl halide. The aUcoxide ion is prepared by the reaction of an alcohol with a strong base such as sodium hydride. 

In the case of ethane, this mechanism cannot occur since the resulting metal-ethyl intermediate does not display any alkyl group in the P-position. Consequently, with tantalum hydride(s), 3, which cleave ethane, another process must take place, involving only one carbon atom at a time. Among various reasonable possibilities, we assume a carbene deinsertion from a tantalum-ethyl species because the reverse step is known in organometallic chemistry (Scheme 3.4) [22]. Note that this reverse step has been postulated as the key step in Fischer-Tropsch synthesis [23]. 

We then studied group 5 metals, especially tantalum-for which the laboratory already had great experience. Because of the studied reaction, alkyl or hydride-type compounds such as those developed in the laboratory could not be employed. Consequently, we became interested in alkoxo-type derivatives, either synthesized by reaction of the grafted complex with an alcohol or obtained by direct synthesis starting from an alkoxy-tantalum compound grafted on silica. In all cases, resulting complexes have been characterized by surface organometallic chemistry techniques, especially EXAFS and solid-state NMR (ID and 2D with C-labeled compounds). Indeed various compounds bonded by one, two or three surface bonds have been prepared and characterized. 

In biochemistry, metal hydrides such as NaBH4 have been widely used in synthesis. For example, NaBH4 has been used in the preparation of alkyl cobalamins from cyanocobalamin , and in the synthesis of the chiral [/3,y- 0 7 0, 0]ATPyS from the 5 -aldehyde of adenosine . Lithium triethylborohydride (also known as Su-... 

Eliminations from Os(CO)4RR occur by dinuclear mechanisms only if either R or R is H. A hydride on one metal is necessary to interact with a vacant coordination site on the other in the dinuclear transition state. With Os(CO)4H2, the vacant site is created by dissociation of CO. With Os(CO)4-(H)CH the vacant site is created by a facile rate-determining isomerization which we suggest is to an acetyl hydride. The unique instability of hydridoalkyl carbonyls thus is explained. The synthesis and properties of Os(CO)4(H)C2H and various polynuclear ethyl osmium derivatives show that (3-hydrogens have no significant effect on these elimination mechanisms. Dinuclear hydridoalkyls are excellent starting points for the synthesis of more complex polynuclear alkyls. 

Another application of the direct alkylation of metal-14 anions is the synthesis of polymer-supported organotin hydrides. These were prepared by the reaction of stannyl group was separated from the phenyl ring of polystyrene by two, three or even four carbon spacers. These polymers were found to contain 0.8-1.4 mmol of Sn-H per gram. The reducing ability of the polymer-supported organotin hydrides was monitored by reactions with haloalkanes (Scheme 22)142. 

It should be pointed out that intermolecular hydrostannation of acetylene compounds which can be induced by radical initiators, transition metal catalysts, base catalysts, or Lewis acids to form vinylstannanes has also been known for many years. However, the first synthesis of 1-benzostannepines 34 was described in 1998 from (Z)-l-(t>-bromo-phenyl)but-l-en-3-ynes 32 via the tin hydride intermediate 33 by the intramolecular 1 -endn-dig-n ng closure at the sp carbon atom of ethynyl moiety (Scheme 3) <1998CC767, 2000J(PI)1965>. 2-Alkyl-l-benzostannepins thus obtained are stable (not sensitive to air, light and moisture, and colorless oils). 

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