Alkanes reagents

The first type of mechanism involves a redox chain process. As shown in Eqs. (1-3), it begins with the abstraction of a halogen atom from a polyhalo-alkane reagent by the metal complex. This generates a radical species that further adds to an olefin. A chain-transfer reaction ensues and yields back the reduced metal species, hence the acronym ATRA, for the sequence. 

According to Rudakov and co-workers (see reviews [41] and original papers [42]), due to the difficulty of the C-H bond rupture, the alkane activation usually begins from the preactivation of a reagent M (step a), followed by the formation of a weak adduct alkane-reagent (step b) which then is transformed into the first product Ti ... 

Insertion of zinc into the carbon-halogen bond of organozinc carbenoids is accelerated by the presence ofa catalytic amount of Pb(II) salts [40]. By this means, several l,l-(biszincio)alkane reagents such as 14,15 [41], or 16 [42] can be prepared (Scheme 4.4). The commercially available Nysted reagent 17 is also prepared through zinc insertion with zinc-lead couple [43]. In the case of aUcyl-substituted l,l-(biszincio)aIkanes, P-hydride elimination is prevented with the adjunction of TMEDA (N,N,N, N -tetramethylethylenediamine) [44]. 

Fig. 11. Cartoon representation of a molecular version of the "Chinese Finger Puzzle" constructed from a,co-bis(basic)alkane reagents and cyclodextrins modified by tosylation at each of their primary hydroxyl sites.

The coupling of alkyl Grignard reagents with alkyl iodides to afford alkanes by use of dppf as a ligand has been reported[449], but re-examination of the reaction has shown that only reduction takes place, and no coupling was observed[450]. 

The most frequently used organocuprates are those m which the alkyl group is pri mary Steric hindrance makes secondary and tertiary dialkylcuprates less reactive and they tend to decompose before they react with the alkyl halide The reaction of cuprate reagents with alkyl halides follows the usual 8 2 order CH3 > primary > secondary > tertiary and I > Br > Cl > F p Toluenesulfonates are somewhat more reactive than halides Because the alkyl halide and dialkylcuprate reagent should both be primary m order to produce satisfactory yields of coupled products the reaction is limited to the formation of RCH2—CH2R and RCH2—CH3 bonds m alkanes... 

Hydroperoxides have been obtained from the autoxidation of alkanes, aralkanes, alkenes, ketones, enols, hydrazones, aromatic amines, amides, ethers, acetals, alcohols, and organomineral compounds, eg, Grignard reagents (10,45). In autoxidations involving hydrazones, double-bond migration occurs with the formation of hydroperoxy—azo compounds via free-radical chain processes (10,59) (eq. 20). 

A catalyst, usually acid, is required to promote chemoselective and regioselective reduction under mild conditions. A variety of organosilanes can be used, but triethylsilane ia the presence of trifiuoroacetic acid is the most frequendy reported. Use of this reagent enables reduction of alkenes to alkanes. Branched alkenes are reduced more readily than unbranched ones. Selective hydrogenation of branched dienes is also possible. 

Physieal properties are similar to alkanes and the ehemistry is dietated by the earbon triple bond. This bond is less reaetive than the olefin double bond towards eleetrophilie reagents, but more... 

Numerous half-sandwich compounds of the type [M()7 -C5R5)L2], M = Rh, Ir R = H, Me L = CO, phosphine etc.) are known and are useful reagents. [Ir()7 -C5Me5)(CO)2] for instance is an excellent nucleophile and is also used in the photochemical activation of C-H in alkanes. It is particularly effective in the latter role when supercritical CO2 is the solvent. ... 

Physical properties of alkynes [49, p. 251] are essentially similar to those of alkanes and alkenes. These compounds are weakly polar and are insoluble in water, but they are quite soluble in organic solvents of low polarity (e.g., ether, benzene, CCl ). Chemically, alkynes are more reactive than alkanes but behave like alkenes. The triple bond appears to be less reactive than the double bond in some reagents while more reactive in others. In a chemical reaction, the triple bond is usually broken into a double bond, which may eventually split into single bonds. 

Alkanes are sometimes referred to as paraffins, a word derived from the Latin parum affinis, meaning "little affinity." This term aptly describes their behavior, for alkanes show little chemical affinity for other substances and are chemically inert to most laboratory reagents. They are also relatively inert biologically and are not often involved in the chemistry of living organisms. Alkanes do, however, react with oxygen, halogens, and a few- other substances under appropriate conditions. 

Structurally simple alJkyl halides can sometimes be prepared by reaction of an alkane with Cl2 or Br2 through a radical chain-reaction pathway (Section 5.3). Although inert to most reagents, alkanes react readily with Cl2 or Br2 in the presence of light to give alkyl halide substitution products. The reaction occurs by the radical mechanism shown in Figure 10.1 for chlorination. 

The a-lithiated sulfides 33 are another class of chiral organometallic reagent, readily available by deprotonation of the parent l-(phcnylthio)alkanes 32 with butyllithium in tetrahydrofuran at - 78 °C. 

It is very interesting, however, that in alkane potassium diazoate alkylations with Meerwein s reagent (triethyloxonium tetrafluoroborate, Et30+BF4) in CH2C12 suspensions or with alkyl halides in hexamethylphosphoric triamide solutions, azoxy compounds (6.4) are formed, i.e., alkylation takes place at the (3-nitrogen (Moss et al., 1972). 

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