Alkanes from Grignard reactions

The Grignard reaction is often one of the first reactions encountered for the preparation of organometallic compounds. As such it provides a method for the conversion of an alkyl bromide to an alkane. From the example shown below it is seen that the overall oxidation level change from the organic reactants to the products is from 0 to —2, so a reduction has occurred. Magnesium is the reductant and is itself oxidized from 0 to +2 oxidation state. The actual reduction takes place in the first step of the process in which the C-Br bond is converted to a C-Mg-Br bond. The reaction with water is merely a hydrolysis that does not change the oxidation state of carbon. 

The synthesis of jS-sitosteryl acetate (384) has been achieved by Grignard reaction between the resolved bromo-alkane (382), prepared from the ester (379), and 3/ -acetoxypregn-5-en-20-one (383) (Scheme 19). After acetylation and... 

Finally, it should be mentioned that Braun and Sobecki reported in 1911 [95] about the preparation of oligoalkanes. They extended the Grignard reaction to a,(o-dibromoalkanes. Starting out from 1,5-dibromopentane, decane, pentadecane and eicosane were isolated by fractionated distillation and characterized by elemental analyses and cryoscopy in benzene. The analogous dimer were obtained from 1,7-dibromoheptane and 1,10-dibromodecane. Twenty years later Carothers et al. [96] replaced magnesium by sodium and isolated pure alkanes up to heptakontane. 

Alkanes can also be prepared from alkyl halides by reduction, directly with Zn and acetic acid (AcOH) (see Section 5.7.14) or via the Grignard reagent formation followed by hydrolytic work-up (see Section 5.7.15). The coupling reaction of alkyl halides with Gilman reagent (R 2CuLi, lithium organocuprates) also produces alkanes (see Section 5.5.2). 

Considering the problem retrosynthetically, we can see that a key intermediate having the carbon skeleton of the desired product is 3-methyl-3-pentanol. This becomes apparent from the fact that alkanes may be prepared from alkenes, which in turn are available from alcohols. The desired alcohol may be prepared from reaction of an acetate ester with a Grignard reagent, ethylmagnesium bromide. 

Alkanes, RH (Sec. 7.7) (Sec. 10.7) (Sec. 10.8) (Sec. 19.9) from alkenes by catalytic hydrogenation from alkyl halides by protonolysis of Grignard reagents from alkyl halides by coupling with Gilman reagents from ketones and aldehydes by Wolff-Kishner reaction... 

Alcohols react with HX to form alkyl halides, but the reaction works well only for tertiary alcohols, R,COH. Primaiy and secondary alkyl halides are normally prepared from alcohols using either SOClj or PBr ). Alkyl halides react with magnesium in ether solution to form organomagnesium halides, or Grignard ret ents (RM O- Since Grignard reagents are both nucleophilic and basic, they react with acids to yield hydrocarbons. The overall result of Grignard formation and protonation is the conversion of an alkyl halide into an alkane (RX— RM RH). 

Organolithium and Grignard reagents are prepared from alkyl (or aryl) halides. The reactions are carried out under anhydrous conditions because reaction with water leads to the formation of alkanes. 

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