Grignard reagents general mechanism

Sodium borohydride and lithium aluminum hydride react with carbonyl compounds in much the same way that Grignard reagents do except that they function as hydride donors rather than as carbanion sources Figure 15 2 outlines the general mechanism for the sodium borohydride reduction of an aldehyde or ketone (R2C=0) Two points are especially important about this process... 

Quite generally addition to unsaturated systems takes place, e.g. to >C=0, >C=N—, — G=N, — N=0, but >C=C< and —C=C— do not react. The mechanism of the addition is as follows the Grignard reagent is added on in the form of its two components R and MgHal in the case of the 0=0-double bonds the Mg-containing constituent always unites with the oxygen, R always with the carbon. 

The addition of RLi and other nucleophiles to carbonyl functions in general proceeds via one of the two possible reaction pathways, polar addition (PL) and electron transfer (ET)-radical coupling (RC) sequence (equation 5). Current reaction design for the synthetic purpose of additions of common nucleophiles to aldehydes and ketones is mostly based on the polar mechanism, but apparently the ET process is involved in some reactions of, for example, Grignard reagents Mechanistically there are three possible variations the PL pathway, the ET rate-determining ET-RC route and the RC rate-determining ET-RC route. 

The general reaction of a Grignard reagent with a carbonyl is shown in Figure 14-1. The product of the first step in the mechanism in Figure 14-1 is a salt, which undergoes hydrolysis in the next step. The hydrolysis (an acid-base reaction) is shown in Figure 14-2. 

Any aldehyde other than formaldehyde reacts with a Grignard reagent to produce a secondary alcohol (see Figure 14-5). The general mechanism is the same for any aldehyde (see Figure 14-6). As shown in Figure 14-7, more than one path may lead to the same alcohol. 

The generality of the carbon monoxide insertion reaction is clear from reports that methylcyclopentadienyliron dicarbonyl (16), ethylcyclopentadienylmolylbde-num tricarbonyl (66), alkylrhenium pentacarbonyls (50), alkylrhodium dihalo carbonyl bisphosphines (34), allylnickel dicarbonyl halides (35), and mono-and di-alkyl derivatives of the nickel, palladium, and platinum bisphosphine halides (P), also undergo the reaction. The reaction of Grignard reagents (24), and of boron alkyls (51) with carbon monoxide probably takes place by the same mechanism. 

The reaction of Grignard reagents and organolithium compounds with aldehydes and ketones is perhaps the most useful method for the preparation of alcohols. The reaction is conducted under basic conditions and proceeds according to the following general mechanism ... 

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