Reducing agents Grignards

In equation 1, the Grignard reagent, C H MgBr, plays a dual role as reducing agent and the source of the arene compound (see Grignard reaction). The Cr(CO)g is recovered from an apparent phenyl chromium intermediate by the addition of water (19,20). Other routes to chromium hexacarbonyl are possible, and an excellent summary of chromium carbonyl and derivatives can be found in reference 2. The only access to the less stable Cr(—II) and Cr(—I) oxidation states is by reduction of Cr(CO)g. 

There were two potential starting materials for Trost s Mo chemistry (Scheme 2.7). The first approach utilized the commercially available 3-fluorocinnamic acid (4). However, reduction of 4 did not proceed well with various reducing agents and provided the desired allylic alcohol 28 in only mediocre yield. Alternatively, 3-fluorobenzaldehyde (29) was used as the starting material. Vinyl Grignard addi-... 

Pathway 2 of Scheme 9 corresponds to one of the most interesting developments in the Beckmann rearrangement chemistry. By trapping of the electrophilic intermediate with a nucleophile (Nu ) other than water, an imine derivative 227 is produced that may be used for further transformations. Carbon or heteroatom nucleophiles have been used to trap the nitrilium intermediate. Reducing agents promote the amine formation. More than one nucleophile may be added (for example, two different Grignard reagents can be introduced at the electrophilic carbon atom). Some of the most used transformations are condensed in Scheme 11. 

Other reducing agents that have been reported to afford enamines are sodium hydrosulfite [142], dialkyl aluminohydrides [143], and Grignard reagents [144] for quaternary isoquinoline salts sodium borohydrides [145] and Grignard reagents for 3,5-dicyanopyridines and electroreduction [146, 147] of iV-methylglutarimide. 

Grignard reagents and lithium tetrahydridoaluminate achieved SN2 displacement of fluoride anion (Eq. 100). Displacement of a second fluoride ion occurred with excess reducing agent, and upon the action of butyllithium. These reactions have not found extensive use in target synthesis. 

Aldehydes and ketones have also been prepared by nucleophilic cleavage of resin-bound O-alkyl hydroxamic acids (Weinreb amides [744]) with lithium aluminum hydride [745] or Grignard reagents (Entries 1 and 2, Table 3.41). Similarly, support-bound thiol esters can be cleaved with Grignard reagents to yield ketones [272], or with reducing agents to yield aldehydes (Entry 3, Table 3.41). Polystyrene-bound sele-nol esters (RCO-Se-Pol) react with alkynyl cuprates to yield alkynyl ketones [746]. 

---