Magnesium amides chiral

Magnesium amides have also found good utility in enantioselective deprotonation processes. A range of chiral amines has been prepared by Henderson and coworkers and it was found after conversion to their Mg-bisamide derivatives that it react with 4- and 2,6-substituted cyclohexanones with good to excellent selectivities (see Section m). Structures of some chiral magnesium amides are given in Chart 1. 

The concept of chiral magnesium amides for the preparation of magnesium enolates has been extended to chiral magnesium bis(sulfonamide) complexes as catalysts for the enolization of A-acyloxazolidines ° (equation 63). 

Recently, Henderson has investigated the effect of Lewis base additives such as HMPA in enantioselective deprotonation of ketones mediated by chiral magnesium amide bases. In almost all reactions investigated, the additive HMPA could be replaced by DMPU without any undue effect on either selectivity or conversion (equation 69) ... 

The chemistry of magnesium bisamides has been reviewed" . They can be used for the regio- and stereoselective formation of enolates", while chiral magnesium amides are applied in asymmetric synthesis for enantioselective enolisations ". 

The chemistry of magnesium bis-amides has been reviewed.21 Magnesium bis-amides have been used for the region- and stereoselective formation of enolates.2 a Enantioselective enolization with chiral magnesium amides has been applied in asymmetric synthesis.23 233... 

Novel and readily accessible polymer-supported chiral magnesium amide reagents have been prepared and shown to be effective in the asymmetric deprotonation of a series of prochiral cyclohexanones, affording good to excellent conversion and enantiomeric ratio (up to 93 7) the Merrifield-based chiral amine species has been shown to be readily recyclable (Scheme 3.39) [29]. 

The chemistry of magnesium bisamides has been reviewed. They can also be used for the regioselective and stereoselective enolization, and chiral magnesium amides can be advantageously used for enantioselective enolizations. ... 

Chiral organomagnesium amides form an efficient method to realize enantioselective conjugate addition. Sibi and Asano have reported the Michael addition of cr-bound magnesium reagents derived from bisoxazolines to enamidomalonates (equation 41). The enantioselectivity of the addition is discussed in Section El. This method allows the preparation of chiral -amino acid derivatives. ... 

Aldol reactions of simple amide enolates give poor stereoselection. Stimulated by the interest in /3-lactams, the stereochemistry of aldol reactions of chiral magnesium enolates of /3-lactams has been studied . The best results have been obtained with 6,6-dibromopenams 85 (equation 108). After bromine-magnesium exchange with MeMgBr,... 

Other chiral magnesium enolates derived from amides are known to react with aldehydes. For example, the aldol-type reaction of magnesium enolate of —)-trans-2-N,N-diethylacetamide-l,3-dithiolanes-5 -oxide with isobutyraldehyde affords a single diastere-omer in 82%. The relative stereochemistry of the adduct originates from a rigid transition state 87 where the oxygen atoms of the enolate and the aldehyde are coordinated to the magnesium atom. ... 

Novel chiral bases introduced recently include the magnesium amide base 7 and analogs reported by Henderson [48] and Knochel s urea base 8 (mono and di-anions) [49]. 

Intramolecular cycloaddition reactions of chiral tertiary amides derived from auxiliary 27 have been studied by Mukaiyama et al. In thermal reactions moderate results were observed. However, useful results were obtained by treatment with nBuMgBr via the magnesium alkoxide formed as an intermediate. 

A review article has discussed die zirconocene dichloride catalysed cyclometalation of olefins and alkynes with alkylalanes. A usefiil thesis of zirconocme methylamides has been documoited from the sequential reactimi of magnesium amide and a mediyl Grignand reagent to zirconocene dichloride. Chiral ethylene bridged onsa-zirconocene dichlorides have... 

In various cases, Koga s bases 75 and 76 performed better with respect to enantioselectivity [77]. The groups of Williard [78] and Majewski etal. [79] introduced - soluble or cross-linked - polymer-supported lithium bases, as illustrated by the chiral deprotonating agent 77. Aside from lithium amides that were used by far most frequently, chiral magnesium bisamide 74 and polymer-supported regents derived thereof were also applied in enantioseiective deprotonations of prochiral ketones [80]. 

Silyl enol ethers react with aldehydes in the presence of chiral boranes or other additives " to give aldols with good asymmetric induction (see the Mukaiyama aldol reaction in 16-35). Chiral boron enolates have been used. Since both new stereogenic centers are formed enantioselectively, this kind of process is called double asymmetric synthesis Where both the enolate derivative and substrate were achiral, carrying out the reaction in the presence of an optically active boron compound ° or a diamine coordinated with a tin compound ° gives the aldol product with excellent enantioselectivity for one stereoisomer. Formation of the magnesium enolate anion of a chiral amide, adds to aldehydes to give the alcohol enantioselectively. 

Sodium amide, o LDA, l.illMI)S. " NaHMDS, KHMDS, ° sodium and magnesium methoxide or ethoxide, and DBU are less frequently used. The nature of the cation present in the Homer-Wadsworth-Emmons reaction depends on the chosen base and greatly influences the stereoselectivity of the reaction. The chiral lithium 2-aminoalkoxides (17 ,2S) have been used as chiral bases for the enantioselective reaction between diethyl cyanomethylphosphonate and 4-terz-butylcyclohexanone." ... 

---