Isopinocampheyl ligands

Use of diisopinocampheyl boron chloride in place of the triflate affords E(0)-enolates, but the isopinocampheyl ligands were ineffective for anti aldol reactions [48]. Encouraged by the molecular mechanics analysis of the ZfOj-enolate additions, Gennari and Paterson used computational methods to design a new boron ligand for use with E(G)-enolates [97]. The design was cued by Still s comment [98] that cis-2-... 

Aldols Based on Chiral Enolates Besides the predetermined selectivity observed in aldol reactions enforced by the earlier described factors, chiral auxiliaries on the enolate part can interfere and overrule the facial selection. One example is the use of isopinocampheyl ligands as introduced by Paterson. In both cases, the 2,3-syn and the 2,3-flnft-aldehyde can be converted to their diastereoisomers in good to excellent selectivities (Scheme 2.105). 

However, if the isopinocampheyl residue is used instead of achiral ligands (such as butyl or cyclohexyl) in the enolborinate, the diastereoselectivity is slightly improved (93 7). Thus, the combination of substrate-induced and auxiliary-induced stereoselectivity has only a marginal effect in this case56. 

Advantages of Brown s chiral allylboranes (isopinocampheyl and later caranyl borane) are the easy access to the ligands (a-pinene is a natural product, chiral pool), the availability of both enantiomers and their low price. Excellent selectivities (96-99% ee) can be obtained at reaction temperatures of -100 °C. Other important mediators for enantioselective allylation of aldehydes are shown below.14... 

Table 5.6. Asymmetric aldol additions of ketone enolates using chiral ligands on boron (Ipc = isopinocampheyl MeMn = methylmenthyl). See Schemes 5.16 and 5.17.

Asymmetric allylation and crotylation, synthetically equivalent to the aldol reaction, have been extensively studied and have become a very useful procedure for preparation of propionate units. Among various chiral ligands on boron-developed, isopinocampheyl- and tartrate-derived reagents, 51 and 52, which were developed by Brown et al. [18] and Roush et al. [19], respectively, are the most commonly used (Scheme 7). Reaction of aldehyde with (Sl-Sla or 52a gave anu -adduct 54, while that using (Z)-51b or 52b afforded syn-adduct 53 with high asymmetric selectivity. 

A chiral alkyltris(pyrazolyl)borate ligand has been used in the synthesis of [(p-cymene)Ru(K -(Ipc)BpZ3)]PFg (Ipc = isopinocampheyl) (Fig. 21.8a) [21]. The complex [(p-cymene)RuCl(/c -(dmap)Bpz3)]PFg has been recently reported... 

Again, cbiral ketones can be combined with Paterson s isopinocampheyl boron concept. The great advantage of Paterson s strategy is the fact that the Ipc ligands overrule the inherent selectivity and thus take over control of stereoselectivity (Scheme 2.108). A common subseqnent step is the snbseqnent reduction to yield either the anti- or syn-diol. These strategies will be discussed in detail in the snbseqnent chapter. 

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