Ketones boron reagents

Entry Ketone Boron reagent Solvent Reaction conditions Isolated Configuration yield (%) Ee(%)... 

Asymmetric Reduction of Unsymmetrical Ketones Using Chiral Boron Reagents Review Synthesis 1992, 605. 

Chiral diamino carbene complexes of rhodium have been merely used in asymmetric hydrosilylations of prochiral ketones but also in asymmetric addition of aryl boron reagents to enones. 

Although both boronates and alanates react with allylic bromides, aldehydes and C02 to afford allenic products in satisfactory yield, the alanates are more efficient in additions to ketones (Table 9.14). Boronate reagents do not require a B-C alkyl migration for their preparation. Thus the starting acetylene possesses the structural elements of the product. Additionally, the issue of dummy ligands is irrelevant. 

The CBS-reduction [137] of prochiral ketones is a well-known process which employs a chiral oxazaboroHdine as catalyst and BHs-THF or catecholborane as stoichiometric reductants. It is believed that the active catalytic species is a LLA, resulting from coordination of the oxazaborolidine nitrogen with the boron reagent to render the oxazaborolidine boron atom highly Lewis acidic [87]. Similarly, Corey... 

Reviews on stoichiometric asymmetric syntheses M. M. Midland, Reductions with Chiral Boron Reagents, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 2, Chap. 2, Academic Press, New York, 1983 E. R. Grandbois, S. I. Howard, and J. D. Morrison, Reductions with Chiral Modifications of Lithium Aluminum Hydride, in J. D. Morrison, ed.. Asymmetric Synthesis, Vol. 2, Chap. 3, Academic Press, New York, 1983 Y. Inouye, J. Oda, and N. Baba, Reductions with Chiral Dihydropyridine Reagents, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 2, Chap. 4, Academic Press, New York, 1983 T. Oishi and T. Nakata, Acc. Chem. Res., 17, 338 (1984) G. Solladie, Addition of Chiral Nucleophiles to Aldehydes and Ketones, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 2, Chap. 6, Academic Press, New York, 1983 D. A. Evans, Stereoselective Alkylation Reactions of Chiral Metal Enolates, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 3, Chap. 1, Academic Press, New York, 1984. C. H. Heathcock, The Aldol Addition Reaction, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 3, Chap. 2, Academic Press, New York, 1984 K. A. Lutomski and A. I. Meyers, Asymmetric Synthesis via Chiral Oxazolines, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 3, Chap. 

Ketone reduction is achieved by using a boron reagent. 

As above (eq 1), a major drawback of this reagent is the lack of a readily available enantiomer. There are many alternative methods for the enantioselective propionate aldol reaction. The most versatile chirally modified propionate enolates or equivalents are N-propionyl-2-oxazolidinones, a-siloxy ketones, boron enolates with chiral ligands, as well as tin enolates. Especially rewarding are new chiral Lewis acids for the asymmetric Mukaiyama reaction of 0-silyl ketene acetals. Most of these reactions afford s yw-aldols good methods for the anri-isomers have only become available recently. ... 

Boron reagents such as ( + )- or (-)-(Ipc)2BOTf are chiral promoters in aldol condensations. " Enolization of an achiral ketone with (Ipc)2BOTf forms a chiral enolate and thus imparts diastereofacial selectivity (DS) for condensation with a chiral aldehyde. If the ketone is chiral, the DS of the reagent may be matched or mismatched with the... 

Reaction with ketones. The reagent reacts very sluggishly with ketones, and usci ally the product is an enol ether. However, in the presence of boron trifluoride,... 

It is all very well knowing what enolates give rise to what aldol products, but this is not much use unless we can choose to have cis or trans enolates. Clearly with cyclopentanone there is no option but to form a trans enolate. But with an acyclic ketone we will need a little more care. Boron enol ethers provide an answer. Two common boron reagents used are 9-BBN chloride 46 (derived from 9-BBN 45) and another is dicyclohexylboron chloride 47. Other reagents include the corresponding triflates. There are also the chiral boron reagents that we shall meet later chapters. 

Paterson et have prepared the enolate of 3-pentanone, an achiral ketone, with (-( )- or (-)-IpcaBOTf and have found that its aldol reactions with various aldehydes proceed with high syn.anti ratios (>9 1) and respectable enantioselectivities (5 1-20 1) (Scheme 44). High degrees of asymmetric induction are noted with unhindered aldehydes, llie combination of the chiral ethyl ketone (104) and (-t-)-Ipc2BOTf constitutes a matched pair, which enhances the diastereofacial selectivity of the resulting enolate (compared to that obtained with an achiral boron reagent), and provides via aldol reactions high... 

Table 10 Asymmetric Aldol Reaction of Ketones with Various Boron Reagents"...

Unless otherwise noted, the enolates are prepared by adding the boron reagent (1.2 equiv.) to a mixture of the ketone (1.0 equiv., 0.15-0.20 M) and PHjNEt (1.2 equiv.), isobutyraldehyde (1.5 equiv.) is then added to the enolate. "Prepared from (a)-(+)-pinene. 

Ketone enolates have also been investigated in the asymmetric boron-mediated aldol reaction. The chiral boron reagents (+)- or (-)-diisopinocampheylboron tri-flate [(lpc)2BOTf], derived from a-pinene, allow the formation of the m-enolate and promote enantioselective aldol reactions with aldehydes to give either enantiomer of the syn aldol product. For example, the asymmetric aldol reaction between pentan-3-one and 2-methylpropenal takes place in the presence of (-)-(Ipc)2BOTf and diisopropylethylamine to give the syn aldol product 74 as the major enantiomer (1.84). 

When the ketone or the aldehyde contains a chiral centre, then the use of a chiral boron reagent can result in a matched or a mismatched pair. The two chiral groups will either both favour the same stereoisomer of the product, or will work in opposition to one another. Normally, the reaction is carried out first in the absence of the chiral reagent in order to assess the extent of stereoselectivity afforded by the chiral ketone (or aldehyde) alone. One or both enantiomers of the chiral boron reagent can then be used to promote the reaction and to determine the relative influence of the chiral groups. The matched pair enhances the stereoselectivity, whereas the... 

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