Alkenes stoichiometric chiral reagents

E. Stereoselective Cyclopropanation of Alkenes using Stoichiometric Chiral Reagents... 

The cyclopropanation of alkenes using external stoichiometric chiral additives can be divided according to their general mechanistic scheme into two classes. The enantios-elective cyclopropanation of allylic alcohols, in which a pre-association between the corresponding zinc alkoxide and the zinc reagent probably takes place, constitutes the first class. The second class involves the enantioselective cyclopropanation of unfunctionalized alkenes. The latter implies that there will be no association between the reagent and the alkene through alkoxide formation. 

The chiral sulfur(vi) reagent, A -[[J-(/i-toluenesulfonimido)-6 -(/i-tolyl)]sulfonyl]amide 431 upon reaction with iodosylbenzene 426 affords in situ the chiral iminoiodane 432. In the presence of Cu(MeCN)4PF6 as catalyst, iminoiodane 432 forms the complex 433 that very efficiently transfers the nitrene moiety together with the stereogenic information under stoichiometric conditions to a variety of alkenes 414 the corresponding aziridines 434 were obtained with diastereoselectivities up to 60% (Scheme 112) <20040L4503>. 

The chiral nitridomanganese complex 545 represents a novel self-contained asymmetric nitrogen-transfer reagent which has been used to convert alkenes to scalemic aziridines directly, although a stoichiometric amount of transfer reagent is required. This protocol makes use of A -2-(trimethylsilyl)ethanesulfonyl chloride (SESCl) (546) as an activator, providing A -SES-aziridines 547 that are easily deprotected under mild conditions using... 

The chiral ruthenium(II) carbene complex 8, prepared from diazo(trimethylsilyl)methane, (p-cymene)2ruthenium(II) chloride, and 2,6-bis(4-isopropyloxazolinyl)pyridine, has been introduced as catalyst for the enantioselective cyclopropanation of alkenes with ethyl diazoacetate. The carbene complex 8 also serves as a transfer reagent for trimethylsilylcarbene and cyclopro-panates styrene in 34% yield. This reaction demonstrates the similarities between catalytic and stoichiometric cyclopropanations and between in situ generated and isolated transition metal carbenes. 

In the last few chapters we have started with a prochiral molecule having enantiotopic features the faces of an alkene for instance - and differentiated these faces with enantiomerically pure reagents or catalysts. In this chapter we explore the alternative approach. The enantiotopic faces are transformed into diastereotopic faces by the covalent attachment of a chiral auxiliary. It seems inevitable that this auxiliary must be stoichiometric but you will even see that catalytic substrate methodology is starting to emerge. It also seems inevitable that two extra reactions will be needed the attachment and the removal of the chiral auxiliary. In spite of this, one of the most important methods of asymmetric synthesis, centred around Evans s amino acid-based chiral auxiliaries, uses this approach. 

Preparation of enantiopure chiral molecules by transformation of prochiral substrates can offer the most elegant of available approaches, especially when the source of chirality is a man-made chemical catalyst rather than a reagent used in stoichiometric quantitites. Tremendous effort has been devoted to the development of asymmetric synthesis methodology, with notable success in the fields of asymmetric hydrogenation [98], hydride reduction of ketones [99], epoxidation [100] and dihydroxylation [101] of alkenes. In constrast to the enzymes which are used in organic synthesis, man-made chiral catalysts [102] are much simpler molecular entities and are routinely available in both enantiomeric forms. Since reactions employing such catalysts usually follow a predictable course, the correct form can be chosen for the desired product configuration. 

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