Olefins acyclic chiral

Chiral olefins (racemic or enantiomerically pure) can provide two diastcreomers when carbene (carbenoid) CX2 is added, or four diastereomers when carbene (carbenoid) CXY is employed. Cyclic alkenes (on-ring stereoselection) are considered before acyclic olefins (acyclic stcrcosclcction) are discussed. 

For acyclic chiral alkenes the diastereofacial discrimination is often low due to their high conformational mobility. However, in certain allylsilane derivatives the diastereoselectivity of the methylenation is high for (Z)-olefins (Table 3), but is considerably lower for the corresponding ( )-olefins38,93. This effect can be explained by the allylic 1,3-strain model39. 

The stereocontrol in acyclic chiral olefins in which the basic directing group is not the stereogenic centre is generally quite poor. One exception to this rule however, was reported by Panek et al., who demonstrated that a stereogenic bulky silicon group in the allylic position of acyclic substrates can induce good diastereoselectivities. ... 

From the 1980s on, many efforts were directed toward asymmetric induction of nitrile oxide cycloadditions to give pure (dia)stereoisomeric isoxazolines, and acyclic products derived from them (17,18,20-23). The need to obtain optically active cycloaddition products for use in the synthesis of natural products was first served by using chiral olefins, relying on 1,2-asymmetric induction, and then with optically active aldehydes or nitro compounds for the nitrile oxide part. In the latter case, insufficient induction occurs using chiral nitrile oxides, a problem still unsolved today. Finally, in the last 5 years, the first cases of successful asymmetric catalysis were found (29), which will certainly constitute a major area of study in the coming decade. 

For many years acyclic stereoselection has been one of the main interests in organic chemistry35. The most studied case is 1,2-induction, usually with a-chiral carbonyl compounds or olefins. 

From among the variety of non-carbohydrate precursors, acetylenes and alkenes have found wide application as substrates for the synthesis of monosaccharides. Although introduction of more than three chiral centers having the desired, relative stereochemistry into acyclic compounds containing multiple bonds is usually difficult, the availability of such compounds, as well as the choice of methods accessible for their functionalization, make them convenient starting-substances for the synthesis. In this Section is given an outline of all of the synthetic methods that have been utilized for the conversion of acetylenic and olefinic precursors into carbohydrates. Only reactions leading from dialkenes to hexitols are omitted, as they have already been described in this Series.7... 

An example is the rhodium catalyzed hydroformylation reaction, which is an industrially important homogenous catalytic process [3]. In contrast, it is amazing that such an important transition-metal catalyzed C/C bond-forming process has been employed only rarely in organic synthesis [4]. Part of the reason stems from the difficulty in controlling stereoselectivity. Even though some recently developed chiral rhodium catalysts allow for enantio- and diastereoselective hydroformylation of certain specific classes of alkenes [5, 6], only little is known about the diastereoselective hydroformylation of acyclic olefins [7, 8]. 

In recent years, most of the attention has focused on the stereocontrolled synthesis of cyclopropanes. The isolation of several structurally intriguing natural product has revived the interest of the scientific community for the development of new methods. Several efficient and practical chiral auxiliaries have been developed for the enantioselective cyclopropanation of olefins. The most efficient chiral auxiliaries have been specifically designed for the cyclopropanation of acyclic allylic alcohols (Table 13.3, entry 1, 2, Protocol 8),24 alkenones,25 cycloalkenones26 (Table 13.3, entry 3,4, Protocol 9), vinyl ethers27 (Table 13.3, entry 5, Protocol 10), and vinylboronate esters28 (Table 13.3,... 

Considerable work was done to induce chirality via chiral auxiliaries. Reac tions with aromatic a-ketoesters like phenylglyoxylates 21 and electron-rich al kenes like dioxoles 22 and furan 23 were particularly efficient (Scheme 6). Yield up to 99% and diastereoselectivities higher than 96% have been observed whet 8-phenylmenthol 21a or 2-r-butylcyclohexanol 21b were used as chiral auxiliarie [14-18]. It should be noted that only the exoisomers 24 and 25 were obtained from the reaction of dioxoles 22. Furthermore, the reaction with furan 23 wa regioselective. 24 were suitable intermediates in the synthesis of rare carbohydrate derivatives like branched chain sugars [16], Other heterocyclic compounds liki oxazole 28 [19] and imidazole 29 [20] derivatives as well as acyclic alkenes 3fl 31, and 32 [14,15,21,22] were used as olefinic partners. Numerous cyclohexane derived alcohols [18,21-24] and carbohydrate derivatives [25] were used as chiri... 

In contrast to olefins, little is known on catalytic hydroboration of conjugated dienes. Suzuki and Miyaura20 described a 1,4-addition of catecholborane to acyclic 1,3-dienes, catalyzed with tetrakis(triphenylphosphine)pa]ladium(0). An interesting Markovnikov type regioselectivity was observed in the enantioselective dihydroboration of (E)-1-phenyl-1,3-butadiene with catecholborane, catalyzed by chiral rhodium complexes.21 However, the scope of these reactions is not well known, and the choice of catalysts is very limited. 

Asymmetric aziridination can also be accomplished via chiral salen ligands. Shi has synthesized a number of axially dissymmetric binaphthyldiimine salen complexes that have shown excellent facility in catalytic asymmetric aziridination reactions <2001TA3105>. Although yields were generally good with acyclic electron-deficient olefins, the chemical yield with electron-rich olefin indene was relatively low (25%). A reasonable enantiomeric excess of 73% was achieved at —20°C over a 24h reaction period (Equation 9). 

Yang s chiral ketones 75 have also been used as catalysts in the kinetic resolution of acyclic secondary allyl silyl ethers <2001JOC4619>. Dioxiranes generated in situ from dehydrocholic acid derivatives 122 and Oxone have been used in the asymmetric epoxidations of cinnamic acid derivatives with product ee s up to 95% <2001TA1113, 2002JOC5802> and unfunctionalized olefins (up to 98% ee) <2006T4482>. 

Charette, A B, Cote, B, Marcoux, J E, Carbohydrates as chiral auxiliaries asymmetric cyclopropanation reaction of acyclic olefins, J. Am. Chem. Soc., 113, 8166-8167, 1991. 

---