Allyl organometallics

L3.3.3.3.1.1.1. From Allylic Organometallic Reagents and Electrophilic Boranes (a) Allylboranes and Allylboronates... 

One of the most general preparative routes to allyl- and 2-butenylboranes involves the reaction of an allylic organometallic species and an electrophilic borylating reagent. Various esters of allylboronic acid have been prepared in this way2,4-5. 

A potentially useful route to substituted (Z)-allylboron derivatives involves the selective cis hydrogenation of propynylboron derivatives. One recent report applied this approach in the synthesis of a (Z)-3-trimethylsilyl-2-propenylboronate, which cannot be prepared by the allyl-organometallic route discussed in Section 1.3.3.3.3.1.1.1. The selectivity for the Z-isomer was only 9 1 21. The scope of this method remains to be fully documented35. 

Further reactions of allyl organometallics with a-alkoxyaldimines 1, prepared from (S)-2-(methoxymcthoxy)propionaldehyde and (R)- and (S)-l-phenylethylamine, illuminate the difference in the influence of the nitrogen chiral auxiliary and the x-alkoxy center7. 

As the results show, the chirality of the a-alkoxy center, as well as the type of allyl metal employed, are the two most important determinants for the stereochemical outcome of the reaction. In other words, the 1,2-asymmetric induction combined with the right choice of the allyl organometallic overrides the influence of the chiral nitrogen substituent. 

For a review of reactions of allylic organometallic compounds, see Courtois, G. Miginiac, L. J. Organomet. Chem., 1974, 69, 1-44. 

Oximes are also good substrates for allylsamarium bromide addition.25 The Beckmann rearrangement product 44 was produced from oxime 43 in good yields when the ratio of allysamarium bromide to oximes was more than 3 1 (Equation (8)). This type of product was also obtained when the other allylic organometallic compounds were used. The reaction mechanism was proposed as shown in Scheme 13. 

As a result, allylic organometallic reactions have attracted much attention, and the allylation or crotylation reactions via the corresponding organometallic compounds have been extensively studied. This method has become one of the... 

Silicon-based Lewis acids have been known for some time, and the related chemistry in catalysis has recently been reviewed [24]. Most examples in the literature are mainly based on achiral species and will be discussed only briefly in this section. In general, a broad variety of reactions can be catalyzed with compounds like MejSiOTf, MejSiNTf or MOjSiClO. One advantage over some metal Lewis acids is that they are compatible with many carbon nucleophiles like silyl enol ethers, allyl organometallic reagents and cuprates. 

From Hard Allylic Organometallics. The most common preparation of allylic boranes and boronates is the addition of a reactive allylic metal species to a borinic or boric ester, respectively (Eqs. 10 and 11). Preparations from allyllithium, " " allylmagnesium, and allylpotassium " ° reagents are all well known. These methods are popular because the required allylic anions are quite easy to prepare, and because they generally lead to high yields of products. 

Compared to the reaction of benzylic systems, the situation with allylic organometallic compounds is much more complicated. 

Nitrogen-Substituted Allylic Organometallic Compounds From Allylantines... 

For the (S-elimination to occur smoothly the reaction mixtures must be warmed to room temperature otherwise the intermediate allylic organometallic 49 undergoes a second homologation (equation 22). 

The allene synthesis strategy outlined in equation 21 can be rendered asymmetric if non-racemic sulfoxides such as 47a are employed26. The diastereoselective step is an equilibration of the sp3-allylic organometallic species 53 before the -elimination step (equation 23). 

Intramolecular metallo-ene reactions are thermodynamically favored and are thus more efficient than the intermolecular versions. The classical distinction of the cyclization modes, suggested by Oppolzer, depends on the carbon of the allylic organometallic (ene-component) to which the alkene or the alkyne (enophile) is linked (equation 77)5,6. 

By analogy with allylic organometallic compounds (see Section . ), the possibility of achieving intramolecular related zinc-ene reactions involving allenylzinc species acting as ene-components has been investigated. Such reactions benefit from favorable thermodynamics and were thus expected to proceed more readily than the related intermolecular additions of allenylzincs to alkynes or alkenes. 

Enantiomerically-pure sulfoxides are readily available. Ilan Marek of Technion-Israel Institute of Technology reports (1. Am. Chem. Soc. 125 11776, 2003) that alkyne-derived sulfoxides such as 8 can be used to direct the addition of an allylic organometallic, prepared in situ, to an aldehyde 9. Both the secondary alcohol, from the aldehyde, and the adjacent quaternary center of 10 are formed with >99% stereocontrol. 

Vara Prasad and Rich [35] examined the diastereoselectivity of addition of allylic organometallics 19 to N,N-diprotected a-amino aldehydes 4 (Scheme 8). 

Dithioacids themselves (R CSSH) could be prepared in about a 40-50% yield by addition of Grignard reagents to caibon disulfide in THF as the solvent, followed by acidification in the presence of ether or pentane [143], Hartke has reported that the yields of dithioacids could be improved by reverse addition of the Grignard reagent to carbon disulfide [144], a protocol previously used by Julia [145] in the preparation of dithioesters from allylic organometallics. A 70-80% yield of dithioacetic acid was thus achieved on a molar scale. 

For reactions of allylic organometallics that proceed with allylic transposition, see page 351, Section 5. 

The a or y carbon atom in an allylic organometallic compound could also be attacked by an electrophile in a single bimolecular encounter the latter would give rise to mechanism SE2 (open) if the transition state was of the open type and is again illustrated for a crotyl compound, viz. 

Denmark, S. E. Fu, J. Catalytic enantioselec-tive addition of allylic organometallic reagents to aldehydes and ketones. Chem. Rev. 2003,... 

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