Allyltitaniums

A large number of publications appeared on these aspects5, but most of these studies did not address stereochemical questions. In most cases, a given synthetic problem can be better solved by other allylmetals. Grignard reagents have some importance as intermediates for the preparation of allylboronates (Section D.1.3.3.3.3.2.1.), allylsilanes (Section D.1.3.3.3.5.2.L), allyl-stannanes (Section D. 1.3.3.3.6.2.1.1.), or allyltitanium derivatives (Section D.I.3.3.3.8.2.). 

In general, there are three main types of allyltitanium reagent, usually prepared in situ from allyllithium or allylmagnesium derivatives ... 

Hydroxyalkylation of these allyltitanium derivatives by carbonyl compounds proceeds at the more highly substituted terminus. Exceptions are observed for those hetero-substitutcd derivatives in which the chelating substituent Y ties the cation to the 1-position. 

Thus, allyltitanium(IV) reagents offer a number of advantages which make their utilization very appealing ... 

I.3.3.3.8.2. Stereoselective Carbonyl Addition Reactions of Allyltitanium and Allylzirconium Reagents... 

The regio- and diastereoselectivity in carbonyl addition of unsymmetrically di- or higher substituted allyltitanium intermediates depend largely on the type of attached groups. 

An isopropyl group on both cyclopentadienyl residues causes a further increase in the antijsyn ratio84. Cyclic fy3-allyltitanium complexes also add to aldehydes with good results85 the cyclo-pentenyl derivative, as may be expected from its enforced cis configuration, affords adducts of the syn series83. 

Only few allyltitanium reagents bearing a removable chiral auxiliary at the allylic residue are known. The outstanding example is a metalated 1-alkyl-2-imidazolinone14, derived from (—)-ephedrine, representing a valuable homoenolate reagent. After deprotonation by butyllithium, metal exchange with chlorotris(diethylamino)titanium, and aldehyde or ketone addition, the homoaldol adducts are formed with 94 to 98% diastereoselectivity. 

Table 9. Enantioselective Alkenylations with Chiral 3-Substituted Allyltitanium Complexes...

The combination of metal tuning and double stereodifferentiation helps to prepare chelation and nonchelation products in the imine series7. In the case of an alkoxy substituent adjacent to the aldimino, the chelation product 10 is predominantly obtained with allylmagnesium chloride, chloromagnesium allyltriethylaluminate or allylzinc bromide, while the use of allyl-boronates or allyltitanium triisopropoxide, which lack the requisite Lewis acidity for chelation, gives 11 with good Cram selectivity. 

Gais and co-workers recently reported the preparation and use of chiral sulfoximine-substituted allyltitanium(iv) complexes of type 78 for the asymmetric synthesis of 3-substituted unsaturated proline derivatives 79 (Scheme 31).108... 

Asymmetric carboxylation was also achieved by use of 7r-allyltitanium complexes bearing chiral ligands (Scheme 13).26 The enantiomeric selectivity was significantly influenced by the chiral auxiliary ligands used. In the case of 45, the chiral /3,7-unsaturated carboxylic acid 46 was obtained with excellent ee.27,28... 

Preparation of allyltitaniums of the type (allyl)Ti(OiPr) 3 from the corresponding allyl-lithium or -magnesium compounds and ClTi(OiPr)3 by transmetallation and their subsequent synthetic utilization have attracted considerable interest because of the advantageous reactivity of the allyltitaniums as compared to other allylmetal complexes in terms of chemo-, regio-, and diastereoselectivity [3], The preparation of certain allyllithium or -magnesium reagents, however, is not necessarily easy, which would seem to limit the utility of this method. 

The titanium reagent 1 reacts with allyl alcohol derivatives, such as halides, acetates, carbonates, phosphates, or sulfonates to afford allyltitanium complexes of the type (r 1-al-lyl)TiX(OiPr)2, as shown in Eq. 9.21 [42], As a variety of allylic alcohols are easily obtained,... 

The following synthetically attractive features of the reaction are apparent. The reaction allows the preparation of allyltitaniums bearing functional groups, thus providing easy access to functionalized compounds (Eq. 9.22). 

Alkylidenecydopropane derivatives can readily be prepared by the reaction of 1 with vinylcyclopropyl carbonates and subsequent trapping of the resulting allyltitaniums with aldehydes or ketones (Eq. 9.24) [44], It should be noted that, in this case, the carbon—carbon bond formation occurs at the less substituted allylic terminus, and not at the more substituted end of the titanium reagent, the latter being the position at which addition to substituted allyltitanium reagents is usually observed. 

Reaction of the allyltitaniums with D20 and NCS proceeds with excellent regioselectiv-ity, and thus a new one-pot method for converting ally] alcohol derivatives to 1-alkenes having D and Cl at the allylic position is opened up (Eq. 9.26) [46]. 

The preparation of chiral allyltitanium reagents having a stereogenic center and their utilization in asymmetric synthesis have been pursued. As shown in Eq. 9.27, chiral allyl-... 

The synthesis of optically active compounds by the diastereoselective reaction of allyltitanium reagents with chiral electrophiles has also been reported. The reaction of allyltitanium reagents with chiral imines proceeds with excellent diastereoselectivity, as shown in Eq. 9.28, thus providing a new method for synthesizing optically active homoallylic amines with or without a P-substituent [51,52],... 

In contrast to the allyltitaniums derived from acrolein cyclic acetals, such as 1,2-dicyclo-hexylethylene acetal shown in Scheme 9.8, those derived from acrolein acyclic acetals react with ketones and imines exclusively at the y-position. As shown in Eq. 9.29, the reaction with chiral imines having an optically active 1-phenylethylamine moiety proceeds with high diastereoselectivity, thus providing a new method for preparing optically active 1-vinyl-2-amino alcohol derivatives with syn stereochemistry [53], The intermediate allyltita-nium species has also found use as a starting material for a carbozincation reaction [54],... 

The cyclization of dienynes was found to proceed equally well, as shown in Eq. 9.55, and the resulting intermediate titanacycle reacts regiospecifkally with aldehydes at the remote position of the most likely allyltitanium system [101]. 

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