Titanate enolates

Fig. 39 Ruthenium-catalyzed radical addition reactions to titanate enolates...

Titanium Complexes of Unsaturated Alcohols. TetraaHyl titanate can be prepared by reaction of TYZOR TPT with aHyl alcohol, followed by removal of the by-product isopropyl alcohol. EbuUioscopic molecular weight determinations support its being the dimeric product, octaaHoxydititanium. A vinyloxy titanate derivative can be formed by reaction of TYZOR TPT with vinyl alcohol formed by enolization of acetaldehyde (11) ... 

The reaction is believed to proceed via a six-membered cyclic transition state, analogously to the carbonyl addition of enolates, but the energy differences between boat- and chair-like arrangements are lower for x-sulfinyl carbanions69. Tor tert-butyl sulfoxides only anti- and, vn-products are obtained, arising from the approach onto the same diastereotopic face of the anion, but with different relative topicity. The exchange of lithium by zinc causes an increase of the anft-produci, but attempts to titanate the anion failed (see Table 3)69. 

Covalently bonded chiral auxiliaries readily induce high stereoselectivity for propionate enolates, while the case of acetate enolates has proved to be difficult. Alkylation of carbonyl compound with a novel cyclopentadienyl titanium carbohydrate complex has been found to give high stereoselectivity,44 and a variety of ft-hydroxyl carboxylic acids are accessible with 90-95% optical yields. This compound was also tested in enantioselective aldol reactions. Transmetalation of the relatively stable lithium enolate of t-butyl acetate with chloro(cyclopentadienyl)-bis(l,2 5,6-di-<9-isopropylidene-a-D-glucofuranose-3-0-yl)titanate provided the titanium enolate 66. Reaction of 66 with aldehydes gave -hydroxy esters in high ee (Scheme 3-23). 

What is needed in the synthesis of C is a two-carbon nucleophile (or its equivalent) which is less basic titan an enolate so elimination is not competitive. If product C is recognized as an acetic acid derivative, then the following analysis can be made. A malonate ion used as the carbon nucleophile is much less basic than a simple ester enolate and hence undergoes substitution readily but does not promote elimination effectively, particularly in secondary systems. 

In contrast to titanium enolates of ketones, titanium enolates of aldehydes exhibit practically no stereoselectivity in aldol reactions. However, titanation of dimethylhy-drazones of aldehydes with 1 results in substrates (2) that show high eryt/iro-selecti v i ty in aldol-type reactions with aldehydes (equation I). Bromotitanium tris(diethylamide) can be used in place of 1, but is less efficient, as is Ti(IV) isopropoxide.6... 

Titanium chelates are formed from tetraalkyl titanates or halides and bi- or polydentate ligands. One of the functional groups is usually alcoholic or enolic hydroxyl, which interchanges with an alkoxy group, RO, on titanium to liberate ROH. If the second function is hydroxyl or carboxyl, it may react similarly. Diols and polyols, CC-hydroxycarboxylic acids and oxalic acid are all examples of this type. P-Keto esters, P-diketones, and alkanolamines are also excellent chelating ligands for titanium. 

Methylation of the lithium enolates derived fn>m cyclohexen-2-one and l,l-bis(methylseleno)etiiylli-thium takes place regioselectively but requires the use of more titan one equivalent of HMPA (S equiv., 0 C, 1 h). The product consists quite exclusively of the Irons stereoistxner (99%) vrith a trace (1%) of the cis (Scheme 140, d and e). The latter can be obtained as a single stereoisomer on hydrolysis of the adduct resulting fixnn the reaction of the same organometallics with 2-methylcyclohexen-2-one... 

Enol-Phosphate werden durch Titan(O) unter Substitution zu den entsprechenden un-substituierten Alkenen reduziert z.B.1 ... 

One of the interesting specific methods of MCM synthesis is the interaction of metal alkoxy derivatives with enolizing aldehydes and ketones [28]. It has been useful in producing tetrastyryloxy-, dihexadienyloxy-, and divinyloxy-diisopropoxy titanates. The scheme of the reaction can be represented as shown in Eq. 4-4. 

In the case of the use of aryltitanium reagents in the rhodium-catalyzed asymmetric 1,4-addition to cyclic and acyclic a, 3-unsaturated enones, only one report was found in literature. Hayashi and coworkers [63] reported a study concerning the use of aryltitanium reagents. The intermediate chiral titanium enolates formed were isolated as silyl enol ethers by way of titanate-type enolates generated by the addition of lithium isopropoxide to the titanium enolates. High enantioselectivities were achieved using (5)-BINAP (Scheme 5.19). 

Homoenolate Dianion Equivalent. The intermediate in the reaction is believed to be an enol silane and/or enol titanate which gives the expected product with carbon electrophiles such as acetals (eq 3). ... 

Scheme 2.30 Generation of titanium enoiates by deprotonation equiiibrium of titanate 106 and titanium enolate 107.

The Duthaler aldol reaction [21] utilizes the transmeta-lation of relatively stable lithium enolates of tert-butyl acetate with chloro(cyclopentadienyl)-bis(l,2 5,6-di-0-isopropylidene-a-D-glucofuranos-3-0-yl)titanate. The so-obtained titanium enolate adds to the re-side of aldehydes and can be converted under acid or basic conditions into its corresponding acid. As often performed for the Braun aldol reaction, recrystallization of the primary aldol product is used as a convenient way for purification and improvement of selectivities. 

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