Propargylic titanium reagent

Table 9.20 Additions of chiral carbamoy propargylic titanium reagents to aldehydes.

Dienyl Titanium and Zirconium Reagents 133.6 Propargylic Titanium Reagents... 

Propargylic ethers undergo directed lithiation and subsequent transmetallation to afford oxygenated allenyl titanium reagents. Subsequent addition of aldehydes gives rise to various homopropargylic alcohol adducts as mixtures favoring the anti dia-stereomers (Tables 9.18 and 9.19) [29, 30]... 

An alternative, but related, route to allenic titanium reagents from propargylic esters has been reported recently. Reaction of titanocene dichloride with BuMgCl and Mg yields a reactive titanocene intermediate, formulated as Cp2Ti. This reduced Ti species reacts in situ by oxidative addition to propargylic acetates. The allenyltitanium reagents thus produced add to aldehydes and ketones, as expected, to afford homopropargylic alcohols (Table 9.27) [43]. 

It has been found that allenyltitaniums (134), prepared in situ by the reaction of optically active secondary propargyl phosphates (135) with a divalent titanium reagent, react readily with alkenylidenemalonates with excellent regio- and dia-stereoselectivity to afford the Michael addition products (136) with high optical purity (Scheme 33). ... 

Enantio-enriched axially chiral allenyltitanium compounds have been synthesized from optically active propargyl alcohol derivatives by the reaction with a divalent titanium reagent, TKOPfj MgXPr1, and their reactions with a variety of electrophiles have been studied.285... 

Me) reacts efficiently to give a-allenic alcohols as major products only with aldehydes and in the presence of titanium tetrachloride at low temperatures (Scheme 17). In the other reports propargyl organochromium reagents (pre-... 

Alkoxypropargylation. The ambident character of the propargylic anion, which may be in equihbrium with the aUenic form, is responsible for its limited use in synthesis. In general the structure and reactivity of the ambident anion depend on the nature of the substrate, the counter cation, and the solvent. There is also an erythro-threo stereoselectivity problem when alkylated propargylic anions react with aldehydes or unsymmetrical ketones. In contrast, the zinc and titanium reagents derived from the title compound possess the allenic structure and, upon reaction with aldehydes, lead almost exclusively to the )8-acetylenic alcohol (eq 1), presumably by a chelate transition state (Sei process). The reaction also leads preferentially to the erythro diastereomer. The stereoselectivity is highest with titanium as the metal and THE as the solvent. ... 

Allenyllithium reagents are commonly prepared through lithiation of propargylic halides or by deprotonation of alkynes or certain allenes (Eq. 9.1). Lithiated allenes often serve as precursors to stable allenylmetal compounds such as stannanes or silanes. They can also be employed for the in situ synthesis of allenylzinc, -titanium and -boronate compounds, which can be further transformed to substitution products not accessible from their allenyllithio precursors. 

Alkynylvanadium(III) chloride, 219 Chromium(II) chloride, 84 Lithium methoxyacetylide, 166 Organocerium reagents, 206 Titanium(IV) chloride, 304 Propargyl groups... 

Analogous to the allylation with allylsilanes and -stannanes, the transformations, vinylallylation, propargylation, allenylation, alkenylation, alkynylation, and arylation, are viable by the use of an appropriate reagent in the presence of a titanium Lewis acid these are surveyed in the review articles cited both in the Introduction and in this section. The stereochemistry of the reaction of a (vinylallyl)silane in the presence of TiCU has been reported [234]. Equation (113) shows that the major reaction of this silane and isobutyraldehyde occurred mainly in the anti sense with a ratio of anti to syn attack of 90 10 at the terminus remote from the silyl group. Essentially the same stereochemical outcome was observed for the same reaction with the corresponding trimethylsilyl derivative. The intramolecular reaction with an acetal, however, proceeded less selectively the anti syn ratio was 60 40 (Eq. 114) [234]. 

Allenylsilanes and -stannanes combined with a titanium salt are versatile reagents for propargylation of aldehydes (Eq. 115) [297], ketones (Eq. 116) [298], (A,0)-acetals (Eq. 117) [299], and a,/ -unsaturated ketones in a conjugate fashion (Eq. 118) [300]. Intramolecular reaction has also been reported (Eq. 119) [301] in which a Bu3Sn-carbon bond was cleaved exclusively in the presence of a TBS-carbon bond. That the isomeric starting material, propargylstannane, did not give the desired product (Eq. 120) demonstrates that the direct scission of the carbon-Sn bond by the electrophile under these reaction conditions is not a feasible path [301]. 

Optically active allylstannanes, R3SnC HR (CH=CHR"), are formed from active secondary propargyl phosphates from the sequence of reactions shown in equation 9-4,8 using the Ti(OPr1)4/Pr1MgBr reagent.9 Presumably the titanium/tin transmetallation involves an Sn2 process. 

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