Aryltitanium

As well as undergoing carbonyl addition, titanacyclopentadiene intermediates generated from two unsymmetrical acetylenes have been shown to react with ethynyl para-tolyl sul-fone to afford an aryltitanium compound of the structure shown in Scheme 9.6 [34], The reaction may proceed according to path a or path b, as shown in Scheme 9.7. In path a, the first step should be regioselective [4+2] cycloaddition of the titanacyclopentadiene with the sulfonylacetylene to afford the bicyclic titanacycle, at least in an equilibrium concen-... 

Scheme 2.57 Rhodium-catalyzed enantioselective 1,6-addition of aryltitanium reagents to 3-alkynyl-2-cycloalkenones 180 (ee values referto the corresponding allenic enol pivalates).

A rhodium-catalyzed asymmetric synthesis of chiral titanium enolates by 1,4-addition of aryltitanium reagents to a,p-enones under aprotic conditions has... 

When a,p-unsamrated sulfones are employed as substrates for the rhodium-catalyzed 1,4-addition of aryltitanium reagents, they provide cine-substitution products instead of regular 1,4-addition products [Eq. (3.16)]. This new process... 

It was found that aryltitanium triisopropoxides 52 (ArTi(0 Pr)3) also participate in asymmetric conjugate addition to a,yS-unsaturated ketones in an aprotic solvent [38] (Scheme 3.20). The addition of 52 to 2-cyclohexenone la was complete within 1 h in the presence of 3 mol% of [Rh(OH)((S)-BINAP)]2 in THF at 20°C to give high yields of the titanium enolates 53 as conjugate addition products. The enantioselectivity is very high, 99.5, 99.0, and 99.8% enantiomeric excess, for Ar = Ph, 4-FC6H4, and... 

Scheme 3.23 Rhodium-catalyzed c/ne-substitution of alkenyl sulfones with aryltitanium reagents [41].

Arylmetallic reagents other than arylboronic acid were shown to be applicable in the rhodium-BINAP-catalyzed arylation. Aryltitanium reagent 93 con-... 

Success has been obtained using other organometallic reagents and some high enantioselectivities in the rhodium-catalysed addition of aryl and alkenylsilanes and also aryltitanium species to cyclic and acyclic enones has been achieved. 

The synthesis of biaryls using aryltitanium and arylindium reagents are under extensive investigations which will obviously be a significant contribution to the aryl-aryl bond forming methodology. 

Synthesis of biaryls based on aryltitanium Preparation of 2-phenylnaphthalene (214) I143J... 

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). 

Scheme 5.19 Rhodium-catalyzed asymmetric 1,4-addition of aryltitanium reagents reported by Hayashi and co orkers [63] - Generation and isolation of silyl enol ethers and derivatives.

Although the enantioselective stoichiometric addition of chiral aryltitanium reagents have been reported in 1987 [66], the catalytic enantioselective aryl addition to carbonyl compounds with titanium catalyst was realized by Walsh using the catalyst prepared from bis(sulfonamide) ligand and Ti(OTr)4 with diarylzinc as the nucleophiles, to afford the tertiary alcohol in good to excellent enantioselectivities for a range of ketone substrates [67]. Very recently, Gau reported that... 

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