Titanium low-valent

Low valent titanium McMurry carbonyl coupling is believed to go through the vic-diol. vic-diols are smoothly converted to the corresponding olefins under these conditions. JOC 1976, 41, 896... 

Formation o( oleltns by coupling or cross coupling of ketones, mediated by low valent titanium Also coupling ol enol ethers of 1,3-dicarbonyl compounds. 

The reductive coupling of the dicarbonyl compound 6 induced by a low-valent titanium species gives initially the dihydroporphycenes 7 which then spontaneously aromatize in the presence of oxygen to yield the porphycenes 8. 

Low-valent titanium was prepared from TiCl4 (9.5 g, 50 mmol) and activated zinc (6.55 g, 100 mmol) in the presence of pyridine (2.5 mL) in THF (400 mL) To this mixture was added all at once the corresponding bipyrroledicarbaldehyde (6 1.36 g, 5 mmol) and the mixture was refluxed for 30 min. After hydrolysis with a 10% aq K.2C03 (125 mL), aqueous extraction and chromatography (silica gel, CH2Cl,/hexane 1 1), the porphycene was obtained as the only nonpolymerized product. Recrystallization (hexane) gave red-violet needles of the porphycene yield 120 mg (10%). 

The titanium(IV) chloride catalyzed addition of allylic silanes to (E)-(2-nitroethenyl)benzene affords y,<5-unsaturated nitronates which, on treatment with low valent titanium species [generated in situ from titanium(IV) and zinc], give y,<5-unsaturated nitriles. For example, [(Zs)-2-butenyl]-(dimethyl)phenylsilane underwent reaction with ( )-(2-nitroethenyl)benzene to give 3-methyl-2-phenyl-4-pentenenitrile in 65 % yield as a 3 1 mixture of diastereomers of unassigned configuration22. 

J. E. McMurry, Organic Chemistry of Low valent Titanium, Accounts Chem. Res. 7, 281 (1974). 

It is interesting to see that amines can be deaminated to give the corresponding methylene compounds with low-valent titanium (TiCE/Li/THF). " ... 

The synthesis of soluble PPV derivatives (82), via McMurry-type coupling of solubilized aromatic dialdehydes in the presence of low-valent titanium reagents, was described by Schliiter et al. [105]. The products are characterized... 

As already discussed, low-valent titanium can be generated in situ from catalytic amounts of TiCls, by use of excess Zn powder and TCS 14 in acetonitrile, to cy-clize reductively 2-benzoylaminoacetophenone 2091 to the indole 2092, in 80% yield, in an elegant version of the McMurry reaction [23, 64]. Replacement of the Zn metal powder by Ti powder and TCS 14 is very effective - 2092 is obtained in 97% yield [64]. In these reactions the intermediate Ti(0)Cl is apparently recycled by MesSiCl 14 into TiCh [64]. In these Fiirstner versions of the McMurry reaction... 

The most general procedures are based on low-valent titanium. Good yields of diols are obtained from aromatic aldehydes and ketones by adding catechol to the TiCl3-Mg reagent prior to coupling.250... 

Reductive elimination from 2-en-l,4-diol derivatives has been used to generate 1,3-dienes. Low-valent titanium generated from TiCl3-LiAlH4 can be used directly with the diols. This reaction has been used successfully to create extended polyene conjugation.305... 

Another well-known transformation of carbonyl derivatives is their conversion to pinacols (1,2-diols) via an initial one-electron reduction with highly active metals (such as sodium, magnesium, aluminum, samarium iodide, cerium(III)/ I2, yttrium, low-valent titanium reagents (McMurry coupling), etc.), amines, and electron-rich olefins and aromatics as one-electron donors (D).43 Ketyl formation is rapidly followed by dimerization44 (equation 22). 

Allylic silanes react with dichlorocarbenes, generated from dechlorination of carbon tetrachloride with low valent titanium species, to furnish dichlorocyclopropanes, which in turn get desilylated with CsF in DMF to generate 3-chloro-l,3-butadienes (equation 30)63. 

A short five-step synthesis of a bifuran, namely ( )-2,2 -bis(diphenylphosphino)-3,3 -binaphtho[2,l-I>]furan (BINAPFu) from naphthofuranone via a low-valent titanium-mediated dimerization was reported. The newly developed resolution procedure for phosphines was utilized to provide the optically active bidentate phosphine ligands (BINAPFu), which consistently outperforms BINAP in the asymmetric Heck reaction between 2,3-dihydrofuran and phenyl triflate . Another way in which a benzofuranone can be converted into benzo[7 ]furan is by treatment of the former with 1-BU2AIH at -78°C followed by an acidic work up <00TL5803>. 

Scheme 14.28. Carbonyl alkylidenation utilizing gem-dihalides and low-valent titanium reagents.

Reductive elimination of an allylic diol group. A new synthesis of vitamin A involves reduction of the allylic diol 1, prepared in several steps from JJ-ionone, with a low valent titanium formed from TiCl3 and LiAlH, in the ratio 2 1. Thus, the allylic diol group of 1 [either (E) or (Z)] is reduced to an (E,E)-1,3-diene group to form the silyl ether (2) of vitamin A.1 When the primary hydroxyl group is protected as an acetate, the reduction gives a mixture of (E)- and (Z)-2. 

Reduction of enedicarboxylates. A low-valent titanium reagent obtained from TiCL, and LiAlH, (about 2 1) can reduce enedicarboxylates in the presence of a trace of triethylamine. 

A review on the chemistry of low-valent titanium has appeared which deals with some aspects of the chemistry of titanium sulfoxide complexes (397). Titanyl complexes of the type [Ti0L5][C104]2 have... 

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