Titanium-induced reactions

Beller and coworkers [312] have recently published a very useful domino reaction using a Ti-based Lewis acid as catalyst to prepare indoles. The new procedure consists of a titanium-catalyzed amination of a chloroalkylalkyne to give an aryl hydra- 

---

This one-pot procedure for titanium-induced reactions is also applicable to the synthesis of crowded products 5 to completely chemo- and regioselective "zipper-type" polycyclizations,7 to bimolecular reductions of alkynes,5 and to conventional McMurry reactions of aldehydes or ketones.5 Some representative examples are compiled in the Table. 

Excellent chelation control was observed using tributyl(2-propenyl)stannane and a-benzyloxy-cyclohexaneacetaldehyde with magnesium bromide or titanium(IV) chloride, whereas useful Cram selectivity was observed for boron trifluoride-diethyl ether complex induced reactions of the corresponding ferr-butyldimethylsilyl ether89. 

The titanium(IV) chloride induced reaction of an a-methyl-y-carbamoyloxyallylstannane, which is believed to involve the in situ generation of a chiral a-carbamoyloxytitanium species, gave products 3 and 4 in yields of ca. 85 %, whose configuration was determined by the reagent, not by the aldehyde118. 

Titanium tetrachloride induces reaction with dichloromethyl methyl ether to give a, (3-unsaturated aldehydes.127... 

McMurry, J. E. 1983. Titanium-induced dicarbonyl-coupling reactions. Accounts Chemical Research 16 405-11. 

JCS Chem Comm, (1986), 1665 Review "Titanium Induced Dicarbonyl-Coupling Reactions" McMurry, J.E. Accts Chem Res, (1983), 405... 

In 1977, McMurry and Kees [152] developed a titanium-induced intramolecular coupling procedure to form cycloalkenes from dicarbonyl compounds. Mechanistically, as shown in Scheme 85, the coupling reaction proceeds by an initial pinacol dimerization of the dicarbonyl 253 to 254, followed by titanium-induced deoxygenation to afford alkene 255. 

The first total synthesis of ( )-sarcophytol B (5) from S./i-famesal (92), which was reported by McMurry et al. in 1989, used a low-temperature titanium-induced pinacol coupling reaction of 1,14-dialdehyde as the key step. They concluded that the natural sarcophytol B has the stereochemistry of a trans diol (Scheme 6-5). Li et al. reported on a concise total synthesis of ( )-sarcophytol B (5) from /i./i-famesol (91) by a low-valent titanium-mediated intramolecular McMurry... 

Yue et al. reported on the total synthesis of / -(—)-cembrene A by using the mod-ihed Wittig reaction and titanium-induced intramolecular carbonyl coupling as key steps the overall yield is 29% starting from -geranylacetone (164) and R-(+)-limonene (168) (Scheme 6-15). ... 

An alternative strategy to prepare ( )-cembrene A (59) by this group is described below. The key step here is a titanium-induced intramolecular carbonyl coupling reaction starting from -geranylacetone and geraniol (137) (Scheme 6-16). ... 

From a synthetic point of view, the titanium-induced mframolecular cyclization of dicarbonyl compounds is especially attractive in that carbocyclic rings of all sizes may be prepared in good yields, competitive with those obtained using the Dieckmann and acyloin cyclization methods. The synthesis of medium- and large-ring cycloalkenes by the McMurry reaction requires high-dilution conditions (< 0.0 IM) to repress intermol-ecular polymerization. ... 

Aluminum chloride is the most fiequently used Lewis acid in aliphatic Friedel-Crafts acylations, and is one of the strongest. Its complexes with acyl halides are strong, producing very active acylating species. Titanium and tin tetrachlorides also find use as catalysts, and are powerful enough to induce reaction at low temperatures. 

Poly(ferrocenylene vinylene) derivatives 68 with values of 3,000-10,000 and polydispersities of ca. 2.2-2.8 (determined by GPG) were synthesized in 1995 in high yields via a titanium-induced McMurry coupling reaction of the corresponding alkylferrocenyl carbaldehyde monomers (Equation (26)). " Gharacterization of these soluble polymers by NMR and IR revealed the presence of trans-Yinylcnc units. The UV-VIS spectra of the polymers are similar to those of the monomers and this indicates a fairly localized electronic structure in the former. The relatively limited electron localization is also reflected in the electrical and optical properties. For example, the values for iodine-doped conductivity a= 10 Scm ) and non-linear third-order optical susceptibility (x = 1-4 x 10 esu) are lower than those of linear conjugated polymers such as poly(l,4-phenylene-vinylene) (a = 2.5x 10 Scm" = 8 X 10 esu). 

Titanium-induced Intramolecular Carbonyl Coupling Reactions... 

Table 9-1 Effect of the ring size on intramolecular, low-valent titanium-induced carbonyl coupling reactions.

---