Carbonyl Olefination with Higher Alkylidenes

Similarly, neither zirconium, tantalum, molybdenum, nor tungsten carbene complexes have been applied extensively by organic chemists for carbonyl olefination [609,727-729], probably because of the difficulty of their preparation and the high price of some of these compounds. These reagents can, however, have appealing chemo- and stereo-selectivity (Table 3.11). 

A series of reagents have been developed which are prepared in situ from a geminal dihalide or a dithioacetal [635,730] and a transition metal complex. Titanium-based reagents of this type olefinate a broad range of carbonyl compounds, including carboxylic acid derivatives (Table 3.12), and are a practical alternative to the use of isolated carbene complexes. 

Experimental Procedure 3.2.5. Alkylidenation of an Ester 5-Methoxy-5-undecene [737] 

Experimental Procedure 3.2.6. Alkylidenation of a Ketone 4-Methyl-1-phenyl-octa-1,3-diene [633] 

A mixture of finely powdered molecular sieves (4 A, 150 mg), magnesium turnings (43 mg, 1.8 mmol), titanocene dichloride (374 mg, 1.5 mmol), THE (4 mL), and triethyl phosphite (0.51 mL, 2.9 mmol) is stirred under argon at room temperature for 3 h. A solution of ( )-2-(2-phenylethenyl)-l,3-dithiane (122 mg, 0.55 mmol) in THE (1 mL) is added, followed er 5 min by dropwise addition of a solution of 2-hexanone (50 mg, 0.50 mmol) in THE (2.5 mL). The resulting mixture is stirred at room temperature for 30 min, diluted with hexane (30 mL), and filtered through Celite. Concentration of the filtrate and purification of the crude product by preparative TLC (hexane/ethyl acetate 98 2) yields 74 mg (74% Z/ 60 40) of the title compound. 

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As noted above, titanocene-alkylidenes can be prepared using various methods and starting materials. Like the methylidene complex, higher alkylidene complexes are useful for the transformation of carbonyl compounds to highly substituted olefins. Ketones and aldehydes are converted into substituted allenes by treatment with titanocene-alkenylidenes prepared by olefin metathesis between titanocene-methylidene and substituted allenes (see Scheme 14.7) [17]. Titanocene-alkenylidene complexes can also be prepared from... 

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