Wittig-like olefination

The results presented in Section 6.4.3 revealed that the McMurry reactions of ster-ically hindered ketones can in fact be viewed as Wittig-like olefination reactions. While these two reactions were for a long time thought to be mechanistically different, formation of carbenoid species by reduction of ketones with low-valent titanium complexes is not really surprising in view of the highly oxophilic and reducing character of titanium. Carbonyl olefinations by means of titanium carbe-... 

Tlie starting material for this Wittig-Horner-like olefination was easily accessible by reaction of benzo-l,3-dithiolium-tetrafluoroborate with trimethylphosphite in the presence of an equimolar amount of sodium iodide in dry acetonitrile under nitrogen at room temperature (93% yield). 

The carbonyl olefination is in fact a Wittig-like reaction of carbene complex. Some disputable, even unknown, reactions are also noticed in Chart 4 and that is why the three cycles are examined separately and some literature data supporting our hypothesis are quoted. 

Many reactions can be envisaged using this scheme. For some of them (OM (X = Y = Z = C) and carbonyl olefination via Wittig-like reaction of the transition metal carbene complexes (Z = O, X = Y C)) the vaUdity of Eq. 12 is well documented. For others (X = O, Y = Z = C) it is only suggested. The other combinations are still unknown reactions. 

The carbene mechanism of COER according to our hypothesis consists of forward (Z = O, X = Y = C) and backward (X = O, Y = Z = O) Wittig-like reactions. A transition metal-carbene complex reacts with a carbonyl compound generating an olefin and transition metal oxo-complex. Then the oxo-complex reacts with another olefin generating a new carbonyl compound and regenerating the transition metal-carbene complex. It is known that oxo-alkylidene complexes can be generated via oxidative addition reaction between some tungsten complexes with carbonyl compounds. 

The Wittig-type olefination of carbonyl compounds is one of the characteristic reactions of carbene complexes. High-valent carbene complexes of early transition metals show ylide-like reactivity to vards carbonyl compounds. In 1976, Schrock first demonstrated that niobium and tantalum neopentylidene complexes 1 and 2, the typical nucleophilic Schrock-type carbene complexes, olefinate various carbonyl compounds including carboxylic acid derivatives [4]. 

The Peterson olefination reaction involves the addition of an a-silyl substituted anion to an aldehyde or a ketone followed by the elimination of silylcarbinol either under acidic (awP -elimination) or basic (syn-elimination) conditions to furnish olefins178. Thus, Peterson olefination, just like Wittig and related reactions, is a method for regioselective conversion of a carbonyl compound to an olefin. Dienes and polyenes can be generated when the Peterson reaction is conducted using either an ,/l-unsaturated carbonyl compound or unsaturated silyl derivatives as reaction partners (Table 20)179. 

The overall sequence of three steps may be called the Wittig reaction, or only the final step. Phosphonium salts are also prepared by addition of phosphines to Michael olefins (like 5-7) and in other ways. The phosphonium salts are most often converted to the ylides by treatment with a strong base such as butyllithium, sodium amide,640 sodium hydride, or a sodium alkoxide, though weaker bases can be used if the salt is acidic enough. For (PhjP CHj, sodium carbonate is a strong enough base.641 When the base used does not contain lithium, the ylide is said to be prepared under "salt-free conditions.642... 

In many of their reactions, these carbenes behave like the familiar Wittig reagent, PhjPCH,. Schrock carbenes are important intermediates in olefin metathesis.68... 

The acid-catalyzed Peterson olefination is presumably an E2-elimination, that is, a one-step reaction. On the other hand, the base-induced Peterson olefination probably takes place via an intermediate. In all probability, this intermediate is a four-membered heterocycle with a pentavalent, negatively charged Si atom. This heterocycle probably decomposes by a [2+2]-cycloreversion just like the oxaphosphetane intermediate of the Wittig reaction (Section 4.7.3). 

A different reaction mode of lithiobetaines is used in the Schlosser variant of the Wittig reaction. Here, too, one starts from a nonstabilized ylide and works under non-salt-free conditions. However, the Schlosser variant is an olefination which gives a pure frans-alkene rather than a trans.cis mixture. The experimental procedure looks like magic at first ... 

Fig. 11.8. trans-Selective Wittig olefination of aldehydes II—Synthesis of /J-carotene from a dialdehyde. The ylide used here is already known from Figure 11.2. In a way, it is "(semi)stabilized" since it is prepared in situ like a semista-bilized phosphonium ylide, but reacts as trans-selectively as a stabilized ylide. 

The detailed mechanism of pyrolysis of Cp iR compounds has been studied (307—313). A useful titanocycle is formed from Cp2TiCl2 and trimethylaluminum triethylaluminum gives a different product (314). The titanocycle adds to terminal olefins in the presence of 4-dimethylaminopyridine the adduct expels olefin above 0°C to yield a bistitanocyclobutane (315). The titanocycle can also behave like a Wittig reagent, reacting with aldehydes and ketones to give olefins (314,316). 

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