McMurry alkenation

Accounts of die reductive coupling of two molecules of ketone via the McMurry alkene synthesis have been described66-68 earlier under Miscellaneous Aldols. 

The main advantage of this reaction over the McMurry reaction (see section on McMurry alkenation ) is the notion that the reaction can take place with two different ketones. In this regard the diazo-thioketone coupling is a cross-coupling rather than a homocoupling. 

Titanium forms sufticiently strong bonds to oxygen to remove the oxygen from the dimeric species as titanium dioxide this results into an alkene rather than a diol (see McMurry alkenation under section 4.3.4). 

Other reports of kinetic studies deal with mechanisms of thermal oxidation of a variety of simple ketones monitored via gas evolution (CO, CO2, H2, etc.), alkaline oxidation of aldehydes with copper and silver tellurates, [M (H2Te06)2] , and oxidation of acetals of simple aldehydes in aqueous acetic acid with (i) N-chlorobenzamide (H2OCI+ is the oxidant inferred) and (ri) W-chlorosaccharin. Accounts of the reductive coupling of two molecules of ketone via the McMurry alkene synthesis have been described earlier under Miscellaneous Aldols. 

The McMurry alkene synthesis has been reviewed, considering the mechanistic alternatives of carbene or pinacolate intermediates. ... 

Activation of low-valent titanium species is highly desirable in the case of McMurry alkene syntheses where the required high temperature is detrimental for compounds containing thermolabile functional groups. External addition of group I and II metal halides, iodine, or arenes to the TiCls-Ii system in THF dramatically enhances its activity (Table 6.11). 

Other Systems for the McMurry Alkene Synthesis Organotitanium Complexes, Titanium Oxides, Titanium Metal... 

Among the appHcations of lower valent titanium, the McMurry reaction, which involves the reductive coupling of carbonyl compounds to produce alkenes, is the most weU known. An excellent review of lower valent titanium reactions is available (195). Titanium(II)-based technology is less well known. A titanium(II)-based complex has been used to mediate a stetio- and regio-specific reduction of isolated conjugated triple bonds to the corresponding polyenes (196). 

The intermolecular McMurry reaction is first of all a suitable method for the synthesis of symmetrical alkenes. With a mixture of carbonyl compounds as starting material, the yield is often poor. An exception to this being the coupling of diaryl ketones with other carbonyl compounds, where the mixed coupling product can be obtained in good yield. For example benzophenone and acetone (stoichiometric ratio 1 4) are coupled in 94% yield. ... 

The McMurry procedure is a valuable method for the synthesis of highly substituted, strained alkenes such compounds are difficult to prepare by other methods. Diisopropyl ketone 6 can be coupled to give tetraisopropylethene 7 in 87% yield attempts to prepare tetra-t-butylethene however were not successful. ... 

The olefin metathesis of 3-hydroxy-4-vinyl-l,2,5-thiadiazole 112 and a McMurry coupling reaction (Ti3+ under reductive conditions) of the aldehyde 114 were both unsuccessful <2004TL5441>. An alternative approach via a Wittig reaction was successful. With the use of the mild heterogenous oxidant 4-acetylamino-2,2,6,6-tetramethyl-piperidine-l-oxoammonium perfluoroborate (Bobbitt s reagent), the alcohol 113 was converted into the aldehyde 114. The phosphonium salt 115 also obtained from the alcohol 113 was treated with the aldehyde 114 to give the symmetrical alkene 116 (Scheme 16) <2004TL5441>. 

Metal alkylidene complexes find application in the metathesis of alkenes, the cvclopropanation of alkenes (Grubbs, Schrock), Wittig type reactions, and the McMurry reaction. In suitable complexes a-elimination can occur twice yielding alkylidync complexes. See Figure 2.21 for an example with tungsten. 

Macrolactone synthesis 6,51, 71, 72, 94, 124,131,163,187,195 McMurry coupling 43 Medium ring synthesis 43,45,75, 77 Metathesis, Alkene (see Grubbs) Metathesis, Alkyne (see alkyne metathesis) Michael addition Intramolecular 166,166, 167,201 Intermolecular 57,84, 153, 166,204... 

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