Reductive coupling reactions carbonyl olefination

The twofold extrusion of nitrogen and sulfur and selenium and sulfur from 1-thia- and l-selena-3,4-diazolines(77), readily available from the reaction of diazoalkanes with thio- and selenaketones, respectively, as well as from the reductive coupling of carbonyl compounds such as 78, is the preferred method for preparation of olefins of type I and II (98,99). Examples of the extrusion method, developed by Barton (100), and the McMurry reaction (101) are given in Scheme 1. 

One of the most powerful methods for constructing a carbon-carbon bond is the reductive coupling of carbonyl compounds giving olefins and/or 1,2-diols [71]. Of these methods, the pinacol coupling [72], which was described in 1859, is still a useful tool for the synthesis of vicinal diols. The corresponding products of this reaction can be used as intermediates for the preparation of ketones and alkenes [73]. More importantly, this methodology has been applied to the synthesis of biologically active natural compounds [74]. 

A titanium(Il) species formed from titanium trichloride and lithium aluminum hydride is a useful reagent for the reductive coupling of carbonyl compounds to olefins (McMurry, 1974 McMurry and Fleming, 1974). Both aliphatic and aromatic ketones can be converted to tetrasubstituted olefins in excellent yields. Reductive dimerization of retinal (CCLXXFV) affords j6-carotene (CCLXXV) in 85% yield. The course of the reaction can be accounted for by assuming pinacol formation followed by loss of titanium dioxide. 

Keywords carbon-carbon bond formation reactions, carbonyl-olefin exchange reaction, olefin metathesis, reductive coupling of carbonyl compounds, carbonyl... 

PET reaction of carbonyl compounds with olefins form either oxetanes (Paterno-Buchi reaction, Eq. 31) by direct coupling or a radical pair reaction leading to coupling product or reduction. The carbonyl-olefin radical pairs are formed by proton transfer within their radical ion pairs (Eq.32). Both these aspects of ketone-olefin photoreaction have been recently rationalized by Mattay et al. [167] from the photoreactions of 2,3-butanedione (208) with different olefins such as 209 and 210 as shown in Scheme 39. Photoprocesses of... 

During the past decade, considerable progress has been made in the area of transition metal-catalyzed cleavage and functionalization of the inert C-Cl bond in nonactivated chloroaromatic compounds. This new and important field of chemistry is reviewed in the present chapter, which describes both mechanistic and synthetic aspects of C-Cl activation. Oxidative addition reactions of chloroarenes to complexes of catalytic metals are discussed, along with their applications in a wide variety of reductive dechlorination, nucleophilic displacement, olefin arylation, coupling, and carbonylation reactions. 

Serval reactions occurred evidenced by a complex desorption products. First, acetaldehyde (m/e 29, 15, 44) desorbed at 390 K followed by traces of ethanol at 415 K (2 % of carbon selectivity, table 2). Three other products were observed. Butadiene and butene desorbed at 540 and 673 K respectively with a combined carbon selectivity of 21.1 %. This reaction pathway follows a reductive coupling mechanism which has been observed previously on the surfaces of Ti02 single crystal and powder [19-21]. The formation of C4 olefins is a clear example of the capacity of UO2 surfaces to abstract large amounts of oxygen from surface carbonyls, via pinacolates [19], as follow... 

The formation of C—N bonds is an important transformation in organic synthesis, as the amine functionality is found in numerous natural products and plays a key role in many biologically active compounds [1]. Standard catalytic methods to produce C—N bonds involve functional group manipulations, such as reductive amination of carbonyl compounds [2], addition of nucleophiles to imines [3], hydrogenation of enamides [4—8], hydroamination of olefins [9] or a C—N coupling reaction [10, 11]. Recently, the direct and selective introduction of a nitrogen atom into a C—H bond via a metal nitrene intermediate has appeared as an attractive alternative approach for the formation of C—N bonds [12-24]. 

The radicals R" generated by reduction of the halides RX (R = alkyl or aryl X = Cl, Br, I, etc.) by Sml2 (see Chap. 12.4.2) can be reduced (formation of RH by trapping an H atom from the solvent) or couple onto a carbonyl (aldehydes, ketones Barbier reaction), an olefin or an alkyne. For coupling reactions, the competitive reduction to RH indicated above must be avoided. The intramolecular coupling with formation of 5- or 6-atom rings, of synthetic interest, are promoted by the entropic factor. ... 

As inert as the C-25 lactone carbonyl has been during the course of this synthesis, it can serve the role of electrophile in a reaction with a nucleophile. For example, addition of benzyloxymethyl-lithium29 to a cold (-78 °C) solution of 41 in THF, followed by treatment of the intermediate hemiketal with methyl orthoformate under acidic conditions, provides intermediate 42 in 80% overall yield. Reduction of the carbon-bromine bond in 42 with concomitant -elimination of the C-9 ether oxygen is achieved with Zn-Cu couple and sodium iodide at 60 °C in DMF. Under these reaction conditions, it is conceivable that the bromine substituent in 42 is replaced by iodine, after which event reductive elimination occurs. Silylation of the newly formed tertiary hydroxyl group at C-12 with triethylsilyl perchlorate, followed by oxidative cleavage of the olefin with ozone, results in the formation of key intermediate 3 in 85 % yield from 42. 

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