Reductively dimerized

Formic acid behaves differently. The expected octadienyl formate is not formed. The reaction of butadiene carried out in formic acid and triethylamine affords 1,7-octadiene (41) as the major product and 1,6-octadiene as a minor product[41-43], Formic acid is a hydride source. It is known that the Pd hydride formed from palladium formate attacks the substituted side of tt-allylpalladium to form the terminal alkene[44] (see Section 2.8). The reductive dimerization of isoprene in formic acid in the presence of Et3N using tri(i)-tolyl)phosphine at room temperature afforded a mixture of dimers in 87% yield, which contained 71% of the head-to-tail dimers 42a and 42b. The mixture was treated with concentrated HCl to give an easily separable chloro derivative 43. By this means, a- and d-citronellol (44 and 45) were pre-pared[45]. 

Two classes of charged radicals derived from ketones have been well studied. Ketyls are radical anions formed by one-electron reduction of carbonyl compounds. The formation of the benzophenone radical anion by reduction with sodium metal is an example. This radical anion is deep blue in color and is veiy reactive toward both oxygen and protons. Many detailed studies on the structure and spectral properties of this and related radical anions have been carried out. A common chemical reaction of the ketyl radicals is coupling to form a diamagnetic dianion. This occurs reversibly for simple aromatic ketyls. The dimerization is promoted by protonation of one or both of the ketyls because the electrostatic repulsion is then removed. The coupling process leads to reductive dimerization of carbonyl compounds, a reaction that will be discussed in detail in Section 5.5.3 of Part B. 

We have noted above that benzocrowns may be nitrated quite readily. This approach was used in the formation of a photoresponsive bis-crown (see Sect. 3.8) wherein the nitrobenzo crowns reductively dimerize to the corresponding azobenzene. Kikukawa, Nagira and Matsuda have utilized 4-nitrobenzo-15-crown-5 in a somewhat different way during the synthesis of 4 -vinylbenzo-l 5-crown-5Nitration is effected using nitric acid in a mbcture of chloroform and acetic acid. 

Another interesting variation on the linkage holding together two benzocrowns was reported by Shinkai and coworkers In their case, the benzo groups on the crowns were also part of a photolabile azobenzene system. The azo-linked bis-crown was prepared as follows (see Eq. 3.34). 4 -Nitrobenzo-l 5-crown-5 was reductively dimerized... 

Pyrroline-N-oxides (12) are sometimes isolated when using zinc-ammonium chloride (19,20), iron-sulfuric acid (14) or hydrazine-Raney nickel (21) as reducing agents. During the reduction, dimerization has been often observed (22). 

Steroids are heavily modified triterpenoids that are biosynthesized in living organisms from farnesyl diphosphate (Cl5) by a reductive dimerization to the acyclic hydrocarbon squalene (C30), which is converted into lanosterol (Figure 27.12). Further rearrangements and degradations then take place to yield various steroids. The conversion of squalene to lanosterol is among the most... 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

Treatment of UCI4 with the lithium complex obtained from dicyclohexylcar-bodiimide followed by crystallization from pyridine afforded a dinuclear uranium(rV) oxalamidinate complex in the form of dark green crystals in 94% yield (Scheme 191). The same compound could also be obtained by first reducing UCI4 to LiUCli (or UQs+LiCl) followed by reductive dimerization of di(cyclo-hexyl)carbodiimide as shown in Scheme 191. The molecular structure of this first oxalamidinato complex of an actinide element is depicted in Figure 31. ° ... 

Ethers have also been prepared by the reductive dimerization of two molecules of an aldehyde or ketone (e.g., cyclohexanone dicyclohexyl ether). This was accomplished by treatment of the substrate with a trialkylsilane and a catalyst. ... 

McMurry coupling reaction. The mechanism of the McMurry coupling reaction consists of two defined steps the reductive dimerization of the... 

COVALENT COMPOUNDS, METAL IONS OXIDATION-REDUCTION dimeric products and propose a scheme... 

Shey J, CM McGinley, KM McCauley, AS Dearth, BT Young, WA van der Donk (2002) Mechanistic investigation of a novel vitamin Bjj-catalyzed carbon-carbon bond forming reaction, the reductive dimerization of arylalkenes. J Org Chem 67 837-846. 

Fx Zn dust/NH4Cl (30%)/THF/reflux Fz When 3-chloromethyl-Fx derivative was submitted to these reduction conditions a fcisfurazan was generated as a reductive dimerization Zn-promoted process [38]... 

Schemes 6-29 A plausible reaction pathway for Rh(l)-catalyzed reductive dimerization of DMAD...

Zinc-mediated reductive dimerization cyclization of 1,1-dicyano-alkenes occurs to give functionalized cyclopentenes in good yields under saturated aqueous NH4CI-THF solution at room temperature. The trans isomers are the major products (Eq. 10.38).89... 

To our knowledge, the first examples of catalytic non-hydrogen-mediated reductive C-C bond formation involve the reductive dimerization (telomeriza-tion) of butadiene mediated by silane, see Ref. 6-12. 

A vanadium catalyst is essential although the combination of Zn and MejSiCl is capable of reductive dimerization of aldehydes [20]. A reversible redox cycle for the in situ generated low-valent vanadium species mediating the electron transfer is achieved in the presence of Zn as the stoichiometric co-reductant (Scheme 4). 

Another method for reductive dimerization has been developed in hy-drosilylation. NiCl2-SEt2 is an effective catalyst in silylative dimerization of aromatic aldehydes with a hydrosilane (Scheme 12) [40]. A catalytic thiolate-bridged diruthenium complex [Cp RuCl(/ 2-SPrI)2RuCp ][OTf] also induces the conversion to 1,2-diaryl-1,2-disiloxyethane [41]. A dinuclear (siloxyben-zyl)ruthenium complex is considered to be formed, and the homolytic Ru - C bond fission leads to the siloxybenzyl radicals, which couple to the coupling product 14. 

Alkyl, allyl, and aryl bromides are dehalogenated mainly with the formation of R R dimers in the presence of polypyridyl complexes of the metals of Group VIII. It has been demonstrated that the complexes [Co(bpy)3] + 203-204 [Ni(bpy)3]2+,205 and [Ni(phen)3]2+206 catalyze the reductive dimerization of allyl and alkyl bromides in organic 203 205 206 and aqueous micellar 204 solution. 

The octacyclic dimer (+)-94 could be obtained in short order from the tetracyclic bromide (+)-93 via a Co(I)-mediated reductive dimerization protocol first implemented in our prior syntheses of (+)-chimonanthine (7), (+)-folicanthine (8), and (—)-calycanthine (9) [7]. Simple exposure of intermediate (+)-93 to tris (triphenylphosphine)cobalt(I) chloride [48] in acetone under anaerobic conditions rapidly afforded dimer (+)-94 in 46 % yield. While higher yields (52 % yield) could be obtained in tetrahydrofuran on small scale, performing the reaction in acetone reproducibly afforded higher yields on gram scales. Notably, the product was obtained in similar efficiency on multi-gram scale (43 % yield on 8-g scale)... 

Direct reduction of chloro cation 292 in non-aqueous media to diphenylcyclopro-pene 293 is possible by means of dimethylamine borane211. The chloro cation 292 is easily prepared from the dichlorocyclopropene 154 and Lewis acids like AICI3 or SbCls11S. The reductive dimerization of cyclopropenones has already been mentioned (p. 58). 

An analogy with reductive dimerization of diphenyl cyclopropenone (p. 58) was found on polarography of l,2-diphenyl-4,4-dicyano triafulvene (64)289. In a one-electron reduction step the cyclopropenyl radical anion 469 is likely to be generated and dimerized to the dianion of tetraphenyl-l,4-dicyanomethyl benzene (470) the dianion 471 could be successively oxidized via the anion radical 472 to the 1,4-quinodimethane derivative 473. 

Reductive dimerization of the above type is not observed in the 4,4-diacyl triafulvene series88. Instead, l,2-diphenyl-4,4-diacetyl and -4,4-dibenzoyl triafulvene are readily reduced by means of zinc/acetic acid to monomeric products, which are likely to possess structure 474 from their spectral data. 

Derivatives 47-51 were prepared from the iV-oxide 46 using zinc as the deoxygenating agent (Scheme 11). The use of Zn in NH4CI solution led to the deoxy derivatives 47 and 48 in moderate yields. The reduction was clearly observed by NMR HETCOR experiments (HMQC and HMBC). When the reduction of the derivative 46 (R = CH2C1) was carried out under the same conditions, compound 51 was generated via a Zn-promoted reductive dimerization process <2001EJM771>. 

Heck tried the reductive dimerization of isoprene in formic acid in the presence of triethylamine at room temperature using 1% palladium phosphine catalysts to give dimers in up to 79% yield (95). Better selectivity to the head-to-tail dimer was obtained by using Pd(OAc)2 with 1 1 ratio of arylphosphines. THF as solvent showed a favorable effect. In a scaled-up reaction with 0.5 mole of isoprene using 7r-allylpalladium acetate and o-tolyphosphine, the isolated yield of the dimers was 87%. The dimers contained 71% of the head-to-tail isomers. The mixture was converted into easily separable products by treatment with concentrated hydro-... 

The mechanism of the reductive dimerization was studied by using both dideuterated and carboxyl-deuterated formic acid. The carboxyl deuterium was found on carbon 6, and the other deuterium on carbon 3 of 2,6-dimethyl-1,7-octadiene. 

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