Sterically induced reduction

The synthesis of Cp 3U Cp = p -CsMes carved a new path for the researchers to go one step ahead in the electrochemical studies of organoac-tinide complexes. The reduction reaction involving more than two electrons are not common for metal complexes containing just one metal. However, the synthesis of complexes of the type Cp 3M led to the development of sterically induced reduction (SIR) chemistry in which sterically crowded complexes of redox inactive metals act as reductants [87,88]. Evans et al. showed that the sterically induced reduction couple, U(III)/U(IV), can act as a multielectron reductant [89]. As an example, Cp 3U reacts as a three-electron reductant with 1,3,5,7-C8H8, (Eq. 12) in which one electron arises from U(III) (Eq. 13) and two result from two CsMes /CsMes half reactions (Eq. 14) presumably via SIR. This phenomenon was further corroborated by the stepwise reduction of phenyl halide with Cp 3lJ (Eq. 15)  

With KCg in benzene it gave [ U(T) -C5Me5)2 2([Jt-ii i1 -C6H6)] [106], which itself proved to be also quite interesting in the area of multi-electron reduction chemistry, as shown by its reaction with azobenzene in which it functions as an eight-electron reductant [104]. Thus, it is clear that the complex (ii -CsMes) shows quite an interesting two- to four-electron reduction chemistry in a similar way to that of the com- 

Further investigating the SIR, Evans et al. [106] reported that [(Cp )2U]2 ( x-Y] ri -C6H6) functions as a six-electron reductant in its reaction with three equivalents of cyclooctatetraene to form [(Cp )(C8H8)U]2( x-Ti Ti -C8H8), (C5Me5)2, and benzene (Eq. 16)  

This multi-electron transformation can be formally attributed to three different sources two electrons from two U(III)/U(IV) reaction, two electrons from sterically induced reduction by two (C5Me5) /(C5Me5) ligands, and two electrons from a bridging (C6H6) /(C6H6) process. 

Apart from these SIR processes, there are few other examples where the U(III) center undergoes one- or two-electron oxidation. It was found that alkyl halides can also be oxidized by U(III) to generate equimolar mixtures of U(IV)-R and U(IV)-X as shown in Eq. 17 [108]  

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It is worth noting that the sterically crowded tris(pentamethylcyclopentadienyl) lanthanide complexes (C5Mes)3Ln have similar reductive reactivity to the divalent samarium complex. This phenomenon has been termed sterically induced reduction (Section 8.2.1.2). 

Evans, W.J. (2002) The expansion of divalent organolanthanide reduction chemistry via new molecular divalent complexes and sterically induced reduction reactivity of trivalent complexes. Journal of Organometallic Chemistry, 647, 2. 

This reaction is referred to as a sterically induced reduction . Write a balanced equation for the overall reaction. What is being reduced in the reaction, and which two species undergo oxidation ... 

Scheme 4 Sterically induced reduction chemistry of [(MesCsisU] [89,104-107]... 

More recently, reductive elimination of aryl ethers has been reported from complexes that lack the activating substituent on the palladium-bound aryl group (Equation (55)). These complexes contain sterically hindered phosphine ligands, and these results demonstrate how steric effects of the dative ligand can overcome the electronic constraints of the reaction.112,113 Reductive elimination of oxygen heterocycles upon oxidation of nickel oxametallacycles has also been reported, but yields of the organic product were lower than they were for oxidatively induced reductive eliminations of alkylamines from nickel(II) mentioned above 215-217... 

Bauerle and coworkers have adapted the Cu+ template (48) approach to [2]cate-nane synthesis using an intermediate platinum diacetylide linkage to macrocyclize each of the two rings (49, Scheme 10.9) [38], Oxidation of the platinum centers in the macrocyclic rings of SO with I2 induces reductive elimination of the two acetylides to form the 1,3-butadiyne linked macrocycles (51). Unfortunately, the authors could not remove the copper template in this example, likely as a result of steric congestion about the metal ion in the interlocked product. 

Hartwig and Wolkowski have synthesized arylpalladium complexes of mal-onates, and have shown that the greater steric hindrance of bulky ligand FcP(t-Bu)2 (VIII-2), relative to that of phenylphosphines, induces reductive elimination from... 

Similar trends were observed in a complementary study using the palladium complexes 14 and 15 comprising sterically identical normal and abnormal bis(imidazolylidene) ligands (Scheme 5.4). ° In the presence of chlorine, complex 14 was stable and did not react, whereas the abnormal carbene complex decomposed to [PdCU] " and a doubly chlorinated bisimidazolium dication 16. This outcome was explained by oxidative CI2 addition to complex 15, followed by reductive Ccarbene Cl bond formation. Obviously, this process was unfavourable with normally bound imidazolylidenes. It is worth noting that an analogue of complex 15 with no alkyl substituents at the C5 and C5 positions induced reductive Ccarbene-Ccarbene bond formation. The higher propensity of abnormal carbenes to be reductively cleaved was rationalised by the enhanced electron donor properties of the non-classical carbenes, which made them more susceptible towards elimination processes. Evidently, steric factors could be ruled out in these systems. 

The use of the zinc-copper couple to effect the reduction of the methanesulfonate 168 with rearrangement furnished 169 (Scheme 20.34) [10]. Treatment of 168 with methylmagnesium bromide in the presence of copper(I) cyanide to induce an SN2 -type reaction produced the methylated adduct 170. The half-life of the Myers-Saito cyclization of 169 is 66 h at 37 °C, whereas that of 170 is 100 min. The faster rate of cyclization for 170 has been attributed to a steric effect favoring the requisite s-cis or twisted s-cis conformation. 

Other disulfones like (32) and (33) were studied (see Ref [6]). They exhibit surprisingly stable anion radicals (and even a stable dianion with (32)). Such charged species could be used as redox mediators, tiowever, reduction of alkyl halides RX by (33) did not permit the formation of alkylated products presumably because of the strong steric strain induced by alkylation of the corresponding carbon atoms in the cyclobutene ring. 

A more traveled route to the absolute configuration represented by cyclohexa-1,4-diene 8 involves Birch reduction-alkylation of benzoxazepinone 9.2.5 heterocycle is best prepared by the base-induced cyclization of the amide obtained from 2-fiuorobenzoyl chloride and (5)-pyrrolidine-2-metha-nol. o The molecular shape of enolate 10 is such that the hydrogen at the stereogenic center provides some shielding of the a-face of the enolate double bond. Thus, alkylation occurs primarily at the 3-face of 10 to give 11 as the major diastereomer. The diastereoselectivity for alkylation with methyl iodide is only 85 15, but with more sterically demanding alkyl halides such as ethyl iodide, allyl bromide, 4-bromobut-1-ene etc., diastereoselectivities are greater than 98 2. 

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