Transition metal halides reduction, mechanism

Mechanism of Meta Hydride-Transition Metal Halide Reductions... 

The possible mechanisms which one might invoke for the activation of these transition metal slurries include (1) creation of extremely reactive dispersions, (2) improved mass transport between solution and surface, (3) generation of surface hot-spots due to cavitational micro-jets, and (4) direct trapping with CO of reactive metallic species formed during the reduction of the metal halide. The first three mechanisms can be eliminated, since complete reduction of transition metal halides by Na with ultrasonic irradiation under Ar, followed by exposure to CO in the absence or presence of ultrasound, yielded no metal carbonyl. In the case of the reduction of WClfc, sonication under CO showed the initial formation of tungsten carbonyl halides, followed by conversion of W(C0) , and finally its further reduction to W2(CO)io Thus, the reduction process appears to be sequential reactive species formed upon partial reduction are trapped by CO. 

All of the proposed mechanisms for the reduction of alkynes with metal hydride-transition metal halide combinations involve an initial hydrometallation of the ir-system by the transition metal hydride, formed by the reaction of the original metal hydride with the transition metal halide, to form the vi-nylmetallic intermediate (99 equation 38). For the reduction of alkenes, similar alkylmetallic intermediates are implied to be formed. In the case of the reduction of alkenes with NaBH4 in the presence of Co" in alcohol solution, the hydrometallation reaction appears to be reversible as evidenced by the incorporation of an excess of deuterium when NaBD4 was used in the reduction. ... 

The general mechanism of coupling reactions of aryl-alkenyl halides with organometallic reagents and nucleophiles is shown in Fig. 9.4. It contains (a) oxidative addition of aryl-alkenyl halides to zero-valent transition metal catalysts such as Pd(0), (b) transmetallation of organometallic reagents to transition metal complexes, and (c) reductive elimination of coupled product with the regeneration of the zero-valent transition metal catalyst. 

Secondary bromides and tosylates react with inversion of stereochemistry, as in the classical SN2 substitution reaction.24 Alkyl iodides, however, lead to racemized product. Aryl and alkenyl halides are reactive, even though the direct displacement mechanism is not feasible. For these halides, the overall mechanism probably consists of two steps an oxidative addition to the metal, after which the oxidation state of the copper is +3, followed by combination of two of the groups from the copper. This process, which is very common for transition metal intermediates, is called reductive elimination. The [R 2Cu] species is linear and the oxidative addition takes place perpendicular to this moiety, generating a T-shaped structure. The reductive elimination occurs between adjacent R and R groups, accounting for the absence of R — R coupling product. 

There has been some exploration of the mechanism of reduction of d transition metal complexes by M2+(aq) (M = Eu, Yb, Sm). Both inner- and outer-sphere mechanisms are believed to operate. Thus the ready reduction of [Co(en)3]3+ by Eu2+(aq) is necessarily outer-sphere. 2 However, the strong rate dependence on the nature of X when [Co(NH3)5X]2+ or [Cr(H20)5X]2+ (X = F, Cl, Br or I) are reduced by Eu2+(aq) possibly suggests an inner-sphere mechanism.653 The more vigorous reducing agent Yb2+ reacts with [Co(NH3)6]3+ and [Co(en)3]3+ by an outer-sphere route but with [Cr(H20)5X]2+ (X = halide) by the inner-sphere mechanism.654 Outer-sphere redox reactions are catalyzed by electron-transfer catalysts such as derivatives of isonicotinic acid, one of the most efficient of which is iV-phenyl-methylisonicotinate, as the free radical intermediate does not suffer attenuation through disproportionation. Using this catalyst, the outer-sphere reaction between Eu2+(aq) and [Co(py)(NH3)5]3+ proceeds as in reactions (18) and (19). Values found were ki = 5.8 x KFM-1 s 1 and k kx = 16.655... 

The transition metal catalyzed synthesis of arylamines by the reaction of aryl halides or tri-flates with primary or secondary amines has become a valuable synthetic tool for many applications. This process forms monoalkyl or dialkyl anilines, mixed diarylamines or mixed triarylamines, as well as N-arylimines, carbamates, hydrazones, amides, and tosylamides. The mechanism of the process involves several new organometallic reactions. For example, the C-N bond is formed by reductive elimination of amine, and the metal amido complexes that undergo reductive elimination are formed in the catalytic cycle in some cases by N-H activation. Side products are formed by / -hydrogen elimination from amides, examples of which have recently been observed directly. An overview that covers the development of synthetic methods to form arylamines by this palladium-catalyzed chemistry is presented. In addition to the synthetic information, a description of the pertinent mechanistic data on the overall catalytic cycle, on each elementary reaction that comprises the catalytic cycle, and on competing side reactions is presented. The review covers manuscripts that appeared in press before June 1, 2001. This chapter is based on a review covering the literature up to September 1, 1999. However, roughly one-hundred papers on this topic have appeared since that time, requiring an updated review. 

The relatively high reactivity of the allyl-NF complexes toward aryl halides, especially bromides, is unusual among traditional organic mechanisms and represented one of the first observations of this special feature in transition metal reactions. An Sn2 mechanism for activation of the halide is clearly ruled out, and the reactivity correlates better with an electron-transfer mechanism. The rates are roughly correlated with ease of reduction of the organic halide. This also is consistent with the low reactivity of / -toluenesulfonate esters, compared to bromides. ... 

This mechanism is quite general for this substitution reaction in transition metal hydride-carbonyl complexes [52]. It is also known for intramolecular oxidative addition of a C-H bond [53], heterobimetallic elimination of methane [54], insertion of olefins [55], silylenes [56], and CO [57] into M-H bonds, extmsion of CO from metal-formyl complexes [11] and coenzyme B12- dependent rearrangements [58]. Likewise, the reduction of alkyl halides by metal hydrides often proceeds according to the ATC mechanism with both H-atom and halogen-atom transfer in the propagation steps [4, 53]. 

Organocuprates and a few other carbon nucleophiles sometimes react with allylic halides (or acetates) to give the product of what looks like an SN2 reaction. However, the mechanism is completely different— preliminary coordination by the copper or other transition metal to the C=C n bond is the first step, and the coordination of the copper changes from rj-2 to rj-1 or rj-3 before a reductive elimination step establishes the C—C bond.394 Each of these steps affects the overall regiochemistry (and the stereochemistry discussed in the next chapter), which may look like an SN2 or an SN2 reaction, while mechanistically being neither. 

As early as 1901, Ullmann discovered the copper promoted coupling of aryl halides to biaryls, one of the first uses of transition metals in synthesis. The mechanism is still not entirely clear, but a binuclear reductive elimination of Ar—Ar from CuAr is possible. 

The possibilities for the formation of carbon-carbon bonds involving arenes have been dramatically increased in recent years by the use of transition metal catalysis. Copper-mediated reactions to couple aryl halides in Ulknann-type reactions [12, 13] have been known for many years, and copper still remains an important catalyst [14, 15]. However, the use of metals such as palladium [16,17] to effect substitution has led to such an explosion of research that in 2011 transition metal-catalyzed processes comprised more than half of the reactions classified as aromatic substitutions in Organic Reaction Mechanisms [18]. The reactions often involve a sequence outlined in Scheme 6.6 where Ln represents ligand(s) for the palladium. Oxidative addition of the aryl halide to the paiiadium catalyst is followed by transmetalation with an aryl or alkyl derivative and by reductive elimination to give the coupled product and legeuCTate the catalyst. Part 6 of this book elaborates these and related processes. 

Preparations of all these organic materials involve the constmction of new carbon-carbon bonds and have prompted the development of many catalytic cross-coupling reactions. One of the most reliable synthetic methods to form carbon-carbon bonds is transition metal-catalyzed cross-coupling between organo-metallic nucleophiles and electrophilic organic halides or pseudohalides, respectively (Scheme 2a). The mechanisms of common cross-coupling reactions such as the Suzuki, Negishi, or Stille catalysis can be described by a catalytic cycle, differ in detail, but all include three main steps in the order oxidative addition, transmetallation, and reductive elimination (Scheme 1). 

Systems are known which will reduce most of the organic functions (amides, carboxylic acids, nitriles, nitro, halides, double and triple bonds, aromatic rings, for instance). In most cases, the true catalyst is formed in situ by reduction of the metal with the hydride. Actually, very little is known of the mechanisms of those transition metal-assisted hydride reductions. Catalyzed reductions with NaBH4 have been reviewed [7]. 

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