Hydrides reaction with organic halides

A feature absent from the chemistry of the [Cr(CO)s] ion is the reaction with organic halides to form R2Cr(CO)5 derivatives. No doubt such derivatives would be very unstable due to the heptacoordinate chromium atom. In an attempt to make the hydride H2Cr(CO)s an aqueous solution of [Cr(CO)5] was acidified in the presence of diethyl ether 121). The ether became yellow, but no H2Cr(CO)s or other species of interest could be isolated from this unstable solution. 

As with other palladium-catalyzed reactions of organic halides, if the group R is such as to permit a /3-hydride elimination, reaction (7) may occur preferentially. 

The proposed scheme of interactions in films is supported by the reaction of organopolymagnesium halides and organopolymagnesium hydrides with organic halides. Indeed, the concept of competition between the molecular... 

The anionic hydrido carbonyls have been much studied because of their intermediacy in catalytic processes like the water gas shift reaction, reduction of organic substances, and other reactions. Thus chromium group hydrides [HM(CO)4PR3] are highly efficient H-transfer agents and for reactions with alkyl halides a reactivity order was established cii-[HW(CO)4PR3] > m-[HCr(CO)4PR3] > [HW(CO)5]... 

Binary ionic or covalent hydride reduction of organic halides is important in organic syntheses . Reactions of metal hydrides with alkyl halides in ethers occur ... 

The reaction of organic halides with hydrido complex is less readily analyzed than the reaction with pentacyanocobaltate(II) since the two complex species are in equilibrium in a hydrogen atmosphere (Reaction 1). The question of whether alkanes are formed via hydride reduction of a radical (Reaction 11, Mechanism I) or via hydride-halide exchange (Reaction 13, Mechanism III) must be determined indirectly. 

If one of the reactions in a radical chain sequence is too slow to compete effectively with radical-radical reactions, the chain will collapse. Slow reactions of simple silanes such as Et3SiH with alkyl radicals precludes their use in the tin hydride method. Although quite reactive with alkyl radicals, thiols and selenols fail in the tin hydride method because the thiyl and selenyl radicals do not react rapidly with organic halide precursors. Nonetheless, it is possible to use thiols and selenols in tin hydride sequences when a Group 14 hydride is used as a sacrificial reducing agent. The thiyl or selenyl radical reacts with the silane or stannane rapidly, and the silicon- or tin-centered radical thus formed reacts rapidly with the organic halide [8], In practice, benzeneselenol in catalytic amounts has been used in radical clock studies where BusSnH served as the sacrificial reductant [9]. 

Fukuyama T, Kobayashi M, Rahman MT et al (2008) Spurring radical reactions of organic halides with tin hydride and TTMSS using microreactors. Org Lett 10 533-536... 

Acyloins (la) [29,30] and bis-silylated derivatives of their enediols (lb) [31] have been demonstrated to be useful acyl anion equivalents (RCO ) and have been employed in an indirect but facile way for the preparation of ketones (3) (Equation 7.1). The hydroxy ketones (2), which are the key intermediates in this method, have been prepared by the reaction of acyloins (la) with organic halides in the presence of sodium hydroxide in dimethyl sulfoxide [29, 30] according to the Hein s procedure [32] or using sodium hydride in 1,2-dimethoxyethane (DME) (glyme) [33]. They also have been prepared by the alkylation of lithium enedi-olates of acyloins prepared from the corresponding lb and methyllithium [31]. These reactions proceed with C-alkylation however, the reaction conditions employed are quite basic which would limit substrate choice. 

In an attempt to resolve this dilemma, the reaction rates were measured for a series of organic bromides with sodium anthracene (Table I) and correlated with two model systems. The model for the first possibility, a one-step process with bond dissociation, is the tri-n-butyltin hydride reaction with these same halides (6 ). Correlation of the reaction rates would indicate that the transition state for anthracene radical anion reduction is similar to the transition state for radical formation by tri-n-butyltin radical. On the other hand, the model for the second possibility, the organic halide reduction potential, is a measure of organic halide radical anion formation (7). 

In contrast, for reaction of organic halides with metallic magnesium, Whitesides ( 7) found a poor correlation with the rates of tri-n-butyltin hydride, and a reasonable correlation, especially with primary bromides, with reduction potentials, suggesting the formation of RXT. Sodium naphthalene reductions also correlate with reduction potentials for primary halides (4b). Moreover, reaction rates are faster in solvents favoring loose ion pairs over tight, further evidence for an early transition state involving election transfer and little bond dissociation. (4c)... 

The dehalogenation of organic halides by organotin hydrides takes place in most cases with a free-radical mechanism [1, 84, 85], The stereospecific reduction of 1,1-dibromo-l-alkenes with Bu3SnH discovered by Uenishi and coworkers [86-89], however, did not occur in the absence of palladium complexes and did not involve radicals. For the synthesis of (Z)-l-bromo-l-alkenes, [(PPh3)4Pd] proved to be the most effective catalyst which could also be generated in situ. The reaction in Eq. (7) proceeded at room temperature and a wide range of solvents could be used. 

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