Mild hydride donor

The naphthalene complex of [Os] (3) reacts with triflic acid to form a /3-lH-naphthalenium species, which has been characterized at —40 °C [16]. Unfortunately, most nucleophiles react as bases with this species and return the naphthalene complex. However, MMTP and the mild hydride donor triethylsilane (Table 4, entries 1 and 2) both add to C4 of the complex and yield 1,4-dihydronaphthalenes in moderate overall yield following decomplexation [17]. 

The reduction of Ceo and Cvo has normally been achieved by the use of strong re-ductants such as BH3, which yield not only CeoHv and CV0H2 but also polyhydride mixtures [104, 230-237]. The use of the triplet excited state of Ceo has made it possible to attain the selective two-electron reduction of Ceo to l,2-CeoH2 by 10-methyl-9,10-dihydroacridine (AcrH2) which is a mild hydride donor (Eq. 14) [225, 238],... 

Because systematic variations in selectivity with reactivity are commonly quite mild for reactions of carbocations with n-nucleophiles, and practically absent for 71-nucleophiles or hydride donors, many nucleophiles can be characterized by constant N and s values. These are valuable in correlating and predicting reactivities toward benzhydryl cations, a wide structural variety of other electrophiles and, to a good approximation, substrates reacting by an Sn2 mechanism. There are certainly failures in extending these relationships to too wide a variation of carbocation and nucleophile structures, but there is a sufficient framework of regular behavior for the influence of additional factors such as steric effects to be rationally examined as deviations from the norm. Thus comparisons between benzhydryl and trityl cations reveal quite different steric effects for reactions with hydroxylic solvents and alkenes, or even with different halide ions... 

If, for example, the mild Lewis acid LiAlH4—it is mildly acidic because its acidic moiety is Li+—is used as a hydride donor, the diastereoselectivity of the reduction of a-chiral carbonyl compounds with a heteroatom in the a position can be controlled by... 

Interestingly, hydride donors react with the diiodide, 14, to give the Hl-adduct (17) which in turn may be reduced further to l and H2 (eq. 43,44). Presumably the latter reaction proceeds through a di-hydride intermediate, 19, which loses H2 to give 1. That the presumed dihydride loses H2 is consistent with the fact that 1 does not react with H2 under mild conditions. 

The addition of one or even two hydride ions to a coordinated arene can be accomplished depending upon the degree of activation induced by the metal center and the reducing ability of the hydride donor. Commonly, addition of the first nucleophile to a coordinated arene proceeds under relatively mild conditions. For example, addition of hydride ion to [ML(ti -... 

Ti hydride/K perchlorate mixts have been found to be good donor materials (Ref 28) when initiated with a bridgeware or mild detonating fuze, affording the possibility of utilization ini electric detonators which function with delays ranging from 25 msec to 3 sec. Whereas no fundamental studies have been found on the shock initiation of Zr hydride, its response must be considered comparable with that of Ti hydride for whose shock initiation an excellent study was recently published (Ref 31). Methods for estimating internal energies of pyrotechnic mixts in the absence of shock initiation studies were described by Hardt and Martinson (Ref 22)... 

Tetraalkylborates are mild and selective alkylation reagents [186, 187], and they are commonly considered as sources of nucleophilic alkyl groups (R ) just as borohy-drides are depicted as hydride (H ) sources. However, since organoborates represent excellent electron donors (see Table 5, Section 2.2.1), the question arises as to what role electron donor-acceptor interactions play in the nucleophilic alkyl transfer. Phenyl- and alkyl-substituted borate ions form highly colored charge-transfer salts with a variety of cationic pyridinium acceptors [65], which represent ideal substrates to probe the methyl-transfer mechanisms. Most pyridinium borate salts are quite stable in crystalline form (see for example Figure 5C), but decompose rapidly when dissolved in tetrahydrofuran to yield methylated hydropyridines (Eq. 65). 

Unsymmetric alkynylaluminum derivatives can be prepared by a) the reaction of sodium acetylides with dialkyl- or diaryluminum halides [92-94] (6) the reaction of terminal acetylenes with aluminum hydrides in donor solvents [95,96] and (c) the reaction of terminal acetylenes with triorganoaluminums, especially with trimethyl- or triphenylaluminum [91,97]. Where applicable, the last method gives an unsolvated product under mild, simple conditions. 

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