Metal hydrides, addition

Charge-transfer complexes as intermediates in metal hydride additions to tetracyanoethylene (TCNE). Strong charge-transfer colors are observed when a colorless solution of TCNE is exposed to various metal hydrides owing to the formation of the [D, A] complex188 (equation 49). 

There are two possible modes of metal hydride addition to carbon-oxygen double bonds ... 

In reaction (11) the metal-hydride addition suggests a protonation reaction whereas, in reaction (12) the addition appears to be a hydride transfer reaction. If the reaction is indeed a hydride transfer reaction then the introduction of p-electron donating substituents, which place more electron density at the carbonyl carbon, (the site of hydride attack) will inhibit hydride addition. The data in Table 2 show that the introduction of p-electron donating substituents reduces the turnover frequency. This is consistent with hydride attack at the benzaldehyde carbonyl carbon, (12). 

Some experimental evidences are in agreement with this proposed mechanism. For example, coordinating solvents like diethyl ether show a deactivating effect certainly due to competition with a Lewis base (149). For the same reason, poor reactivity has been observed for the substrates carrying heteroatoms when an aluminum-based Lewis acid is used. Less efficient hydrovinylation of electron-deficient vinylarenes can be explained by their weaker coordination to the nickel hydride 144, hence metal hydride addition to form key intermediate 146. Isomerization of the final product can be catalyzed by metal hydride through sequential addition/elimination, affording the more stable compound. Finally, chelating phosphines inhibit the hydrovinylation reaction. 

Besides ring cyclization reactions, reductive methods can also provide access to reduced forms of pyrimidines, quin-azolines, and perimidines. Both hydrogenation and metal hydride addition can be used <1994HC(52)1, 1996HC(55)1>. 

Catalytic reduction of olefins by heavy metal catalysts probably involves metal hydride addition reactions also. If this is correct, the observed inhibition of the reduction by carbon monoxide, phosphines, sulfur compounds, and other materials with unshared electrons is exactly what would be expected if a vacant orbital on the hydride is required before addition can take place. 

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 327). Electrocyclic and sigmatropic rearrangements are treated at 8-29 to 8-37. Double-bond migrations have also been accomplished photochemically,67 and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts.68 In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the metal hydride addition-elimination mechanism ... 

The two mechanisms may result in substantial and characteristic differences in deuterium distribution. The metal hydride addition-elimination mechanism usually leads to a complex mixture of labeled isomers.195 198 208-210 Hydride exchange between the catalyst and the solvent may further complicate deuterium distribution. Simple repeated intramolecular 1,3 shifts, in contrast, result in deuterium scram-bling in allylic positions when the ir-allyl mechanism is operative. ... 

This stereochemistry results from syn metal hydride addition and CO insertion with retention of configuration thus, the net syn addition is not the result of double anti addition.57... 

Once metal hydride addition (alkene insertion) has taken place, for example (3) —> (4), -elimination (4) - (3) and readdition can occur (Scheme 2). Accordingly, alkene isomerization can take place in the hydroformylation process (equation 3). 

This stereochemistry is a result of syn metal hydride addition across the alkene followed by CO insertion with retention of configuration at the carbon bound to the metal (e.g. steps 3 — 4 and 5 — 6 Scheme 2). 

The most perspective on the technology, meeting the requirements of high heat conductivity, are methods of formation of porous matrix metal hydrides addition of the metal filler, cellular bodies or a preliminary incapsulation of a powder of an initial alloy. 

Isomerization of allylic alcohol to ketone has been extensively studied [13], and two different pathways have been established, including tt-allyl metal hydride and the metal hydride addition-elimination mechanisms [5,14]. McGrath and Grubbs [ 15] investigated the ruthenium-catalyzed isomerization of allyl alcohol in water and proposed a modified metal hydride addition-elimination mechanism through an oxygen-functionality-directed Markovnikov addition to the double bond. 

The nickel/chromium-catalyzed cyclizations of 1,2,7-trienes (270) to (272) were postulated to proceed via a metal hydride addition to the allene unit of (270X generating an allylmetal intermediate (271) which undergoes a metallo-ene/p-elimination sequence. It is worth noting that these cyclizations can be stereocontrolled by the presence of resident stereogenic centers, as demonstrated by the transformations (273) (274) and (275) (276) (Scheme 57). 

The two established pathways for transition metal-catalyzed alkene isomerization are the jr-allyl metal hydride and the metal hydride addition-elimination mechanisms. The metal hydride addition-elimination mechanism is the more common pathway for transition metal-catalyzed isomerization. In this mechanism, free alkene coordinates to a metal hydride species. Subsequent insertion into the metal-hydride bond yields a metal alkyl. Formation of a secondary metal alkyl followed by y3-elimination yields isomerized alkene and regenerates the metal hydride. The jr-allylhydride mechanism is the less commonly found pathway for alkene isomerization. Oxidative addition of an activated allylic C-H bond to the metal yields a jr-allyl metal hydride. Transfer of the coordinated hydride to the opposite end of the allyl group yields isomerized alkene. 

The fundamental differences between these two mechanisms are that 1) the jr-allyl metal hydride mechanism involves a 1,3-hydrogen shift while the metal hydride addition-elimination mechanism involves a 1,2-hydrogen shift and 2) the hydrogen shift in the Jt-allylhydride mechanism proceeds in an intramolecular fashion while that in the metalhydride addition-elimination mechanism proceeds in an intermolecular fashion. 

The crossover product, propionaldehyde-l,3-d-3- C 12, clearly demonstrated that the isomerization occurred via intermolecular 1,3-hydrogen shift. These results are consistent with a modified metal hydride addition-elimination mechanism which involves exclusive 1,3-hydrogen shift through oxygen-directed Markovnikov addition of the metal hydride to the carbon-carbon double bond (Scheme 12.2). The directing effect of functional groups on the selectivity of transition metal catalysis is well presented [9], and an analogous process appears to be operative in the isomerization of allylamines to enamines [10]. 

Metal-hydride addition reactions can be used to generate triene derivatives directly ... 

A few studies of metal hydride additions are also available. Houk et al. briefly reported location of the transition-state structure for LiH addition to acetone (3-2IG) [145, 151]. Subsequently, alternative transition-state structures for addition of LiH to cyclohexanone, 3-fluorocyclohexanone, l,3-dioxan-5-one and 1,3-dithian-5-one were reported [147, 152, 153]. The general features of these cyclic transition states are similar to those of the analogous structures reviewed earlier. 

Ti o.9i jji most cases the organometallic undergoing insertion is formed in situ by a metal hydride addition (insertion) with an olefin rather than by the exchange reaction. The olefin reacting with the hydride to form the alkyl may be the same one that undergoes the insertion with the metal alkyl, or a different one. This sequence with a single olefin produces olefin dimers after the final / -hydride elimination. A mechanism for the RhClj-catalyzed dimerization of ethylene is ... 

The addition of transition metal alkyls to alkynes is less common and often less facile than the corresponding metal-hydride additions, although this reaction is probably k r to many metal-catalyzed alkyne polymerizations (see below). In one case where an alkyne adduct has been established, i.e., the reaction of L2PtClMe with electrophilic alkynes, cts-M—C addition is observed (Scheme 4-24) [94]. 

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