Addition 1,2-hydride

Synthetically useful stereoselective reductions have been possible with cyclic carbonyl compounds of rigid conformation. Reduction of substituted cyclohexanone and cyclopentan-one rings by hydrides of moderate activity, e.g. NaBH (J.-L. Luche, 1978), leads to alcohols via hydride addition to the less hindered side of the carbonyl group. Hydrides with bulky substituents 3IQ especially useful for such regio- and stereoselective reductions, e.g. lithium hydrotri-t-butoxyaluminate (C.H. Kuo, 1968) and lithium or potassium tri-sec-butylhydro-borates or hydrotri-sec-isoamylborates (=L-, K-, LS- and KS-Selectrides ) (H.C. Brown, 1972 B C.A. Brown, 1973 S. Krishnamurthy, 1976). 

In principle, the direct hydride addition or catalytic hydrogenation, which did not give chlorins, was replaced by an electrocyclic intramolecular addition which is much easier with the above system. Complete regioselectivity was also achieved since electrocyclization did not occur with the resonance-stabilized ring C. 

Unsymmetrical functional tetraorganotins are generally prepared by tin hydride addition (hydrostaimation) to functional unsaturated organic compounds (88) (see Hydroboration). The realization that organotin hydrides readily add to atiphatic carbon—carbon double and triple bonds forming tin—carbon bonds led to a synthetic method which does not rely on reactive organometatiic reagents for tin—carbon bond formation and, thus, allows the synthesis of... 

The formation of 88 is postulated to be occurring by the nucleophilic attack of a hydride ion (47), abstracted from the secondary amine, on the a-carbon atom of the iminium salt (89). The resulting carbonium ion (90) then loses a proton to give the imine (91), which could not be separated because of its instability (4H). In the case of 2-methyIhexamethylenimine, however, the corresponding dehydro compound /l -2-methylazacyclo-heptene (92) was isolated. The hydride addition to the iminium ion occurs from the less hindered exo side. 

The reduction of the double bond of an enamine is normally carried out either by catalytic hydrogenation (MS) or by reduction with formic acid (see Section V.H) or sodium borohydride 146,147), both of which involve initial protonation to form the iminium ion followed by hydride addition. Lithium aluminum hydride reduces iminium salts (see Chapter 5), but it does not react with free enamines except when unusual enamines are involved 148). 

Hydride addition to methylcyclohexanone leads either to cis or trans alcohols, depending on which face is attacked preferentially. 

Formation of 84 upon reaction of 82 with KH in DMF without using NaOMe is explained in terms of the initial hydride addition to 82 at the 3 position, forming 101 and a methoxide ion (Scheme 14). The newly bom methoxide ion then adds to another molecule of 82 to generate 102 and another methoxide ion. The process is repeated as a chain reaction, while 101 and 102 collapse to 83 and 84, respectively. 

Enantiomerically pure 3-tolyl-2-sulfinyl-2-cyclopentenone 37 undergoes smooth, mild and diastereoselective conjugate hydride addition with lithium tri(sec-butyl)borohydride to afford ultimately 3-tolylcyclopentanone 38 in 93% enantiomeric purity (equation 35)78. The absolute stereochemistry of product 38 is consistent with a chelated intermediate directing hydride addition from that diastereoface containing the sulfoxide lone pair. 

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 421). Electrocyclic and sigmatropic rearrangements are treated at 18-27-18-35. Double-bond migrations have also been accomplished photochemically, and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts. In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the nwtal hydride addition-elimination mechanism ... 

The regioselectivity of the hydride addition step has been probed by searching for deuterium exchange into isomerized alkenes that have undergone partial reduction.15 The results suggest that Rh is electrophilic in the addition step and that the hydride transfer is nucleophilic. 

Epoxides are converted to alcohols by LiAlH4 in a reaction that occurs by nucleophilic attack, and hydride addition at the less hindered carbon of the epoxide is usually observed. 

The overall transformation of alkenes to alcohols that is accomplished by epoxi-dation and reduction corresponds to alkene hydration. Assuming a nucleophilic ring opening by hydride addition at the less-substituted carbon, the reaction corresponds to the Markovnikov orientation. This reaction sequence is therefore an alternative to the hydration methods discussed in Chapter 4 for converting alkenes to alcohols. 

Rhodium hydride addition to the enone s-cis conformer through a six-centered transition structure accounts for stereospecific Z(0)-enolate forma-... 

Hydride addition to the cationic Os(O) carbyne complex 10 occurs at the para position of the aryl ring rather than at the carbyne carbon, affording the vinylidene complex 11 (33) ... 

Hydride addition to [CoCp(CsH5BPh)]+ (1) occurs at C-2 and C-4 as well as at the Cp ligand. The products 67 with an (r/5-l-bora-2,4-... 

Addition of electrophiles to a coordinated benzenoid ligand is a characteristic reaction of 20-e complexes. [Co(C5H5BPh)2] in 59 is readily protonated at C-2, producing complex 79 (60) which alternatively can be obtained by hydride addition to cation 61 (69). Formal electrophilic addition is also observed in the reaction of [Co(C5H5BMe)2] in 58 with Phi which affords the two isomeric phenylation products 80 and 81 (60). 

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). 

The 10-57-5-hydridosiliconate ion 62 is known in association with lithium,323 tetrabutylammonium,101 and bis(phosphoranyl)iminium93 cations. It is synthesized by hydride addition to the 8-.S7-4-silane 63, which is derived from hexafluoroacetone.101 Benzaldehyde and related aryl aldehydes are reduced by solutions of 62 in dichloromethane at room temperature101 or in tetrahydrofuran at 0°96 within two hours. The alkyl aldehyde, 1-nonanal, is also reduced by 62 in tetrahydrofuran at O0.96 Good to excellent yields of the respective alcohols are obtained following hydrolytic workup. The reactions are not accelerated by addition of excess lithium chloride,96 but neutral 63 catalyzes the reaction, apparently through complexation of its silicon center with the carbonyl oxygen prior to delivery of hydride from 62.101... 

The reversal of the insertion reaction [Eq. (10)] is not normally observed [in contrast to nickel hydride addition to olefins, Eq. (9)]. An exception is the skeletal isomerization of 1,4-dienes (88, 89). A side reaction—the allylhydrogen transfer reaction [Eq. (5)]—which results in the formation of allylnickel species such as 19 as well as alkanes should also be mentioned. This reaction accounts for the formation of small amounts of alkanes and dienes during the olefin oligomerization reactions (51). 

The essential steps in the nickel-catalyzed 1 1 codimerization reaction, which involve hydride addition to butadiene and ethylene coordination to the metal atom, were first proposed by Kealy, Miller, and Barney (35) and were later demonstrated by Tolman (40) using a model complex. Tolman prepared the complex H—Ni+L PFe [L = (EtO)3P] and showed that, after prior dissociation to form H—NiL3, it can react with butadiene to form a 7r-crotyl complex 19. 

Reactions a and b in Scheme 8 represent different ways of coordination of butadiene on the nickel atom to form the transoid complex 27a or the cisoid complex 27b. The hydride addition reaction resulted in the formation of either the syn-7r-crotyl intermediate (28a), which eventually forms the trans isomer, or the anti-7r-crotyl intermediate (28b), which will lead to the formation of the cis isomer. Because 28a is thermodynamically more favorable than 28b according to Tolman (40) (equilibrium anti/syn ratio = 1 19), isomerization of the latter to the former can take place (reaction c). Thus, the trans/cis ratio of 1,4-hexadiene formed is determined by (i) the ratio of 28a to 28b and (ii) the extent of isomerization c before addition of ethylene to 28b, i.e., reaction d. The isomerization reaction can affect the trans/cis ratio only when the insertion reaction d is slower than the isomerization reaction c. 

Furthermore, the data from Table 2 allow us to draw several valuable conclusions regarding the mechanism of 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). 

We have demonstrated that a series of first row, Group 8 organometallic hydride complexes effect intermolecular hydride addition to coordinated n2-alkene, n2-vinyl ether, and a-alkoxyethy-lidene compounds (64). For example, one equivalent of CpFe(C0)PPh3(H) quantitatively reduces CpFe(CO)2(T12-CH2=CH2)+ to CpFe(C0)2CH2CH3 within one-half hour and leaves... 

A mechanism has been proposed for this, and related transformations, involving a chelation assisted C-H bond functionalization. Following hydride addition to the solvent, acetone, and a transmetallation reaction, reductive elimination yields the ketimine. Hydrolysis of the latter affords the ketone (Equation (131)).114 114a... 

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. 

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