Intramolecular hydride migration

The steps by which this metal hydride forms the observed organic products are perhaps similar to those already discussed for cobalt catalysts. Steps which may be involved are intramolecular hydride migration to produce a formyl ligand ... 

The first intramolecular hydride migration from the metal to BT may lead to either one of the two possible isomeric 2- or 3-benzothienyl intermediates, and the available experimental evidence does not allow a clear distinction between them. However, semi empirical (CNDO) [83-85] as well as ab initio [93] calculations indicate that the C(2) atom of free benzothiophene is more negatively charged than C(3) and thus it should be more susceptible to electrophilic attack by the metal therefore a 2-benzothienyl intermediate is more likely to be involved in the catalytic cycle. 

Rh( 73-S(C6H4)CH=CH2)] (6), which can be isolated as the potassium salt. As in monophase systems [4], 6 would form by regioselective insertion of rhodium into the C2—S bond from BT, followed by intramolecular hydride migration to the a-car-bon atom of a metallathiacycle intermediate [14, 16]. Reaction of 6 with H2 finally yields the (2-ethylthiophenolate)dihydride complex [(SULPHOS)Rh(H)2(o-S(C6H4)-C2H5)]- (7). 

Wilkinson s catalyst mediates stoichiometric intramolecular C-C bond forming reactions with certain substrates containing acidic C-H bonds via an intramolecular hydride migration yielding a 1,3-diketone (eq 79). ... 

The alkylidene complex [CpRe(=CHEt)(PPh3)(NO)]PFg rearranges to the propene crunplex [CpRe(CH2=CHMe)(PPh3)(NO)]PF5. The mechanism involves intramolecular hydride migration, no PPh3 dissociation and retention of configuration at ihwiium e... 

Scheme 16.2 illustrates the catalytic mechanism proposed by Muetterties and coworkers [13]. Salient features of this mechanism are the coordination of benzene in the -fashion, to give a transient Col I( 4-C, iH, i)(PR3)2 complex, and the intramolecular hydride transfer to form the allylic intermediate Co(//3-Ctl l7) (PR3)2. Hydrogen addition would give an 4-1,3-cyclohexadiene complex that ultimately releases cyclohexane via H2 addition/hydride migration steps. Complete cis stereoselectivity of hydrogen addition was demonstrated by replacing H2 with D2. 

In acceptor-substituted carbene complexes with hydrogen at Cp fast hydride migration to the carbene will usually occur [1094,1095]. The resulting olefins are often formed with high stereoselectivity. 1,2-Hydride migration will also occur in P-hydroxy carbene complexes, ketones being formed in high yields (Table 4.2). Intramolecular 1,5-C-H insertion can sometimes compete efficiently with 1,2-insertion [1096]. 

The intramolecular reaction of carbenoids with pyrroles is extremely effective and leads to the formation of heterobicyclic systems in high yield, as can be seen in the simple systems shown in equation (44).163 The reaction is so favorable that even an alkylcarbenoid capable of hydride migration could be utilized to generate a bicyclic system (218), but formation of (219) was a competing reaction in this case (equation 45).166... 

A number of intramolecular coupling reactions that involve reductive cyclization of a (cyclohexadiene)Fe(CO)3 complex that bears a pendant alkene or diene have been reported. Under CO atmosphere, this unusual cyclization presumably produces an intermediate r] -7t-d y complex that then rearranges via hydride migration. In the case of the pendant diene, a second such process can occur (Scheme 68). 

An iminium ion-alkene cyclization has been employed to assemble the phenylmorphan ring system (Scheme 26). The conversion of enamine (64) to (66) was suggested to arise by 1,5-hydride migration of an initially formed bicyclic cation (65). Direct intramolecular ene cyclization of the iminium ion (67) produced by protonation of (64) provides an alternative rationale for the net cis addition to the terminal alkene that occurs in this transformation, and avoids postulating the intervention of a relatively unstable fully formed secondary carbocation. 

Other reports were more fortuitous, as in the unexpected formation of 565 (Scheme 55) upon alkylation of Bp Rh(CO)(py) (566,) with methyl iodide. The mechanism for this was established to be intramolecular (from cross-over experiments with and H-labelled 566), and proposed to proceed by oxidative addition of Mel, with subsequent migratory insertion of CO. The nature of the hydride migration process from boron remains to be determined. On prolonged heating (45 °C), 565 evolves into 567 a rare example of reverse a-hydride migration from metal to carbene. 

It is important to note the similarities and differences between the benzylic acid rearrangement and the Cannizzaro reaction (Scheme 9.4). The aryl group migrates in the benzylic acid rearrangement. The hydride migrates in the Cannizzaro reaction. Further, the Cannizzaro reaction is intermolecular (second-order in aldehyde and first-order in base), while the benzylic acid rearrangement is intramolecular and first-order both in base and in diketone. 

The initial carbocation formation bears the potential for various reactions including intramolecular attack of an olefinic double bond, hydride migrations, or Wagner-Meerwein rearrangements. The transformation of a linear polyisoprenoid... 

Hurd et al. elegantly elaborated this methodology in the key step of total synthesis of PNU-286607 (Scheme 4) [88]. The benzylidene intermediate 5 was prepared in situ and [1,5]-hydride migration readily proceeded under thermal conditions to give zwitterionic intermediate 6. Via trans-cis isomerization of methyl group, the zwitterion 6 was converted to thermodynamically more favorable zwitterion 7, and a subsequent intramolecular equatorial attack of the enolate on the iminium subunit furnished cis (-)-PNU-286607 in 74 % yield and >99 1 er. 

Harmata et al. developed an intramolecular redox C-H activation process of alkenyl sulfoximines to synthesize 4- and 6-membered heterocycles 106 and 105 (Scheme 41) [120]. Terminal alkene activated by sulfoximines worked as hydride acceptor and allylic C-H bond served as hydride donor. Notably, the reaction time strongly influenced the formation of final products. When 104 was refluxed in toluene for 3.5 h, the 4-membered cyclic species 106 could be obtained in 41 % yield as the major product whereas if the reaction was refluxed for around 24 h, 6-membered thiazines 105 were isolated as a mixture of diastereomers in 40 % yield. Mechanistically, an intramolecular [1,5]-hydride migration operates initially, leading to zwitterionic intermediate I. Subsequent ring closure can be formulated as the intramolecular collapse of the zwitterionic intermediate I or II. The formation of 4-membered product 106 might be kinetically favorable and reversible. Although intermediate II might be less stable than intermediate I for the reason that the allylic... 

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