Hydride shift-ring closure reaction

Scheme 3.23 Cascade hydride shift-ring closure reaction.

Intramolecular Redox Reaction. Polycyclic tetrahydro-quinolines were prepared by a scandium triflate-catalyzed 1,5-hydride shift/ring-closure sequence (eq 27). While thetitlecom-pound readily catalyzes this intramolecular neutral C-H bond functionalization, Gd(OTf)3 proved to be more efficient. 

Diols generally react with dichlorocarbene to produce a mixture of alkenes and chlorinated cyclopropanes or chloroalkanes, depending on the reaction conditions whereas, under phase-transfer catalysed conditions, the major products are the alkenes and epoxides produced by ring closure of the initial adduct (Scheme 7.20) [14]. When an excess of chloroform is used, further reaction of the alkenes with dichlorocarbene produces the cycloadducts. In addition to the formation of the alkene and epoxide, 1,2-dihydroxycyclooctane yields cyclooctanone, via a 1,2-hydride shift within the intermediate carbenium ion. 

The reaction path depicted in Scheme 5.14 involves Wagner-Meerwein shifts of the methyl group prior to cyclization followed by hydride shift to a number of cationic intermediates. The second scheme (Scheme 5.15) depicts ring closure before methyl migration. The first step involves protolysis of the C—H bond next to the methyl-bearing carbon. The corresponding ion can then rearrange by a 1,2-methyl shift and yield 1,16-dimethyldodecahedrane 28 by hydride abstraction from a hydride donor. 

Presumably this reaction proceeds via an allylic carbocation intermediate 301 formed by Lewis acid-initiated C-S cleavage, then ring closure to a new carbocation 302 stabilized by the silyl group. As illustrated in Scheme 55, the subsequent steps, leading to final product 300, involve disulfide bond formation, double-bond shift, and hydride migration. 

The mechanistic details of this reaction have been examined by several workers. Absorption of a photon produces the enamine excited singlet state EA, which undergoes inter system crossing to the excited triplet state EA. Cyclization leads to an excited zwitterion ZW, which relaxes to the zwitterionic ground state (Scheme 8) . A suprafacial hydride shift produces the observed product. The presence of the zwitterionic intermediate has been detected by laser flash photolysis . In studies on the related sulfide ring closure, the zwitterion was trapped by a dipolarophile ... 

A 5-endo reaction takes place when 4-methyl-4-dcccnal is treated with tin(IV) chloride. After ring closure by nucleophilic attack of the olefin on the polarized carbonyl group, the resulting tertiary cation is stabilized by a sequence of two consecutive suprafacial [1,2] hydride shifts and ra . -2,3-dialkylated cyclopentanone 1 is formed stereoselectively2Q. 

Equation 16 illustrates a celebrated example where ring closure competes with vicinal hydride shift (a common form of atom transfer in cations, which does not take place in free radicals or anions). The gas phase reaction was explored by preparing the dimethylfluoronium ion, (CH3)2F" , by y-radiolysis of fluoromethane. Exothermic methylation of a sample of C-yd-phenethyl chloride (where the asterisk in equation 16 symbolizes the labeled position) in the gas mixture gives a vibrationally excited ion that loses chloromethane to form two isomeric ions, a-phenethyl cation and spirooctadienyl cation (sometimes called ethylenebenzenium). Nucleophilic attack by methanol in the reaction mixture yields PhCH(CH3)OCH3, whose isotopic label remains almost entirely at the methyl group. The recovered PhCH2CH20CH3 contains equally distributed between the two methylene positions. The spirooctadienyl ion does not isomerize to a-phenethyl cation, even though DFT calculations predict the latter to be 55 kJ/mol more stable. 

A relay catalytic hydroamination/redox reaction for the synthesis of cyclic aminals 83 from tertiary amine-substituted 3-en-l-yne derivatives 81 and various aniline derivatives 82 was recently developed by Gong and coworkers (Scheme 12.40) [43]. The gold carbene complex [(IPrlAuNTfj] and TfOH were found to be optimal cocatalysts for this cascade reaction. In their preliminary studies, the authors found that this reaction could be carried out in a highly enantioselective manner, despite that 2 equiv. of an expensive chiral phosphoric acid are required. Interestingly, the diastereoselectivity for a particular substrate appears to be independent of the catalyst or ee. This suggests that the enantioselectivity may result from the ring-closure step, rather than the hydride shift step. 

The stannic chloride-catalyzed cyclization of acyl cyclopropane (195) to the tricyclic ketone (196) affords a precedent for the biogenesis of the tricyclo[4.4.0.0 ]nucleus of copaene and ylangene 270). The reaction pathway is thought to proceed via ring opening, cyclization, hydride shift, and ring closure. 

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