Six-membered cyclic transition state

When fragmentation is accompanied by the formation of a new bond as well as by the breaking of an existing bond, a rearrangement process is involved. These will be even mass peaks when only C, H, and O are involved. The migrating atom is almost exclusively hydrogen six-membered cyclic transition states are most important. 

Substitution, addition, and group transfer reactions can occur intramolecularly. Intramolecular substitution reactions that involve hydrogen abstraction have some important synthetic applications, since they permit functionalization of carbon atoms relatively remote from the initial reaction site. ° The preference for a six-membered cyclic transition state in the hydrogen abstraction step imparts position selectivity to the process ... 

A mechanism of the 1,2-addition of Grignard reagents to ketones has been suggested by Swain involving a six-membered cyclic transition state. Ashby proposed a detailed mechanism as shown in (l)-(2). 

It has been proposed that oxygen adds to the excited keto group [- (112)]. The rearrangement of the resulting hydroxyhydroperoxy diradical (112) could then proceed by intramolecular hydrogen abstraction involving a six-membered cyclic transition state, followed by fission of the former C —CO bond to form the unsaturated peracid (113) as the precursor of the final product. Such a reaction sequence demands a hydrogen atom in the J -position sterically accessible to the intermediate hydroperoxy radical. 

Flash vacuum thermolysis (FVT) of 2-substituted 4//-pyrido[l,2-n]pyrimidin-4-ones 126 above 800 °C afforded (2-pyridyl)iminopropadie-none (130) (99JCS(P2)1087). These reactions were interpreted in terms of reversible ring opening of 4//-pyrido[l,2-n]pyrimidin-4-ones to imidoyl-ketenes 127. A 1,5-H shift in 127 generated the N(l)H-tautomeric methylene ketene 128, in which facile elimination of HX took place via a six-membered cyclic transition state 129 to yield 130. In the case of 2-methoxy derivative 126 (X = OMe) another competing pathway was also identified at lower temperature, which resulted in the formation C3O2 and 2-methylaminopyr-idine via mesoionic isomer 131 (Scheme 9). The products were identified by IR spectroscopy. 

The thermolysis (pyrolysis) is generally carried out at temperatures ranging from 100-250 °C. Similar to the closely related ester pyrolysis the reaction mechanism is of the Ei-type, which involves a six-membered cyclic transition state 7 ... 

The diene passes through a six-membered cyclic transition state 3 preferentially of chair-like conformational geometry ... 

The reaction proceeds by an Ei-mechanism. The /3-hydrogen and the carboxy-late are cleaved synchronously from the substrate molecule, while forming a new bond. This elimination reaction belongs to the class of -eliminations in the case of the ester pyrolysis, the substrate molecule passes through a six-membered cyclic transition state 4 ... 

Since the -elimination mechanism requires formation of a six-membered cyclic transition state, this reaction is not possible for five- or six-membered lactones, but may be applied to higher homologs. 

Grignard reagents that contain a /3-hydrogen—e.g. 15—can reduce a carbonyl substrate by transfer of that hydrogen as a side-reaction. The so-called Grignard reduction is likely to proceed via a six-membered cyclic transition state 16 the alkyl group of alkylmagnesium compound 15 is thereby converted into an alkene 17. 

The aldehyde or ketone, when treated with aluminum triisopropoxide in isopropanol as solvent, reacts via a six-membered cyclic transition state 4. The aluminum center of the Lewis-acidic reagent coordinates to the carbonyl oxygen, enhancing the polar character of the carbonyl group, and thus facilitating the hydride transfer from the isopropyl group to the carbonyl carbon center. The intermediate mixed aluminum alkoxide 5 presumably reacts with the solvent isopropanol to yield the product alcohol 3 and regenerated aluminum triisopropoxide 2 the latter thus acts as a catalyst in the overall process ... 

Thus one of the transferred hydrogens conies from the aluminum reagent, and the other one from the solvent. In addition to the mechanism via a six-membered cyclic transition state, a radical mechanism is discussed for certain substrates. ... 

Like the Diels-Alder reaction discussed in Sections 14.4 and 14.5, the Claisen rearrangement reaction takes place through a pericyclic mechanism in which a concerted reorganization of bonding electrons occurs through a six-membered, cyclic transition state. The 6-allyl-2,4-cyclohexadienone intermediate then isomerizes to o-allylpbenol (Figure 18.1). 

The reaction is believed to proceed via a six-membered cyclic transition state, analogously to the carbonyl addition of enolates, but the energy differences between boat- and chair-like arrangements are lower for x-sulfinyl carbanions69. Tor tert-butyl sulfoxides only anti- and, vn-products are obtained, arising from the approach onto the same diastereotopic face of the anion, but with different relative topicity. The exchange of lithium by zinc causes an increase of the anft-produci, but attempts to titanate the anion failed (see Table 3)69. 

No stereoselectivity was observed in the formation of a 1 1 diastereomeric mixture of 2-hydroxy-2-phenylethyl p-tolyl sulfoxide 145 from treatment of (R)-methyl p-tolyl sulfoxide 144 with lithium diethylamide . However, a considerable stereoselectivity was observed in the reaction of this carbanion with unsymmetrical, especially aromatic, ketones The carbanion derived from (R)-144 was found to add to N-benzylideneaniline stereoselectivity, affording only one diastereomer, i.e. (Rs,SJ-( + )-iV-phenyl-2-amino-2-phenyl p-tolyl sulfoxide, which upon treatment with Raney Ni afforded the corresponding optically pure amine . The reaction of the lithio-derivative of (-t-)-(S)-p-tolyl p-tolylthiomethyl sulfoxide 146 with benzaldehyde gave a mixture of 3 out of 4 possible isomers, i.e. (IS, 2S, 3R)-, (IS, 2R, 3R)- and (IS, 2S, 3S)-147 in a ratio of 55 30 15. Methylation of the diastereomeric mixture, reduction of the sulfinyl group and further hydrolysis gave (—)-(R)-2-methoxy-2-phenylacetaldehyde 148 in 70% e.e. This addition is considered to proceed through a six-membered cyclic transition state, formed by chelation with lithium, as shown below . ... 

The pyrolysis of xanthates (84)—the Chugaev reaction—and of carboxylic esters (85) differ from the above in proceeding via six-membered, cyclic transition states, e.g. (86) and (87), respectively ... 

From a mechanistic point of view, two very general pathways can be envisaged for hydrogen transfer direct hydrogenation transfer, consisting of a concerted process that involves a six-membered cyclic transition state in which both the hydrogen donor and the acceptor are coordinated to the metal (1 in Scheme 22) and a hydridic route (2 in Scheme 22).116... 

Since many of these processes occur through six-membered cyclic transition states, they can be highly stereoselective. However, for these reactions to proceed, not only high steric demands but also good leaving groups X are required. In this respect, silyl and acyl nitronates are a priori most preferable. 

In Scheme 3.194, this process is represented as proceeding through the six-membered cyclic transition state, although C,C-coupling of two ions generated from the starting substrates cannot be ruled out. However, the structure of the nitronate can be varied over a wide range attempts to modify the siloxymethylene component failed. 

The report suggests that the actual reagent may be a pentacoordinate allyl-siliconate, such as CH,CH=CHCH2Si F4Cs+, which reacts with an aldehyde via a six-membered cyclic transition state. 

They envisaged the propagation as monomer insertion into the C-0 bond of the ester via a four- or six-membered cyclic transition state (II). 

Transfer of hydrogen occurs exclusively in a syn manner and it has been concluded that the reduction of a multiple bond by diimide involves a synchronous transfer of a pair of hydrogens to a single face of the carbon-carbon double bond via a six-membered cyclic transition state to give a syn adduct (equation 22)77,81. 

The smooth allyloboration of alkynes is known to proceed via an allyl rearrangement, probably including a six-membered cyclic transition state. Thermal treatment of the product initiates a second allyloboration step and a vinyloboration thereafter the whole procedure [Eq. (37)] opens a synthesis of boraadamantanes by further reaction steps 76). 

Reaction of aldehydes and ketones with iminoboranes has been widely investigated. Conditions for the [2 + 2]-cycloaddition between XBNR and R R"CO are relatively good stability of the iminoborane and lack of enolic protons in the oxo compound [Eq. (46)] 14, 19). Relatively less stable iminoboranes, but in some cases the stable ones too, may react with 0X0 compounds by a total opening of the B=N triple bond [Eq. (43)], presumably via a [2 + 2]-cycloaddition [Eq. (42)] (Section V,D). A relatively stable iminoborane and a ketone containing enolic protons may yield an open-chain product, probably through a six-membered cyclic transition state [Eq. (46b)] 19). 

In this section, we will focus on intramolecular functionalization. Such reactions normally achieve selectivity on the basis of proximity of the reacting centers. In acylic molecules, intramolecular functionalization normally involves hydrogen-atom abstraction via a six-membered cyclic transition state. The net result is introduction of functionality at... 

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