Bonding isomers

The coupling of alkenylboranes with alkenyl halides is particularly useful for the stereoselective synthesis of conjugated dienes of the four possible double bond isomers[499]. The E and Z forms of vinylboron compounds can be prepared by hydroboration of alkynes and haloalkynes, and their reaction with ( ) or (Z)-vinyl iodides or bromides proceeds without isomerization, and the conjugated dienes of four possible isomeric forms can be prepared in high purity. 

Includes cis, trans, and terminal double-bond isomers. 

While the reductive procedure appears simpler, it also provides a higher proportion of the A -double bond isomer on dehydration than is obtained from the tertiary carbinol. This may be a consequence of the lower steric requirements of a A -olefin having only one alkyl group attached to C-20. In either case, all other double bonds must be protected before the ozonolysis and other free hydroxyls should be esterified prior to dehydration at C-20. 

Valence Bond Isomers of Aromatic Compounds Stabilized by Trifluoromethyl Groups Kohayashi Y Kumadaki, I Acc Chem Res 14 76-82 46... 

UV irradiafion of perfluoro o, m-, and p-xylenes in the gas phase gives a mixture of all possible Dewar benzene isomers Prismane valence bond isomers are proposed to be intermediates [148]... 

Perfluocoalkyl groups thermodynamically destabilize double bonds and small rings, but they can kineiically stabilize highly stramed molecules [75]. This remarkable perfluoroalkyl effect has made possible the isolation of stmctures that are uncommon m hydrocarbon chemistry, especially valence-bond isomers of aromatics and heteroaromatics such as 1, 2, and 3 [108],... 

It is interesting to see that the addition of methyl acrylate to the pyrrolidine enamine derivative of 2-methylcyelohexanone in benzene gave equal amounts of 2-methyl-2-carbomethoxyethyl and 2-methyl-6-earbomethoxy-ethylcyclohexanone even though the less substituted double-bond isomer predominates in the starting enamine (199,200,237). In contrast, the methylation of the same enamine mixture led only to 2,6-dimethyleyclo-... 

Synthetic work in the imidazole series is complicated by the ready tautomeric equilibrium that can be set up among the double bond isomers (A, B). Thus, even i a reaction designed to afford the 5 isomer (A) is accomplished rcgiospecifically, tautomeriza-tion of the product affords either the 4 isomer (B) or a mixture of the two. (This tautomerism is, of course, no problem in azomycin because of the symmetry of that molecule.)... 

Kobayashi, Y., and Kumadaki, I. Valence-Bond Isomer of Aromatic Compounds. 123, 103-150... 

If we were to measure what are called heats of hydrogenation (A/Phydrog)tor the two double-bond isomers and find their difference, we could determine the relative stabilities of cis and Ivans isomers without having to measure an equilibrium position. In fact, the results bear out our expectation. For c/s-2-butene, AEP y rog -120 kj/mol (-28.6 kcal/mol) for the trans isomer, AFP]lvdrfM, = -116 kj/mol (-27.6 kcal/mol). 

Problem 23,4 Aldol condensation of 3-methylcyclohexanone leads to a mixture of two enone products, not counting double-bond isomers. Draw them. 

For product 1 (X = Y = H) an equilibrium between the two possible double-bond isomers has been detected by NMR spectroscopy. For the dibromo derivative (X = Y = Br) a tub conformation has been determined by X-ray structural analysis. 

Trifluoromethyl-containing valence bond isomers have been reviewed (81ACR76). 

A valence bond isomer of pentakis-(trifluoromethyl)-l, 3-diazepine (44) was prepared from (43) (81TL1113) (44) can be transformed thermally or photochemically to a 2,4-diazabicyclo(3.2.0)hepta-2,6 diene (45), which was subsequently photolysed to an imidazole in an anionic process. Compound (45) is highly acidic arising out of the bishomoaromaticity of the anion and forms a salt with Et3N (81TL1369). 

Sporadic reports of the 2//-thiopyran transformation are still available. The electrocyclic ring-opening of 2-benzyl-2,4,6-triphenyl-2//-thiopyran (131, X = PhCH2) was detected by laser flash spectroscopy (86MI1). 2-Amino-2//-thiopyran was predicted to be more stable than its valence-bond isomer on the basis of semiempirical MINDO/3 and MNDO calculations (84JPR955). 

Reaction of PdCl4 with KNCS leads successively to a precipitate of Pd(SCN)2 and the soluble salt K2Pd(SCN)4 (square planar, Pd-S 2.31-2.39 A). This reacts with Ph3As to form the S,S-bonded Pd(SCN)2(AsPh3)2 (kinetic product), which on heating gives the thermodynamically more stable N,N-bonded isomer ... 

The first carbocationic photolysis to be investigated in detail was that of the tropylium ion (1) (van Tamelen et al., 1968, 1971), in which generation of the [3,2,0] valence bond isomer (the Dewar tropylium ion ), (2) was the dominant reaction. When irradiated for 10 minutes in 5% aqueous sulfuric acid, two major products were formed in a total... 

We have seen that carbonium ions can undergo a variety of photoreactions, affording products which often vary considerably from those obtained in the photolysis of the corresponding uncharged compounds. The predominant mode of reaction encountered would seem to be isomerization to one or more valence bond isomers, which occurs via a symmetry-allowed disrotatory electrocyclic closure, rather than a [<72a-f 7r2a] cycloaddition in the case of alkylbenzenium ions and pro-... 

In some cases, the two stereoisomers can interconvert. In cis- and trans-disubstituted cyclopropanones, for example, there is reversible interconversion that favors the more stable trans isomer. This fluxional isomerization occurs via ring opening to an unseen oxyallyl valence bond isomer. ... 

All the above-mentioned acyclic 1,3-diradicals are less stable than the a-bonded isomers. Therefore, in addition to using various substituents, other factors should be further considered in our design of persistent singlet 1,3-diradicals. In Sect. 5.2, ring structure is taken into account. Strain prevents the ring closure in the singlet state. Two linkers between the radical centers multiply the through-bond interactions. 

Fig. 20 The optimized geometries of the lowest singlet (S) and triplet (T) states and the a-bonded isomer (S ) for some selected silicon-centered diradicals...

Fig. 24 Reaction barriers for transformations from singlet 1,4-diradicals 61 (E = Si, Ge, Sn, Pb) to their o-bonded isomers at the level of UB3LYP/LANL2DZ...

Pi oblem The pheromone gossyplure (21) of the pink bollworm moth is a mixture of double bond isomers at bond (a). The c-is double bond (b) can be made from an acetylene so disconnection (24c) is suggested, Hou would you make both cis and trans (23) ... 

Owing to the concerted mechanism, chirality at C(3) [or C(4)] leads to enantiospecific formation of new stereogenic centers formed at C(l) [or C(6)].203 These relationships are illustrated in the example below. Both the configuration of the new stereocenter and the new double bond are those expected on the basis of a chairlike TS. Since there are two stereogenic centers, the double bond and the asymmetric carbon, there are four possible stereoisomers of the product. Only two are formed. The Zs-double bond isomer has the 5-con figuration at C(4) and the Z-isomer has the -configuration. These are the products expected for a chair TS. The stereochemistry of the new double bond is determined by the relative stability of the two chair TSs. TS B is less favorable than A because of the axial placement of the larger phenyl substituent. 

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