Intermolecular Reaction Pattern

As mentioned above, the double dilithium bridge in 1,4-dilithio-l,3-butadiene 1 exists both in the solid states and in solution. When these di-lithio reagents are allowed to react with other substrates, the two C-Li bonds would work together to show cooperative effect, and a variety of cyclic or acyclic compounds could be thus generated. Formation of cyclic compounds from the reactions of polylithium reagents has been reported. For example, bis(2-lithioallyl) amines as 

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Scheme 1 Preparation of carbocyles mediated by stoichiometric amounts of metallacycles via intermolecular reaction patterns...

The reactions of carbon dioxide and carbon disulfide with organoUthium reagents have attracted much attention and have been applied in various organic synthesis. Reaction of carbon dioxide with organolithium compounds normally affords carboxylic acids after hydrolysis [50]. The formation of unsymmetrical ketones was reported from the reaction of CO2 and two organolithium compounds via an intermolecular reaction pathway [51]. When 1,4-dilithio-l,3-dienes 1 was treated with CO2, cyclopentadienone derivatives 20 with various substituents could be prepared in high yields in one-pot within several minutes via cleavage of one of the C=0 double bonds (Scheme 10) [52]. The experimental results indicate that this intermolecular reaction pattern affords cyclopentadienones in the reaction mixture before hydrolysis. 

Pyrolysis MS (PyMS) has been applied to the characterisation and identification of a variety of microbial systems over a number of years (for reviews see [25-27]) and, because of its high discriminatory ability [28-30], presents a powerful fingerprinting technique applicable to any organic material. Whilst the pyrolysis mass spectra of complex organic mixtures may be expressed in the simplest terms as sub-patterns of spectra describing the pure components of the mixtures and their relative concentrations [24], this may not always be true because during pyrolysis intermolecular reactions can take place in the pyrolysate [31-33]. This leads to a lack of superposition of the spectral components and to a possible dependence of the mass spectrum on sample size [31]. However, suitable numerical methods (or chemometrics) can still be employed to measure the concentrations of biochemical components from pyrolysis mass spectra of complex mixtures. 

The regio- and stereoselective intramolecular silaformylation of alkynes is the result of an exo-dig patterned pathwayThe intermolecular reaction is still syn-selective, but the formyl group is placed in the more hindered position. 

Both intramolecular and intermolecular reaction pathways are available to the conjugated dieneketenes. Which product components emerge victoriously out of the competing processes depends on the substitution pattern and the reaction conditions. 

Before we really take off, we would like to outline the reaction patterns that have been observed for this reaction type. They fall into three categories (i) completely intermolecular reactions, (ii) partially intramolecular reactions, and (iii) completely intramolecular reactions (Figure 9.1) [Id]. 

Patterns of [2 - - 2 - - 2] cycloaddition for the synthesis of cyclophane are depicted in Scheme 8.1. Intermolecular reaction of diyne and monoyne can provide ortho, meta, and para isomers as dipodal cyclophanes (pattern A). Linear triyne can be transformed into ortho-ortho and ortho-meta isomers by an intramolecular reaction (pattern B). In the reaction of branched triyne, symmetrical 1,3,5- and unsymmetrical 1,2,4-isomers can be obtained as tripodal cyclophanes (pattern C). The choice of catalyst and tether is very important for induction of the aforementioned regioselectivities. 

In the stochastic theory of branching processes the reactivity of the functional groups is assumed to be independent of the size of the copolymer. In addition, cyclization is postulated not to occur in the sol fraction, so that all reactions in the sol fraction are intermolecular. Bonds once formed are assumed to remain stable, so that no randomization reactions such as trans-esterification are incorporated. In our opinion this model is only approximate because of the necessary simplifying assumptions. The numbers obtained will be of limited value in an absolute sense, but very useful to show patterns, sensitivities and trends. 

This chapter deals with [2 + 2]cycloadditions of various chromophors to an olefinic double bond with formation of a four-membered ring, with reactions proceeding as well in an intermolecular as in an intramolecular pattern. Due to the variety of the starting materials available (ketones, enones, olefins, imines, thioketones, etc.. . .), due to the diversity of products obtained, and last but not least, due to the fact that cyclobutanes and oxetanes are not accessible by such a simple one-step transformation in a non-photo-chemical reaction, the [2+2]photocycloaddition has become equivalent to the (thermal) Diels-Alder reaction in importance as for ring construction in organic synthesis. 

Trost and others have extensively studied the ruthenium-catalyzed intermolecular Alder-ene reaction (see Section 10.12.3) however, conditions developed for the intermolecular coupling of alkenes and alkynes failed to lead to intramolecular cycloisomerization due the sensitivity of the [CpRu(cod)Cl] catalyst system to substitution patterns on the alkene.51 Trost and Toste instead found success using cationic [CpRu(MeCN)3]PF6 41. In contrast to the analogous palladium conditions, this catalyst gives exclusively 1,4-diene cycloisomerization products. The absence of 1,3-dienes supports the suggestion that the ruthenium-catalyzed cycloisomerization of enynes proceeds through a ruthenacycle intermediate (Scheme 11). 

Needless to say, the Buchwald-Hartwig reaction can also be usefully employed in ways other than the efficient preparation of diphenylamines. Given the respective substitution, it should be possible to bring about the phenazine skeleton by Pd-catalyzed ring formation as well. There are two ways to proceed either the substituent pattern required by the intramolecular Buchwald-Hartwig reaction is elaborated after the formation of the diphenylamine (121 124), or the starting material already contains the substituents necessary for the two JV-arylations. A reasonable starting point is the intermolecular JV-arylation of an o-haloaniline... 

In contrast to the intermolecular cyclopropanation, the dirhodium tetraprolinates give modest enantioselectivities for the corresponding intramolecular reactions with the do-nor/acceptor carbenoids [68]. For example, the Rh2(S-DOSP)4-catalyzed reaction with al-lyl vinyldiazoacetate 32 gives the fused cyclopropane 33 in 72% yield with 72% enantiomeric excess (Eq. 4) [68]. The level of asymmetric induction is dependent upon the substitution pattern of the alkene cis-alkenes and internally substituted alkenes afford the highest asymmetric induction. Other rhodium and copper catalysts have been evaluated for reactions with vinyldiazoacetates, but very few have found broad utility [42]. 

We have demonstrated that intermolecularly, amidyl radicals preferentially abstract an allylic hydrogen rather than add to a TT bond of olefins such as cyclohexene and 1,3-pentadiene (33). This reactivity pattern is completely reversed in intramolecular reactions as shown in the following examples of alkenyl mitro-samide photolysis. In every case, the amidyl radicals generated from photolysis preferentially attack the ir bonds intramolecu-... 

As mentioned at the beginning of this section, intermolecular [2 + 2] photocycloadditions quite often afford product mixtures, depending on the alkene used as a ground state partner. A complete overview of such reactions described in the last twenty years would be far beyond the scope of this section and therefore attention will be directed to examples where the symmetric substitution pattern of the alkene allows for the formation of a specific cyclobutane derivative. 

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