Isomerization regioselective

The possible sources of isomeric aldehyde formation include olefin isomerization, regioselectivity of the addition of the hydridocobalt carbonyl to the olefin, isomerization of the alkylcobalt carbonyl, and isomerization of the acylco-balt carbonyl species. There is no evidence for an isomerization of the alkylcobalt carbonyl species under the conditions of industrial oxo synthesis (high pressure) [96]. In contrast, the isomerization of a coordinated olefin is well known and a plethora of studies have proven this behavior [4]. 

It is interesting to note that these considerations suggest that if the intermediate 34a were to occur, it would isomerize regioselectively in the manner shown in Eq. 

Fustero, S. Sanchez-Rosello, M. Jimenez, D. Sanz-Cervera,J.F. DelPozo,C. Acena,J. L. Role of the gem-difluoro moiety in the tandem ring-closing metathesis-olefin isomerization regioselective preparation ofunsaturated lactams. 7. Org. Chem. 2006, 77(7), 2706-2714. 

Various terminal allylic compounds are converted into l-alkenes at room temperature[362]. Regioselective hydrogenolysis with formate is used for the formation of an exo-methylene group from cyclic allylic compounds by the formal anti thermodynamic isomerization of internal double bonds to the exocyclic position[380]. Selective conversion of myrtenyl formate (579) into /9-pinene is an example. The allylic sulfone 580 and the allylic nitro compound... 

Arylthiols (but not alkylthiols) add to terminal alkynes regioselectively to afford a Markovnikov-type adduct 212 in good yield using Pd(OAc)2 as a catalyst[120]. This result is clearly different from the an/i-Markovnikov addition induced by a radical initiator. The hydroselenation of terminal alkynes with benzeneselenol catalyzed by Pd(OAc)2 affords the terminal alkene 213, which undergoes partial isomerization to the internal alkene 214[121]. 

In the case of the 2-dimethylamino- (or 2-amino-)methylene derivatives, the products were at first thought to be pyrimido[4,5-c]isoquinolines (267), but later work with 6-(N-substituted amino)uracils assigned the structures of the products (268) as belonging to the isomeric pyrimido[4,5-f>]quinoline system (74CB2537), in agreement with the regioselection rules above. 

The fact that the isomeric structure of azolides is thermodynamically controlled has been used by Olofson and Kendall to prepare 1-alkylazoles regioselectively (70JOC2246). An asymmetric pyrazole yields two alkylated derivatives (Scheme 21 see Section 4.04.2.1.3 (viii)), but the alkylation with a powerful alkylating agent of the acetylated derivative leads to the less abundant isomer via the salt (249), which is too unstable to be isolated. 

Diazo compounds react with alkenes to afford A -pyrazolines, which in turn izomerize to A -pyrazolines if there is a hydrogen atom a to the N=N bond (Scheme 54). In those cases where two possible ways of isomerization exist, the more acidic hydrogen migrates preferentially. The alkene configuration is conserved on the A -pyrazoline (stereospecificity) but the regioselectivity depends on the substituents of both the alkene and the diazo compound. 

The photochemical addition of azirines to the carbonyl group of aldehydes, ketones, and esters is also completely regiospecific (77H(6)143). Besides the formation of the isomeric oxazolines (50) from (39) and ethyl cyanoformate, there is also formed the imidazole (51) from addition to C=N in the expected regioselective manner. Thioesters lead to thiazolines (52), while isocyanates and ketenes produce heterocycles (53). 

Perfluoroalkylation of substituted benzenes and heterocyclic substrates has been accomplished through thermolysis of perfluoroalkyl iodides in the presence of the appropriate aromatic compound Isomeric mixtures are often obtained W-Methylpyrrole [143] and furan [148] yield only the a-substituted products (equation 128) Imidazoles are perfluoroalkylated under LTV irradiation [149] (equation 129). 4-Perfluoroalkylimidazoles are obtained regioselectively by SET reactions of an imidazole anion with fluoroalkyl iodides or bromides under mild conditions [150] (equation 130) (for the SET mechanism, see equation 57)... 

When N,A/ -dimethyl-Af,N -diphenyl-p-phenylenediamine (231) underwent irradiation in die presence of oxygen, a remarkably regioselective reaction was observed, from which the /V,A/ -dimediylindolo[2,3-f ]carbazole (190) was obtained, without coformation of die other possible isomeric product, indolo[3,2-/)]carbazole (4). The expected intermediate 232, where only one ring closure had taken place, could not be detected. The authors attributed this fact to a two-photon autosensitization mechanism in which die conversion of 231 into 190 is sensitized by 232, permitting accumulation of 232 only in extremely small concentrations (83JA6268). 

Substituted TMMs also participate smoothly in imine cycloaddition to generate more structurally elaborate pyrrolidines. The regioselectivity of these reactions is similar to that of olefin addition, although subsequent isomerization of the initial adduct is often observed. For example, the cyano system produced the thermody-... 

In their synthesis of (+)-cerulenin, Mani and Townsend employed lithiated epoxysilane 157, which they trapped with (4E,7 )-nonadienal to give a 77% yield of 158, which was further manipulated to give the natural product (Scheme 5.37) [58], as-ot, 3-Epoxy-Y,S-vinylsilanes 159 are regioselectively lithiated at the a-silyl position, and can subsequently be stereo selectively trapped with a range of electrophiles to give a-substituted epoxyvinylsilanes 160, which can in turn be isomerized to a-silyl-P-vinylketones 161 (Scheme 5.38) [59]. 

Interesting regioselectivities have been known for more than 100 years for azo coupling reactions with aminonaphthol derivatives such as 6-amino-4-hydroxy-2-naphthalenesulfonic acid (12.136, y-acid), 7-amino-4-hydroxy-2-naphthalenesulf-onic acid (J-acid), and 4-amino-5-hydroxy-2,7-naphthalenedisulfonic acid (H-acid). They give two types of isomeric azo compounds depending on the pH-value of the... 

In 2000, it was proposed that the regioselectivity of the [3 + 2] cycloaddition of fullerenes could be modified under microwave irradiation. Under conventional heating, N-methylazomethine yhde and fullerene-(C7o) gave three different isomeric cycloadducts because of the low symmetry of C70 vs. Ceo. Using microwave irradiation and o-dichlorobenzene as a solvent, only two isomers were obtained, the major cycloadduct 114 being kinetically favored (Scheme 39) [75]. The same authors had previously reported the 1,3-dipolar cyclo addition of pyrazole nitrile oxides, generated in situ, to Geo under either conventional heating or microwave irradiation. The electrochemical characteristics of the cycloadduct obtained with this method made this product a candidate for photophysical apphcations [76]. 

Terminal alkenes react with selenenyl halides with Markovnikov regioselectivity.64 However, the (J-selenyl halide addition products readily rearrange to the isomeric products.65... 

The regioselectivity of the hydride addition step has been probed by searching for deuterium exchange into isomerized alkenes that have undergone partial reduction.15 The results suggest that Rh is electrophilic in the addition step and that the hydride transfer is nucleophilic. 

Scheme 12.15 gives some examples of both acid-catalyzed and nucleophilic ring openings of epoxides. Entries 1 and 2 are cases in which epoxidation and solvolysis are carried out without isolation of the epoxide. Both cases also illustrate the preference for anti stereochemistry. The regioselectivity in Entry 3 is indicative of dominant bond cleavage in the TS. The reaction in Entry 4 was studied in a number of solvents. The product results from net syn addition as a result of phenonium ion participation. The ds-epoxide also gives mainly the syn product, presumably via isomerization to the... 

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