Diene regiospecific

Reactions of trifluoroacetonitnle oxide with conjugated dienes also lead to regiospecific additions [36] (equation 36) Its addition to the strain-activated double... 

The rearrangement proceeds from the Si-state of the 1,4-diene 1. The Ti-state would allow for different reactions like double bond isomerization. Rigid systems like cyclic dienes, where EfZ -isomerization of a double bond is hindered for steric reasons, can react through the Ti-state. When the rearrangement proceeds from the Si-state, it proves to be stereospecific at C-1 and C-5 no -isomerization is observed. Z-l,l-Diphenyl-3,3-dimethyl-l,4-hexadiene 5 rearranges to the Z-configured vinylcyclopropane 6. In this case the reaction also is regiospecific. Only the vinylcyclopropane 6 is formed, but not the alternative product 7. ... 

Hoomaert has studied Diels-Alder reactions of pyridine oquinodimethane analogs generated from functionalized o-bis(chloromethyl)pyridines <96T(52)11889>. The photochemical cycloaddition of 2-alkoxy-3-cyano-4,6-dimethylpyridine with methacrylonitrile gives a bicyclic azetine, 6-alkoxy-3,5-dicyano-2,5,8-trimethyl-7-azabicyclo[4.2.0]octa-2,7-diene, in moderate yield <96CC1349>. Regiospecific hydroxylation of 3-(methylaminomethyl)pyridine to 5-(methylaminomethyl)-2-(17/)-pyridone by Arthrobacter ureafaciens has been reported <96MI173>. 

Cycloaddition of unsymmetrical 1,3-dienes, such as isoprene 41b, proceeded regiospecifically to give only a single regioisomer 44 (see Equation (13) and Table 7). 

A simple two-step protocol for the generation of a terminal diene is to add allyl magnesium bromide to an aldehyde or a ketone and subsequent acid or base catalysed dehydration (equation 34)72. Cheng and coworkers used this sequence for the synthesis of some indole natural products (equation 35)72a. Regiospecific dienones can be prepared by 1,2-addition of vinyllithium to a,/l-unsaturated carbonyl compounds and oxidative rearrangement of the resulting dienols with pyridinium dichromate (equation 36)73. 

By regiospecific [4+2] cycloadditions of functional alkenyltin derivatives with dienes such as 1,3-butadiene, 2,3-dimethyl-l,3-butadiene or 1-substituted 1,3-butadienes, polyfunctional cyclic vinyltin compounds 80 are obtained. 

The intramolecular cyclization of l,2-dien-7-ynes and l,2-dien-6-ynes regiospecifically affords the corresponding titanacycles, which react with protons, carbon monoxide, aldehydes, or imines to give single products, as shown in Eqs. 9.56 and 9.57 [102], As the formation of titanacycles and their subsequent reaction with externally added reagents such as carbon monoxide (Eq. 9.56) or an aldehyde (or imine) (Eq. 9.57) proceeds with excellent chirality transfer, this represents a new method for synthesizing optically active cyclopentane derivatives from optically active allenes [102]. 

Electron-withdrawing groups attached to the dienophile accelerate the rate of the reaction, donating groups decrease it. For the diene, the opposite is true. Because of the high regiospecificity and the tolerance of a wide number of functional groups, the Diels-Alder reaction is of considerable synthetic importance. 

Depending on the linear, branched or cyclic structure of the unsaturated compound, a variety of dialkyl sulfides has been obtained in the reaction with H2S (equations 16-18). The regiochemistry depended markedly on the structure of the diene. For a mechanistic purpose, some experiments have been carried out using deuterium sulfide, D2S. The results have been interpreted in terms similar to those of Nordlander and coworkers9 (vide infra). The thiylation of 1,3-dienes was assumed to start with a regiospecific addition of a proton or a deuteron to one of the two double bonds to form two isomeric ion pairs as in equation 14 which, in the poorly dissociating solvent, collapse into products with equal probability. 

The explanation of the regiospecificity of Diels-Alder reactions requires knowledge of the effect of substituents on the coefficients of the HOMO and LUMO orbitals. In the case of normal electron demand, the important orbitals are the HOMO on the diene and the LUMO on the dienophile. It has been shown that the reaction occurs in a way which bonds together the terminal atoms with the coefficients of greatest magnitude and those with the coefficients of smaller magnitude [18]. The additions are almost exclusively cis and with only a few exceptions, the relative configurations of substituents in the components is kept in the products [19]. 

Reaction of pentafluoro-substituted benzene derivatives with xenon difluoride/boron trifluoride results in 1,4- and 1,2-fluorine addition with regiospecificity depending on the substituent.41 For example, the heptafluorocyclohexa-1,4-dienes 1 can be obtained in this way in high yield. 

RCH—CHi - RCHiCHO. This reaction can be carried out by hydroboration followed by oxidation of the resulting organoborane with pyridinium chlorochro-mate (PCC). In first reports of the transformation BH3 S(CII3)2 was used, but this hydroboration is not regiospecific in the case of terminal alkenes. Use of disiamylborane is superior since this borane is highly selective in hydroboration of both alkenes and dienes.1... 

The latter transformation requires the use of a small amount of an acid or its ammonium salt. By using [Cp2TiMe2] as the catalyst, primary anilines as well as steri-cally hindered tert-alkyl- and sec-alkylamines can be reacted.596 Hydroamination with sterically less hindered amines are very slow. This was explained by a mechanism in which equlibrium between the catalytically active [L1L2Ti=NR] imido complex and ist dimer for sterically hindered amines favors a fast reaction. Lantha-nade metallocenes catalyze the regiospecific addition of primary amines to alkenes, dienes, and alkynes.598 The rates, however, are several orders of magnitude lower than those of the corresponding intramolecular additions. 

Dihydropyrans and tetrahydropyrans add on alkenes when irradiated but some of the products are unstable. 3,3-Dimethylpyran-2,4-dione (664) and an unsymmetrical diene such as 2-methylprop-l-ene (665) give regiospecifically a 1 1 mixture of cis- and frans-fused pyran-4-one (666) (73T1317). Tetrahydropyran (667) does not undergo photoaddition unless a sensitizer such as benzophenone is present, but then it reacts with diethyl maleate (668) (67T3193). 

Cycloadditions of nitroso compounds and 1,3-dienes lead to 3,6-dihydro-2//- 1,2-oxazines this is a well tried and documented procedure (B-67MI22700), a comparatively modern example of which is the regiospecific synthesis of 2,3,4-trisubstituted derivatives (155) from the addition of nitrosobenzenes to 1,2-disubstituted dienes (154) (78T697). 

Coupling of 1-alkenylboranes with 1-bromoalkenes or -alkynes.4,s This reaction fails when catalyzed by Pd(0) alone, but proceeds in high yield in the presence of an added sodium alkoxide or sodium hydroxide in refluxing benzene (or THF). This coupling can be used to prepare (E,E)-, (E,Z)-, (Z,E)-, or (Z,Z)-1,3-dienes stereo- and regiospecifically. 

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