Dienes nucleophilic attack

If racemization were observed, it could only be due to a cleavage of one of die bonds to the chiral center prior to carbon-nitrogen bond formation or subsequent to it. This could occur by (a) enolization of the starting tridate, (b) an ionization of tridate to a carbocation and dien nucleophilic attack by die azide, or (c) enolization in the azido product. The fact that clean inversion occurs means not only that the substitution by azide occurs with inversion but also diat none of diese odier processes is signidcant under the reaction conditions since they would lead to racemized product. 

This thesis has been completely devoted to catalysis by relatively hard catalysts. When aiming at the catalysis of Diels-Alder reactions, soft catalysts are not an option. Soft catalysts tend to coordinate directly to the carbon - carbon double bonds of diene and dienophile, leading to an activation towards nucleophilic attack rather than to a Diels-Alder reaction . This is unfortunate, since in water, catalysis by hard catalysts suffers from a number of intrinsic disadvantages, which are absent for soft catalysts. 

Section 10 10 Protonation at the terminal carbon of a conjugated diene system gives an allylic carbocation that can be captured by the halide nucleophile at either of the two sites that share the positive charge Nucleophilic attack at the carbon adjacent to the one that is protonated gives the product of direct addition (1 2 addition) Capture at the other site gives the product of conjugate addition (1 4 addition)... 

On the basis of these findings, the reaction of acyl imines with methanesulfony 1 chloride-triethylamine is not expected to proceed via a sulfene intermediate as previously proposed [99]. Again, a carbanion intermediate accounts nicely for the experimental facts. The electrophihcity of the hetero-l,3-diene is exdemely high, therefore the carbanion, formed on reaction of triethylamme with methanesulfonyl chloride, should undergo nucleophilic attack at C-4 of the hetero-1,3-diene faster than sulfene formabon by chloride elimination. 

Five-membered ring systems can be obtained from hetero-l,3-dienes on reaction with oxiranes and thiiranes. To avoid competition from a possible 1,4-addition, the nucleophilic attack of the terminal heteroatom of the diene has to be sterically or electronically hindered by incorporation of the heteroatom into... 

Formation of five-membered ring systems (1,2-addition) can compete with formation of the seven-membered heterocycles (1,4-addition). If the first step of the reaction sequence, namely the nucleophilic attack of the terminal heteroatoin of the diene, is hindered by steric or electronic effects, the five-membered ring product is formed exclusively. 

The steric crowding introduced in the latter by the four ethyl substituents inhibits nucleophilic attack at platinum, so that complexes of this type tend to undergo substitution by a dissociative mechanism [89]. The complex of the more rigid ligand, 2,2, 2"-terpyridyl, Pt(terpy)Cl+, is found to be about 103 to 104 times more reactive to substitution than the dien analogue this is ascribed to steric strain [90], which is reflected in the short Pt—N bond to the central nitrogen (Pt-N some 0.03 A shorter than the other two Pt-N bonds) and N—Pt—N bond angles of 80-82°). 

Comparison of results for complexes of tridentate amines R2N(CH2)2-NH(CH2)2NR2 show similar effects. With dien (R = H), rapid substitution of chloride in Pt(dien)Cl+ by bases occurs at room temperature however with Et4dien (R = Et) the reaction is considerably slowed, since the four ethyl groups crowd the metal above and below the plane of the molecule (Figure 3.82) making nucleophilic attack harder. Such a complex can be attacked more easily by a small nucleophile rather than a better nucleophile which happens to be larger [89],... 

Selenoaldehydes 104, like thioaldehydes, have also been generated in situ from acetals and then directly trapped with dienes, thus offering a useful one-pot procedure for preparing cyclic seleno-compounds [103,104], The construction of a carbon-selenium double bond was achieved by reacting acetal derivatives with dimethylaluminum selenide (Equation 2.30). Cycloadditions of seleno aldehydes occur even at 0 °C. In these reactions, however, the carbon-selenium bond formed by the nucleophilic attack of the electronegative selenium atom in 105 to the aluminum-coordinated acetal carbon, may require a high reaction temperature [103], The cycloaddition with cyclopentadiene preferentially gave the kinetically favorable endo isomer. 

Palladium-catalyzed oxidation of 1,4-dienes has also been reported. Thus, Brown and Davidson28 obtained the 1,3-diacetate 25 from oxidation of 1,4-cyclohexadiene by ben-zoquinone in acetic acid with palladium acetate as the catalyst (Scheme 3). Presumably the reaction proceeds via acetoxypalladation-isomerization to give a rr-allyl intermediate, which subsequently undergoes nucleophilic attack by acetate. This principle, i.e. rearrangement of a (allyl)palladium complex, has been applied in nonoxidative palladium-catalyzed reactions of 1,4-dienes by Larock and coworkers29. Akermark and coworkers have demonstrated the stereochemistry of this process by the transformation of 1,4-cyclohexadiene to the ( r-allyl)palladium complex 26 by treatment... 

In general, allenyl cations 38 attack at the sp2-carbon atom of 1,3-dienes and form vinyl cations 39 and 40 (R = H, alkyl) or (R = aryl). Although a concerted cycloaddition mechanism is possible, a stepwise mechanism is preferred34. If a nucleophilic attack at the sp-carbon atom of the allenyl cation takes place, then cation 41 and the resulting cations 42 and 43 are formed. Some examples of bicyclic products obtained from cyclic 1,3-dienes and propargyl chlorides are given in equation 1534. 

When the hydroxy group is converted to a leaving group, the allene moiety can accept the nucleophilic attack leading to the formation of 1,3-dienes 380, 381, 383 and 386 [174],... 

The reaction of an allenoxylsilane, generated in situ from the reaction of propar-gyltrimethylsilane with carbonyl compounds, with TiCl4 led to the formation of 2-chloro-l,3-diene via the Ti-mediated nucleophilic attack of the chlorine atom [182]. 

It is noteworthy that Br2 addition to 3 in aprotic and protic solvents gives exclusively the anti 1,2-addition product. For diene 55, the intramolecular nucleophilic attack of the Z-double bond on the cationic center is exclusive, even in the presence of Et3N-3HF. This has been ascribed to a large strain release in the formation of the d.v-dccalin system from the highly strained medium-sized system. 

In contrast, spectroscopic and crystal structure analysis indicates that nucleophilic attack of hydride on 72 occurs on the face of the ligand which is coordinated to the metal (Scheme 17). No intermediate species could be detected for this latter reaction. Monitoring of the reduction of the rhenium analog 74 with sodium borohydride indicated the intermediacy of a rhenium formyl complex 75, presumably formed by attack on a coordinated carbon monoxide. Signals for 75 eventually disappear and are replaced by those of the (diene)rhenium product 76 (Scheme 18)95. 

Examination of the reactivity of acyclic (diene)Fe(CO)3 complexes indicates that this nucleophilic addition is reversible. The reaction of (C4H6)Fe(CO)3 with strong carbon nucleophiles, followed by protonation, gives olefinic products 195 and 196 (Scheme 49)187. The ratio of 195 and 196 depends upon the reaction temperature and time. Thus, for short reaction time and low temperature (0.5 h, —78 °C) the product from attack at C2 (i.e. 195) predominates while at higher temperature and longer reaction time (2 h, 0 °C) the product from attack at Cl (i.e. 196) predominates. This selectivity is rationalized by kinetically controlled attack at the more electron-poor carbon (C2) at low temperature. Nucleophilic attack is reversible and, under conditions where an equilibrium is established, the thermodynamically more stable (allyl)Fe(CO)3" is favored. The regioselectivity for nucleophilic attack on substituted (diene)Fe(CO)3 complexes has been reported187. The... 

Liebeskind and coworkers have examined the reactivity of (2//-pyran)Mo(CO)2Cp+ cations 210, which may be prepared in optically active form from carbohydrate precursors. Nucleophilic attack on cation 210 occurs at the diene terminus bonded to the ring oxygen to give jr-allyl complexes 51 (Scheme 53)85. Hydride abstraction from 51 gives the cation 54 addition of a second nucleophilie occurs regioselectively to give... 

Nucleophilic addition to acyclic (diene)Mo+ cations has been examined. For (isoprene) Mo(CO)2L (L = Cp, Cp, In), the regioselectivity for nucleophilic attack has been found to depend on the nature of the nucleophile, the ligand L, the reaction solvent and the temperature21,813 833 193. The generation and in situ reactivity of transoid acyclic (diene)molybdenum and tungsten cations with nucleophiles has been previously mentioned (Section IV.C.2). 

Besides direct nucleophilic attack onto the acceptor group, an activated diene may also undergo 1,4- or 1,6-addition in the latter case, capture of the ambident enolate with a soft electrophile can take place at two different positions. Hence, the nucleophilic addition can result in the formation of three regioisomeric alkenes, which may in addition be formed as E/Z isomers. Moreover, depending on the nature of nucleophile and electrophile, the addition products may contain one or two stereogenic centers, and, as a further complication, basic conditions may give rise to the isomerization of the initially formed 8,y-unsaturated carbonyl compounds (and other acceptor-substituted alkenes of this type) to the thermodynamically more stable conjugated isomer (Eq. 4.1). 

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