Diene 1,3-disubstituted

Cycloocta-1,3-diene merely rearranges to the 1,5-isomer, norbornadiene is polymerised, and no reaction occurs with 2,5-dimethylhexa-2,4-diene. Disubstituted alkenes, RCH=CHR, afford isomerised terminal 1 1 adducts. Similarly, terminal alkenes RCH CH2 gave RCH2CH2SiX2 (X = Ph, OEt) or a mixture of... 

A major difficulty with the Diels-Alder reaction is its sensitivity to sterical hindrance. Tri- and tetrasubstituted olefins or dienes with bulky substituents at the terminal carbons react only very slowly. Therefore bicyclic compounds with polar reactions are more suitable for such target molecules, e.g. steroids. There exist, however, several exceptions, e. g. a reaction of a tetrasubstituted alkene with a 1,1-disubstituted diene to produce a cyclohexene intermediate containing three contiguous quaternary carbon atoms (S. Danishefsky, 1979). This reaction was assisted by large polarity differences between the electron rich diene and the electron deficient ene component. 

The dienyne 394 undergoes facile polycyclization. Since the neopentylpalla-dium 395 is formed which has no hydrogen /J to the Pd after the insertion of the disubstituted terminal alkene, the cyclopropanation takes place to form the tt-allylpalladium intermediate 396, which is terminated by elimination to form the diene 397(275]. The dienyne 398 undergoes remarkable tandem 6-e. o-dig. 5-cxo-trig. and -exo-trig cyclizations to give the tetracycle 399 exclu-sively(277]. 

Part structure A is recognised to be a 2,5-disubstituted cyclohexa-1,3-diene on the basis of its chemical shift values. The ethyl group is one substituent, the other is a carboxy function judging by the chemical shift value of 8c = 174.1. The CH multiplicities which follow from the DEPT subspectra, 2C, 4CH, 5CH2 and CHj, lead to the CH part formula C2 + C4H4 + CsHw + CH3 = C12///7. Comparison with the given molecular formula, Ci2/7j 03, indicates an OH group. Since... 

The FMO approach shown in Figure 15.19 again indicates that the 4s - - 2s interaction should lead directly to formation of two new bonding a-bonds, i.e. this is an allowed reaction. The preference for a concerted 4s + 2s reaction is experimentally supported by observations which show that the stereochemistry of the diene and dieneophile is carried over to the product, e.g. a trans,frans-l,4-disubstituted diene... 

The addition of benzyl and t-butylmercaptans to diacetylene and its symmetric disubstituted homologs 57 affords l,4-di(benzylthio)- (59) or l,4-di(t-butylthio)-buta-1,3-dienes (61), respectively, from which 1,2-dithiins 55 are formed (65ZC352 67AG685 85KGS1443 96T12677 96USP5453500). 

Yamamoto et al. were probably the first to report that chiral aluminum(III) catalysts are effective in the cycloaddition reactions of aldehydes [11]. The use of chiral BINOL-AlMe complexes (R)-S was found to be highly effective in the cycloaddition reaction of a variety of aldehydes with activated Danishefsky-type dienes. The reaction of benzaldehyde la with Danishefsky s diene 2a and traws-l-methoxy-2-methyl-3-(trimethylsilyloxy)-l,3-pentadiene 2b affords cis dihydropyrones, cis-3, as the major product in high yield with up to 97% ee (Scheme 4.6). The choice of the bulky triarylsilyl moiety in catalyst (J )-8b is crucial for high yield and the en-antioselectivity of the reaction in contrast with this the catalysts derived from AlMe3 and (J )-3,3 -disubstituted binaphthol (substituent = H, Me, Ph) were effective in stoichiometric amounts only and were less satisfactory with regard to reactivity and enantioselectivity. 

The Diels-Alder cycloaddition reaction (Section 14.4) is a pericvclic process that takes place between a diene (four tt electrons) and a dienophile (two tr electrons) to yield a cyclohexene product. Many thousands of examples of Diels-Alder reactions are known. They often take place easily at room temperature or slightly above, and they are stereospecific with respect to substituents. For example, room-temperature reaction between 1,3-butadiene and diethyl maleate (cis) yields exclusively the cis-disubstituted cyclohexene product. A similar reaction between 1,3-butadiene and diethyl fumarate (trans) yields exclusively the trans-disubstituted product. 

Early work on the microstructurc of the diene polymers has been reviewed.1 While polymerizations of a large number of 2-substituted and 2,3-disubstituted dienes have been reported,88 little is known about the microstructure of diene polymers other than PB,89 polyisoprene,90 and polychloroprene.91... 

The reaction of 1,3-diamino-1,3-dienes with aryl or a,/J-disubstituted alkenylcarbene complexes leads to the formation of formal [4S+1C] cyclopen-tenones [25a] (Scheme 35). In the case of alkenylcarbene complexes, the substitution of the double bond of the complex in both a- and /J-carbons seems to play a fundamental role as reactions performed in the same conditions but using alkenylcarbene complexes with other substitution patterns leads to compounds of a different nature ([4+3], [4+2] and [2+1] cycloadducts). 

Scheme 3 RCM of a diene with trisubstituted and gem-disubstituted double bond en route to iridoid natural products [23]...

Scheme 86 Regioselective enyne RCM for the synthesis of 1,3-disubstituted macrocyclic 1,3-dienes 433 and 434, main fragments in Shair s total synthesis of longithorone A [180]...

With respect to 1,4-disubstituted dienes, fewer cases have been investigated, but here too the reaction is stereospecific and syn. Thus, transjrans-lA-diphenylbutadiene gives cis- 1,4-diphenylcyclohexene derivatives. 

Butadiene with an electron donating group at the 2-position has the largest HOMO amplitude on C, (Scheme 15). A bond forms between Cj of the dienes and Cp of the dienophiles (Scheme 17b). This is in agreement with the exclusive formation of 1,4-disubstituted cyclohexene in the reaction of 2-ethoxybutadiene with methyl acrylate (Scheme 13b) [13, 14]. 

If the reaction of the 2,3-disubstituted-l, 3-butadienes is with a symmetric dienophile, such as maleic anhydride, the 2 and 3 positions in the diene are equivalent, and therefore... 

For these transition states, equations analogous to eqs. (92) and (93) may be written. For case (c), we may obtain equations analogous to eqs. (85) and (88) which are not capable of further simplification. From the above discussion, we see that in the event of the concerted mechanism, the rate constants for the reaction of the disubstituted dienophiles with symmetric dienes should be successfully correlated by eq. (30), which should result in values of a and /3 equal to those obtained from the correlation of the rate constants of substituted dienophiles (with the same diene under the same reaction conditions) with eq. (2). In the event of the two-step mechanism, the rate constants for the reaction of the disubstituted dienophiles with symmetric dienes should not be correlated by eq. (30) unless the two substituents are identical. In addition, the values of a and (3 obtained from the correlation of rate constants for disubstituted dienophiles with eq. (30) should not be equal to the values of a and 3 obtained for the correlation of the rate constants of substituted dienophiles with eq. (2). 

Complete diastereoselection is observed in the HDA reaction of Danishefsky s diene with o-substituted benzaldehyde chromium tricarbonyl complexes. Decomplexation is facile and good yields of 2-aryl-2,3-dihydropyran-4-ones result <96SL258>. Cis-2,3-disubstituted pyranones are accessible from the Lewis-acid catalysed HDA reaction between (triisopropylsilyloxy) dienes and aldehydes and dehydrogenation of the resulting dihydropyrans <96JOC7600>. 

N-silylated imines 509 react with the Li salts of tosylmethylisonitriles to give 4,5-disubstituted imidazoles in moderate yields [93]. Acetylation of N-trimethylsilyl imines 509 with acetyl chloride and triethylamine affords 72-80% of the aza-dienes 510 these undergo readily Diels-Alder reactions, e.g. with maleic anhydride at 24 °C to give 511 [94] or with dimethyl acetylenedicarboxylate to give dimethyl pyridine-3,4-dicarboxylates [94] (Scheme 5.29). 

Both aliphatic and aromatic terminal alkynes reacted with aliphatic aldehydes giving exclusively a mixture of ( ,Z)-1,5-dihalo-1,4-dienes and disubstituted ( )-a,p-unsaturated ketones, the former being the major products in all cases. When nonterminal aromatic acetylenes were used, the trisubstituted ( )-a,p-unsat-urated ketones were the exclusive compounds obtained. The procedure was not valid for ahphatic and unsaturated alkymes. However, the catalytic system was found to be compatible with alcohols and their corresponding acetates although limited yields were obtained. 

In 2009, Ritter and coworkers reported a selective Fe-catalyzed hydroboration of 1,3-dienes to produce linear ( )-y-disubstituted allylboranes under mild conditions when a combination of Ll-FeCl2 and magnesium metal as a catalyst was used. Tbe branched ( )-allylboranes were obtained by using L2-FeCl2 instead of Ll-FeCl2 (Scheme 26) [88]. For the synthesis of 2-borylallylsilanes, this method was superior to the previously reported silaboration of allenes [89]. 

The reactivity of different alkenes toward mercuration spans a considerable range and is governed by a combination of steric and electronic factors.24 Terminal double bonds are more reactive than internal ones. Disubstituted terminal alkenes, however, are more reactive than monosubstituted cases, as would be expected for electrophilic attack. (See Part A, Table 5.6 for comparative rate data.) The differences in relative reactivities are large enough that selectivity can be achieved with certain dienes. 

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