2-Alkyl-l ,3-butadiene

Table 13 Polymerization of 2-alkyl-l,3-butadienes with various influence on the micro structure. 

The 3-alkyl-l,3-butadiene-2-carboxylate (2-vinylacrylate) 850 is obtained in a high yield by the carbonylation of the 2-alkyl-2,3-butadienyl carbonate 849 under mild conditions (room temperature, 1 atm)[522]. The corresponding acids are obtained in moderate yields by the carbonylation of 2,3-alkadienyl alcohols under severe conditions (100°C, 20 atm) using a cationic Pd catalyst and p-TsOH[523],... 

AIkylthio)allylritanium reagentS, RSCH=CHCH2TiL (l).9 The reagents are prepared by deprotonation of allylic alkyl (aryl) sulfides with sec- or r-butyllithium followed by addition of Ti(0-/-Pr)4 at - 78°. They can react with carbonyl compounds at the a- or "/-position. a-Adducts predominate in reactions with a- and /1-mono- and disubstituted sulfides, whereas /-adducts predominate in reaction with /-substituted sulfides. The a-adducts show high eryr/iro-selectivity. The products are useful precursors to alkenyl oxiranes and to 2-(arylthio)-l,3-butadienes. 

Butadienes with alkyl substituents in the 2-position favor the formation of the so-called para-products (Figure 15.25, X = H) in their reactions with acceptor-substituted dienophiles. The so-called mefa-product is formed in smaller amounts. This regioselectivity increases if the dienophile carries two geminal acceptors (Figure 15.25, X = CN). 2-Phenyl-1,3-butadiene exhibits a higher para -selectivity in its reactions with every unsymmetrical dienophile than any 2-alkyl-1,3-butadiene does. This is even more true for 2-methoxy- 1,3-butadiene and 2-(trimethylsilyloxy)-l,3-butadiene. Equation 15.2, which describes the stabilization of the transition states of Diels-Alder reactions in terms of the frontier orbitals, also explains the para "/"meta "-orientation. The numerators of both fractions assume different values depending on the orientation, while the denominators are independent of the orientation. 

Butadienes with alkyl substituents in the 2-position favor the formation of the so-called para products (Figure 12.24, X = H) in their reactions with acceptor-substituted dienophiles. The so-called meta product is formed in smaller amounts. This orientation selectivity increases if the dienophile carries two geminal acceptors (Figure 12.24, X = CN). 2-Phenyl-l,3-butadiene exhibits a higher para selectivity... 

Substitution of complexed dienols (244) or dienol acetates with carbon or heteroatom nucleophiles, in the presence of a Lewis acid, occurs with retention of configuration (Scheme 69). (Alkyl aluminum reagents act as both nucleophile and Lewis acid in this process). This reaction is believed to proceed via stereospecific ionization, with anchimeric assistance from the iron, to generate the transoid pentadienyl cation (247) followed by attack of the weak nucleophile on the face opposite to iron. The cross-conjugated pentadienyl cation can also be generated the substitution of (2-acetoxymethyl-l,3-butadiene)Fe(CO)3 (193) has previously been discussed (Section 6.1.1). 

Allylic and dienyl sulfones have been prepared by conjugate addition to 1,3-dienes ". Phenylsulfonyhnercuration of conjugated dienes gives mercury adducts which can be treated with base to afford phenylsulfonyldienes. 2-(Phenylsulfonyl)-l,3-dienes can be stereo- and regioselectively functionalized via Michael addition of nucleophiles to give allylic sulfones. A key intermediate in the synthesis of a Monarch butterfly pheromone 4 was prepared by BackvaU and Juntunen by alkylation and subsequent palladium-catalyzed substitution of the allylic sulfone formed by Michael addition of dimethyl malonate to 2-(phenylsulfonyl)-l,3-butadiene (equation 10). 

The 1,2-dienes (allenes) 622 and 623 were obtained by the reaction of soft carbon nucleophiles such as malonate [222]. On the other hand, reaction of the phosphate 624 with Grignard reagent provided the 1,3-diene 625 [223]. Carbony-lation of the carbonate 626 proceeded smoothly under mild conditions (rt, 1 atm) and 3-alkyl-l,3-butadiene-2-carboxylate 627 was obtained in high yield [224]. The 2,3-dienylamine 628 was carbonylated under harsh conditions to provide the a-vinylacrylamide 629 in the presence of DPPP and TsOH [225]. 

Corey and coworkers , in a synthesis of prostaglandins, prepared diene 34 by alkylation of the lithiodithiane 32 with 2-bromomethyl-l,3-butadiene (equation 41). A synthesis of jasmone (35), in an overall yield of 50%, has been reported by Ellison and Woessner in which the bisdithianylethane 33 was sequentially alkylated, followed by hydrolysis and cyclization (equation 42). A similar route for preparation of 4-hydroxy-2-cyclopenten-l-ones has been reported . This method appears to provide a general route to 1,4-diketones via 1,3-dithianes. 

Sterically hindered, 2-alkyl-substituted dienes form high 1,4 microstructure in polar media as well as in hydrocarbon media (115). Butyllithium-initiated polymerization of 2-isopropyl-l,3-butadiene in diethyl ether produces a polymer with 81% cis-1,4 and 19% trans-1,4 microstructure. Similarly, >90% 1,4 microstructure is observed in THF for butyllithium-initiated polymerization of 2-triethylsilyl-l,3-butadiene, 2-trimethoxysilyl-l,3-butadiene, l-phenyl-l,3-butadiene,l-pyridyl-l,3-butadiene, and 2-phenyl-l,3-butadiene. 

In Table 4.1 the reactivities of some 2-substituted and 2,3-disubstituted-butadienes from the work of Craig et are compared. Note that 2-alkyl substitution in each case increases the rate by facilitating a cisoid configuration. Furthermore, the larger the alkyl group, the higher the acceleration rate. Rate decrease in 2-chloro-l,3-butadiene is due to the importance of the electronic effect. 

One of the features of Diels-Alder reactions with most alkyl and aryl nitriles that has made them rather unattractive as dienophiles is the requirement of very high reaction temperatures Again, only when electron-withdrawing substituents are directly bonded to the nitnle function do [4+2] cycloaddition reactions occur at reasonably low temperatures [ 48, 231, 232] A high yield [4+2] cycloaddition was observed on reaction of 4,4-bis(trifluoromethyl) 1 thia-3-aza-l,3-butadienes with trifluoroacetonitrile at 150 °C [225]... 

Access to non-terminal ( ,2)-dienes and ( ,Z, )-trienes 61 is provided analogously through deprotonation of ( , )-4-alkyl-l-chloro-l,3-butadienes followed by insertion of the resultant carbenoid 60 into alkyl- and alkenyl-zirconocene chlorides (Scheme 3.14) [38], The corresponding internal (Z,Z)-dienes and (Z,Z, )-trienes are also readily obtained by insertion of (3-alkynyl carbenoids 62 [44] into alkyl- and alkenylzirconocene chlorides, respectively (Scheme 3.14). Reduction of the triple-bond moiety in the products 63 to afford the cis-alkenes is well known [45—47]. 

The presence of electron-donating substituents in the diene enables it to react with simple aldehydes thus both acetaldehyde and benzaldehyde add to 1-methoxy-1,3-butadiene at 50-65 °C under high pressure (20 Kbar) to give dihydropyrans as 70 30 mixtures of cis- and frans-isomers (equation 5)4. The combination of electron-rich diene/electron-poor dienophile makes it possible to perform the reaction under milder conditions. 2-Alkyl-l-ethoxy-1,3-butadienes and diethyl mesoxalate afford dihydropyrans almost quantitatively (equation 6)5. 

Bei der Palladium(ll)-chlorid bzw. Palladiumkomplex-katalysierten Reaktion von Allen mit Ammoniak oder primaren Aminen erfolgt sowohl C,C-Verkniipfung von zwei Molekii-len Allen als auch Ammoniak- bzw. Amin-Addition. Das hierbei entstehende primare bzw. sekundare Amin kann seinerseits eine weitere Addition eingehen, so daB man ein Gcmisch von N-unsubstituiertem (im Fall von Ammoniak) bzw. N-Alkyl-substituiertem (im Fall von Alkylamin) 2-Aminomethyl-3-methyl-l,3-butadien und dem entsprcchenden Bis-[3-me-thyl-2-methylen-3-butenyl -amin erhalt, dessen Zusammensetzung von den Reaktionsbe-... 

Reaction of 112b with methyllithium followed by alkylation with methyl iodide gives a germophilic product 194. The [2+4]cycloaddition of 112b with 2-methyl-1,3-pentadiene affords 195 regioselectively. When a hexane solution of 195 is heated at 140 °C in a sealed tube in the presence of excess 2,3-dimethyl-l,3-butadiene, a dimethylbutadiene adduct 196... 

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