Allylic halides 4 + 3 cycloaddition reactions

A similar transformation results when trimethylsilyloxy-substituted allylic halides react with silver perchlorate in nitromethane. The resulting allylic cation gives cycloaddition reactions with dienes such as cyclopentadiene. The isolated products result from desilyla-tion of the initial adducts ... 

The suprafacial thermal addition of an allylic cation to a diene (a 3 + 4 cycloaddition) is allowed by the Woodward-Hoffmann rules (this reaction would be expected to follow the same rules as the Diels-Alder reaction1095). Such cycloadditions can be carried out1096 by treatment of a diene with an allylic halide in the presence of a suitable silver salt, e.g,1097... 

Allyl complexes of the type ri1-allyl-Fp are prepared by reaction of [Cp(CO)2Fe] Na+ with allyl halides or, alternatively, by deprotonation of [Ti2-alkene-Fp]+ complexes. The most important reaction of V-allyl-Fp complexes is the [3+ 2]-cycloaddition with electron-deficient alkenes [33]. The reaction proceeds via a non-concerted mechanism, to afford Fp-substituted cydopentanes (Scheme 1.10). 

On the other hand, silver salts can act as a mild Lewis acid, promoting various reactions such as allylations, aldolizations, cycloadditions, and cyclizations. Silver salts can also be used as halide scavengers, acting as cocatalysts in cross-coupling reactions catalyzed by other metals, especially palladium. In the latter context, the exact role of silver salts is far from clear and may be more complex than just halide... 

Nadal and colleagues recently reported a Ni-catalyzed carbonylative Pauson-Khand-like [2+2+1] cycloaddition of allyl halides and alkynes in the presence of carbon monoxide and iron as the stoichiometric reducing agent [148]. The reaction was proposed to occur via reductively generated Ni(I)-radical like species free radicals were, however, considered unlikely. 

An interesting Darzens-like formal cycloaddition of azirines has also been reported <2004EJ02421>. While this is not a true cycloaddition reaction, two bonds are formed in the same reaction sequence. Reaction of sulfone 114 with lithium diisopropylamide (LDA) generates a carbanion which then adds to the azirine to generate intermediate 115. Cyclization of this intermediate with the allyl halide provides the product aziridine 116. This reaction proceeds in only 13% yield but is nonetheless an interesting route to fused-ring aziridines (Equation 27). 

Cycloaddition reactions of 18-electron transition metal ti -allyl complexes with unsaturated electrophiles to form five-membered rings have been extensively investigated. These transformations constituted a family of metal-assisted cycloaddition reactions in which the metal functions as an electron-donor center. These are typically two-step processes that involve the initial formation of a dipolar metal r) -alkene intermediate (2) and subsequent internal cyclization (equation 2). The most extensively investigated application of this methodology has been with dicarbonyl-ii -cyclopentadienyliron (Fp) complexes from the laboratory of Rosenblum. These (ri -allyl)Fp complexes are available either by metallation of allyl halides or tosylates with a Fp anion, or by deprotonation of (alkene)Fp cations. ... 

Silver(I) compounds are known to promote different kinds of cycloaddition. Reactions of 2-alkoxyallyl halides with 1,3-dienes in the presence of silver(I) compounds provide a beneficial route to cycloheptanones [2,3]. When a mixture of 2-(trimethyl-siloxy)allyl chloride 1 and cyclopentadiene (2) is treated with 2 equiv. AgC104 in THF-ether (1 2) at 0 °C, bicyclo[3.2.1]oct-6-en-3-one 3 is produced in 91 % yield [3] (Sch. 1). The 2-(trimethylsiloxy)allyl cation 4 is believed to be involved as a reactive species in the reaction. 

Studies on the control of product selectivity and regioselectivity in tetracarbonylnickel-promot-ed carbonylative cycloaddition of allylic halides and acetylenes reveal an influence of the solvent and the substitution pattern in both the acetylenes and the allylic halides. Thus, by appropriate choice of the reaction conditions, cyclopentenone derivatives can be obtained in satisfactory yield46,419,12°. 

Cycloaddition of allyl cations to conjugated dienes provides a route to seven-membered carbocycles. One of several methods can be used to generate the allyl cation, such as from an allyl halide and silver trifluoroacetate, or from an allyl alcohol by way of its trifluoroacetate or sulfonate. Cycloaddition of the allyl cation proceeds best with a cyclic diene, particularly for intermolecular reactions. Thus, cyclohexadiene and methylallyl cation gave the bicyclo[3.2.2]nonadiene 187 (3.125). Many intramolecular examples are known, such as the formation of the... 

Although nickel-catalyzed carbonylative Pauson-Khand cycloadditions have not been broadly developed, related doubly carbonylative cycloadditions involving allyl halides have been demonstrated as an entry to functionalized cyclopentenones. In recent catalytic versions, iron powder was used as the terminal reductant and dehalogenating agent (Scheme 3-38). [Caution Ni(CO)4, which could potentially be liberated in this reaction, is a highly toxic gas.]... 

Although 2-[(trimethylsilyl)methyl]allyl esters and halides react with imines to give pyrrolidines (Scheme 22),the cycloaddition reaction with carbonyl compounds requires much more drastic conditions. On the other hand, 2-[(trialkyIstannyl)methyl]allyl acetate can react with aldehydes to give furanyl compound, where the presence of trialkyltin acetate, as a by-product, is crucial to compel the reaction successfully (Scheme 23). In fact, in the presence of the hialkyltin acetate, even the 2-[(trimethyl silyl)methyl]allyl ester undergoes [3 + 2] cycloaddition reactions with aldehydes (Scheme 24) and ketones successfully.Similarly, InCls is also an efficient additive for the cycloaddition reaction and both aldehydes and ketones are suitable as sub-strates. ... 

The common methods for the S5mthesis of p-lactams are cycloaddition reactions such as the Staudinger s ketene-imine cycloadditions, ester enolate-imine cycloadditions, alkyne-nitrone cycloadditions (Kinugasa reaction), alkene-isocyanate cycloadditions, and Torii s cyclocarbonylation of allyl halides with imines. Several cyclizahon reactions of p-amino esters, p-amino acids, p-hydroxamate esters, and a-diazocarbonyls have been developed for the formation of p-lactam ring. N,N-Disubstituted a-haloamides cyclize by C3-C4 bond formation leading to the formation of P-lactam ring. 

Using Ni(CO)4 as a catalyst (or, perhaps more correctly, as a promoter) the carbonylative cycloaddition of allyl halides and alkynes has been investigated with a thorough expose of the reaction mechanism. 

Shimizu and Tsuno employed the less reactive chloro analogue 4b, which was converted to the siloxyallyl cation by the silver salt method [14] (Scheme 18.8), used previously in the dehalogenation of other allyl halides to generate allyl cations [18]. Excellent cycloaddition yields of 6 and 8 were obtained, again in a relatively short reaction time. 

---