Cycloaddition four-membered rings formation

Several relevant papers and review articles have appeared recently. These contain reports on the mechanism and kinetics of the ene reaction of ADC compounds,243-245 examples of four-membered ring formation,246-247 other cycloadditions of ADC compounds,248-252 the synthesis of azoalkanes,253 the use of chiral l,2,4-triazole-3,5-diones,254 and the use of the DEAZD/PI13P reagent in organic synthesis.255... 

The thermal [2-1-2] cycloaddition of cumulenes with alkenes, imines or carbonyl compounds is one of the most useful methods of four-membered ring formation. The cycloaddition of ketenes with alkenes to give cyclobutanones represents a reaction of general importance. According to Woodward and Hoffmann, these reactions proceed via a [ttIs+ttIi,] pathway [24]. Dihaloketenes are more reactive than simple ketenes and readily react with electron-rich olefins [25]. 

Intramolecular [2 + 2] photocycloadditions of alkenes is an important method of formation of compounds containing four-membered rings.184 Direct irradiation of simple nonconjugated dienes leads to cyclobutanes.185 Strain makes the reaction unfavorable for 1,4-dienes but when the alkene units are separated by at least two carbon atoms cycloaddition becomes possible. 

Summary The formation, reactivity, and cycloaddition behavior of neopentylsilenes towards suitable reaction partners is described. Especially l,l-dichloro-2-neopentylsilene. Cl2Si=CHCH2Bu (2) - easily obtained from vinyltrichlorosilane and LiBu - is a useful building block for the synthesis of SiC four membered ring compounds. These can be converted into the isomeric Diels-Alder and retro ene products upon thermolysis reactions. The mode of the silenes cycloaddition reactions ([4+2] vs [2+2] addition) can be directed by either the substitution pattern at the Si=C moiety, the choice of reaction partners or the conditions. Furthermore the products resulting from cycloaddition reactions open up a wide variety of following reactions, which possibly will lead to new organosilicon materials or pharmaceutical compounds. 

This chapter deals with [2 + 2]cycloadditions of various chromophors to an olefinic double bond with formation of a four-membered ring, with reactions proceeding as well in an intermolecular as in an intramolecular pattern. Due to the variety of the starting materials available (ketones, enones, olefins, imines, thioketones, etc.. . .), due to the diversity of products obtained, and last but not least, due to the fact that cyclobutanes and oxetanes are not accessible by such a simple one-step transformation in a non-photo-chemical reaction, the [2+2]photocycloaddition has become equivalent to the (thermal) Diels-Alder reaction in importance as for ring construction in organic synthesis. 

Some selected reactions of 21b were investigated [32]. In the reaction with [(Ph 0)4W=0j the dinuclear compound 24 is formed (Eq. 16) containing an almost planar W2OP four-membered ring system. The structure of 24 reveals that after the formal cycloaddition reaction a reductive W-W bond formation occurs under loss of OPh moieties. 

As exemplified in Scheme 30, the formation of four-membered ring systems via inter- or intramolecular [2-1-2] cycloadditions between the Co,=Cp moiety and C=N or C=C double bonds have also been described [89, 129, 189]. 

Photochemical reaction with disiliranes leads to a [3-i-2]-cycloaddition (Section 4.3.9). Disilylcyclobutanes [67, 68] and cyclotetrasilanes [68, 69] react in a similar fashion. In both cases the four-membered ring is photolytically cleaved and a diradical is formed. This diradical adds in a formal cycloaddition to the [6,6] double bond of CgQ. The cycloadduct 20 (R = 4-MeCgH4) can be obtained from 19 in 13% yield, but it is not the major product (Scheme 6.12). Rearrangement of an H atom leads to 21 in 46% yield. The product distribution depends strongly on R. Changing R from 4-MeCgH4 to phenyl leads to an exclusive formation of 21. 

The stereoselective or stereospecific formation of these compounds and their interaction with butyllithium was studied with the help of NMR. Paquette and Freeman first applied asymmetric induction to the synthesis of four-membered rings, especially with the sulfene-enamine 2 -F 2 cycloaddition. The in situ generation of sulfene 68 by dehydrochlorination with butyllithium of the sulfonyl chloride allowed the formation of cycloadduct 69 in 88 % yield. In a variation, the sulfene may be generated by base-induced... 

Cyclobntane. A few examples of cyclobutane derivatives have been described in the carbohydrate series. Formation of this type of ring involves a 2+2 cycloaddition. Relevant examples of cycloaddition of dichloroketene on glucals, explored by Redlich [195] and Lallemand [196,197], and significant transformations of the four-membered ring, such as ketone 165a, are given in Scheme 56. 

Only a few papers on the formation of compounds with small rings have been published. One example is the [2 + 2]-cycloaddition of electron-rich enamines to Schiff bases under high pressure (1.4 GPa) (87JOC365). The reaction leads to substituted azetidines (1). Four-membered ring heterocycles, thietane derivatives (4), are formed by interaction of sulfene (2) with enamines (3) (86CB257 93JOC3429). 

The formation of four-membered rings through 2 + 2 cycloaddition is a well-established reaction and the most generally effective synthetic approach to cyclobutanes. Most olefins cannot be induced to undergo this reaction thermally, a finding that is readily rationalized by the forbidden nature of the 2s + 2s addition and the steric difficulties associated with the allowed 2s + 2a pathway. There are nevertheless exceptions. Olefins substituted by two or more fluorine atoms undergo thermal 2 + 2 additions under relatively mild conditions,16 as do ketenes and allenes. 

The reaction of 1-disilagermirene 22 with ketones is similar to the benzaldehyde case. Thus, reaction with butane-2,3-dione gives a final bicyclic product 41, which also has a norbornane type skeleton (Scheme 15, Figure 13)50. Formation of this compound can be reasonably explained by the initial [2 + 2] cycloaddition of one carbonyl group across the Si=Si bond to form the three- and four-membered ring bicyclic compound 42, followed by the isomerization of disilaoxetane 42 to an enol ether derivative 43. The intramolecular insertion of the second carbonyl group into the endocyclic Si—Ge single bond in 43 completes this reaction sequence to produce the final norbornane 41. In this case, C=0 insertion occurred into the Si—Ge bond rather than the Si—Si bond, which is reasonable due to the weakness of Si—Ge bond. 

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