Stereochemistry of ene reactions

Fig. 5-3. Stereochemistry of ene reaction a-olefin with MA. From Reference 47.

The stereochemistry of the reaction varies. For example, irradiation of E,Z,Z- and ,Z, -l,2,6-triphenylhexatriene (E,Z,Z- and ,Z, -142, respectively) proceeds with formal [tt4s + rr2a] stereochemistry to yield the exo,endo- and o,ejto-bicyclo 3.2.0 hcx-2-ene derivatives (143 equation 53), in chemical yields in excess of 75%221. Irradiation of the Z,Z,E- and Z,Z,Z-isomers leads to the same two products in nearly the same yields, via 2-photon processes of which the first is selective E,Z-isomerization to the E,Z,E- and ,Z,Z-isomers, respectively. In contrast, irradiation of E,Z,Z- and E,Z,E-144 affords the endo,endo- and enclo,exo-isomer 145, the products corresponding to formal jr4a + jr2a cycloaddition (equation 54)191,192. 

Two papers have been devoted to stereochemical aspects of the synthesis of phosphetans from 2,4,4-trimethylpent-2-ene (38). The stereochemistry of the reaction of (38) with dichlorophosphines is determined by... 

Neighboring carbon-carbon 7t bonds can influence the rate and control the stereochemistry of a reaction just as a heteroatom can. For example, the tosylate of anti-7-hydroxybicyclo[2.2.1]hept-2-ene (7-hydroxynorbornene) reacts in acetic acid 10 ... 

Treatment of the enantiopure substrate 4-341 containing a sila-ene moiety with trimethylsilyl trifluoromethanesulfonate (TMS OTf) gave 4-342 in a highly stereoselective fashion and 52% yield. It can be assumed that the reaction passes through the two chairlike transition structures 4-345 and 4-346 (Scheme 4.76). It is of interest that the stereochemistry of the two C-C-double bonds in 4-341 did not influence the configuration of the product. Moreover, when using EtAlCl2 as mediator, a 3 l-mixture of 4-343 and 4-344 is obtained in 40% yield. This can be explained by an axial orientation of the carbonyl moiety in the transition state. 

Incorporation of the carboxylic acid group into the substrate also had an effect on the stereochemistry of the Alder-ene products. Trost and Gelling60 observed diastereoselectivity in the palladium-catalyzed cycloisomerization of 1,7-enynes when the reactions were conducted in the presence of A,A-bis(benzylidene)ethylene diamine (BBEDA, Figure 2). They were able to synthesize substituted cyclohexanes possessing vicinal (Equation (53)) and... 

The formation of bicyclo[3.1.0]hex-2-enes is a common reaction of cyclic and acyclic cZt-l,3,5-trienes. While it has been frequently depicted as a [n4 + tt2]-cycloaddition and it often exhibits the stereochemistry expected of a concerted [zr4s + rr2a] process109, it is well known that it just as often does not. The stereochemistry, conformational requirements and scope of the reaction have been thoroughly studied by a number of workers, and have been extensively reviewed116,128,170 172. 

In principle, the approach outlined above for the a-oxoamides can be applied to any reaction, ground or excited state, which converts an achiral reactant into a chiral product, and Toda, Tanaka, and coworkers have investigated a wide variety of such processes [ 15,16]. A complete discussion of their work is beyond the scope of this review, and we illustrate the general approach taken with one final example. As shown in Scheme 4, irradiation of crystalline complexes of ene-diones 20a-f with chiral host (R,R)-(-)-9b led to cyclized products 21a-f in the variable yields and ee values indicated in Table 1 [22]. Remarkably, for reasons that were not clear (there was no accompanying X-ray crystallography), the R=n-propyl derivative 20g was found to give a completely different photoproduct, spiro compound 22 (69% yield, 97% ee, stereochemistry unknown), a result that once again illustrates the rather capricious nature of the use of chiral hosts for asymmetric induction. 

PTAD undergoes the ene-reaction with hindered alkenes and the stereochemistry of the products has been determined in the case 4-r-butylcyclohexylidene isomers <9iJCS(P2)72i>. 

In marked contrast, we used the cyclopropanation reaction of (phosphino)(silyl)-carbene (la) with methyl acrylate to demonstrate the carbene nature of our compound. More recently, we studied in detail the stereochemistry of this type of reaction.The (phosphino)(silyl)carbene (la) reacted efficiently with 3,3,4,4,5, 5,6,6,6-nonafluorohex-l-ene and (Z)- and ( )-2-deuteriostyrene giving the corresponding cyclopropanes in good yields. The stereochemical outcome was such that all monosubstituted alkenes gave exclusively the syn isomer (with respect to the phosphino group), and the addition of disubstituted alkenes was totally stereospecific (Scheme 8.16). 

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