Ene products formation

The singlet oxygen ene reaction of allylic stannane, 16, is a synthetically useful procedure that leads to quantitative formation of the metal ene (M-ene) product [61-64]. However, other allylic stannanes, (e.g., 20 and 21) with less electropositive tin centers generate both hydrogen ene (H-ene) and novel cycloaddition products along with the M-ene product. The complete absence of a M-ene product in the photo-oxygenation of 22 has also led to the speculation that the M-ene reaction is less important for allylic stannanes in which the tin is bonded to a 2° rather than to a 1° carbon [65-67]. On the other hand, the use of more polar solvents can be used to enhance M-ene product formation [68]. 

In the photo-oxygenation of enol ethers, where the ene reaction and [2 + 2]-cycloaddition compete, polar solvents favour cycloaddition whereas nonpolar solvents favour ene product formation [681, 683-685]. For example, 2,3-dihydro-4-methyl-4/f-pyran reacts with singlet oxygen to yield both a 1,2-dioxetane and an allylic hydro-... 

The regioselectivity is controlled by the coefficients at the intermediately formed dienophile moiety. Thus, aldehydes of type 2-746 favor the formation of annulated compounds. However, with aldehyde 2-750 the bridged cycloadduct 2-752 is formed predominantly, in addition to small amounts of the ene product 2-753 via the 1-oxa-... 

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. 

Trost et al 1 have observed product distribution to be dependent in part on the steric and electronic properties of the substrate. For example, linear enyne 48 (Equation (30)) cyclized exclusively to the Alder-ene product 49, whereas branching at the allylic position led to the formation of 1,3-diene 50 (Equation (31)) under similar conditions. Allylic ethers also give 1,3-dienes this effect was determined not to be the result of chelation, as methyl ethers and tert-butyldimethylsilyl ethers both gave dialkylidene cyclopentanes despite the large difference in coordinating ability. 

Malacria and co-workers76 were the first to report the transition metal-catalyzed intramolecular cycloisomerization of allenynes in 1996. The cobalt-mediated process was presumed to proceed via a 7r-allyl intermediate (111, Scheme 22) following C-H activation. Alkyne insertion and reductive elimination give cross-conjugated triene 112 cobalt-catalyzed olefin isomerization of the Alder-ene product is presumed to be the mechanism by which 113 is formed. While exploring the cobalt(i)-catalyzed synthesis of steroidal skeletons, Malacria and co-workers77 observed the formation of Alder-ene product 115 from cis-114 (Equation (74)) in contrast, trans-114 underwent [2 + 2 + 2]-cyclization under identical conditions to form 116 (Equation (75)). 

Weinreb86 has reported the Alder-ene cyclization of enallenes under thermal conditions (Equation (85)). Varying the substitution pattern of alkene and allene groups had little effect on the yield of cyclized product. One exception was a,/ -unsaturated ester 130(Equation (86)) cycloisomerization under thermal conditions led to the formation of the Alder-ene product 131 and the unexpected hetero-Diels-Alder product 132 in a 3 1 ratio. 

Rh s(CO)iis they revealed the presence of an unidentified complex which was suggested to be the previously unknown species Rh4(a-CO)i2-The BTEM protocol is an extremely powerful technique to recover pure component spectra of unknown species, even when present at very low concentrations. This was illustrated by a detailed mechanistic study of the promoting effect of HMn(CO)5 on the Rh4(CO)i2 catalyzed hydroformylation of 3,3-dimethylbut-l-ene [22], A dramatic increase in the hydroformylation rate was found when both metals were used simultaneously. Detailed in situ FTIR measurements using the BTEM protocol indicated the presence of homometallic complexes only during catalysis. The metal complexes that were identified under catalytic conditions were RC(0)Rh(C0)4, Rh4(CO)i2, Rh5(CO)i5, HMn(CO)s, and Mn2(CO)io (see Figure 6.6). The kinetics of product formation showed an overall product formation rate, Eq. (3) ... 

Scheme 6.13 Mechanistic proposal for the catalytic effect of hydrogen-bonding thiourea 9 and the product formation resulting from an equilibrium between the hydrazone and its nucleophilic ene-hydrazine form.

However, the barrier to rotation does not always predict the regioselectivity of the ene reaction of O2 with alkenes. As shown latef, it is the non-bonded interactions in the isomeric transition states that control product formation and barriers to rotation are rather irrelevant. The calculated rotational barrier values, with the HF-STO-3G method, for the allylic methyls in a series of trisubstituted alkenes, as well as the experimentally observed ene regioselectivity of a series of selective substrates, are shown in Table 9. ... 

Initial thermal sigmatropic 1,5-carbon shifts are believed to account for the product formation from spiro[2.4]hepta-4,6-diene (l) 200 and spiro[cyclopropyl-l, 2 -2 //-indene] (6),201 obtained by photodecarbonylation of dione 5. In the first case, a 1,5-hydrogen shift and an electrocyclic ring opening completes the formation of 3,4-dimethylenecyclopent-l-ene (4),200 and in the second case, the initially formed benzoannulated bicyclo[3.2.0]hepta-l,3-diene 7 dimerizes to 8 as mixture of two stereoisomers.201... 

For example, the reaction of ethylene at 30°C yields both 1 and 2, but the major portion of the product mixture is due to hydrosilylation. At 80°C, a higher selectivity for double silylation was observed. Similar temperature dependence was observed in the reaction of oct-l-ene, although formation of 1 is never observed. In contrast, the major pathway for reaction of styrene is 1,1-double silylation, independent of temperature. In general, internal olefins do not react with o-bis(dimethylsilyl)benzene even on heating of the reaction mixture, possibly as a result of steric hindrance. However, 1,2-double silylation does occur for the double bond of dimethyl maleate, which is presumably activated by the two electronegative ethoxy groups. 

Lactone 5 can be obtained in both enantiomeric forms or as a racemate according to the described procedure. The reaction sequence includes the in situ formation of an alkylidene-1,3-dicarbonyl system 7 which can act as a heterodiene in an intramolecular hetero-Diels-Alder addition. A small amount of the ene product 4 with de > 98% is formed at room temperature as well. The remarkable selectivity in formation of diastereomer 3 is explained by an energetically more favorable exo transition state 8 with a pseudo-chair arrangement having the methyl group quasi-equatorial. Polycyclic cis-fused compounds can also be synthesized by the procedure above,9 and a related sequence to the cannabinoid skeleton has been described using appropriate 1,3-dicarbonyl reactants.10... 

Solvent effects on the reaction of ethyl glyoxylate and 2,3-dimethyl-1,3-butadiene catalyzed by the cationic bisoxazoline-Cu complex have also been reported (Scheme 8C.9) [26]. In a less polar solvent, CH2C12, the ene product is obtained predominantly, whereas the reaction in a more polar solvent, CH3N02, leads to the preferential formation of the HDA product. 

With dihydropyranyl-substituted phenyldisilane 240 Ishikawa and coworkers found the formation of both possible types of silenes 241 and 242134. Both were trapped by acetone, but 242, however, only through an ene reaction to give 549. With isobutene only the silene 242 gives an ene product 548 (equation 189). 

The yield of the [2 + 3] cycloadducts lies between 15 and 100%. The siladihydrotetra-zoles 344a are thermally quite stable and decompose in solution at temperatures higher than 130 °C in the reversal of their formation reaction to give silanimines and azides. The Si=N species dimerize to 687 or may be trapped by, e.g., acetone to give the ene product 688 (equation 228)310. 

The regioselectivity is controlled by the coefficients at the dienophile moiety. Thus, using aldehydes of type 9 or 10 favors the formation of bridged instead of fused compounds (Scheme 5.2). As an example, the reaction of 9 with 2 gave the 1-oxa-1,3-butadiene 11, which underwent a cycloaddition to afford the cycloadduct 12 and in addition, a small amount of the ene product 13. Interestingly, the ratio of 12 and 13 can be altered by applying high pressure (Scheme 5.3) [9]. 

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