Bicyclic alkenes, selectivity

Unfortunately, these reactions tend to afford mixtures. Similar mixtures are observed when using 2-cy-clohexen-l-ol,315 3-cyclohexen-l-ol,31s,32s or the corresponding ethers or esters.326-327 The regio- and stereo-selectivity of the hydration of bicyclic alkenes has also been carefully examined (equation 2 1 3).328-330... 

A highly regio- and enantio-selective hydroformylation of alkenes, such as PhCH= CH2, CH2=CHCH2CN, and CH2=CHOAc, catalysed by ruthenium complexes with (g) 2,5-disubstituted phospholane ligands has been reported. With (83) as the ligand, the turnover rates over 4000 h-1 at 80 °C, have been attained.108 (Acac)Rh(CO)2-TangPhos [Tangphos = (84)] has been developed as a new enantioselective catalyst for asymmetric hydroformylation of norbornene and other [2.2.1]-bicyclic alkenes (55-92% ee).109... 

The structure of the duodecamethyl derivative (27) has been reported <93OM803>, selected geometrical parameters are shown in Table 3 and confirm the structure as a bicyclic alkene. 

Reactions involving Monocyclic and Fused Bicyclic Alkenes Chemo- and Regio-selectivity... 

Reactions involving Bridged Bicyclic Alkenes Regio- and Diastereo-selectivity... 

Stereoselectivity in addition reactions of borane with bicyclic alkenes is similar to that observed for reduction of bicyclic ketones with hydride reducing agents. Boranes add to norbornene derivatives from the exo face (27 gave 85% exo attack upon reaction with diborane). In 27-30, all values are percentage yields of the organoborane products obtained by reaction from that face. Interestingly, only 80% exo attack was observed when 27 reacted with disiamylborane, and the size of the borane appears to have little effect on selectivity. 

Bicyclic alkenes, like norbomene 519, can be also used in the Povarov reaction and give the exo-exo product 521 with good yields and excellent selectivity, in general (Scheme 13.117) [205]. 

Ring-Opening Metathesis. The ring-opening metathesis (ROM) reaction has also been employed in solid-supported chemistry, but to a lesser extent. However, a sohd-supported ROM reaction has been used in the selective formation of bicyclic lactams. Since the bicyclic alkene 53 is immobilized on the resin, any potential side products formed from the solution-phase metathesis homodimerization of the styrene 54 are simply removed in the filtration and washing of the resin (Scheme 6.14). In addition. 

Addition to Alkenes and 1,3-Dienes. Silylstannanes add to selected alkenes under Pd-catalysis. The applicable alkenes include ethylene, strained bicyclic alkenes, cyclopropylidene-cyclopropane, or cyclopropenes (eq 12). 

Reactions with bicyclic alkenes are exo-selective [167b]. An interesting result has been noted in a computational study of a cycloaddition between cyclopropene and oxadia-zoles [186]. Secondary orbital interactions have been found between methylene of cyclopropene and oxygen-atom (not the diene itself) of the oxadiazole. Together with smaller steric interactions with this atom, this may provide an additional explanation why the exo-transition structure is sometimes favored. 

Asymmetric tandem cycloaddition of a chiral carbohydrate nitroalkene with ethyl vinyl ether in the presence of electron-withdrawing alkenes produces a facile assembly of bicyclic systems, which can further be selectively cleaved to give homologated carbohydrates (Eq. 8.110).176... 

Yttrocene complexes catalyze the cascade cyclization/hydrosilylation of trienes to form saturated silylated bicyclic compounds.For example, reaction of the 4-silyloxy-4-vinyl-l,6-hexadiene 69 and phenylsilane catalyzed by Gp 2YMe(THF) at room temperature for 1 h followed by oxidation of crude 70a gave [3.3.0]bicyclic diol 70b in 73% yield over two steps as a single diastereomer (Scheme 18). Selective conversion of 69 to 70a presumably requires initial 1,2-hydrometallation of one of the less-hindered G=G bonds to form alkylyttrium alkene complex II (Scheme 18). Selective S-exo carbometallation of II in preference to -exo carbometallation would form cyclopentyl-methylyttrium complex III (Scheme 18). Gyclization of III via a chairlike transition state would form the strained /r< /75 -fused alkylyttrium complex IIIl, which could undergo silylation to form 70a. 

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