Alkane stereoselective

Addition of. Sh-phenyl 4-methylphenylselenosulfonate to alkenes can be performed in the presence of boron trifluoride-diethyl ether complex or, alternatively, can be thermally induced (Table 11)69. The boron trifluoride-diethyl ether procedure affords tram-l-arylsulfonyl-2-(phenylseleno)alkanes stereoselectively, however, the thermally induced procedure, when applied to ( )- and (Z)-l-phenyl-l-propene, is a nonstereoselective process when indene is used as the alkene it gives the tram-adduct as the major product. 

In general, hydroboration—protonolysis is a stereoselective noncatalytic method of cis-hydrogenation providing access to alkanes, alkenes, dienes, and enynes from olefinic and acetylenic precursors (108,212). Procedures for the protonolysis of alkenylboranes containing acid-sensitive functional groups under neutral or basic conditions have been developed (213,214). 

The addition of 2-propenyltris(diethylamino)titanium to 2-(dibenzylamino)alkanals, readily prepared from amino acids, yields the anti-amino alcohols with high stereoselectivity and free of racemization91. Allylsilanes, with Lewis acids under the conditions of chelation control, lead to the iyn-diastereomers91. 

Stereoselective Alkene Synthesis via 1-Chloro-1-[(dimethyl)-phenylsilyl]-alkanes and a-(Dimethyl)phenylsilyl Ketones. 

In agreement with Schemes 2.211b and 2.211c, intramolecular cycloadditions of nitrones to 5-allyl- (Scheme 2.225) or 5-homoallylproline (Scheme 2.226), are fully regio- and stereoselective. These reactions are the key steps in the synthesis of functionalized azaoxobicyclo[X.3.0] alkane amino acids, mimics of a homoSer-Pro dipeptide (721). 

In another conceptually novel [5 + 2]-process, Tanino and co-workers synthesized cycloheptene derivatives by stereoselective [5 + 2]-cycloadditions involving hexacarbonyldicobalt-acetylene complexes as the five-carbon component and enol ethers as the two-carbon component (Schemes 22 and 23).60 61 The role of the dicobalthexacarbonyl complex is to facilitate formation and reaction of the propargyl cation putatively involved as an intermediate in this reaction. The dicobalthexacarbonyl moiety can be removed using various conditions (Scheme 24) to provide alkane 60, alkene 62, and anhydride 63. 

Stereoselective hydrogenation ofl -diketones. Hydrogenation of 1,3-alkane-diones catalyzed by Ru2Cl4[(R)-l][N(C2H5)3] results in anf/-l,3-diols with high dias-tereoisomeric and enantiomeric excesses (equation I). Under the same conditions l-phenyl-l,3-butanedione (2) is reduced mainly to the (3-hydroxy ketone 3 in 98%... 

On the other hand, the use of [Rh(CO)2Cl]2 as a catalyst results in ring opening of the siloxycyclopropanes 13 to the silyl enol ethers 14 with high stereoselectivity [10]. The 2-siloxyrhodacyclobutane 15a is proposed to undergo j8-elimination to give jr-allylrhodium 16a followed by reductive elimination to the silyl enol ether 14a. 1-Trimethylsiloxybicyclo[n.l.0]alkanes serve as / -metallo-carbonyl compounds via desilylation with a variety of transition metals [11]. The palladium-catalyzed reaction of the siloxycyclopropanes 17 under carbon monoxide in chloroform provides a route to the 4-keto pimelates 18. In the presence of aryl triflates, the 1,4-dicarbonyl compounds 19 are... 

Carbenes and transition metal carbene complexes are among the few reagents available for the direct derivatization of simple, unactivated alkanes. Free carbenes, generated, e.g., by photolysis of diazoalkanes, are poorly selective in inter- or intramolecular C-H insertion reactions. Unlike free carbenes, acceptor-substituted carbene complexes often undergo highly regio- and stereoselective intramolecular C-H insertions into aliphatic and aromatic C-H bonds [995,1072-1074,1076,1085,1086],... 

In regard to the stereoselectivity of the insertion process, Murray and coworkers have shown that the CH oxidation of substituted cyclohexanes by dioxiranes is, like the already discussed epoxidation, highly stereo-controUed . A specific case is c -decalin, which gives only the cis alcohol, as exemplarily displayed in equation 27. A similar stereoselective retention of configuration was also obtained for frawi-decalin and cis- and frawi-dimethylcyclohexanes"°. In fact, complete retention of configuration was demonstrated in the CH oxidation of chiral alkanes ". For example, the optically active (f )-2-phenylbutane was converted by either DMD or TFD" to (5 )-2-phenylbutan-2-ol (equation 28) without any loss of the enantiomeric purity (ep) in the product. 

Chlorofluorination of alkenes and alkynes using the A -chlorosaccharin-hydrogen fluoride/ pyridine system (Method G)50 results in vicinal chlorofluoro-substituted alkanes or alkenes, respectively. Since this system is more reactive than with other A -chloroamides, its use allows shorter reaction times and milder reaction conditions to prepare chlorofluorinated products in higher yields. With phenyl-substituted alkenes, Markovnikov-type adducts are formed exclusively. The additions proceed predominantly anti, with varying degrees of stereoselectivity. 

Two new reactive, very powerful organic peroxides, dimethyldioxirane and methyl(trifluoromethyl)dioxirane (4), have been introduced.81-83 The latter is more reactive and can be used more conveniently.84 85 Acyclic alkanes give a mixture of isomeric ketones on oxidation with methyl(trifluoromethyl)dioxirane,84,85 while cyclohexanone is the sole product in the oxidation of cyclohexane (99% selectivity at 98% conversion).85 With the exception of norbomane, which undergoes oxidation at the secondary C-2 position, highly selective tertiary hydroxylations can be carried out with regioselectivities in the same order of magnitude as in oxidations by peracids.85-87 A similar mild and selective tertiary hydroxylation by perfluorodialkyloxaziridines was also reported.88 Oxidation with dioxiranes is highly stereoselective 85... 

Ozone reacts slowly with saturated hydrocarbons usually to give alcohols.92 93 The reactivity of alkanes toward ozone is several orders of magnitude less than that of alkenes. Oxidation of saturated hydrocarbons takes place preferentially at the tertiary carbon. In liquid-phase ozonation94 the order of reactivity of the primary, secondary and tertiary C—H bonds is 1 13 110. The formation of tertiary alcohols occurs with high degree (60-94%) of stereoselectivity.94-96... 

It has been shown experimentally that attack by strong nucleophiles also occurs regio-selectively at this C atom, stereoselectively from the face opposite to the metal [287]. Since the alkyl group a bonded to the metal is very carbanion-like, it is susceptible to protonation by acids, yielding an alkane. The overall reaction provides the mechanism for homogeneous hydrogenation of alkenes. It may be extended to hydrogenation of C=N and C =0 pi bonds. 

NR = nonreactive toward hydrocarbons PO = oxidation of phosphines to phosphine oxides MF — peroxometallacyclic adduct formation with cyanoalkenes NSE — nonstereoselective epoxidation SE=stereoselective epoxidation AE = asymmetric epoxidation HA- hydroxylation of alkanes HB=hydroxylation of arenes OA = oxidation of alcohols to carbonyl compounds K = ketonization of Lermina 1 alkenes SO oxidation of S02 to coordinated S04 MO = metallaozonide formation with carbonyl compounds I = oxidation of isocyanides to isocyanates. 

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