Moderately hindered

Unsymmetrical ketones can be made by using either thexylborane or thexylchloroborane.12 Thexylborane works well when one of the desired carbonyl substituents is derived from a moderately hindered alkene. Under these circumstances, a clean monoalkylation of thexylborane can be accomplished, which is then followed by reaction with a second alkene and carbonylation. 

Dehydrohalogenation reactions appeared to be a convenient route to double bond germanium nitrogen species and were commonly carried out with an organolithium compound as a base.3,4 5b 6 7 96,97 123 Accordingly, the synthesis of two moderately hindered stable germanimines Mes2Ge = NR 136 and 137 has been reported.59 Stabilization in these cases is achieved... 

Wittig methylenation4 Potassium f-butoxide has seen limited use for generation of stabilized ylides. Actually, it is a superior base for generation of meth-ylenetriphenylphosphorane. For methylenation of moderately hindered ketones, the phosphorane is generated and used in ether. For reactions of strongly hindered ketones, the ylide is generated in benzene and the reaction is conducted at 90-120° in benzene. 

Methylenation of sulfones.1 This reagent reacts rapidly with the anion (LDA) of even moderately hindered sulfones to form (3-stannyl sulfones, which undergo desulfonylstannylation on treatment with Bu4NF to form alkenes. 

Styrene and 1-decene are selectively epoxidized, as shown in Table XXVII (293). The rates observed for PWM-Co are greater than those observed for Ni(dmp)2 and Fe(dmp)3, and the selectivities are comparable or higher for the former (295). It is also remarkable that PWn—Co polyanion exhibits a steric effect comparable to that of a moderately hindered TTMPP ligand in the... 

Water was preferred to acetonitrile as the solvent, allowing moderately hindered and anionic amines to self-assemble. Acetonitrile is a much better ligand for copper(I) than water, making it more difficult for hindered ligands (such as the one formed from serinol, third entry in Table 1.1) to form complexes in competition with the solvent. More hindered amines as well as cationic amines were not incorporated in either solvent, which we attribute to steric and Coulombic repulsion, respectively. 

Only one case concerning the synthesis of a miktoarm star quaterpolymer has appeared in the literature. It consists of four different branches, namely PS, po-ly(4-methyl styrene) (P4MeS), PI and PB [35]. The reaction sequence for the preparation of this miktoarm star is presented in Scheme 23. The procedure was similar to the one adopted for the synthesis of the ABC-type terpolymers by the chlorosilane method. The characteristic of this method is that two of the arms were incorporated to the linking agent by titration. Consequently the order of addition plays an important role for the preparation of well defined products. PS was chosen to react first with an excess of SiCl4, followed after the evaporation of the excess silane, by the titration with the more sterically hindered P4MeS so that only one arm can be incorporated in the star. The moderately hindered PILi anion was then added by titration, followed by the addition of the fourth arm, which is the least sterically hindered PBLi anion so that complete linking can be achieved. The reaction sequence was monitored by SEC and these results in combination with the molecular and spectroscopic characterization data showed that well defined quaterpolymers were prepared. 

Under second-order conditions (strong base/nucleophile), S 2 and E2 reactions may occur simultaneously and compete with each other. Show what products might be expected from the reaction of 2-bromo-3-methylbutane (a moderately hindered 2° alkyl halide) with sodium ethoxide. 

The other group of moderately hindered cyclohexanones which has been examined in detailed is a group of substituted 1-decalones, which gave results similar to those of the 12-keto steroids on Li-NHs reduction. These decalones were all derived from protected diketone (51) and included two different 2-monosubstituted derivatives (52) and (53). These decalones were reduced both in the presence of a proton donor (NH4CI) and under anhydrous conditions in yields of 69-99%. The axial alcohol was the major product in the presence of a proton donor, while the equatorial alcohol was predominant in its ab-... 

Anti-Markownikov addition may also result from steric hindrance Addition of iodine azide to the moderately hindered... 

Moderately hindered alkenes and certain cycloalkenes also can form R3B. However, control of the stoichiometry and reaction conditions permits the preparation... 

The reductive desulfurization of thiiranes has been accomplished with Raney nickel, Li in EtNHa, Zn in HOAc, BusSnH, and P2I4 in DMF but the reaction has seldom been applied to organic synthesis. A noteworthy procedure for preparing moderately hindered tetrasubstituted alkenes, which proceeds via a thiirane intermediate, was developed independently by Barton and Kellogg. The procedure, illustrated in Scheme 14 for the preparation of (34), involved reaction of thiocamphor (30) with diazoalkane... 

For moderately hindered olefins, the redistribution equilibria may be favorable to the monoalkylborane, e.g., for a-pinene . The equilibrium is reached after 120 h at 25°C and the mixture is composed of monoisopinocampheylborane (IpcBH2, 91%), BH3 (4.5 %) and diisopinocampheylborane (IPC2BH, 4.5 %). The small amount of BH3 can be removed by pumping off the solvent and addition of pentane, followed by an equivalent amount of tetramethylethylenediamine (TMED). The TMED-BH3 complex precipitates and can be filtered. By further addition of the amine, the TMED-2 IpcBH2 complex is formed as a white solid. 

Thexylmonoalkylboranes are not unique in their behavior toward TMED. Dialkyl-boranes derived from moderately hindered olefins may also undergo dehydroboration by the action of TMED e.g., IpCjBH is transformed into the TMED-2 BHjIpc bis adduct and the reaction provides another convenient access to this chiralhydroborating... 

With moderately hindered alkenes, e.g., cyclohexene, 2-methyl-2-butene, isobutene, norbornene, etc., the reaction can be stopped at the thexylmonoalkylborane stage . Subsequent addition of an unhindered alkene gives mixed trialkylboranes ... 

This reaction is sensitive to steric hindrance22-24. Moderately hindered alkyl groups, e.g., 1-methyl-1-propyl, 3-methyl-2-butyl, 3-hexyl, 2-c.vo-norbornyl and tm .s-2-methylcyclopentyl. are transferred, whereas the /rimv-2-mcthylcyclohcxyl group is not. As the substrate, chloroace-tonitrile is more reactive than ethyl bromoacetate. Reaction yields are in the range 45-66% (see Table 3). T he procedure also works with a mixture of 9-alkyl-9-borabicyclo[3.3.1]nonane and 9-alkyl-9-borabicyclo[4.2.1]nonane since these initial hydroboration products can be thermally isomerized solely to 9-alkyl-9-borabicyclo[3.3.1]nonane. 

If the alkyl halide is not sterically hindered and the base is only moderately hindered, the major product will still be the more stable product. For example, the major... 

Sterically unhindered and certain moderately hindered a-chloronitroso compounds (142) react with trimethylaluminium to give the methyl nitrones (143) (Scheme 228).Relatively hindered a-chloronitroso compounds such as (144) can undergo fragmentation followed by ring closure, in an overall sequence which leads to a ring expansion (Scheme 228). 516... 

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