Page 833 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Entry 5 is an example of use of an -trimethylsilylallyl group to prepare a vinylsilane. 809
The stereochemistry is consistent with a cyclic TS having the trimethylsilyl substituent
in a quasi-axial position to avoid interaction with the bridgehead hydrogen of the SECTION 9.2
bicyclic ring. Organosilicon
Compounds

H H H CH 3
CH 3 R Si(CH 3 ) 3
CH 3 B OH Si(CH )
3 3
R O Si(CH ) R H OH
3 3
Entries 6 and 7 involve functionalized allyl groups, with a Z- -methoxy group in
Entry 6 and a Z- -chloro group in Entry 7. Both give syn products; in the case of Entry 7
the chlorohydrin was cyclized to the cis epoxide, which is a pheromone (lamoxirene)
of a species of algae. Entry 8 is another example of the use of a chloro-substituted
allylic borane. Entry 9 involves one of the alternatives to Ipc BH for enantioselective
2
allylation. In Entry 10, both the aldehyde and allyl group contain chiral centers, but
the borane is presumably the controlling factor in the stereoselectivity. Entries 11
to 13 demonstrate several enantioselective reactions using the tartrate-derived chiral
auxiliaries. Entry 14 is an example of reactant-controlled stereochemistry involving the
achiral -allyl pinacol borane. This reaction proceeded with low stereochemical control
to give four isomers in a ratio of 18:3.4:1.4:1. Entry 15 shows high diastereoselectivity
and enantioselectivity in a reaction with a Z- -methoxyallyl- Ipc -borane. Entries 16
2
to 19 are examples of the use of allylboration in multistage syntheses. Entry 16 involves
magnesium-free conditions (see p. 799). Entry 17 was used to construct balanol, a
PKC inhibitor, and demonstrates reagent control of stereochemistry by allyl-B Ipc 2
without interference from the protected -amino and -hydroxy substituents. Entries
18 and 19 also involve functionalized aldehydes.

9.2. Organosilicon Compounds

9.2.1. Synthesis of Organosilanes
Silicon is similar in electronegativity to carbon. The carbon-silicon bond is
quite strong ∼75kcal and trialkylsilyl groups are stable to many of the reaction
conditions that are used in organic synthesis. Much of the repertoire of synthetic
organic chemistry can be used for elaboration of organosilanes. 65 For example, the
Grignard reagent derived from chloromethyltrimethylsilane is a source of nucleophilic
CH Si CH units. Two of the most general means of synthesis of organosilanes
3 3
2
are nucleophilic displacement of halogen from a halosilane by an organometallic
reagent and addition of silanes at multiple bonds (hydrosilation). Organomagnesium
andorganolithiumcompoundsreactwithtrimethylsilylchloridetogivethecorresponding
tetrasubstituted silanes.

CH 2 CHMgBr + (CH ) SiCl CH 2 CHSi(CH ) Ref. 66
3 3
3 3
65 L. Birkofer and O. Stuhl, in The Chemistry of Organic Silicon Compounds, S. Patai and Z. Rappoport,
eds., Wiley-Interscience, 1989, New York, Chap. 10.
66
R. K. Boeckman, Jr., D. M. Blum, B. Ganem, and N. Halvey, Org. Synth., 58, 152 (1978).