Page 876 - Advanced Organic Chemistry Part B - Reactions & Synthesis

852 Scheme 9.7. Summary of Stereoselectivity of Allylic Reagents in Carbonyl Addition
Reactions
CHAPTER 9
Monocyclic TS Open TS Chelation TS Stereoconvergent
Carbon-Carbon
Bond-Forming Reactions Allylboration with Lewis acid–catalyzed Lewis acid–catalyzed SnCl 2 - mediated
of Compounds of Boron,
Silicon, and Tin -allylic boranes and addition of allylic silanes addition of allylic addition of allylic to
boronates silanes and stannanes stannanes aryl methyl
- and -oxy ketones
aldehydes
Addition of allylic Lewis Acid-catalyzed
trihalo stannanes to addition of allylic
aldehydes stannanes

TS, which is usually based on the chair (Zimmerman-Traxler) model. This
pattern is particularly prevalent for the allylic borane reagents, where the Lewis
acidity of boron promotes a tight cyclic TS, but at the same time limits the
possibility of additional chelation. The dominant factors in these cases are the
E-or Z-configuration of the allylic reagent and the conformational preferences
of the reacting aldehyde (e.g., a Felkin-type preference.)
2. Reactions proceeding through open TS: In this group, exemplified by BF -
3
catalyzed additions of allylic silanes and stannanes, the degree of stereo-
chemical control is variable and often moderate. The stereoselectivity depends
on steric factors in the open TS and can differ significantly for the E- and
Z-isomers of the allylic reactant.
3. Reactions through chelated TS: Reactions of -or -oxy-substituted aldehydes
often show chelation-controlled stereoselectivity with Lewis acids that can
accommodate five or six ligands. Chelation with substituents in the allylic
reactant can also occur. The overall stereoselectivity depends on steric and
stereoelectronic effects in the chelated TS.
4. Stereoconvergence owing to reactant or product equilibration: We also saw
several cases where the product composition was the same for stereoisomeric
reactants, e.g., for E- and Z-allylic reactants. This can occur if there is an
intermediate step in the mechanism that permits E- and Z-equilibration or if
the final stereoisomeric product can attain equilibrium.
Scheme 9.7 gives examples of each of these types of stereoselectivities. The
analysis of any particular system involves determination of the nature of the reactant,
e.g., has transmetallation occurred, the coordination capacity of the Lewis acid, and
the specific steric and stereoelectronic features of the two reactants.

General References

Organoborane Compounds

H. C. Brown, Organic Synthesis via Boranes, Wiley, New York, 1975.
A. Pelter, K. Smith, and H. C. Brown, Borane Reagents, Academic Press, New York, 1988.
A. Pelter, in Rearrangements in Ground and Excited States, Vol. 2, P. de Mayo, ed., Academic Press,
New York, 1980, Chap. 8.
B. M. Trost, ed., Stereodirected Synthesis with Organoboranes, Springer, Berlin, 1995.