Some Diels-Alder Reactions in 
            water solution
   1.  Faster in water than in other solvents.
   2.  Even in a case in which reaction is faster in isooctane 
    than in methanol.
   3.  Reaction speeds with additives that increase the 
    hydrophobic effect (LiCl) but slows with an 
    antihydrophobic additive (guanidinium chloride).
   4. More selective for endo addition in water, and ON 
    WATER.  
        Selective Diels-Alder Reactions in 
      Aqueous Solutions and Suspensions
         R. Breslow, U. Maitra, and D. Rideout  
         Tetrahedron Lett. 1983, 24, 1901-1904
        Abstract:  Diels-Alder reactions show high 
       endo/exo selectivities in aqueous suspensions
      “Even with a considerable layer of “neat” diene-
       dienophile solution the selectivities suggest 
       that much of the reaction occurs in or at the 
       water phase.  This is consistent with our rough 
             rate measurements.”
        “Thus water as a medium for Diels-Alder 
       reactions, and other processes, is of practical 
       interest even with poorly soluble substrates.”
         Hydrophobic Effects on Simple 
           Organic Reactions in Water
         R. Breslow, Acts. Chem. Res. 1991, 24, 159-164
         Interestingly, a high preference for endo 
             addition was found even with 
               suspensions in which the 
             cyclopentadiene was over 90% 
                   undissolved.
                    Selectivity Produced by Hydrophobic Packing 
                           of Reagents and Substrate in Water 
                                   O                                H OH                    O
                                           +       -
                                         Li  RBH
                                                  3                     +
                                        H O
                                          2
             O SO                             O SO                    O SO
              3                                 3                       3         H OH
                          O                                 O
                                                          A                       B
                                                A:B ratio
                          R                   D O           4M LiCl/D O                1:1 CD OD/D O
                                        2               2                3     2
                          H                  13:87              14:86                          10:90
                          Ph                60:40              69:31                          32:68
                          C F            78:22               85:15                         46:54
                            6 5 
                                                Mark R. Biiscoe, Christopher Uyeda, and RB
                                                Org. Lett. 2004, 6, 4331-4334
                Considerations in Choosing a Hydrophobic 
                             Oxidant:  T.S. Geometry
                   O                  O O                                  O
                     O O H
                             Or
              Peracid            Dioxirane
           Hydrophobic packing is not permitted        Hydrophobic packing is permitted
           in peracid transition state.                in dioxirane transition state.
       Houk, K. N.; Liu, J.; DeMello, N. C.; Condroski, K. R.  J. Am. Chem. Soc. 1997, 119, 10147-10152