Allylic anions phosphine-substituted

Alternatively, the ambident oi-hetero substituted allyl anions have been utilized as homoenolate equivalents. For example, in the presence of HMPA, allyl phenyl sulfides (251),192 allyl phenyl sulfones (252)192b c and allyl phenyl selenides (253)192d e add to a,(3-enones in a l,4(0)-mode, while allyl phenyl sulfoxides (254) and allyl phosphine oxides (255) afford 1 A j-addition exclusively, irrespective of solvent used.193 Hua has shown that additions of either chiral sulfoxide (254 R1 = R2 = R3 = R4 = H, R5 = p-tolyl) or allyl oxazaphospholidine oxide (256) occur with excellent enantioselectivity (>95% ee).194 Similarly, Ahlbrecht reports that the a-azaallyl (257) adds exclusively in a 1 A j-mode to acceptor (59) to afford 1,5-diketones (Scheme 86).195... 

The titanium- ate complexes of a-methoxy allylic phosphine oxides, generated in situ by reaction of the corresponding lithium anion and Ti(0-i-Pr)4, condense with aldehydes exclusively at the a-position to produce homoallylic alcohols in a diastereose-lective fashion. The overall result is the three-carbon homologation of the original aldehyde, and this protocol has been used in a synthesis of (-)-aplysin-20 from nerolidol. The titanium- ate complex produced by reaction of the chiral lithium anion of an ( )-crotyl carbamate with Ti(0- -Pr)4 affords -y-condensation products (homoaldols) on reaction with aldehydes. Allyl anions produced by the reductive metalation of allyl phenyl sulfides condense with a,p-unsaturated aldehydes in a 1,2-manner at the more substituted (a) allyl terminus in the presence of Ti(0-i-Pr)4. 1,2-Addition of dialky Izincs to a,p-unsaturated aldehydes can be achieved with useful levels of enantiocontrol when the reaction is conducted using a chiral titanium(IV) catalyst in the presence of Ti(0- -Pr)4 (eq 20). Higher ee values are observed when an a-substituent (e.g. bromine) is attached to the substrate aldehyde, but a -substituent cis-related to the carbonyl group has the opposite effect. 

As mentioned in the chapter on the reaction mechanism, the anion, especially of Ni-salts, is important in affecting the reaction course. The catalytic efficiency of the nickel halides strongly increases in the series fluoride, chloride, bromide, iodide [374—376]. The molar ratio of cobalt or nickel to iodine is also very important [414]. As in the hydroformylation reaction, metal carbonyls substituted by phosphine ligands are very reactive [377, 1009], and especially modified rhodium and palladium catalysts [1021, 1045] allow reactions under mild conditions. Thus, the nickel bromide triphenylphosphine allyl bromide complex shows an increased reactivity in the carbonylation of acetylenes. On the other hand, carbonyls substituted by phosphine ligands are also readily soluble in the reaction mixture [345, 377]. 

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