Kinetic protectors

Triplet diphenylcarbene is chosen as a prototypical triplet system as it has been the most extensively studied of diarylcarbenes and abundant basic data are available. Alkyl, halo and fluoroalkyl groups have been used as kinetic protectors for DPCs (Scheme 9.36). 

The fert-butyl group is known as one of the most effective kinetic protectors and many reactive molecules have been stabilized and even isolated by using this group. For example, the divalent center of silylenes and germylenes, the heavy atom analogues of carbenes, have been shown to be blocked by tcrt-butyl groups. 

The methyl group is the smallest alkyl group and is generally not considered as an effective kinetic protector. However, Zimmerman and Paskovich " showed that decay pathways of dimesitylcarbene are suppressed both inter- and intramolecu-larly by nearby methyl groups (Scheme 9.35). 

Bicyclohexyl groups act as an ideal kinetic protector of triplet carbene not only by quenching the intramolecular hydrogen-donating process but also by inhibiting dimerization of the carbene center. 

Alkyl groups are attractive kinetic protectors for triplet carbenes. However, they are potentially reactive toward triplet carbenes and, hence, will not be able to shield the reactive center completely. In this respect, we need to explore protecting groups that are almost completely unreactive toward triplet carbenes. Halogens are generally reactive toward the singlet state of carbene but are not reactive with the triplet. 

Kinetic protectors we first investigated were alkyl groups since it was our initial hope to stabilize and hopefully isolate triplet carbene consisting of only hydrocarbons. However, the use of alkyl groups as effective kinetic protectors was supposed to be very difficult in the light of the strong affinity of carbenes for electrons since they react even with very weak sources of electrons, e. g., C-H bond a electrons. 

Isopropyl group appears to be an attractive kinetic protector for earhene in this respect since it is not expected to be in too much close contact with earhene center when introduced at the ortho position of DPC but it is still able to block the center from external reagents. 

The comparison of the data between two carbenes reveals intriguing and unique nature required for kinetic protectors of triplet carbenes. Thus, significant E values of 26, as opposed to the essentially zero E values for 23 under identical conditions, clearly suggest that steric congestion around the carbene center in 26 is less severe than that in 23, as expected. Nevertheless, o-isopro-pyl groups can act as kinetic protector of triplet DPC comparable to the sterically bulkier tert-butyl group. 

It has been observed that anthryl groups can act as excellent reservoirs for the unpaired electrons as well as relatively effective kinetic protectors for carbene. 

A new method of kinetically controlled generation of the more substituted enolate from an unsymmetrical ketone involves precomplexation of the ketone with aluminium tris(2,6-diphenylphenoxide) (ATPH) at —78°C in toluene, followed by deprotonation with diisopropylamide (LDA) highly regioselective alkylations can then be performed.22 ATPH has also been used, through complexation, as a carbonyl protector of y./)-unsaturated carbonyl substrates during regioselective Michael addition of lithium enolates (including dianions of /i-di carbonyl compounds).23... 

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