7i-Donor effects

The 4e -a, MO 16 of 1 can interact both with a n- and a er-donor substituent. Hence, a-donor substituents have the same effects as 7i-donor substituents, namely a lengthening of vicinal and a shortening of the distal CC bond of 1. 

The lone pair of planar tricoordinate phosphorus in II is an excellent building block for 7i-systems. However, the phosphorus environment is usually strongly pyramidal (the inversion barrier in phosphine is about 35 kcal mok1 [43]) and the lone pair has a considerable -character. Accordingly, its interaction with 7i-systems is small. If planarized, however, the interaction becomes effective [44, 45], It was Schleyer who first made the explicit statement the 7i-donor ability of planar tricoordinate phosphorus rivals that of nitrogen [45], Planarizing effects on the tricoordinate phosphorus... 

We also note that when ion radical reactivity is slowed, ex-ciplex chemistry (which is completely different) becomes competitive. Thus, in the reaction with acenaphthene several products corresponding to the expected pattern for benzylation (see Scheme 12) are formed. However, here product II (which does not arise from secondary photocycloaddition from one of the previously mentioned products) Is also produced. This can be explained by an initial [2-f2] photocycloaddition between the two naphthalene rings which precedes alkylation (Scheme 3). Thus, when the donor is a naphthalene rather than a benzene derivative, the twofold effect of the diminished acidity of the benzylic position and of the increased 7i-donor-acceptor interaction makes the exciplex path sufficiently important to compete with the ion pair chemistry. Going further this way, i.e. toward more positive AG , cycloaddition via exciplex completely dominates, as indicated in Section 2. 

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