Borylative cyclisation of diynes using BCl3 and borocations.

Andrew J Warner,Kang Yuan,Michael J Ingleson,John M Cole,John S Mcgough,Kieron M Enright

doi:10.1039/c9ob00991d

Abstract

The borylative cyclisation of 1,2-dialkynyl benzenes with BCl3 leads to dibenzopentalenes (via intramolecular SEAr) or benzofulvenes (via chloride addition) depending on substituents, with stabilised vinyl cation intermediates (e.g. with a p-MeO-C6H4-group) favouring the latter. The use of borocations leads to more selective dibenzopentalene formation, while other diyne frameworks undergo intramolecular SEAr selectively even with p-MeO groups.

Highlights

The borylative cyclisation of 1,2-dialkynyl benzenes with BCl3 leads to dibenzopentalenes or benzofulvenes depending on substituents, with stabilised vinyl cation intermediates favouring the latter
The catalyst-free version of this reaction[1,2] requires sufficiently electrophilic boranes to proceed with pioneering work performed using B(C6F5)3.3 While notable, borylative cyclisation with B(C6F5)[3] generally leads to [RB(C6F5)3]− containing products (though a carboboration step occurs in some cases generating RB(C6F5)[2] containing cyclised products).[4]
We present our results into the borylative cyclisation of a range of diynes along with the development of methods to control, to some extent, the product distribution principally by varying the electrophile from BCl3 to a borocation

Summary

Results and discussion

Scheme 1 Previous work on borylative cyclisation and bottom inset this work. methane (DCM). [NBu4][BCl4] with the dibenzopentalene product still formed in excellent conversion, presumably due to the intermediacy of a highly electrophilic vinyl cation that leads to rapid intramolecular SEAr. increasing the chloride donor concentration using [BCl4]− has only a modest effect on selectivity. We attribute the absence of any chlorine incorporated product in the borylative cyclisation of 14 with BCl3 to a lower barrier to SEAr due to the vinyl cation intermediate containing a six membered ring (ring A, inset Fig. 4) in contrast to a five membered ring in the vinyl cation derived from 1a (ring B) The latter may result in a higher barrier for the intramolecular SEAr step due to a less favourable geometry derived from the relative bond angles around rings A and B

Conclusions

For a recent example of borylative cyclisation using CatBCl see

14 For a discussion on how low basicity media effect SEAr see