Sarah M. Crawford

BippyPhos: A Single Ligand With Unprecedented Scope in the Buchwald–Hartwig Amination of (Hetero)aryl Chlorides

Over the past two decades, considerable attention has been given to the development of new ligands for the palladium-catalyzed arylation of amines and related NH-containing substrates (i.e., Buchwald-Hartwig amination). The generation of structurally diverse ligands, by research groups in both academia and industry, has facilitated the accommodation of sterically and electronically divergent substrates including ammonia, hydrazine, amines, amides, and NH heterocycles. Despite these achievements, problems with catalyst generality persist and access to multiple ligands is necessary to accommodate all of these NH-containing substrates. In our quest to address this significant limitation we identified the BippyPhos/[Pd(cinnamyl)Cl]2 catalyst system as being capable of catalyzing the amination of a variety of functionalized (hetero)aryl chlorides, as well as bromides and tosylates, at moderate to low catalyst loadings. The successful transformations described herein include primary and secondary amines, NH heterocycles, amides, ammonia and hydrazine, thus demonstrating the largest scope in the NH-containing coupling partner reported for a single Pd/ligand catalyst system. We also established BippyPhos/[Pd(cinnamyl)Cl]2 as exhibiting the broadest demonstrated substrate scope for metal-catalyzed cross-coupling of (hetero)aryl chlorides with NH indoles. Furthermore, the remarkable ability of BippyPhos/[Pd(cinnamyl)Cl]2 to catalyze both the selective monoarylation of ammonia and the N-arylation of indoles was exploited in the development of a new one-pot, two-step synthesis of N-aryl heterocycles from ammonia, ortho-alkynylhalo(hetero)arenes and (hetero) aryl halides through tandem N-arylation/hydroamination reactions. Although the scope in the NH-containing coupling partner is broad, BippyPhos/[Pd(cinnamyl)Cl]2 also displays a marked selectivity profile that was exploited in the chemoselective monoarylation of substrates featuring two chemically distinct NH-containing moieties.

Use of F-BODIPYs as a Protection Strategy for Dipyrrins: Optimization of BF2 Removal

We recently reported the first general method for the deprotection of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes (F-BODIPYs) involving a microwave-assisted procedure for the removal of the BF(2) moiety, and liberation of the corresponding free-base dipyrrin. Further optimization of the reaction has resulted in a more convenient and accessible protocol. The availability of this new methodology enables BF(2)-complexation to be used as a dipyrrin protection strategy. Herein lies a detailed examination of the deprotection reaction, with a view to optimization and gaining mechanistic insight, and its application in facilitating a multistep synthesis of pyrrolyldipyrrins.

Synthesis and Characterization of Fluorescent Pyrrolyldipyrrinato Sn(IV) Complexes

A series of neutral, 5-coordinate pyrrolyldipyrrinato Sn(IV) complexes have been synthesized via reaction of a pyrrolyldipyrrin, or its corresponding hydrochloride salt, with dibutyltin or diphenyltin oxide. The complexes are structurally unique in that all three nitrogen atoms of the pyrrolyldipyrrinato ligand bind to the tin center, making these complexes the first examples of pyrrolyldipyrrins behaving as LX(2) ligands. The complexes are highly fluorescent, exhibiting fluorescence quantum yields between 0.28 and 0.61, and display interesting preliminary biological activity.

Conversion of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes (F-BODIPYs) to dipyrrins with a microwave-promoted deprotection strategy.

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacenes (F-BODIPYs) have been deprotected to give the corresponding free-base dipyrrins by heating a solution of the F-BODIPY in tert-butanol under 600 W of microwave irradiation in the presence of 6 equiv of potassium tert-butoxide for 40 min at 92 degrees C. Investigations of BODIPY modification at the meso position have also been undertaken and a meso-butyl product has been isolated.

An improved method for the synthesis of F-BODIPYs from dipyrrins and bis(dipyrrin)s.

An improved methodology for the synthesis of F-BODIPYs from dipyrrins and bis(dipyrrin)s is reported. This strategy employs lithium salts of dipyrrins as intermediates that are then treated with only 1 equiv of boron trifluoride diethyletherate to obtain the corresponding F-BODIPYs. This scalable route to F-BODIPYs renders high yields with a facile purification process involving merely filtration of the reaction mixture through Celite in many cases.

The first series of alkali dipyrrinato complexes

The first series of alkali dipyrrinato complexes is reported, encompassing lithium, sodium, and potassium salts of meso-unsubstituted and meso-aryl-substituted derivatives. By varying the substitue...

Investigations into the nucleophilic meso-substitution of F-BODIPYs and improvements to the synthesis of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene

A series of three F-BODIPYs, with varying levels of steric crowding about the meso-position were selected to investigate nucleophilic meso-substitution of F-BODIPYs. The synthesis of one of these F-BODIPYs, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (totally unsubstituted dipyrrin skeleton), was optimized to give higher yields over routine literature procedures. This modified procedure involves oxidation of a dipyrromethane using p-chloranil, instead of DDQ, to give a dipyrrin which is then trapped in situ as its BF2 complex. Nucleophilic meso-alkylation of the series of F-BOIDPYs with n-butyllithium gave meso-butyl F-BODIPYs in moderate to good yields. This work represents a new, synthetically viable method for the synthesis of meso-alkylated F-BODIPYs. Extension of the nucleophilic substitution methodology to meso-arylation was possible. However, the reaction was unselective: substitution at boron, to give the boron-diaryl C-BOIDPYs, occurred preferentially to nucleophilic meso-substitution and thus a mixture of products was obtained. NUCLEOPHILIC meso-SUBSTITUTION OF F-BODIPYs Crawford and Thompson Dalhousie University, Halifax, NS, Canada 3 INTRODUCTION Molecules containing the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (F-BODIPY) framework have wide applications as dyes, fluorescent probes in biological systems, and materials for incorporation into electroluminescent devices. 2 Both symmetrical and unsymmetrical F-BODIPYs are routinely synthesized in high yields from the isolated dipyrrins free-bases or HX salts or by trapping the dipyrrin in situ. However, dipyrrin free-bases and salts are historically difficult to manipulate and purify. Furthermore, there are only a few methods available to synthesize meso-substituted dipyrrins. To produce meso-substituted dipyrrins (Figure 1), one can condense two equivalents of an α-unsubstituted pyrrole with a carboxylic acid, an acid chloride or an orthoformate (method 1) or oxidize a 5-unsubstituted dipyrromethane to the corresponding meso-unsubstituted dipyrrin (method 2). The first method is limited to the synthesis of symmetrical dipyrrins, while the second method is currently limited to the synthesis of dipyrrins with meso-aryl substituents and it requires the synthesis of the dipyrromethane starting material. Dipyrrins with meso-alkyl substituents can be synthesized from alkyl acid chlorides (method 1); however, the meso-alkyl dipyrrins are generally unstable and are thus trapped in situ using boron trifluoride diethyletherate and then isolated as their corresponding F-BODIPYs, with overall yields often below 20%.