Single Electron Transfer to Diazomethane–Borane Adducts Prompts C−H Bond Activations

Abstract While (Ph2CN2)B(C6F5)3 is unstable, single electron transfer from Cp*2Co affords the isolation of stable products [Cp*2Co][Ph2CNNHB(C6F5)3] 1 and [Cp*Co(C5Me4CH2B(C6F5)3)] 2. The analogous combination of Ph2CN2 and BPh3 showed no evidence of adduct formation and yet single electron transfer from Cp*2Cr affords the species [Cp*2Cr][PhC(C6H4)NNBPh3] 3 and [Cp*2Cr][Ph2CNNHBPh3] 4. Computations showed both reactions proceed via transient radical anions of the diphenyldiazomethane–borane adducts to effect C−H bond activations.

The activation of small molecules has been amajor driver of organometallic chemistry over the last 60 years.S uch efforts have spawned great interest and important developments.In recent years,s uch inquiries have begun to permeate main group chemistry.O ne avenue of main group chemistry exploited for the activation of small molecules has been frustrated Lewis pair (FLP) chemistry. [1] While this initially emerged from the finding of the heterolytic activation of H 2 by combinations of Lewis acids and bases, [2] subsequent efforts demonstrated reactivity of FLPs with aw ide range of small molecules. [3] Noticeably absence from these investigations have been studies involving dinitrogen.
Main group interactions with N 2 have drawn much less attention. Anumber of computational studies have addressed the interactions of N 2 with Lewis acids,w hile the species (N 2 )BF 3 was spectroscopically characterized upon generation by supersonic expansion at 600 torr and 170 K. [18] The compound Ph 3 PNNPPh 3 [19] although not derived from N 2 , was controversially described as N 2 stabilized by two phosphine donors. [20] However, in at ruly seminal finding, Braunschweig et al. [21] described the first metal-free capture of N 2 using ac AAC-stabilized borylene (cAAC: cyclic (alkyl)(amino) carbene).
In our own efforts towards main group-N 2 chemistry,w e initiated studies of diazomethanes which liberate N 2 .S uch systems may provide insight for the design of main group-N 2 systems. [22] In 2012, we reported the insertion of diazomethanes into B À Cb onds of electrophilic boranes with the liberation of N 2 (Scheme 1). [23] Such insertions were recently exploited in organic synthesis by Melen et al. [24] In recent work, [25] we showed that the sterically-encumbered diazomethane,P h 2 CN 2 ,d oes not insert but rather forms ah ighly reactive,y et isolable borane-adduct, (Ph 2 CN 2 )B(C 6 F 5 ) 3 . Moreover,w ea lso showed weak Lewis acid-base adducts were stabilized by stoichiometric reduction. [26] This notion was also exploited by the Erker group in the isolation of Lewis acid stabilized radicals. [27] Herein, we probe the impact of reduction on the reactivity of the unstable (Ph 2 CN 2 )B(C 6 F 5 ) 3 , demonstrating that single electron transfer to diazomethaneborane adducts stabilizes weak B···N interactions providing reactive transient radicals which effect C À Hbond activation. A1 :1 combination of diphenyldiazomethane (Ph 2 CN 2 ) with B(C 6 F 5 ) 3 in chlorobenzene was stirred at À35 8 8C.
Ph 2 CN 2 was combined with BPh 3 in chlorobenzene at À35 8 8C. Monitoring the solution by multinuclear NMR spectroscopy revealed no evidence of adduct formation. This is consistent with the poor Lewis basicity of the diazomethane and the weaker Lewis acidity of the BPh 3 in comparison to B(C 6 F 5 ) 3 ,i nl ine with the computed free energies (see Supporting Information). Addition of Cp* 2 Cr to amixture of BPh 3 and diazomethane at À35 8 8Cgenerated an orange solution. The 11 BNMR spectrum showed resonances at 23.0 and À3.5 ppm consistent with the formation of two products, 3 and 4 which were isolated by fractional crystallization. An X-ray diffraction study revealed species 3 to be [Cp* 2 Cr][PhC(C 6 H 4 )NNBPh 3 ]( Scheme 2, Figure 2a). While the cation was typical, the anion of 3 was shown to be aborate with as ubstituent derived from the cyclization of the N 2 fragment onto the ortho position of one of the aryl rings on the diazomethane carbon. Ther esulting five membered ring which is fused to the aryl ring is 1,3-disubstituted with and phenyl ring on carbon and BPh 3 bound to nitrogen. The resulting N À Bbond is 1.566(6) ,while the N À Nand new N À Scheme 1. Interactions of main group systemsw ith N 2 -fragments.  Cb ond distances are determined to be 1.312(5) and 1.351-(5) .T he second product 4 was also characterized crystallographically revealing its formulation as [Cp* 2 Cr] [Ph 2 CNNHBPh 3 ]( Scheme 2, Figure 2b). TheB À Na nd N À N distances in the anion of 4 were determined to be 1.562(4) and 1.322(3) ,r espectively.
In contrast, the reaction of 9-diazofluorene ((C 12 H 8 )CN 2 ) with B(C 6 F 5 ) 3 did not form an adduct but led to loss of N 2 (Scheme 3) and the formation of the carboboration product (C 12 H 8 )C(C 6 F 5 )(B(C 6 F 5 ) 2 )asconfirmed spectroscopically and crystallographically (see Supporting Information). This is consistent with observations seen for less sterically encumbered diazomethanes. [23,30] However,monitoring the reaction of (C 12 H 8 )CN 2 with BPh 3 and Cp* 2 Cr by 11  Themechanism of these reactions were probed by density functional theory (DFT) calculations at the PW6B95-D3/def2-QZVP + COSMO-RS// TPSS-D3/def2-TZVP + COSMO level of theory in chlorobenzene solution. [31] Thereaction of Ph 2 CN 2 with BPh 3 and Cp* 2 Cr is initiated by single electron transfer from Cp* 2 Cr to the unstable Ph 2 CN 2 ·BPh 3 adduct (Figure 4) affording the radical anion [Ph 2 CNNHBPh 3 ]C À INT1 (spin on N next to B: 0.53e)i s4 .3 kcal mol À1 endergonic.Alternative pathways involving electron transfers to separated BPh 3 and Ph 2 CN 2 species (14.8 and 8.9 kcal mol À1 )a re significantly less favorable.S imilarly,f urther electron transfer to INT1 is unlikely (12.2 kcal mol À1 endergonic). TheN -centered radical INT1 may then add intramolecularly to the ortho position of ap henyl ring (via transition structure TS1)t og ive INT2 affording delocalization of the spin onto the ring. From here,ahighly exergonic H-transfer to another Ph 2 CN 2 molecule (via TS2)g ives the anion of 3 and the neutral N-radical (Ph 2 CNNH)C (spin on N next to H: 0.54e)w ith am oderate overall barrier of 23.5 kcal mol À1 .T he latter radical is readily reduced by Cp* 2 Cr through electron transfer and trapped by BPh 3 giving the anion of 4 (À66.4 kcal mol À1 ).
Fort he reaction of Ph 2 CN 2 with the stronger Lewis acid B(C 6 F 5 ) 3 and the more reductive Cp* 2 Co, [32] electron transfer from Cp* 2 Co to the reversible adduct (Ph 2 CNN)B(C 6 F 5 ) 3 is À15.3 kcal mol À1 exergonic affording the radical anion [Ph 2 CN 2 B(C 6 F 5 ) 3 ] ·À .T his intermediate is computed to effect H-atom from one methyl group of Cp* 2 Co over ab arrier of 16.6 kcal mol À1 to form the stable anion of 1 (see Supporting Information). Thea lternative intramolecular pathway,a nalogous to that above encounters ah igher overall barrier of 23.3 kcal mol À1 (see Supporting Information).
To garner further support for the computed mechanism, efforts to observe the transient radical adducts in the reactions were undertaken, but were unsuccessful. However, monitoring the reaction of (C 12 H 8 )CN 2 ,Cp* 2 Fe and Al(C 6 F 5 ) 3 in C 6 H 5 Cl at room temperature by EPR spectroscopy revealed ap entet resonance at g(iso) = 2.0039, with ( 14 N) hyperfine couplings of 3.70 Ga nd 3.58 G. This signal was similar to the related N-based radicals [33] and was attributed to the radical species [Ph 2 CN 2 Al(C 6 F 5 ) 3 ]C À .T his signal slowly degrades at room temperature over 5h,l eaving ab road resonance attributed to an organic radical (see Supporting Information). Subsequent addition of Ph 3 SnH generated am ixture of products,f rom which [Cp* 2 Fe]-[(C 12 H 8 )CNNHAl(C 6 F 5 ) 3 ] 8 was identified by NMR spectroscopy while single crystals of [Cp* 2 Fe][(C 12 H 8 )CHAl(C 6 F 5 ) 3 ] 9 were obtained from the reaction mixture ( Figure 5). Com-
In conclusion, we have demonstrated that single electron transfer to unstable diazomethane-borane adducts,a ccesses reactive radical anions that effect H-atom abstraction from CÀHb onds.T he resulting anionic species are significantly more stable than the corresponding neutral adducts,suggesting that in situ reductions may be auseful strategy to infer the presence of weakly bound adducts.W es uggest that this strategy could be exploited in developing main group-N 2 chemistry.A tt he same time,t he potential utility of main group radical anions in C À Hb ond homolysis offers an interesting prospect for CÀHf unctionalization.