Formation of Boron–Main-Group Element Bonds by Reactions with a Tricoordinate Organoboron L2PhB: (L = Oxazol-2-ylidene)

The
reactivity of L2PhB: (1; L = oxazol-2-ylidene)
as well as its transition-metal (chromium and iron) complexes toward
main-group substrates have been systematically examined, which led
to the construction of B–E (E = C, Ga, Cl, H, F, N) bonds.
The combination of 1 and triethylborane smoothly captured
carbon dioxide concomitant with the formation of B–C and B–O
bonds. The soft basic boron center in 1 readily reacted
with soft acidic gallium trichloride (GaCl3) to afford
the extremely stable adduct 4 involving a B–Ga
dative bond. Electrophilic alkylation of a neutral tricoordinate organoboron
was first achieved by the treatment of 1 with dichloromethane
and methyl trifluoromethanesulfonate (MeOTf), both of which afforded
ionic species featuring an additional B–C bond. Comparatively,
redox reactions took place when halides of heavier elements such as
germanium dichloride, dichlorophenylphosphine, and chlorodiphenylbismuth
were employed as substrates, from which cationic species 7 bearing a B–Cl bond was obtained. In addition, reactions
of metal complexes [2, Cr­(1)­(CO)5; 8, Fe­(1)­(CO)4] with cationic
electrophiles were investigated. With HOTf and FN­(SO2Ph)2, the corresponding ionic species featuring a B–H bond
(9) and a B–F bond (10) were formed
via a formal electrophilic substitution reaction, whereas the reaction
of 1 with F·Py-BF4 resulted in the formation
of a dicationic boron species 11 with a newly formed
B–N bond.

L2PhB的活性及其与主族底物（如碳、镓、氯、氢、氟、氮）形成的过渡金属（铬和铁）络合物的反应性已得到系统研究，从而构建了B-E（E代表C、Ga、Cl、H、F、N）键。1与三乙硼烷的结合顺利捕获二氧化碳，同时形成B-C和B-O键。1中的软碱性硼中心易于与软酸性三氯化镓（GaCl3）反应，生成含有B-Ga配位键的极为稳定的加合物4。首次通过1与二氯甲烷和甲基三氟甲磺酸酯（MeOTf）的处理实现了中性三配位有机硼的亲电烷基化，两者均提供了具有额外B-C键的离子物种。相比之下，当使用重元素如二氯化锗、二氯化苯基磷和氯二苯基铋的卤化物作为底物时，发生了氧化还原反应，从中获得了带有B-Cl键的阳离子物种7。此外，对金属络合物[2, Cr-（1）-（CO）5；8，Fe-（1）-（CO）4]与阳离子亲电试剂的反应进行了研究。使用HOTf和FN-（SO2Ph）2，通过正式的亲电取代反应形成了具有B-H键（9）和B-F键（10）的相应离子物种，而1与F·Py-BF4的反应则导致形成了一种带有新形成的B-N键的阳离子硼物种11。