Table 3.

Reactivity of catalytic borenium sources with cyclopentadiene and selected dienophiles

entry	Catalyst	mol%	Dienophile	conc	T	time	Yield	eea	Ref
1	38/39	6	methacrolein	0.2Mb	−95 °C	1h	99%	91%	31
2	168/169	6	methacrolein	0.2Mb	−95 °C	1h	97%	96%	31
3	168/169	20	di-Et fumarate	0.2Mc	−35 °C	1.5h	99%	98%	73
4	168/169	20	cyclopentenone	0.2Mc	−20 °C	14h	99%	92%	73
5	155/156	20	di-Et Fumarate	0.2Md	−60 °C	2h	99%	99%	72
6	155/156	20	di-Me-quinone	0.2Md	−95 °C	2h	99%	>99%	72
7	161/162	10	di-Et fumarate	0.25Mc	−60 °C	8h	99%	96%	74
8	161/162	10	cyclopentenone	0.25Mc	−50 °C	8h	99%	92%	74
9	161/162	10	di-Me-quinone	0.2Mc	−78 °C	1h	97%	98%	74
10	163	4	methacrolein	0.5Mc	−78 °C	2h	99%	93%	75
11	163	4	cyclopentenone	2.0Mc	−40 °C	1h	95%	92%	75
12	163	4	di-Et fumarate	2.0Mc	−78 °C	6h	95%	98%	75
13	163	4	di-Me-quinone	0.2Mc	−78 °C	0.5h	99%	98%	75

^aee’s refer to major cycloadduct

^b10:1 diene:dienophile

^c5:1 diene:dienophile

^ddiene:dienophile not given; a ratio of 1.1:1 is given for a related reaction.