Windows thoroughly confused about this issue.
Fix was to go to a machine with the assemblies and just copy the 100MB over.
Directory
c:\program files(x86)\reference assemblie\microsoft framework\.NETFramework\v4.5\
ack to stack overflow: https://stackoverflow.com/questions/17220615/where-can-i-download-the-net-4-5-multitargeting-pack-for-my-build-server
4/2019: found an alternative, with VS 2017 installer you can right click on the right hand side components and hand pick the various .NET components you want to install. what a mess!
--adam